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Soil nematode community composition and stability under different nitrogen additions in a semiarid grassland
Journal Pre-proof Soil nematode community composition and stability under different nitrogen additions in a semiarid grassland Siwei Liang, Xinchang Kou, Yingbin Li, Xiaotao Lü, Jingkuan Wang, Qi Li PII:
S2351-9894(19)30757-7
DOI:
https://doi.org/10.1016/j.gecco.2020.e00965
Reference:
GECCO 965
To appear in:
Global Ecology and Conservation
Received Date: 11 November 2019 Revised Date:
11 February 2020
Accepted Date: 11 February 2020
Please cite this article as: Liang, S., Kou, X., Li, Y., Lü, X., Wang, J., Li, Q., Soil nematode community composition and stability under different nitrogen additions in a semiarid grassland, Global Ecology and Conservation (2020), doi: https://doi.org/10.1016/j.gecco.2020.e00965. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
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Type of contribution: Research paper
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Date of preparation: Feb 11, 2019
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Number of text pages: 25
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Number of tables: 2
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Number of figures: 6
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Soil nematode community composition and stability under different
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nitrogen additions in a semiarid grassland
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Siwei Liang a, b, 1, Xinchang Kou b, c, 1, *, Yingbin Li b, Xiaotao Lü b, Jingkuan Wang a,
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*
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a
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110866, China
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b
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China
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c
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China
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*Corresponding authors: Prof. Jingkuan Wang and Dr. Xinchang Kou
, Qi Li b College of Land and Environment, Shenyang Agricultural University, Shenyang
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016,
School of Geographical Sciences, Northeast Normal University, Changchun 130024,
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Institute of Applied Ecology,
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Chinese Academy of Sciences
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P. O. Box 417, Shenyang 110016, China
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Tel.: +86-24-83970359; Fax: +86-24-83970300
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E-mail address: [email protected]; [email protected]
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1
These authors contributed equally to this work.
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Abstract
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Anthropogenic input of reactive nitrogen (N) is an environmental problem that
26
threatens the diversity and stability of belowground ecosystems. Soil nematodes are
27
abundant in soil and are occurring at multiple trophic levels in the soil food web.
28
However, how N deposition affects the composition and stability of soil nematode
29
community is largely unknown. Here, we investigated the response of soil nematode
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community composition to N deposition at different sampling seasons and also
31
estimated the stability of nematode community in a semiarid grassland in northern
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China. We found that the addition of N not only reduced the diversity of the nematode
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community, but also reduced the temporal stability of nematode community. The
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stability of different nematode trophic groups had different responses to N addition,
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and the community of plant parasites was more stable than the other trophic groups at
36
a relatively higher N addition level. Moreover, soil pH was closely correlated with the
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stability of bacterivores, fungivores and predators-omnivores and the diversity of
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nematode community under different N additions. Our results highlight that N
39
addition indirectly influence the synchrony and the stability of nematode community
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through change in soil nematode abundance and richness, and the variations of
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nematode community stability under different N additions are closely related to soil
42
pH. These changes in nematode community composition and stability will eventually
43
influence soil ecosystem function and nutrient cycling through biotic interactions in
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the soil food web. 2
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Keywords: N addition; Nematode community; Diversity; Stability; Semiarid grassland
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1. Introduction
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Anthropogenic reactive nitrogen (N) deposition has been increased three- to five-folds
49
over the past century (Galloway et al., 2008; Sutton et al., 2014). Nitrogen deposition
50
can relieve N limitation and it is beneficial for plant growth (Quinn Thomas et al.,
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2009). However, the continuous increase of N deposition is an environmental problem
52
that leads to biodiversity loss and reduction in ecosystem stability (Bai et al., 2010).
53
While an increasing number of studies have focused on the responses of aboveground
54
plant communities to N deposition (Stevens et al., 2004; Simkin et al., 2016; Yang et
55
al., 2019; Hou et al., 2019), more and more attentions have been paid to the
56
belowground subsystems in recent years. For example, Wang et al. (2018), in a
57
meta-analysis, found that microbial diversity showed a robust decrease under N
58
addition and this decrease was associated with the decline in microbial biomass.
59
Changes in microbial biomass could also affect microbial predators, such as
60
microbivorous nematodes. Nitrogen deposition was also reported to reduce the
61
nematode generic richness, and alter community structure by promoting the
62
abundance
63
predators-omnivores (Lokupitiya et al., 2000; Song et al., 2016). However, Chen et al.
64
(2015) found that N-enrichment (17.5, and 28.0 g of N m-2 yr-1) induced soil
65
acidification, reduced concentrations of mineral cations and nematode food resources,
of
bacterivores
and
suppressing
3
that
of
fungivores
and
66
decreased the abundance of bacterivores, fungivores and omnivores in a semiarid
67
grassland. In addition, Shao et al. (2018), in a two-years field experiment, found that
68
N-addition (6 g N m−2 yr−1) significantly decreased nematode richness and
69
Shannon-Wiener diversity in a forest ecosystem. However, the influences of
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N-addition on belowground subsystems still lack consistent conclusions, and highly
71
depend on N addition levels, experimental durations and ecosystem types. Further, N
72
deposition also influence the biodiversity of soil biota by changing plant community
73
composition, soil conditions and N availability, and then influence the structure and
74
stability of soil food web through biotic interactions between above- and belowground
75
communities (de Ruiter et al., 1998; Ferris et al., 2001; Okada and Harada, 2007;
76
Bardgett and Wardle, 2010; Chen et al., 2015).
77
Previous studies investigating the effects of N deposition on the biotic community
78
were mostly in single-time sampling (Parfitt et al., 2012; Sun et al., 2016). Temporal
79
stability measures the degree of constancy in a variable relative to its mean over a
80
time period (Lehman and Tilman, 2000). At present, researches on community
81
stability mostly focus on aboveground vegetation, and relatively few attentions are
82
paid on belowground biota. Stability of biotic community has fundamental
83
importance for understanding the dynamics of belowground communities under
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temporal fluctuations (Yeats, 2003; Viketoft et al., 2011). The stabilities of
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communities depend upon the interspecific competition and compensation, and are
86
mainly influenced by the changes of environmental conditions (Thibaut and Connolly, 4
87
2013), for example, precipitation, temperature and nutrient availability (Griffiths and
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Caul, 1993; Sarathchandra et al., 2001; Landesman et al., 2011; Chen et al., 2015).
89
Moreover, due to the different niches and life histories, changes in the stability within
90
a community will also vary with different trophic levels (Chesson, 2000; Wagg et al.,
91
2018). These complicated influences make it critical to understand the mechanisms
92
that underpin the structure, functioning and stability of biotic community (Tilman et
93
al., 2006; Ives and Carpenter, 2007).
94
Since soil nematodes are abundant in soil and occur at multiple trophic levels in the
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soil food web, we investigated the temporal variations of nematode community
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composition and community stability after long-term N deposition. We collected soil
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samples at different seasons across 2014 and 2015 after six-years of N addition. We
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hypothesized that (i) the abundance and diversity of nematode community decrease
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with the increase of N addition levels; (ii) nematode trophic groups with different life
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history strategies, such as bacterivores and predators-omnivores, will have different
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responses to N addition levels, and these changes will eventually change the stability
102
of nematode communities.
103 104
2. Materials and methods
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2.1 Site description and experimental design
5
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Soil samples were collected from a six-year N addition experiment site conducted in a
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natural steppe ecosystem near the Inner Mongolia Grassland Ecosystem Research
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Station (IMGERS, 116˚14ˈE, 43˚13ˈN) of the Chinese Academy of Sciences. The
109
mean annual temperature and precipitation are 0.9°C and 355 mm, respectively. The
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rainfall is mainly in summer, with 70% falling between May and August (Huang et al.,
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2009). Soil is classified as Haplic Calcisol according to the FAO (Food and
112
Agriculture Organization of the United Nations). The zonal vegetation is typical
113
grassland, with dominant species Leymus chinensis and Stipa grandis accounting for
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more than 60% of the total aboveground biomass in the plant community. No
115
fertilizer was applied before starting the experiment. The ambient atmospheric total N
116
deposition was less than 1.5 g N m−2 yr−1 (Lue and Tian, 2007).
117
The long-term N addition experiment was established in September 2008 (Zhang et
118
al., 2014). There were nine N addition levels (0, 1, 2, 3, 5, 10, 15, 20, 50 g N m−2 yr−1)
119
applied at two frequencies (2 times and 12 times per year). The experimental site was
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designed as a randomized block design with 10 replicate blocks, and each block was
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45 m × 70 m. In each block, there were nine plots treated with nine N addition levels.
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The area of each plot was 8 m × 8 m, and there was 1 m walkway between each plot.
123
For the present study, six N addition levels (0, 2, 5, 10, 20, 50 g N m−2 yr−1, 2 times
124
per year) were selected, and six replicate blocks were randomly selected from 10
125
blocks. Therefore, the N addition treatments are expressed as N0, N2, N5, N10, N20 and
126
N50. We implemented wet and dry deposition to mirror the seasonal patterns of the 6
127
natural N deposition, and the adding method was described in detail by Li et al.
128
(2019). Briefly, in June, NH4NO3 was dissolved in purified water (9.0 L per plot; the
129
N0 treatment received only purified water) and evenly sprayed on the sample land
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with a sprayer to simulate wet deposition. In December, NH4NO3 was mixed with
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clean sand (0.5 kg sand per plot; the N0 treatment received only sand) and sprinkled
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evenly by hand to simulate dry deposition (Li et al., 2019).
133 134
2.2 Soil sampling and analysis
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Soil samples were collected from a depth of 0–10 cm (5 cores with 2.5 cm diameter)
136
and uniformly mixed as a composite sample for each replicate in August and October
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of 2014 and March and May of 2015, corresponding to the summer, autumn, winter,
138
and spring, respectively. The corresponding mean precipitations in each season were
139
66.5mm, 34.1mm, 3.4mm and 24.7mm, respectively, and the mean temperatures were
140
20.2°C, 4.3°C, -4.2°C and 11.1°C, respectively. In total, there were 144 samples (6
141
treatments × 6 replicates × 4 sampling seasons). The fresh soil samples were stored
142
individually in plastic bags and kept at 4°C in a refrigerator until analysis. Then soil
143
pH and soil moisture were measured using standard methods (Wei et al., 2013; Zhang
144
et al., 2014).
145 146
2.3 Soil nematode extraction and identification 7
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A total of 100 g of fresh soil each was used to extract soil nematodes using a modified
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cotton-wool filter method (Liang et al., 2009). Nematode abundance was expressed as
149
individuals per 100 g dry soil and at least 100 nematodes from each sample were
150
identified to genus level using the microscope, according to Bongers (1994), Ahmad
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and Jairjpuri (2010) and Li et al. (2017). If the total number of nematodes was less
152
than 100 individuals in a sample, all nematodes were identified. The nematodes were
153
divided into the following four trophic groups according to their feeding habits and
154
esophagus characteristics: bacterivores (BF), fungivores (FF), plant parasites (PP),
155
and predators- omnivores (OP) (Yeates, 2003).
156 157
2.4 Data analyses
158
Nematode generic richness (number of genera), species abundance (number of
159
nematode 100 g-1 dry soil) and Shannon-Wiener diversity (H′) were calculated for
160
each plot. Since nematode abundance was closely correlated with generic richness and
161
biomass of nematode community (Fig. A1), the destabilization in the abundances of
162
nematode likely reflected the stability in the nematode community. Stability of the
163
nematode community was defined as the ratio of the nematode mean abundance
164
across four sampling seasons (µ) to the temporal variation (σ) (Lehman and Tilman,
165
2000; Wagg et al., 2018). We defined synchrony as the average across genus of the
166
correlation between the abundance of each nematode genus and the total abundance of
167
all other genera in the nematode community (Gross et al., 2014). We also calculated 8
168
the average variation in individual taxa (population CV) as the weighted average CV
169
of taxa in a community by weighting the CV of taxa by its overall average abundance
170
(Gross et al., 2014; Wagg et al., 2018). Following equations were used to calculate the
171
indices:
172
′ = − ∑
173
ℎ
⁄ × ln
= µ / σ
= 1/
⁄
!
"# , ! #% ' %&
174
where, H′= Shannon-Wiener diversity, S = number of species, ni = number of each
175
individual genus identified in samples, N = total number of individuals identified in
176
samples, µ = temporal mean of nematode abundance, σ = temporal variation of
177
nematode abundance, Yi = the abundance of genus i in the nematode community of n
178
genera.
179
Data analysis was carried out using SPSS19 statistical software (SPSS Inc.,
180
Chicago, IL, USA). The repeated-measures analysis was performed to test how N
181
deposition level, sampling time and their interactions affected the soil pH, soil
182
moisture and nematode diversity. This analysis method accounts for the effects of
183
temporal autocorrelation and we used the “Mauchly test” to test whether the data
184
conformed to the analysis requirements. Multiple comparisons were based on a
185
Tukey’s HSD test if the main effect or their interactions were significant at P < 0.05
186
level. One-way analysis of variance (ANOVA) was used to test the differences in 9
187
nematode community stability among N addition levels. Principal component analysis
188
(PCA) was performed to explore soil biotic community composition based on the
189
nematode genus using CANOCO software, version 5.0 (ter Braak, 1988). We
190
constructed a structural equation model (SEM) for their relationship based on our
191
predictions and literature reviews (Grace 2006). Then the model was judged based on
192
the χ2 value, degrees of freedom (df) and modification indices. The Amos 17.0
193
software was used for SEM analysis (Arbuckle 2006).
194 195
3. Results
196
3.1 Changes in precipitation, temperature and soil properties
197
The interactions between N addition and sampling time had significant effects on soil
198
pH and soil moisture. Soil pH showed a decreasing trend with increasing N addition
199
level, and the values were higher in the N0, N2 and N5 treatments than those in the N10,
200
N20 and N50 treatments (P < 0.01). Among different sampling times, soil pH showed a
201
decreasing trend, and all treatments reached their lowest value in May (Fig. 1). Higher
202
values of soil moisture were found in August and October than those in March and
203
May.
204 205
3.2 Soil nematode community composition, synchrony and diversity
10
206
The N addition levels, sampling times and their interactions had significant effects on
207
total nematode abundance, richness and Shannon-Wiener diversity (P < 0.01) (Fig 2).
208
The total abundance, richness and Shannon-Wiener diversity of soil nematodes
209
showed a decreasing trend after N5, and sharply declined at N50, and were
210
significantly lower in March than in other three sampling seasons (Fig. 2). Nematode
211
community compositions in high and low N addition levels were clearly separated in
212
the principal component analysis (PCA) along the axis 1 (PC1, 46.17%), and the
213
different sampling seasons were separated by the axis 2 (PC2, 20.74%) (Fig. 3). Soil
214
nematode synchrony was significantly influenced by N addition with the highest and
215
the lowest values being found in N50 treatment and the N0 treatment, respectively (Fig.
216
4).
217
Nitrogen addition had a significant effect on different nematode trophic groups (P
218
< 0.01) (Table 1). The abundance of different trophic groups showed a gradually
219
decreasing trend with increasing N addition level, and the lowest values were
220
observed in the N50 treatment (Table 2). There was a significant effect of sampling
221
time on the abundance and richness of different nematode trophic groups (P < 0.01)
222
(Table 1), with the highest values observed in October (autumn).
223 224
3.3 Soil nematode community stability and their relations with soil properties
225
The stability of nematode community was about the same among different N addition
226
treatments, but the variability increased until N50. No significant differences were 11
227
observed in the stability of the plant parasites among different N addition levels.
228
However, the stabilities of the bacterivores were significantly higher in the N0 and N20
229
treatments than that in the N50 treatment (P < 0.05), and those of predators-omnivores
230
were significantly higher in the N0 and N5 treatments than that in the N50 treatment (P
231
< 0.05) (Fig. 4). Soil nematode stability was positively correlated with nematode
232
generic richness and Shannon-Wiener diversity (P < 0.05). For different nematode
233
trophic groups, the stabilities of microbivores and predators-omnivores were
234
positively correlated with soil pH (Fig. 5). No significant relationship was found
235
between the stability of nematode trophic groups and soil moisture (Fig. 5).
236
The SEM provided a good fit to reveal how the temporal variation of nematode
237
community was influenced by N addition (χ2 = 3.682, df = 4, P = 0.451, GFI = 0.966).
238
Specifically, temporal variation in nematode abundance (σ) was positively related to
239
the temporal mean in nematode abundance (µ), followed by the population synchrony
240
(η). The nematode richness was negatively associated with the population CV and
241
population synchrony. Nitrogen addition indirectly increased the temporal variation in
242
nematode abundance by reducing the richness and abundance of the nematode (Fig.
243
6).
244 245
4. Discussions
246
4.1 Responses of soil nematode community composition to N addition levels varied in
247
different sampling seasons. 12
248
Consistent with our first hypothesis, we found that nematode diversity decreased with
249
increasing N addition level after six years of N addition, and unsurprisingly, the
250
higher the N addition, the greater the impact on the diversity of nematode community
251
was (Sarathchandra et al., 2001; Chen et al., 2015; Shao et al., 2018). However, in our
252
study, soil nematode diversity showed highly temporal variation, with the lowest
253
value being observed in March (spring). This trend was consistent with the changes in
254
precipitation and temperature. As we know, soil nematodes rely on water to move to
255
their prey (Griffiths and Caul, 1993; Yeates et al., 2009), so their abundance may
256
decline with the decreasing precipitation (Landesman et al., 2011). Moreover,
257
although nematode can survive in the winter by shutting down metabolism into a
258
cryptobiotic state or altering biochemical pathways (Mcsorley, 2003; Yeates et al.,
259
2009; Orgiazzi et al., 2016), the environmental conditions in winter increased the loss
260
of nematode species.
261
Since aboveground plants were senesced at the end of September, it might have
262
resulted in the decrease of food resources for plant parasites, and then led to the lower
263
abundance in the other sampling seasons. Although the aboveground plants have
264
already senesced at the end of September, the rhizosphere priming effect might still
265
exist, providing nutrient resources for soil microorganisms and microbivorous
266
nematodes over a period of time (Cheng, 2009; Kuzyakov, 2010), and then the
267
abundance and richness of microbivorous nematode increased until October.
268
Furthermore, in October, the senescence of plants could also reduce the competition 13
269
of nutrients between plant and microbivorous nematodes (Alphei et al., 1996), which
270
also promoted the abundance and richness of microbivores.
271
Previous studies found that nematode trophic groups had different responses to N
272
addition (Lokupitiya et al., 2000; Chen et al., 2015; Song et al., 2016), and these
273
changes might also influence the nematode community composition. Our study also
274
showed that N addition level suppressed the abundance of different nematode trophic
275
groups (Table 2). Since N enrichment negatively affected the total microbial
276
abundance and the biomass of microbial community components (LeBauer and
277
Treseder, 2008), which may lead to the decline in nematode abundance through
278
bottom-up effects (Ingham et al., 1985; Wardle et al., 2004).
279 280
4.2 Relationship between soil nematode stability, diversity and soil properties.
281
Stability is as important as diversity in affecting community productivity and has a
282
multiplicity of meanings in ecology (Lehman and Tilman, 2000; Gross et al., 2014).
283
Our results showed that the stability of the nematode community varied with N
284
addition levels. At a low N addition level, the stability of the nematode community
285
will be decreased due to the competition for N availability with other soil organisms,
286
especially for the fauna with rapid transformation and utilization of N (Pfisterer and
287
Schmid, 2002). Then, with the increasing of N addition levels, sufficient N was
288
provided for the soil organisms, and the stability of nematode community showed a
289
gradually increasing trend. Finally, when the N addition level reached 50 g N m−2 yr−1, 14
290
the increased availability of N has increased the ammonium and aluminum toxicity
291
(Wei et al., 2013). Hence, the stability of nematode community decreased again. The
292
variation of synchrony in nematode community also evidenced that low N addition
293
level increased the degree of synchrony in population fluctuations in nematode
294
community, thus contributed to the decrease of community stability.
295
We found that the positive richness–stability relationship in the nematode
296
community was largely explained through the negative association between richness
297
and the population synchrony, and this result probably suggested that the increase in
298
richness should decrease population synchrony, thereby preserving the stability of the
299
nematode community (van Klink et al., 2019). Additionally, the N addition has no
300
detectable direct effect on nematode community stability, however, N addition
301
simultaneously reduced the nematode richness and abundance, and then, had a
302
negative effect on the stability of nematode community. These results provide further
303
support for the hypothesis that greater richness provides greater insurance that some
304
nematode genus will benefit over others through environmental variations in a
305
compensatory manner so that the stability of the nematode community is maintained.
306
Previous studies proposed that this compensatory dynamic was irrelevant to the
307
changes of abundance (Lehman and Tilman., 2000; Loreau, 2010; Hallett et al., 2014).
308
For instance, although N addition would restrain the survival ability of some
309
nematodes and decrease their abundance, other surviving nematodes could provide a
310
proximate function for community stabilization (Gonzales and Loreau, 2009). 15
311
However, our results suggested that the contribution of abundance to the stability of
312
nematode community should not be ignored, especially in a N limitation condition.
313
Although N addition has no detectable effect on synchrony in nematode populations,
314
a negative association between richness and the population synchrony, and a positive
315
association between population synchrony and community variation were observed.
316
These results indicated that the variation of synchrony was indirectly influenced by N
317
addition and was associated with the stability of the nematode community (i.e.
318
increase richness, less synchrony, more stable).
319
Our study also in line with previous studies that the soil environment has a great
320
influence on the stability of nematode communities (Andres et al., 2016; Zhang et al.,
321
2015). Positive correlations were found between soil pH and the stability of
322
fungivores and predators-omnivores. Since fungivores and predators-omnivores were
323
particularly susceptible to disturbance (Freckman and Caswell, 1985; Yeates et al.,
324
2009; Zhang et al., 2013), their stability changed with the variation of soil pH, and
325
then affected the stability of nematode community. In this study, we designed the N
326
addition experiment with a large gradient (0–50 g N m-2 yr-1), and the highest N
327
addition treatment exerted some effects on the whole pattern of nematode. Even
328
though the rate of 50 g N m-2 yr-1 was relatively high considering the background
329
value, we assumed that with rapid global change because of N deposition and climate
330
change, it becomes increasingly important to develop strategies under a high N
16
331
deposition condition to predict the effects of future global change on ecosystem
332
functioning and services.
333 334
5. Conclusions
335
Here, we assessed the diversity-stability relationships in soil nematode community
336
under different N addition levels. Our results suggested that N addition indirectly
337
alters the temporal stability of nematode community by reducing the richness and
338
abundance of nematodes. The variations of nematode trophic groups under different N
339
additions were closely related to soil pH. These changes in different nematode trophic
340
groups will eventually change the structure and stability of the soil food web and then
341
influence soil ecosystem function and nutrient cycling through biotic interactions.
342 343
Acknowledgements
344
This research was supported by the Strategic Priority Research Program of the
345
Chinese Academy of Sciences (XDB15010402), National Science & Technology
346
Fundamental Resources Investigation Program of China (2018FY100304), and the
347
National Natural Science Foundation of China (31570519 and 41877047). We are
348
grateful to the Inner Mongolia Grassland Ecosystem Research Station (IMGERS) of
349
the Chinese Academy of Sciences for providing the experimental sites. We also
17
350
would like to thank Prof. MD Mahamood for proofing the English throughout the
351
manuscript.
352 353
Compliance with ethical standards
354
Conflict of interest Authors declare no conflicts of interest.
355 356
References
357
Ahmad, W., Jairjpuri, M.S., 2010. Mononchida: The Predaceous Nematodes. Nematology Monographs
358
and Perspectives. Leiden, Netherlands.
359
Alphei, J., Bonkowski, M., Scheu, S., 1996. Protozoa, nematode and Lumbricidae in the rhizosphere of
360
Hordelymus europaeus (Poaceae): faunal interactions, response of microorganisms and effects on
361
plant growth. Oecologia 106, 111-126.
362
Andrés, P., Moore, J.C., Simpson, R.T., Selby, G., Cotrufo, F., Denef, K., Haddix, M.L., Shaw, E.A.,
363
de Tomasel, C.M., Molowny-Horas, R., Wall, D.H., 2016. Soil food web stability in response to
364
grazing in a semi-arid prairie: the importance of soil textural heterogeneity. Soil Biol. Biochem.
365
97, 131-143.
366
Arbuckle, J.L., 2006. Amos (Version 7.0) [Computer Program]. SPSS, Chicago.
18
367
Bai, Y.F., Wu, J.G., Clark, C.M., Naeem, S., Pan, Q.M., Huang, J.H., Zhang, L.X., Han, X.G., 2010.
368
Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem
369
functioning: evidence from Inner Mongolia grasslands. Global Change Biol. 16, 358-372.
370 371 372 373
Bardgett, R.D., Wardle, D.A., 2010. Aboveground-belowground Linkages: Biotic Interactions, Ecosystem Processes, and Global Change. Oxford University Press, Oxford, UK.
Bongers, T., 1994. De Nematoden van Nederland. In: Vormgeving en technische realisatie. Uitgeverij Pirola, Schoorl, Netherlands.
374
Chen, D.M., Lan, Z.C., Hu, S.J., Bai, Y.F., 2015. Effects of nitrogen enrichment on belowground
375
communities in grassland: relative role of soil nitrogen availability vs. soil acidification. Soil Biol.
376
Biochem. 89, 99-108.
377 378 379 380 381 382 383 384 385 386
Cheng, W.X., 2009. Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C-N budgets. Soil Biol. Biochem. 41, 1795-1801.
Chesson, P., 2000. Mechanisms of maintenance of species diversity. Annu. Rev. Ecol. Evol. S. 31, 343-366.
de Ruiter, P.C., Neutel, A.M., Moore, J.C., 1998. Biodiversity in soil ecosystems: the role of energy flow and community stability. Appl. Soil Ecol. 10, 217-228.
Ferris, H., Bongers, T., Goede, R.G.M.D., 2001. A framework for soil food web diagnostics: extension of the nematode faunal analysis concept. Appl. Soil Ecol. 18, 0-29.
Freckman, D.W., Caswell, E.P., 1985. The ecology of nematodes in agroecosystems. Annu. Rev. Phytopathol. 23, 275-296.
19
387
Galloway, J.N., Townsend, A.R., Erisman, J.W., Bekunda, M., Cai, Z., Freney, J.R., Martinelli, L.A.,
388
Seitzinger, S.P., Sutton, M.A., 2008. Transformation of the nitrogen cycle: recent trends,
389
questions, and potential solutions. Science 320, 889-892.
390 391 392 393 394 395
Gonzalez, A., Loreau, M., 2009. The causes and consequences of compensatory dynamics in ecological communities. Annu. Rev. Ecol. Evol. S. 40, 393-414.
Grace, J.B., 2006. Structural Equation Modeling and Natural Systems. Cambridge University Press, Cambridge, pp. 1-365.
Griffiths, B.S., Caul, S., 1993. Migration of bacterial-feeding nematodes, but not protozoa, to decomposing grass residues. Biol. Fert. Soils 15, 201-207.
396
Gross, K., Cardinale, B.J., Fox, J.W., Gonzalez, A., Loreau, M., Polley, H.W., Reich, P.B., van Ruijven,
397
J., 2014. Species richness and the temporal stability of biomass production: a new analysis of
398
recent biodiversity experiments. Am. Nat. 183, 1-12.
399
Hallett, L.M., Hsu, J.S., Cleland, E.E., Collins, S.L., Dickson, T.L., Farrer, E.C., Gherardi, L.A., Gross,
400
K.L., Hobbs, R.J., Turnbull, L., Suding, K.N., 2014. Biotic mechanisms of community stability
401
shift along a precipitation gradient. Ecology 95, 1693-1700.
402
Hou, S.L., Lü, X.T., Yin, J.X., Yang, J.J., Hu, Y.Y., Wei, H.W., Zhang, Z.W., Yang, G.J., Liu, Z.Y.,
403
Han, X.G., 2019. The relative contributions of intra- and inter-specific variation in driving
404
community stoichiometric responses to nitrogen deposition and mowing in a grassland. Sci. Total
405
Environ. 666, 887-893.
20
406 407
Huang, J., Bai, Y., Jiang, Y., 2009. Case Study 3: Xilingol Grassland, Inner Mongolia. Rangeland degradation and recovery in China’s pastoral lands. CBA Internationsl. Oxfordshire, UK.
408
Ingham, R.E., Trofymow, J.A., Ingham, E.R., Coleman, D.C., 1985. Interactions of bacteria, fungi, and
409
their nematode grazers e effects on nutrient cycling and plant-growth. Ecol. Monogr. 55, 119-140.
410
Ives, A.R., Carpenter, S.R., 2007. Stability and diversity of ecosystems. Science 317, 58-62.
411
Kuzyakov, Y., 2010. Priming effects: Interactions between living and dead organic matter. Soil Biol.
412 413 414 415 416 417 418 419 420
Biochem. 42, 1363-1371.
Landesman, W.J., Treonis, A.M., Dighton, J., 2011. Effects of a one-year rainfall manipulation on soil nematode abundances and community composition. Pedobiologia 54, 87-91.
LeBauer, D.S., Treseder, K.K., 2008. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89, 371-379.
Lehman, C.L., Tilman, D., 2000. Biodiversity, stability, and productivity in competitive communities. Am. Nat. 156, 534-552.
Li, Q., Liang, W.J., Zhang, X.K., Mohammad, M., 2017. Soil nematodes of grasslands in Northern China. Zhejiang University Press, Hangzhou; published by Elsevier Inc, London, UK.
421
Liang, W.J., Lou, Y.L., Li, Q., Zhong, S., Zhang, X.K., Wang, J.K., 2009. Nematode faunal response to
422
long-term application of nitrogen fertilizer and organic manure in Northeast China. Soil Biol.
423
Biochem. 41, 883-890.
21
424
Li, Y.B., Martijn, B.T., Yang, J.J., Lü, X.T., Li, X.Y., Liang, W.J., Han, X.G., Li, Q., 2019. Changes in
425
litter quality induced by N deposition alter soil microbial communities. Soil Biol. Biochem. 130,
426
33-42.
427 428
Lokupitiya, E., Stanton, N. L., Seville, R. S., Snider, J.R., 2000. Effects of increased nitrogen deposition on soil nematodes in alpine tundra soils. Pedobiologia 44, 591-608.
429
Loreau, M., 2010. Stability and complexity of ecosystems: New perspectives on an old debate. In M.
430
Loreau (Ed.), From populations to ecosystems: Theoretical foundations for a new ecological
431
synthesis. Princeton, NJ: Princeton University Press, USA.
432 433
Lue, C., Tian, H., 2007. Spatial and temporal patterns of nitrogen deposition in China: synthesis of observational data. J. Geophys. Res-Atmos. 112, 2156-2202.
434
Mcsorley, R., 2003. Adaptations of nematodes to environmental extremes. Fla. Entomol. 86, 138-142.
435
Okada, H., Harada, H., 2007. Effects of tillage and fertilizer on nematode communities in a Japanese
436
soybean field. Appl. Soil Ecol. 35, 582-598.
437
Orgiazzi, A., Bardgett, R.D., Barrios, E., Behan-Pelletier, V., Briones, M.J.I., Chotte, J.L., De Deyn,
438
G.B., Eggleton, P., Fierer, N., Fraser, T., Hedlund, K., Jeffery, J., Johnson, N.C., Jones, A.,
439
Kandeler, E., Kaneko, N., Lavelle, P., Lemanceau, P., Miko, L., Montanarella, L., Moreira, F.M.S.,
440
Ramirez, K.S., Scheu, S., Singh, B.K., Six, J., van der Putten, W.H., Wall, D.H., 2016. Global Soil
441
Biodiversity Atlas. Publications Office of the European Union, European Union.
22
442
Parfitt, R.L., Couper, J., Parkinson, R., Schon, N. L., Stevenson, B. A., 2012. Effect of nitrogen
443
fertilizer on nitrogen pools and soil communities under grazed pastures. New Zeal. J. Agr. Res. 55,
444
1-17.
445 446 447 448
Pfisterer, A.B., Schmid, B., 2002. Diversity-dependent production can decrease the stability of ecosystem functioning. Nature 416, 84-86.
Quinn Thomas, R., Canham, C.D., Weathers, K.C., Goodale, C.L., 2009. Increased tree carbon storage in response to nitrogen deposition in the US. Nat. Geosci. 3, 13-17.
449
Sarathchandra, S.U., Ghani, A.A., Yeates, G.W., Burch, G., Cox, N. R., 2001. Effect of nitrogen and
450
phosphate fertilisers on microbial and nematode diversity in pasture soils. Soil Biol. Biochem. 33,
451
953-964.
452
Shao, Y.H., Liu, T., Eisenhauer, N., Zhang, W.X., Wang, X.L., Xiong, Y.M., Liang, C.F., Fu, S.L.,
453
2018. Plants mitigate detrimental nitrogen deposition effects on soil biodiversity. Soil Biol.
454
Biochem. 127, 178-186.
455
Simkin, S.M., Allen, E.B., Bowman, W.D., Clark, C.M., Belnap, J., Brooks, M.L., Cade, B.S., Collins,
456
S.L., Geiser, L.H., Gilliam, F.S., 2016. Conditional vulnerability of plant diversity to atmospheric
457
nitrogen deposition across the United States. PNAS. 113, 4086-4091.
458 459 460 461
Song, M., Li, X.M., Jing, S.S., Lei, L.J., Wang, J.L., Wan, S.Q., 2016. Responses of soil nematodes to water and nitrogen additions in an old-field grassland. Appl. Soil Ecol. 102, 53-60.
Stevens, C.J., Dise, N.B., Mountford, J.O., Gowing, D.J., 2004. Impact of nitrogen deposition on the species richness of grasslands. Science 303, 1876-1879.
23
462
Sun, F., Tariq, A., Chen, H., He, Q.J., Gu, Y.X., Pan, K.W., Chen, S.Y., Li, J.T., Zhao, C.C., Wang, H.,
463
Gu, Y.F., 2016. Effect of nitrogen and phosphorus application on agricultural soil food webs.
464
Arch.
Agron. Soil Sci. 63, 1176-1186.
465
Sutton, M.A., Mason, K.E., Sheppard, L.J., Sverdrup, H., Haeuber, R., Hicks, K., 2014. Nitrogen
466
Deposition, Critical Loads and Biodiversity, Springer Dordrecht Heidelberg Press, New York,
467
London.
468
ter Braak, C.J.F., 1988. CANOCO-A fortran program for canonical community ordination by (partial)
469
(detrended) (canonical) correspondence analysis, principal components analysis and redundancy
470
analysis (version2.1). Technical Report LWA-88-02. Agricultural Mathematics Group,
471
Wageningen, Netherland.
472 473 474 475 476 477
Thibaut, L.M., Connolly, S.R., 2013. Understanding diversity-stability relationships: Towards a unified model of portfolio effects. Ecol. Lett. 16, 140-150.
Tilman, D., Reich, P.B. Knops, J.M.H., 2006. Biodiversity and ecosystem stability in a decade-long grassland experiment. Nature 441, 629-632.
van Klink, R., Lepš, J., Vermeulen, R., de Bello, F., 2019. Functional differences stabilize beetle communities by weakening interspecific temporal synchrony. Ecology 100, e02748.
478
Viketoft, M., Sohlenius, B., Boström, S., Palmborg, C., Bengtsson, J., Berg, M.P., Danell, K.H., 2011.
479
Temporal dynamics of soil nematode communities in a grassland plant diversity experiment. Soil
480
Biol. Biochem. 43, 1063-1070.
24
481 482 483 484 485 486
Wang, C., Liu, D., Bai, E., 2018. Decreasing soil microbial diversity is associated with decreasing microbial biomass under nitrogen addition. Soil Biol. Biochem. 120, 126-133.
Wardle, D.A., Bardgett, R.D., Klironomos, J.N., Setala, H., van der Putten, W.H., Wall, D.H., 2004. Ecological linkages between aboveground and belowground biota. Science 304, 1629-1633.
Wagg, C., Dudenhoffer, J.H., Widmer, F., van der Heijden, M.G.N., 2018. Linking diversity, synchrony and stability in soil microbial communities. Funct. Ecol. 32, 1280-1292.
487
Wei, C.Z., Zheng, H.F., Li, Q., Lü, X.T., Yu, Q., Zhang, H.Y., Chen, Q.S., He, N.P., Kardol, P., Liang,
488
W.J., Han, X.G., 2012. Nitrogen addition regulates soil nematode community composition
489
through ammonium suppression. PLoS One 7, e43384.
490
Wei, C.Z., Yu, Q., Bai, E., Lü, X.T., Li, Q., Xia, J.Y., Kardol, P., Liang, W.J., Wang, Z.W., Han, X.G.,
491
2013. Nitrogen deposition weakens plant–microbe interactions in grassland ecosystems. Global
492
Chang Biol. 19, 3688-3697.
493
Yang, G.J., Lü, X.T., Stevens, C.J., Zhang, G.M., Wang, H.Y., Wang, Z.W., Zhang, Z.J., Liu, Z.Y.,
494
Han, X.G., 2019. Mowing mitigates the negative impacts of N addition on plant species diversity.
495
Oecologia 189, 769-779.
496 497
Yeates, G.W., 2003. Nematodes as soil indicators: functional and biodiversity aspects. Biol. Fert. Soils 37, 199-210.
498
Yeates, G.W., Ferris, H., Mones, T., van der Putten, W.H., 2009. The role of nematodes in ecosystem.
499
In Wilson MJ, Kakouli-Duarte T. (eds.) Nematode as Environment Biondicators. Wallingford,
500
UK.
25
501 502
Zhang, X.K., Liang, W.J., Li, Q., 2013. Forest soil nematodes in changbai mountain-morphology and distribution. Chinese Agriculture Press, Beijing, China.
503
Zhang, Y.H., Lü, X.T., Isbell, F., Stevens, C., Han, X., He, N.P., Zhang, G.M., Yu, Q., Huang, J.H.,
504
Han, X.G., 2014. Rapid plant species loss at high rates and at low frequency of N addition in
505
temperate steppe. Global Change Biol. 20, 3520-3529.
506
Zhang, Z.Y., Zhang, X.K., Xu, M.G., Zhang, S.X., Huang, S.M., Liang, W.J., 2015. Responses of soil
507
micro-food web to long-term fertilization in a wheat–maize rotation system. Appl. Soil Ecol. 98,
508
56-64.
26
509
Tables
510
Table 1. Repeated-measures analysis of N addition level and sampling time on different nematode trophic groups. Abundance
511
Richness
Shannon-Wiener diversity (H')
Time (T)
Nitrogen (N)
T×N
Time (T)
Nitrogen (N)
T×N
Time (T)
Nitrogen (N)
T×N
Bacterivores Fungivores Plant-parasites
11.54** 16.27** 13.25**
15.81** 15.12** 12.62**
1.74 1.93* 2.21*
10.14** 13.59** 6.59**
5.76** 15.23** 6.62**
2.15* 0.89 0.40
1.23 10.97** 10.12**
37.24** 8.96** 29.96**
1.29 2.29** 2.36**
Predators-omnivores
10.84**
30.61**
2.34** 13.29**
23.53**
1.96*
2.06
50.35**
1.53
Significant: *, P < 0.05; **, P < 0.01
512
27
513
Table 2. Nematode diversity of different trophic groups in different N addition levels across the four sampling times Bacterivores
Abundance
Richness
514
Fungivores
Plant parasites
Predators-omnivores
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
N0
820.1 ± 164.7a
708.1 ± 92.3ab
408.0 ± 76.0a
574.9 ± 40.7a
218.5 ± 38.9a
238.4 ± 57.1b
115.0 ± 19.3a
284.7 ± 77.5a
1662.0 ± 319.9a
681.5 ± 100.3a
492.8 ± 55.3ab
902.8 ± 181.9a
479.8 ± 97.0a
347.2 ± 53.5a
267.5 ± 49.2a
630.3 ± 68.7a
N2
591.6 ± 187.9a
741.7 ± 50.3ab
295.0 ± 59.7a
590.0 ± 73.0a
226.9 ± 36.7a
387.3 ± 47.6a
84.1 ± 18.5ab
220.8 ± 35.3a
863.1 ± 205.4b
710.5 ± 62.3a
319.3 ± 43.2bc
691.6 ± 98.9a
285.3 ± 100.3b
220.5 ± 26.0b
164.9 ± 26.0b
397.7 ± 49.3b
N5
684.8 ± 73.2a
894.1 ± 102.8a
327.8 ± 57.8a
544.8 ± 115.3a
177.4 ± 44.4a
220.7 ± 29.8b
68.6 ± 20.3b
183.3 ± 50.0ab
935.7 ± 243.6b
659.7 ± 130.6a
408.3 ± 128.2ab
503.5 ± 94.8b
184.8 ± 32.1bc
224.6 ± 51.2b
161.9 ± 29.9b
363.4 ± 105.1b
N10
327.7 ± 117.8b
632.5 ± 156.9ab
395.1 ± 64.1a
468.7 ± 89.9a
74.5 ± 17.3b
227.0 ± 80.0b
54.2 ± 13.6b
168.1 ± 30.7ab
748.3 ± 165.9b
679.7 ± 181.1a
387.6 ± 99.0ab
661.1 ± 45.9ab
64.7 ± 19.0cd
102.7 ± 31.8bc
109.7 ± 29.5b
184.0 ± 18.3c
N20
493.7 ± 108.2b
375.9 ± 75.1b
312.1 ± 38.2a
294.8 ± 51.1ab
77.8 ± 46.4b
100.7 ± 34.0bc
41.2 ± 10.9bc
69.3 ± 18.6bc
819.1 ± 165.2b
534.3 ± 128.0a
617.1 ± 82.8a
392.5 ± 65.5b
115.5 ± 21.5bcd
116.6 ± 46.3bc
88.7 ± 21.8b
N50
89.4 ± 17.0c
150.6 ± 53.9c
82.8 ± 31.4b
13.3 ± 3.3b
3.9 ± 1.6b
20.9 ± 10.5c
0.3 ± 0.3c
0.4 ± 0.4c
181.4 ± 61.4c
103.2 ± 30.5b
177.4 ± 87.1c
46.7 ± 20.8c
11.7 ± 6.5d
4.1 ± 3.4c
1.7 ± 1.1c
1.6 ± 1.2d
N0
26.0 ± 3.3
36.7 ± 3.9
31.2 ± 2.7a
25.7 ±2.9b
7.0 ± 0.9b
12.7 ± 3.0ab
9.0 ± 1.2a
11.0 ± 2.0a
52.0 ± 3.2
34.8 ± 4.0
39.3 ± 3.7
36.2 ± 3.0a
15.3 ± 1.6a
17.3 ± 2.1a
20.5 ± 2.4a
27.3 ± 2.8a
N2
29.7 ± 2.2
38.3 ± 3.1
33.2 ± 5.7a
31.2 ± 2.2ab
13.0 ± 1.9a
19.5 ± 1.4a
10.2 ± 2.3a
11.8 ± 1.9a
45.2 ± 2.5
36.2 ± 2.0
37.5 ± 4.3
36.0 ± 2.8a
13.5 ± 1.5a
11.3 ± 1.3b
18.8 ± 2.1a
21.0 ± 1.6a
N5
35.7 ± 3.2
48.8 ± 2.5
36.8 ± 6.1a
36.3 ± 3.2a
9.7 ± 2.5ab
12.2 ± 1.0ab
6.7 ± 1.1ab
12.0 ± 1.5a
44.5 ± 5.7
35.5 ± 4.3
38.7 ± 4.5
34.8 ± 4.4a
10.5 ± 2.2a
12.0 ± 2.5b
18.8 ± 4.2a
25.0 ± 6.2a
N10
29.3 ± 7.0
40.5 ± 5.3
43.5 ± 3.4a
30.3 ± 2.9ab
6.5 ± 1.2b
16.2 ± 5.5ab
5.7 ± 1.1b
11.5 ± 2.3a
60.3 ± 6.3
42.2 ± 5.5
40.3 ± 3.5
45.7 ± 2.2a
5.7 ± 1.4ab
7.5 ± 2.2b
10.8 ± 1.5b
12.7 ± 1.0b
N20
33.3 ± 3.4
35.8 ± 3.3
30.0 ± 3.4a
36.2 ± 6.0a
4.0 ± 1.5b
9.0 ± 1.7b
3.8 ± 1.0bc
8.3 ± 1.8a
56.8 ± 4.6
49.5 ± 4.8
57.7 ± 4.0
46.8 ± 5.8a
8.3 ± 1.2a
9.5 ± 2.8b
8.5 ± 1.9b
10.3 ± 2.3b
N50
25.8 ± 3.3
37.2 ± 9.5
12.7 ± 6.5b
5.7 ± 1.4c
1.2 ± 0.5c
4.7 ± 1.9c
0.2 ± 0.2c
0.2 ± 0.2b
44.5 ± 11.5
28.5 ± 8.7
28.7 ± 15.4
19.8 ± 8.8b
2.8 ± 1.5b
1.2 ± 0.8c
0.3 ± 0.2c
0.7 ± 0.5c
79.2 ± 17.5cd
Shannon-Wiener
N0
1.6 ± 0.1ab
1.6 ± 0.1a
1.7 ± 0.1a
1.7 ± 0.1ab
0.7 ± 0.1a
0.5 ± 0.2
0.5 ± 0.1a
0.6 ± 0.2a
2.0 ± 0.1ab
2.1 ± 0.1a
2.0 ± 0a
1.9 ± 0.1a
1.7 ± 0.1a
1.6 ± 0.1a
1.5 ± 0.1a
1.8 ± 0.1a
diversity
N2
1.6 ± 0.1ab
1.5 ± 0.1a
1.6 ± 0.1a
1.8 ± 0a
1.0 ± 0.1a
0.4 ± 0.1
0.5 ± 0.1a
0.7 ± 0.1a
2.1 ± 0.1ab
2.1 ± 0.1a
2.1 ± 0.1a
2.0 ± 0.1a
1.5 ± 0.1ab
1.4 ± 0.1a
1.6 ± 0.1a
1.7 ± 0.1ab
N5
1.8 ± 0.1a
1.6 ± 0.1a
1.7 ± 0.1a
1.7 ± 0.1ab
0.9 ± 0.2a
0.7 ± 0.1
0.6 ± 0.1a
0.7 ± 0a
2.2 ± 0.1a
2.1 ± 0.1a
1.7 ± 0.1ab
2.0 ± 0.2a
1.3 ± 0.2ab
1.3 ± 0.1a
1.6 ± 0.1a
1.7 ± 0ab
N10
1.3 ± 0.1abc
1.6 ± 0.2a
1.4 ± 0ab
1.4 ± 0.1ab
1.0 ± 0.1a
0.9 ± 0.1
0.4 ± 0.1a
0.7 ± 0.1a
2.0 ± 0.1ab
1.9 ± 0.1a
1.7 ± 0.1ab
1.9 ± 0.1a
0.9 ± 0.2abc
1.3 ± 0.2a
1.0 ± 0.1a
1.4 ± 0.1bc
N20
1.2 ± 0.2bc
1.0 ± 0.1b
1.1 ± 0.1bc
1.3 ± 0.1b
0.7 ± 0.2ab
0.9 ± 0.1
0.3 ± 0.1a
0.4 ± 0.1a
1.9 ± 0.1ab
1.8 ± 0.2ab
1.4 ± 0.1b
1.8 ± 0.1a
0.8 ± 0.2bc
1.0 ± 0.2a
1.0 ± 0.3a
1.2 ± 0.2c
N50
1.0 ± 0.1c
0.7 ± 0.2b
0.9 ± 0.1c
0.7 ± 0.2c
0.2 ± 0.1b
0.7 ± 0.2
0b
1.7 ± 0.1b
1.2 ± 0.2b
0.5 ± 0.2c
1.2 ± 0.2b
0.5 ± 0.3ac
0.2 ± 0.1b
0b
Different lower-case letters represent significant differences among different N addition levels, as determined by Tukey’s test, P < 0.05.
515 516
28
0b
0d
517
Figure legends
518
Figure 1. Soil pH and moisture in different N addition treatments across different
519
sampling times. Different capital and lower-case letters represent significant
520
differences among different times and N addition levels, respectively, as determined
521
by Tukey’s test. *, P < 0.05; **, P < 0.01.
522
Figure 2. Nematode abundance, richness and Shannon-Wiener diversity in different
523
N addition level treatments across different sampling times. Different capital and
524
lower-case letters represent significant differences among different times and N
525
addition levels, as determined by Tukey’s test. *, P < 0.05; **, P < 0.01.
526
Figure 3. Principle components analysis (PCA) of soil nematode communities in
527
different N addition level treatments across the different sampling times.
528
Figure 4. The stability and synchrony of nematode community in different N addition
529
level. Different lower-case letters represent significant differences among different N
530
addition levels, as determined by Tukey’s test, P < 0.05. BF, bacterivores; FF,
531
fungivores; PP, plant-parasites; OP, predators-omnivores.
532
Figure 5. Linear regressions between nematode stability, diversity and soil properties.
533
Figure 6. Structural equation modeling of the temporal variation of nematode
534
community under N addition level (χ2 = 3.682, df = 4, P = 0.451, GFI = 0.966).
535
Numbers next to the arrows are the standardized path coefficients. The width of the
536
arrows indicates the strength of the causal influence. 29
537 538
Figure. 1
539
30
540 541
Figure. 2
31
542 543
Figure. 3
32
544 545
Figure. 4 33
546 547
Figure. 5 34
548 549
Figure. 6
35
550 551
Supplement
552 553
Figure A1.
36
Conflict of interest The authors declare that they have no conflicts of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S2351-9894(19)30757-7
DOI:
https://doi.org/10.1016/j.gecco.2020.e00965
Reference:
GECCO 965
To appear in:
Global Ecology and Conservation
Received Date: 11 November 2019 Revised Date:
11 February 2020
Accepted Date: 11 February 2020
Please cite this article as: Liang, S., Kou, X., Li, Y., Lü, X., Wang, J., Li, Q., Soil nematode community composition and stability under different nitrogen additions in a semiarid grassland, Global Ecology and Conservation (2020), doi: https://doi.org/10.1016/j.gecco.2020.e00965. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
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Type of contribution: Research paper
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Date of preparation: Feb 11, 2019
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Number of text pages: 25
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Number of tables: 2
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Number of figures: 6
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Soil nematode community composition and stability under different
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nitrogen additions in a semiarid grassland
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Siwei Liang a, b, 1, Xinchang Kou b, c, 1, *, Yingbin Li b, Xiaotao Lü b, Jingkuan Wang a,
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*
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a
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110866, China
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b
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China
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c
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China
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*Corresponding authors: Prof. Jingkuan Wang and Dr. Xinchang Kou
, Qi Li b College of Land and Environment, Shenyang Agricultural University, Shenyang
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016,
School of Geographical Sciences, Northeast Normal University, Changchun 130024,
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Institute of Applied Ecology,
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Chinese Academy of Sciences
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P. O. Box 417, Shenyang 110016, China
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Tel.: +86-24-83970359; Fax: +86-24-83970300
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E-mail address: [email protected]; [email protected]
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1
These authors contributed equally to this work.
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Abstract
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Anthropogenic input of reactive nitrogen (N) is an environmental problem that
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threatens the diversity and stability of belowground ecosystems. Soil nematodes are
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abundant in soil and are occurring at multiple trophic levels in the soil food web.
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However, how N deposition affects the composition and stability of soil nematode
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community is largely unknown. Here, we investigated the response of soil nematode
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community composition to N deposition at different sampling seasons and also
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estimated the stability of nematode community in a semiarid grassland in northern
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China. We found that the addition of N not only reduced the diversity of the nematode
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community, but also reduced the temporal stability of nematode community. The
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stability of different nematode trophic groups had different responses to N addition,
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and the community of plant parasites was more stable than the other trophic groups at
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a relatively higher N addition level. Moreover, soil pH was closely correlated with the
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stability of bacterivores, fungivores and predators-omnivores and the diversity of
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nematode community under different N additions. Our results highlight that N
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addition indirectly influence the synchrony and the stability of nematode community
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through change in soil nematode abundance and richness, and the variations of
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nematode community stability under different N additions are closely related to soil
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pH. These changes in nematode community composition and stability will eventually
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influence soil ecosystem function and nutrient cycling through biotic interactions in
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the soil food web. 2
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Keywords: N addition; Nematode community; Diversity; Stability; Semiarid grassland
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1. Introduction
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Anthropogenic reactive nitrogen (N) deposition has been increased three- to five-folds
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over the past century (Galloway et al., 2008; Sutton et al., 2014). Nitrogen deposition
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can relieve N limitation and it is beneficial for plant growth (Quinn Thomas et al.,
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2009). However, the continuous increase of N deposition is an environmental problem
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that leads to biodiversity loss and reduction in ecosystem stability (Bai et al., 2010).
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While an increasing number of studies have focused on the responses of aboveground
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plant communities to N deposition (Stevens et al., 2004; Simkin et al., 2016; Yang et
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al., 2019; Hou et al., 2019), more and more attentions have been paid to the
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belowground subsystems in recent years. For example, Wang et al. (2018), in a
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meta-analysis, found that microbial diversity showed a robust decrease under N
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addition and this decrease was associated with the decline in microbial biomass.
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Changes in microbial biomass could also affect microbial predators, such as
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microbivorous nematodes. Nitrogen deposition was also reported to reduce the
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nematode generic richness, and alter community structure by promoting the
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abundance
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predators-omnivores (Lokupitiya et al., 2000; Song et al., 2016). However, Chen et al.
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(2015) found that N-enrichment (17.5, and 28.0 g of N m-2 yr-1) induced soil
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acidification, reduced concentrations of mineral cations and nematode food resources,
of
bacterivores
and
suppressing
3
that
of
fungivores
and
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decreased the abundance of bacterivores, fungivores and omnivores in a semiarid
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grassland. In addition, Shao et al. (2018), in a two-years field experiment, found that
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N-addition (6 g N m−2 yr−1) significantly decreased nematode richness and
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Shannon-Wiener diversity in a forest ecosystem. However, the influences of
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N-addition on belowground subsystems still lack consistent conclusions, and highly
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depend on N addition levels, experimental durations and ecosystem types. Further, N
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deposition also influence the biodiversity of soil biota by changing plant community
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composition, soil conditions and N availability, and then influence the structure and
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stability of soil food web through biotic interactions between above- and belowground
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communities (de Ruiter et al., 1998; Ferris et al., 2001; Okada and Harada, 2007;
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Bardgett and Wardle, 2010; Chen et al., 2015).
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Previous studies investigating the effects of N deposition on the biotic community
78
were mostly in single-time sampling (Parfitt et al., 2012; Sun et al., 2016). Temporal
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stability measures the degree of constancy in a variable relative to its mean over a
80
time period (Lehman and Tilman, 2000). At present, researches on community
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stability mostly focus on aboveground vegetation, and relatively few attentions are
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paid on belowground biota. Stability of biotic community has fundamental
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importance for understanding the dynamics of belowground communities under
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temporal fluctuations (Yeats, 2003; Viketoft et al., 2011). The stabilities of
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communities depend upon the interspecific competition and compensation, and are
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mainly influenced by the changes of environmental conditions (Thibaut and Connolly, 4
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2013), for example, precipitation, temperature and nutrient availability (Griffiths and
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Caul, 1993; Sarathchandra et al., 2001; Landesman et al., 2011; Chen et al., 2015).
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Moreover, due to the different niches and life histories, changes in the stability within
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a community will also vary with different trophic levels (Chesson, 2000; Wagg et al.,
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2018). These complicated influences make it critical to understand the mechanisms
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that underpin the structure, functioning and stability of biotic community (Tilman et
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al., 2006; Ives and Carpenter, 2007).
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Since soil nematodes are abundant in soil and occur at multiple trophic levels in the
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soil food web, we investigated the temporal variations of nematode community
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composition and community stability after long-term N deposition. We collected soil
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samples at different seasons across 2014 and 2015 after six-years of N addition. We
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hypothesized that (i) the abundance and diversity of nematode community decrease
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with the increase of N addition levels; (ii) nematode trophic groups with different life
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history strategies, such as bacterivores and predators-omnivores, will have different
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responses to N addition levels, and these changes will eventually change the stability
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of nematode communities.
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2. Materials and methods
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2.1 Site description and experimental design
5
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Soil samples were collected from a six-year N addition experiment site conducted in a
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natural steppe ecosystem near the Inner Mongolia Grassland Ecosystem Research
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Station (IMGERS, 116˚14ˈE, 43˚13ˈN) of the Chinese Academy of Sciences. The
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mean annual temperature and precipitation are 0.9°C and 355 mm, respectively. The
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rainfall is mainly in summer, with 70% falling between May and August (Huang et al.,
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2009). Soil is classified as Haplic Calcisol according to the FAO (Food and
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Agriculture Organization of the United Nations). The zonal vegetation is typical
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grassland, with dominant species Leymus chinensis and Stipa grandis accounting for
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more than 60% of the total aboveground biomass in the plant community. No
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fertilizer was applied before starting the experiment. The ambient atmospheric total N
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deposition was less than 1.5 g N m−2 yr−1 (Lue and Tian, 2007).
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The long-term N addition experiment was established in September 2008 (Zhang et
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al., 2014). There were nine N addition levels (0, 1, 2, 3, 5, 10, 15, 20, 50 g N m−2 yr−1)
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applied at two frequencies (2 times and 12 times per year). The experimental site was
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designed as a randomized block design with 10 replicate blocks, and each block was
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45 m × 70 m. In each block, there were nine plots treated with nine N addition levels.
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The area of each plot was 8 m × 8 m, and there was 1 m walkway between each plot.
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For the present study, six N addition levels (0, 2, 5, 10, 20, 50 g N m−2 yr−1, 2 times
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per year) were selected, and six replicate blocks were randomly selected from 10
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blocks. Therefore, the N addition treatments are expressed as N0, N2, N5, N10, N20 and
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N50. We implemented wet and dry deposition to mirror the seasonal patterns of the 6
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natural N deposition, and the adding method was described in detail by Li et al.
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(2019). Briefly, in June, NH4NO3 was dissolved in purified water (9.0 L per plot; the
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N0 treatment received only purified water) and evenly sprayed on the sample land
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with a sprayer to simulate wet deposition. In December, NH4NO3 was mixed with
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clean sand (0.5 kg sand per plot; the N0 treatment received only sand) and sprinkled
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evenly by hand to simulate dry deposition (Li et al., 2019).
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2.2 Soil sampling and analysis
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Soil samples were collected from a depth of 0–10 cm (5 cores with 2.5 cm diameter)
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and uniformly mixed as a composite sample for each replicate in August and October
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of 2014 and March and May of 2015, corresponding to the summer, autumn, winter,
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and spring, respectively. The corresponding mean precipitations in each season were
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66.5mm, 34.1mm, 3.4mm and 24.7mm, respectively, and the mean temperatures were
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20.2°C, 4.3°C, -4.2°C and 11.1°C, respectively. In total, there were 144 samples (6
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treatments × 6 replicates × 4 sampling seasons). The fresh soil samples were stored
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individually in plastic bags and kept at 4°C in a refrigerator until analysis. Then soil
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pH and soil moisture were measured using standard methods (Wei et al., 2013; Zhang
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et al., 2014).
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2.3 Soil nematode extraction and identification 7
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A total of 100 g of fresh soil each was used to extract soil nematodes using a modified
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cotton-wool filter method (Liang et al., 2009). Nematode abundance was expressed as
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individuals per 100 g dry soil and at least 100 nematodes from each sample were
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identified to genus level using the microscope, according to Bongers (1994), Ahmad
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and Jairjpuri (2010) and Li et al. (2017). If the total number of nematodes was less
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than 100 individuals in a sample, all nematodes were identified. The nematodes were
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divided into the following four trophic groups according to their feeding habits and
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esophagus characteristics: bacterivores (BF), fungivores (FF), plant parasites (PP),
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and predators- omnivores (OP) (Yeates, 2003).
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2.4 Data analyses
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Nematode generic richness (number of genera), species abundance (number of
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nematode 100 g-1 dry soil) and Shannon-Wiener diversity (H′) were calculated for
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each plot. Since nematode abundance was closely correlated with generic richness and
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biomass of nematode community (Fig. A1), the destabilization in the abundances of
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nematode likely reflected the stability in the nematode community. Stability of the
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nematode community was defined as the ratio of the nematode mean abundance
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across four sampling seasons (µ) to the temporal variation (σ) (Lehman and Tilman,
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2000; Wagg et al., 2018). We defined synchrony as the average across genus of the
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correlation between the abundance of each nematode genus and the total abundance of
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all other genera in the nematode community (Gross et al., 2014). We also calculated 8
168
the average variation in individual taxa (population CV) as the weighted average CV
169
of taxa in a community by weighting the CV of taxa by its overall average abundance
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(Gross et al., 2014; Wagg et al., 2018). Following equations were used to calculate the
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indices:
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′ = − ∑
173
ℎ
⁄ × ln
= µ / σ
= 1/
⁄
!
"# , ! #% ' %&
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where, H′= Shannon-Wiener diversity, S = number of species, ni = number of each
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individual genus identified in samples, N = total number of individuals identified in
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samples, µ = temporal mean of nematode abundance, σ = temporal variation of
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nematode abundance, Yi = the abundance of genus i in the nematode community of n
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genera.
179
Data analysis was carried out using SPSS19 statistical software (SPSS Inc.,
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Chicago, IL, USA). The repeated-measures analysis was performed to test how N
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deposition level, sampling time and their interactions affected the soil pH, soil
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moisture and nematode diversity. This analysis method accounts for the effects of
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temporal autocorrelation and we used the “Mauchly test” to test whether the data
184
conformed to the analysis requirements. Multiple comparisons were based on a
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Tukey’s HSD test if the main effect or their interactions were significant at P < 0.05
186
level. One-way analysis of variance (ANOVA) was used to test the differences in 9
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nematode community stability among N addition levels. Principal component analysis
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(PCA) was performed to explore soil biotic community composition based on the
189
nematode genus using CANOCO software, version 5.0 (ter Braak, 1988). We
190
constructed a structural equation model (SEM) for their relationship based on our
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predictions and literature reviews (Grace 2006). Then the model was judged based on
192
the χ2 value, degrees of freedom (df) and modification indices. The Amos 17.0
193
software was used for SEM analysis (Arbuckle 2006).
194 195
3. Results
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3.1 Changes in precipitation, temperature and soil properties
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The interactions between N addition and sampling time had significant effects on soil
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pH and soil moisture. Soil pH showed a decreasing trend with increasing N addition
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level, and the values were higher in the N0, N2 and N5 treatments than those in the N10,
200
N20 and N50 treatments (P < 0.01). Among different sampling times, soil pH showed a
201
decreasing trend, and all treatments reached their lowest value in May (Fig. 1). Higher
202
values of soil moisture were found in August and October than those in March and
203
May.
204 205
3.2 Soil nematode community composition, synchrony and diversity
10
206
The N addition levels, sampling times and their interactions had significant effects on
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total nematode abundance, richness and Shannon-Wiener diversity (P < 0.01) (Fig 2).
208
The total abundance, richness and Shannon-Wiener diversity of soil nematodes
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showed a decreasing trend after N5, and sharply declined at N50, and were
210
significantly lower in March than in other three sampling seasons (Fig. 2). Nematode
211
community compositions in high and low N addition levels were clearly separated in
212
the principal component analysis (PCA) along the axis 1 (PC1, 46.17%), and the
213
different sampling seasons were separated by the axis 2 (PC2, 20.74%) (Fig. 3). Soil
214
nematode synchrony was significantly influenced by N addition with the highest and
215
the lowest values being found in N50 treatment and the N0 treatment, respectively (Fig.
216
4).
217
Nitrogen addition had a significant effect on different nematode trophic groups (P
218
< 0.01) (Table 1). The abundance of different trophic groups showed a gradually
219
decreasing trend with increasing N addition level, and the lowest values were
220
observed in the N50 treatment (Table 2). There was a significant effect of sampling
221
time on the abundance and richness of different nematode trophic groups (P < 0.01)
222
(Table 1), with the highest values observed in October (autumn).
223 224
3.3 Soil nematode community stability and their relations with soil properties
225
The stability of nematode community was about the same among different N addition
226
treatments, but the variability increased until N50. No significant differences were 11
227
observed in the stability of the plant parasites among different N addition levels.
228
However, the stabilities of the bacterivores were significantly higher in the N0 and N20
229
treatments than that in the N50 treatment (P < 0.05), and those of predators-omnivores
230
were significantly higher in the N0 and N5 treatments than that in the N50 treatment (P
231
< 0.05) (Fig. 4). Soil nematode stability was positively correlated with nematode
232
generic richness and Shannon-Wiener diversity (P < 0.05). For different nematode
233
trophic groups, the stabilities of microbivores and predators-omnivores were
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positively correlated with soil pH (Fig. 5). No significant relationship was found
235
between the stability of nematode trophic groups and soil moisture (Fig. 5).
236
The SEM provided a good fit to reveal how the temporal variation of nematode
237
community was influenced by N addition (χ2 = 3.682, df = 4, P = 0.451, GFI = 0.966).
238
Specifically, temporal variation in nematode abundance (σ) was positively related to
239
the temporal mean in nematode abundance (µ), followed by the population synchrony
240
(η). The nematode richness was negatively associated with the population CV and
241
population synchrony. Nitrogen addition indirectly increased the temporal variation in
242
nematode abundance by reducing the richness and abundance of the nematode (Fig.
243
6).
244 245
4. Discussions
246
4.1 Responses of soil nematode community composition to N addition levels varied in
247
different sampling seasons. 12
248
Consistent with our first hypothesis, we found that nematode diversity decreased with
249
increasing N addition level after six years of N addition, and unsurprisingly, the
250
higher the N addition, the greater the impact on the diversity of nematode community
251
was (Sarathchandra et al., 2001; Chen et al., 2015; Shao et al., 2018). However, in our
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study, soil nematode diversity showed highly temporal variation, with the lowest
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value being observed in March (spring). This trend was consistent with the changes in
254
precipitation and temperature. As we know, soil nematodes rely on water to move to
255
their prey (Griffiths and Caul, 1993; Yeates et al., 2009), so their abundance may
256
decline with the decreasing precipitation (Landesman et al., 2011). Moreover,
257
although nematode can survive in the winter by shutting down metabolism into a
258
cryptobiotic state or altering biochemical pathways (Mcsorley, 2003; Yeates et al.,
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2009; Orgiazzi et al., 2016), the environmental conditions in winter increased the loss
260
of nematode species.
261
Since aboveground plants were senesced at the end of September, it might have
262
resulted in the decrease of food resources for plant parasites, and then led to the lower
263
abundance in the other sampling seasons. Although the aboveground plants have
264
already senesced at the end of September, the rhizosphere priming effect might still
265
exist, providing nutrient resources for soil microorganisms and microbivorous
266
nematodes over a period of time (Cheng, 2009; Kuzyakov, 2010), and then the
267
abundance and richness of microbivorous nematode increased until October.
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Furthermore, in October, the senescence of plants could also reduce the competition 13
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of nutrients between plant and microbivorous nematodes (Alphei et al., 1996), which
270
also promoted the abundance and richness of microbivores.
271
Previous studies found that nematode trophic groups had different responses to N
272
addition (Lokupitiya et al., 2000; Chen et al., 2015; Song et al., 2016), and these
273
changes might also influence the nematode community composition. Our study also
274
showed that N addition level suppressed the abundance of different nematode trophic
275
groups (Table 2). Since N enrichment negatively affected the total microbial
276
abundance and the biomass of microbial community components (LeBauer and
277
Treseder, 2008), which may lead to the decline in nematode abundance through
278
bottom-up effects (Ingham et al., 1985; Wardle et al., 2004).
279 280
4.2 Relationship between soil nematode stability, diversity and soil properties.
281
Stability is as important as diversity in affecting community productivity and has a
282
multiplicity of meanings in ecology (Lehman and Tilman, 2000; Gross et al., 2014).
283
Our results showed that the stability of the nematode community varied with N
284
addition levels. At a low N addition level, the stability of the nematode community
285
will be decreased due to the competition for N availability with other soil organisms,
286
especially for the fauna with rapid transformation and utilization of N (Pfisterer and
287
Schmid, 2002). Then, with the increasing of N addition levels, sufficient N was
288
provided for the soil organisms, and the stability of nematode community showed a
289
gradually increasing trend. Finally, when the N addition level reached 50 g N m−2 yr−1, 14
290
the increased availability of N has increased the ammonium and aluminum toxicity
291
(Wei et al., 2013). Hence, the stability of nematode community decreased again. The
292
variation of synchrony in nematode community also evidenced that low N addition
293
level increased the degree of synchrony in population fluctuations in nematode
294
community, thus contributed to the decrease of community stability.
295
We found that the positive richness–stability relationship in the nematode
296
community was largely explained through the negative association between richness
297
and the population synchrony, and this result probably suggested that the increase in
298
richness should decrease population synchrony, thereby preserving the stability of the
299
nematode community (van Klink et al., 2019). Additionally, the N addition has no
300
detectable direct effect on nematode community stability, however, N addition
301
simultaneously reduced the nematode richness and abundance, and then, had a
302
negative effect on the stability of nematode community. These results provide further
303
support for the hypothesis that greater richness provides greater insurance that some
304
nematode genus will benefit over others through environmental variations in a
305
compensatory manner so that the stability of the nematode community is maintained.
306
Previous studies proposed that this compensatory dynamic was irrelevant to the
307
changes of abundance (Lehman and Tilman., 2000; Loreau, 2010; Hallett et al., 2014).
308
For instance, although N addition would restrain the survival ability of some
309
nematodes and decrease their abundance, other surviving nematodes could provide a
310
proximate function for community stabilization (Gonzales and Loreau, 2009). 15
311
However, our results suggested that the contribution of abundance to the stability of
312
nematode community should not be ignored, especially in a N limitation condition.
313
Although N addition has no detectable effect on synchrony in nematode populations,
314
a negative association between richness and the population synchrony, and a positive
315
association between population synchrony and community variation were observed.
316
These results indicated that the variation of synchrony was indirectly influenced by N
317
addition and was associated with the stability of the nematode community (i.e.
318
increase richness, less synchrony, more stable).
319
Our study also in line with previous studies that the soil environment has a great
320
influence on the stability of nematode communities (Andres et al., 2016; Zhang et al.,
321
2015). Positive correlations were found between soil pH and the stability of
322
fungivores and predators-omnivores. Since fungivores and predators-omnivores were
323
particularly susceptible to disturbance (Freckman and Caswell, 1985; Yeates et al.,
324
2009; Zhang et al., 2013), their stability changed with the variation of soil pH, and
325
then affected the stability of nematode community. In this study, we designed the N
326
addition experiment with a large gradient (0–50 g N m-2 yr-1), and the highest N
327
addition treatment exerted some effects on the whole pattern of nematode. Even
328
though the rate of 50 g N m-2 yr-1 was relatively high considering the background
329
value, we assumed that with rapid global change because of N deposition and climate
330
change, it becomes increasingly important to develop strategies under a high N
16
331
deposition condition to predict the effects of future global change on ecosystem
332
functioning and services.
333 334
5. Conclusions
335
Here, we assessed the diversity-stability relationships in soil nematode community
336
under different N addition levels. Our results suggested that N addition indirectly
337
alters the temporal stability of nematode community by reducing the richness and
338
abundance of nematodes. The variations of nematode trophic groups under different N
339
additions were closely related to soil pH. These changes in different nematode trophic
340
groups will eventually change the structure and stability of the soil food web and then
341
influence soil ecosystem function and nutrient cycling through biotic interactions.
342 343
Acknowledgements
344
This research was supported by the Strategic Priority Research Program of the
345
Chinese Academy of Sciences (XDB15010402), National Science & Technology
346
Fundamental Resources Investigation Program of China (2018FY100304), and the
347
National Natural Science Foundation of China (31570519 and 41877047). We are
348
grateful to the Inner Mongolia Grassland Ecosystem Research Station (IMGERS) of
349
the Chinese Academy of Sciences for providing the experimental sites. We also
17
350
would like to thank Prof. MD Mahamood for proofing the English throughout the
351
manuscript.
352 353
Compliance with ethical standards
354
Conflict of interest Authors declare no conflicts of interest.
355 356
References
357
Ahmad, W., Jairjpuri, M.S., 2010. Mononchida: The Predaceous Nematodes. Nematology Monographs
358
and Perspectives. Leiden, Netherlands.
359
Alphei, J., Bonkowski, M., Scheu, S., 1996. Protozoa, nematode and Lumbricidae in the rhizosphere of
360
Hordelymus europaeus (Poaceae): faunal interactions, response of microorganisms and effects on
361
plant growth. Oecologia 106, 111-126.
362
Andrés, P., Moore, J.C., Simpson, R.T., Selby, G., Cotrufo, F., Denef, K., Haddix, M.L., Shaw, E.A.,
363
de Tomasel, C.M., Molowny-Horas, R., Wall, D.H., 2016. Soil food web stability in response to
364
grazing in a semi-arid prairie: the importance of soil textural heterogeneity. Soil Biol. Biochem.
365
97, 131-143.
366
Arbuckle, J.L., 2006. Amos (Version 7.0) [Computer Program]. SPSS, Chicago.
18
367
Bai, Y.F., Wu, J.G., Clark, C.M., Naeem, S., Pan, Q.M., Huang, J.H., Zhang, L.X., Han, X.G., 2010.
368
Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem
369
functioning: evidence from Inner Mongolia grasslands. Global Change Biol. 16, 358-372.
370 371 372 373
Bardgett, R.D., Wardle, D.A., 2010. Aboveground-belowground Linkages: Biotic Interactions, Ecosystem Processes, and Global Change. Oxford University Press, Oxford, UK.
Bongers, T., 1994. De Nematoden van Nederland. In: Vormgeving en technische realisatie. Uitgeverij Pirola, Schoorl, Netherlands.
374
Chen, D.M., Lan, Z.C., Hu, S.J., Bai, Y.F., 2015. Effects of nitrogen enrichment on belowground
375
communities in grassland: relative role of soil nitrogen availability vs. soil acidification. Soil Biol.
376
Biochem. 89, 99-108.
377 378 379 380 381 382 383 384 385 386
Cheng, W.X., 2009. Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C-N budgets. Soil Biol. Biochem. 41, 1795-1801.
Chesson, P., 2000. Mechanisms of maintenance of species diversity. Annu. Rev. Ecol. Evol. S. 31, 343-366.
de Ruiter, P.C., Neutel, A.M., Moore, J.C., 1998. Biodiversity in soil ecosystems: the role of energy flow and community stability. Appl. Soil Ecol. 10, 217-228.
Ferris, H., Bongers, T., Goede, R.G.M.D., 2001. A framework for soil food web diagnostics: extension of the nematode faunal analysis concept. Appl. Soil Ecol. 18, 0-29.
Freckman, D.W., Caswell, E.P., 1985. The ecology of nematodes in agroecosystems. Annu. Rev. Phytopathol. 23, 275-296.
19
387
Galloway, J.N., Townsend, A.R., Erisman, J.W., Bekunda, M., Cai, Z., Freney, J.R., Martinelli, L.A.,
388
Seitzinger, S.P., Sutton, M.A., 2008. Transformation of the nitrogen cycle: recent trends,
389
questions, and potential solutions. Science 320, 889-892.
390 391 392 393 394 395
Gonzalez, A., Loreau, M., 2009. The causes and consequences of compensatory dynamics in ecological communities. Annu. Rev. Ecol. Evol. S. 40, 393-414.
Grace, J.B., 2006. Structural Equation Modeling and Natural Systems. Cambridge University Press, Cambridge, pp. 1-365.
Griffiths, B.S., Caul, S., 1993. Migration of bacterial-feeding nematodes, but not protozoa, to decomposing grass residues. Biol. Fert. Soils 15, 201-207.
396
Gross, K., Cardinale, B.J., Fox, J.W., Gonzalez, A., Loreau, M., Polley, H.W., Reich, P.B., van Ruijven,
397
J., 2014. Species richness and the temporal stability of biomass production: a new analysis of
398
recent biodiversity experiments. Am. Nat. 183, 1-12.
399
Hallett, L.M., Hsu, J.S., Cleland, E.E., Collins, S.L., Dickson, T.L., Farrer, E.C., Gherardi, L.A., Gross,
400
K.L., Hobbs, R.J., Turnbull, L., Suding, K.N., 2014. Biotic mechanisms of community stability
401
shift along a precipitation gradient. Ecology 95, 1693-1700.
402
Hou, S.L., Lü, X.T., Yin, J.X., Yang, J.J., Hu, Y.Y., Wei, H.W., Zhang, Z.W., Yang, G.J., Liu, Z.Y.,
403
Han, X.G., 2019. The relative contributions of intra- and inter-specific variation in driving
404
community stoichiometric responses to nitrogen deposition and mowing in a grassland. Sci. Total
405
Environ. 666, 887-893.
20
406 407
Huang, J., Bai, Y., Jiang, Y., 2009. Case Study 3: Xilingol Grassland, Inner Mongolia. Rangeland degradation and recovery in China’s pastoral lands. CBA Internationsl. Oxfordshire, UK.
408
Ingham, R.E., Trofymow, J.A., Ingham, E.R., Coleman, D.C., 1985. Interactions of bacteria, fungi, and
409
their nematode grazers e effects on nutrient cycling and plant-growth. Ecol. Monogr. 55, 119-140.
410
Ives, A.R., Carpenter, S.R., 2007. Stability and diversity of ecosystems. Science 317, 58-62.
411
Kuzyakov, Y., 2010. Priming effects: Interactions between living and dead organic matter. Soil Biol.
412 413 414 415 416 417 418 419 420
Biochem. 42, 1363-1371.
Landesman, W.J., Treonis, A.M., Dighton, J., 2011. Effects of a one-year rainfall manipulation on soil nematode abundances and community composition. Pedobiologia 54, 87-91.
LeBauer, D.S., Treseder, K.K., 2008. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89, 371-379.
Lehman, C.L., Tilman, D., 2000. Biodiversity, stability, and productivity in competitive communities. Am. Nat. 156, 534-552.
Li, Q., Liang, W.J., Zhang, X.K., Mohammad, M., 2017. Soil nematodes of grasslands in Northern China. Zhejiang University Press, Hangzhou; published by Elsevier Inc, London, UK.
421
Liang, W.J., Lou, Y.L., Li, Q., Zhong, S., Zhang, X.K., Wang, J.K., 2009. Nematode faunal response to
422
long-term application of nitrogen fertilizer and organic manure in Northeast China. Soil Biol.
423
Biochem. 41, 883-890.
21
424
Li, Y.B., Martijn, B.T., Yang, J.J., Lü, X.T., Li, X.Y., Liang, W.J., Han, X.G., Li, Q., 2019. Changes in
425
litter quality induced by N deposition alter soil microbial communities. Soil Biol. Biochem. 130,
426
33-42.
427 428
Lokupitiya, E., Stanton, N. L., Seville, R. S., Snider, J.R., 2000. Effects of increased nitrogen deposition on soil nematodes in alpine tundra soils. Pedobiologia 44, 591-608.
429
Loreau, M., 2010. Stability and complexity of ecosystems: New perspectives on an old debate. In M.
430
Loreau (Ed.), From populations to ecosystems: Theoretical foundations for a new ecological
431
synthesis. Princeton, NJ: Princeton University Press, USA.
432 433
Lue, C., Tian, H., 2007. Spatial and temporal patterns of nitrogen deposition in China: synthesis of observational data. J. Geophys. Res-Atmos. 112, 2156-2202.
434
Mcsorley, R., 2003. Adaptations of nematodes to environmental extremes. Fla. Entomol. 86, 138-142.
435
Okada, H., Harada, H., 2007. Effects of tillage and fertilizer on nematode communities in a Japanese
436
soybean field. Appl. Soil Ecol. 35, 582-598.
437
Orgiazzi, A., Bardgett, R.D., Barrios, E., Behan-Pelletier, V., Briones, M.J.I., Chotte, J.L., De Deyn,
438
G.B., Eggleton, P., Fierer, N., Fraser, T., Hedlund, K., Jeffery, J., Johnson, N.C., Jones, A.,
439
Kandeler, E., Kaneko, N., Lavelle, P., Lemanceau, P., Miko, L., Montanarella, L., Moreira, F.M.S.,
440
Ramirez, K.S., Scheu, S., Singh, B.K., Six, J., van der Putten, W.H., Wall, D.H., 2016. Global Soil
441
Biodiversity Atlas. Publications Office of the European Union, European Union.
22
442
Parfitt, R.L., Couper, J., Parkinson, R., Schon, N. L., Stevenson, B. A., 2012. Effect of nitrogen
443
fertilizer on nitrogen pools and soil communities under grazed pastures. New Zeal. J. Agr. Res. 55,
444
1-17.
445 446 447 448
Pfisterer, A.B., Schmid, B., 2002. Diversity-dependent production can decrease the stability of ecosystem functioning. Nature 416, 84-86.
Quinn Thomas, R., Canham, C.D., Weathers, K.C., Goodale, C.L., 2009. Increased tree carbon storage in response to nitrogen deposition in the US. Nat. Geosci. 3, 13-17.
449
Sarathchandra, S.U., Ghani, A.A., Yeates, G.W., Burch, G., Cox, N. R., 2001. Effect of nitrogen and
450
phosphate fertilisers on microbial and nematode diversity in pasture soils. Soil Biol. Biochem. 33,
451
953-964.
452
Shao, Y.H., Liu, T., Eisenhauer, N., Zhang, W.X., Wang, X.L., Xiong, Y.M., Liang, C.F., Fu, S.L.,
453
2018. Plants mitigate detrimental nitrogen deposition effects on soil biodiversity. Soil Biol.
454
Biochem. 127, 178-186.
455
Simkin, S.M., Allen, E.B., Bowman, W.D., Clark, C.M., Belnap, J., Brooks, M.L., Cade, B.S., Collins,
456
S.L., Geiser, L.H., Gilliam, F.S., 2016. Conditional vulnerability of plant diversity to atmospheric
457
nitrogen deposition across the United States. PNAS. 113, 4086-4091.
458 459 460 461
Song, M., Li, X.M., Jing, S.S., Lei, L.J., Wang, J.L., Wan, S.Q., 2016. Responses of soil nematodes to water and nitrogen additions in an old-field grassland. Appl. Soil Ecol. 102, 53-60.
Stevens, C.J., Dise, N.B., Mountford, J.O., Gowing, D.J., 2004. Impact of nitrogen deposition on the species richness of grasslands. Science 303, 1876-1879.
23
462
Sun, F., Tariq, A., Chen, H., He, Q.J., Gu, Y.X., Pan, K.W., Chen, S.Y., Li, J.T., Zhao, C.C., Wang, H.,
463
Gu, Y.F., 2016. Effect of nitrogen and phosphorus application on agricultural soil food webs.
464
Arch.
Agron. Soil Sci. 63, 1176-1186.
465
Sutton, M.A., Mason, K.E., Sheppard, L.J., Sverdrup, H., Haeuber, R., Hicks, K., 2014. Nitrogen
466
Deposition, Critical Loads and Biodiversity, Springer Dordrecht Heidelberg Press, New York,
467
London.
468
ter Braak, C.J.F., 1988. CANOCO-A fortran program for canonical community ordination by (partial)
469
(detrended) (canonical) correspondence analysis, principal components analysis and redundancy
470
analysis (version2.1). Technical Report LWA-88-02. Agricultural Mathematics Group,
471
Wageningen, Netherland.
472 473 474 475 476 477
Thibaut, L.M., Connolly, S.R., 2013. Understanding diversity-stability relationships: Towards a unified model of portfolio effects. Ecol. Lett. 16, 140-150.
Tilman, D., Reich, P.B. Knops, J.M.H., 2006. Biodiversity and ecosystem stability in a decade-long grassland experiment. Nature 441, 629-632.
van Klink, R., Lepš, J., Vermeulen, R., de Bello, F., 2019. Functional differences stabilize beetle communities by weakening interspecific temporal synchrony. Ecology 100, e02748.
478
Viketoft, M., Sohlenius, B., Boström, S., Palmborg, C., Bengtsson, J., Berg, M.P., Danell, K.H., 2011.
479
Temporal dynamics of soil nematode communities in a grassland plant diversity experiment. Soil
480
Biol. Biochem. 43, 1063-1070.
24
481 482 483 484 485 486
Wang, C., Liu, D., Bai, E., 2018. Decreasing soil microbial diversity is associated with decreasing microbial biomass under nitrogen addition. Soil Biol. Biochem. 120, 126-133.
Wardle, D.A., Bardgett, R.D., Klironomos, J.N., Setala, H., van der Putten, W.H., Wall, D.H., 2004. Ecological linkages between aboveground and belowground biota. Science 304, 1629-1633.
Wagg, C., Dudenhoffer, J.H., Widmer, F., van der Heijden, M.G.N., 2018. Linking diversity, synchrony and stability in soil microbial communities. Funct. Ecol. 32, 1280-1292.
487
Wei, C.Z., Zheng, H.F., Li, Q., Lü, X.T., Yu, Q., Zhang, H.Y., Chen, Q.S., He, N.P., Kardol, P., Liang,
488
W.J., Han, X.G., 2012. Nitrogen addition regulates soil nematode community composition
489
through ammonium suppression. PLoS One 7, e43384.
490
Wei, C.Z., Yu, Q., Bai, E., Lü, X.T., Li, Q., Xia, J.Y., Kardol, P., Liang, W.J., Wang, Z.W., Han, X.G.,
491
2013. Nitrogen deposition weakens plant–microbe interactions in grassland ecosystems. Global
492
Chang Biol. 19, 3688-3697.
493
Yang, G.J., Lü, X.T., Stevens, C.J., Zhang, G.M., Wang, H.Y., Wang, Z.W., Zhang, Z.J., Liu, Z.Y.,
494
Han, X.G., 2019. Mowing mitigates the negative impacts of N addition on plant species diversity.
495
Oecologia 189, 769-779.
496 497
Yeates, G.W., 2003. Nematodes as soil indicators: functional and biodiversity aspects. Biol. Fert. Soils 37, 199-210.
498
Yeates, G.W., Ferris, H., Mones, T., van der Putten, W.H., 2009. The role of nematodes in ecosystem.
499
In Wilson MJ, Kakouli-Duarte T. (eds.) Nematode as Environment Biondicators. Wallingford,
500
UK.
25
501 502
Zhang, X.K., Liang, W.J., Li, Q., 2013. Forest soil nematodes in changbai mountain-morphology and distribution. Chinese Agriculture Press, Beijing, China.
503
Zhang, Y.H., Lü, X.T., Isbell, F., Stevens, C., Han, X., He, N.P., Zhang, G.M., Yu, Q., Huang, J.H.,
504
Han, X.G., 2014. Rapid plant species loss at high rates and at low frequency of N addition in
505
temperate steppe. Global Change Biol. 20, 3520-3529.
506
Zhang, Z.Y., Zhang, X.K., Xu, M.G., Zhang, S.X., Huang, S.M., Liang, W.J., 2015. Responses of soil
507
micro-food web to long-term fertilization in a wheat–maize rotation system. Appl. Soil Ecol. 98,
508
56-64.
26
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Tables
510
Table 1. Repeated-measures analysis of N addition level and sampling time on different nematode trophic groups. Abundance
511
Richness
Shannon-Wiener diversity (H')
Time (T)
Nitrogen (N)
T×N
Time (T)
Nitrogen (N)
T×N
Time (T)
Nitrogen (N)
T×N
Bacterivores Fungivores Plant-parasites
11.54** 16.27** 13.25**
15.81** 15.12** 12.62**
1.74 1.93* 2.21*
10.14** 13.59** 6.59**
5.76** 15.23** 6.62**
2.15* 0.89 0.40
1.23 10.97** 10.12**
37.24** 8.96** 29.96**
1.29 2.29** 2.36**
Predators-omnivores
10.84**
30.61**
2.34** 13.29**
23.53**
1.96*
2.06
50.35**
1.53
Significant: *, P < 0.05; **, P < 0.01
512
27
513
Table 2. Nematode diversity of different trophic groups in different N addition levels across the four sampling times Bacterivores
Abundance
Richness
514
Fungivores
Plant parasites
Predators-omnivores
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
Aug-2014
Oct-2014
Mar-2015
May-2015
N0
820.1 ± 164.7a
708.1 ± 92.3ab
408.0 ± 76.0a
574.9 ± 40.7a
218.5 ± 38.9a
238.4 ± 57.1b
115.0 ± 19.3a
284.7 ± 77.5a
1662.0 ± 319.9a
681.5 ± 100.3a
492.8 ± 55.3ab
902.8 ± 181.9a
479.8 ± 97.0a
347.2 ± 53.5a
267.5 ± 49.2a
630.3 ± 68.7a
N2
591.6 ± 187.9a
741.7 ± 50.3ab
295.0 ± 59.7a
590.0 ± 73.0a
226.9 ± 36.7a
387.3 ± 47.6a
84.1 ± 18.5ab
220.8 ± 35.3a
863.1 ± 205.4b
710.5 ± 62.3a
319.3 ± 43.2bc
691.6 ± 98.9a
285.3 ± 100.3b
220.5 ± 26.0b
164.9 ± 26.0b
397.7 ± 49.3b
N5
684.8 ± 73.2a
894.1 ± 102.8a
327.8 ± 57.8a
544.8 ± 115.3a
177.4 ± 44.4a
220.7 ± 29.8b
68.6 ± 20.3b
183.3 ± 50.0ab
935.7 ± 243.6b
659.7 ± 130.6a
408.3 ± 128.2ab
503.5 ± 94.8b
184.8 ± 32.1bc
224.6 ± 51.2b
161.9 ± 29.9b
363.4 ± 105.1b
N10
327.7 ± 117.8b
632.5 ± 156.9ab
395.1 ± 64.1a
468.7 ± 89.9a
74.5 ± 17.3b
227.0 ± 80.0b
54.2 ± 13.6b
168.1 ± 30.7ab
748.3 ± 165.9b
679.7 ± 181.1a
387.6 ± 99.0ab
661.1 ± 45.9ab
64.7 ± 19.0cd
102.7 ± 31.8bc
109.7 ± 29.5b
184.0 ± 18.3c
N20
493.7 ± 108.2b
375.9 ± 75.1b
312.1 ± 38.2a
294.8 ± 51.1ab
77.8 ± 46.4b
100.7 ± 34.0bc
41.2 ± 10.9bc
69.3 ± 18.6bc
819.1 ± 165.2b
534.3 ± 128.0a
617.1 ± 82.8a
392.5 ± 65.5b
115.5 ± 21.5bcd
116.6 ± 46.3bc
88.7 ± 21.8b
N50
89.4 ± 17.0c
150.6 ± 53.9c
82.8 ± 31.4b
13.3 ± 3.3b
3.9 ± 1.6b
20.9 ± 10.5c
0.3 ± 0.3c
0.4 ± 0.4c
181.4 ± 61.4c
103.2 ± 30.5b
177.4 ± 87.1c
46.7 ± 20.8c
11.7 ± 6.5d
4.1 ± 3.4c
1.7 ± 1.1c
1.6 ± 1.2d
N0
26.0 ± 3.3
36.7 ± 3.9
31.2 ± 2.7a
25.7 ±2.9b
7.0 ± 0.9b
12.7 ± 3.0ab
9.0 ± 1.2a
11.0 ± 2.0a
52.0 ± 3.2
34.8 ± 4.0
39.3 ± 3.7
36.2 ± 3.0a
15.3 ± 1.6a
17.3 ± 2.1a
20.5 ± 2.4a
27.3 ± 2.8a
N2
29.7 ± 2.2
38.3 ± 3.1
33.2 ± 5.7a
31.2 ± 2.2ab
13.0 ± 1.9a
19.5 ± 1.4a
10.2 ± 2.3a
11.8 ± 1.9a
45.2 ± 2.5
36.2 ± 2.0
37.5 ± 4.3
36.0 ± 2.8a
13.5 ± 1.5a
11.3 ± 1.3b
18.8 ± 2.1a
21.0 ± 1.6a
N5
35.7 ± 3.2
48.8 ± 2.5
36.8 ± 6.1a
36.3 ± 3.2a
9.7 ± 2.5ab
12.2 ± 1.0ab
6.7 ± 1.1ab
12.0 ± 1.5a
44.5 ± 5.7
35.5 ± 4.3
38.7 ± 4.5
34.8 ± 4.4a
10.5 ± 2.2a
12.0 ± 2.5b
18.8 ± 4.2a
25.0 ± 6.2a
N10
29.3 ± 7.0
40.5 ± 5.3
43.5 ± 3.4a
30.3 ± 2.9ab
6.5 ± 1.2b
16.2 ± 5.5ab
5.7 ± 1.1b
11.5 ± 2.3a
60.3 ± 6.3
42.2 ± 5.5
40.3 ± 3.5
45.7 ± 2.2a
5.7 ± 1.4ab
7.5 ± 2.2b
10.8 ± 1.5b
12.7 ± 1.0b
N20
33.3 ± 3.4
35.8 ± 3.3
30.0 ± 3.4a
36.2 ± 6.0a
4.0 ± 1.5b
9.0 ± 1.7b
3.8 ± 1.0bc
8.3 ± 1.8a
56.8 ± 4.6
49.5 ± 4.8
57.7 ± 4.0
46.8 ± 5.8a
8.3 ± 1.2a
9.5 ± 2.8b
8.5 ± 1.9b
10.3 ± 2.3b
N50
25.8 ± 3.3
37.2 ± 9.5
12.7 ± 6.5b
5.7 ± 1.4c
1.2 ± 0.5c
4.7 ± 1.9c
0.2 ± 0.2c
0.2 ± 0.2b
44.5 ± 11.5
28.5 ± 8.7
28.7 ± 15.4
19.8 ± 8.8b
2.8 ± 1.5b
1.2 ± 0.8c
0.3 ± 0.2c
0.7 ± 0.5c
79.2 ± 17.5cd
Shannon-Wiener
N0
1.6 ± 0.1ab
1.6 ± 0.1a
1.7 ± 0.1a
1.7 ± 0.1ab
0.7 ± 0.1a
0.5 ± 0.2
0.5 ± 0.1a
0.6 ± 0.2a
2.0 ± 0.1ab
2.1 ± 0.1a
2.0 ± 0a
1.9 ± 0.1a
1.7 ± 0.1a
1.6 ± 0.1a
1.5 ± 0.1a
1.8 ± 0.1a
diversity
N2
1.6 ± 0.1ab
1.5 ± 0.1a
1.6 ± 0.1a
1.8 ± 0a
1.0 ± 0.1a
0.4 ± 0.1
0.5 ± 0.1a
0.7 ± 0.1a
2.1 ± 0.1ab
2.1 ± 0.1a
2.1 ± 0.1a
2.0 ± 0.1a
1.5 ± 0.1ab
1.4 ± 0.1a
1.6 ± 0.1a
1.7 ± 0.1ab
N5
1.8 ± 0.1a
1.6 ± 0.1a
1.7 ± 0.1a
1.7 ± 0.1ab
0.9 ± 0.2a
0.7 ± 0.1
0.6 ± 0.1a
0.7 ± 0a
2.2 ± 0.1a
2.1 ± 0.1a
1.7 ± 0.1ab
2.0 ± 0.2a
1.3 ± 0.2ab
1.3 ± 0.1a
1.6 ± 0.1a
1.7 ± 0ab
N10
1.3 ± 0.1abc
1.6 ± 0.2a
1.4 ± 0ab
1.4 ± 0.1ab
1.0 ± 0.1a
0.9 ± 0.1
0.4 ± 0.1a
0.7 ± 0.1a
2.0 ± 0.1ab
1.9 ± 0.1a
1.7 ± 0.1ab
1.9 ± 0.1a
0.9 ± 0.2abc
1.3 ± 0.2a
1.0 ± 0.1a
1.4 ± 0.1bc
N20
1.2 ± 0.2bc
1.0 ± 0.1b
1.1 ± 0.1bc
1.3 ± 0.1b
0.7 ± 0.2ab
0.9 ± 0.1
0.3 ± 0.1a
0.4 ± 0.1a
1.9 ± 0.1ab
1.8 ± 0.2ab
1.4 ± 0.1b
1.8 ± 0.1a
0.8 ± 0.2bc
1.0 ± 0.2a
1.0 ± 0.3a
1.2 ± 0.2c
N50
1.0 ± 0.1c
0.7 ± 0.2b
0.9 ± 0.1c
0.7 ± 0.2c
0.2 ± 0.1b
0.7 ± 0.2
0b
1.7 ± 0.1b
1.2 ± 0.2b
0.5 ± 0.2c
1.2 ± 0.2b
0.5 ± 0.3ac
0.2 ± 0.1b
0b
Different lower-case letters represent significant differences among different N addition levels, as determined by Tukey’s test, P < 0.05.
515 516
28
0b
0d
517
Figure legends
518
Figure 1. Soil pH and moisture in different N addition treatments across different
519
sampling times. Different capital and lower-case letters represent significant
520
differences among different times and N addition levels, respectively, as determined
521
by Tukey’s test. *, P < 0.05; **, P < 0.01.
522
Figure 2. Nematode abundance, richness and Shannon-Wiener diversity in different
523
N addition level treatments across different sampling times. Different capital and
524
lower-case letters represent significant differences among different times and N
525
addition levels, as determined by Tukey’s test. *, P < 0.05; **, P < 0.01.
526
Figure 3. Principle components analysis (PCA) of soil nematode communities in
527
different N addition level treatments across the different sampling times.
528
Figure 4. The stability and synchrony of nematode community in different N addition
529
level. Different lower-case letters represent significant differences among different N
530
addition levels, as determined by Tukey’s test, P < 0.05. BF, bacterivores; FF,
531
fungivores; PP, plant-parasites; OP, predators-omnivores.
532
Figure 5. Linear regressions between nematode stability, diversity and soil properties.
533
Figure 6. Structural equation modeling of the temporal variation of nematode
534
community under N addition level (χ2 = 3.682, df = 4, P = 0.451, GFI = 0.966).
535
Numbers next to the arrows are the standardized path coefficients. The width of the
536
arrows indicates the strength of the causal influence. 29
537 538
Figure. 1
539
30
540 541
Figure. 2
31
542 543
Figure. 3
32
544 545
Figure. 4 33
546 547
Figure. 5 34
548 549
Figure. 6
35
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Supplement
552 553
Figure A1.
36
Conflict of interest The authors declare that they have no conflicts of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.