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Distinct regulation of microbial processes in the immobilization of labile carbon in different soils
Journal Pre-proof Distinct regulation of microbial processes in the immobilization of labile carbon in different soils Xinxin Wang, Wei Zhang, Feng Zhou, Yan Liu, Hongbo He, Xudong Zhang PII:
S0038-0717(20)30020-1
DOI:
https://doi.org/10.1016/j.soilbio.2020.107723
Reference:
SBB 107723
To appear in:
Soil Biology and Biochemistry
Received Date: 10 October 2019 Revised Date:
16 December 2019
Accepted Date: 14 January 2020
Please cite this article as: Wang, X., Zhang, W., Zhou, F., Liu, Y., He, H., Zhang, X., Distinct regulation of microbial processes in the immobilization of labile carbon in different soils, Soil Biology and Biochemistry (2020), doi: https://doi.org/10.1016/j.soilbio.2020.107723. 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 Ltd.
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Type of contribution: Short Communication
2
Date of resubmission: December 16, 2019
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Total text pages: 12
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Total Figures and tables: 3 figures and 1 table
5 6
Distinct regulation of microbial processes in the immobilization of labile carbon
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in different soils
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Xinxin Wanga,b, Wei Zhanga, Feng Zhoua,b, Yan Liua,b, Hongbo Hea,c*, Xudong
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Zhanga**
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a
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
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b
University of Chinese Academy of Sciences, Beijing 100049, China
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c
National Field Research Station of Shenyang Agroecosystems, Chinese Academy of
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Sciences, Shenyang 110016, China
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* Corresponding author: Hongbo He
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** Co-corresponding author: Xudong Zhang
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E-mail address: [email protected] (Hongbo He), [email protected] (Xudong Zhang).
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Abstract
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A process-based understanding of soil carbon (C) sequestration and stabilization
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has not been precisely characterized due to the lacking of linkage between microbial
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proliferation and mortality. In this study, stable isotope probing of phospholipid fatty
29
acids and amino sugars were used to determine the microbial responses and microbial
30
residue retention in two soils (Mollisol and Ultisol) with 13C-labeled glucose addition.
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The microbial responses stimulated by glucose were greater in C-poor Ultisol than in
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C-rich Mollisol. However, the transformation of labile C to microbial residues in
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Mollisol was more rapid. Therefore, the starvation effect may control microbial
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growth and microbial residue production, and thus resulting in distinct sequestration
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and stabilization process of labile C in different soils.
36 37
Keywords: Phospholipid fatty acids; Amino sugars;
38
response; Microbial residue; Soil C sequestration
13
C-labeled glucose; Microbial
39 40
Microorganisms are essential for soil organic carbon (SOC) turnover (Miltner et al.,
41
2012; Bastida et al., 2013; Schaeffer et al., 2015). However, the linkage between
42
microbial responses and microbial necromass formation is unclear, even though the
43
microbial carbon (C) pump model predicts that microbial turnover controls SOC
44
sequestration and stabilization (Liang et al., 2017). The key obstacle to characterizing
45
this linkage is the inability to accurately estimate both microbial biomass and
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necromass. Methods for tracking microbial biomarker dynamics can resolve this issue
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(Liang et al., 2016; Gunina et al., 2017). Microbial responses to extraneous C
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immobilization could be characterized by stable isotope probing of phospholipid fatty
49
acids (13C-PLFA-SIP) (Ma et al., 2018). Subsequently, accrued C can be stabilized in
50
microbial residues, as determined by stable isotope probing of amino sugars
51
(13C-AS-SIP) (Liang et al., 2010). Therefore, linking snapshot variation in
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phospholipid fatty acids (PLFAs) with the legacy effects of amino sugars (ASs) could
53
reflect the intrinsic biological balance in microbial growth, metabolism and mortality,
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and provide a process-based understanding of microbial functions in mediating SOC
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dynamics (Shao et al., 2019).
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The control of microbial processes by interactions between substrate availability
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and potential decomposers are highly dependent on soil types (Hicks et al., 2019).
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Even for substrates with high availability (i.e. glucose), the amounts of
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and bacterial-PLFAs in soil differ depending on ecosystems and management
60
strategies (Dungait et al., 2011; Lemanski and Scheu 2014; Zhang et al., 2016).
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However, distinct microbial responses in different habitats have not been
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characterized. Soils with higher organic C can generally maintain higher ASs contents
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(Liang et al., 2006), but it is not clear how labile substrates promote the accumulation
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of fungal or bacterial residues. This requires an understanding of both intrinsic
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stability and the transformation from microbial proliferation to mortality.
13
C-fungal-
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Therefore, we added 13C-glucose into two types of soils to explore the dynamics of
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labile C flow from living biomass to necromass of microorganisms by PLFA-SIP and
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AS-SIP. We hypothesized that the response of soil microorganisms to labile C is
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controlled by the intrinsic microbial status and thus the sequestration of extraneous C
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is driven by distinct microbial responses and the accumulation of bacterial and fungal
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debris.
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The two soils (Mollisol and Ultisol) were collected from long-term fertilization
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field experiment of Jilin Academy of Agricultural Sciences (Gongzhuling, Jilin) and
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Ecological Experimental Station of Red soil (Yingtan, Jiangxi), respectively (Table 1).
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Maize (Zea mays L.) was annually sowed in Mollisol and total amounts of 150 kg N
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ha-1, 33 kg P ha-1 and 63 kg K ha-1 was applied annually. Peanut (Arachis hypogaea)
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was annually sowed in Ultisol and 121 kg N ha-1, 40 kg P ha-1 and 112 kg K ha-1 were
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applied annually. The top 0-20 cm of these soils was sampled in 2014 using a
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stainless-steel hand auger. Ten cores were collected and sieved through a 2 mm sieve
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and the composite soil samples were air-dried and stored till 2018. After the soils
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were pre-incubated for 7 days with K2HPO4,
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NH4NO3 were added weekly at rates of 1.0 g C kg-1 and 0.1 g N kg-1 soil during
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8-week incubation at 25
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conducted to obtain natural
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incubated soils were destructively sampled weekly with three replicates. Portions of
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the sampled soil were lyophilized for PLFAs analyses (Bligh and Dyer, 1959;
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Frostegård et al., 1991) or air-dried for ASs analyses (Zhang and Amelung, 1996).
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The concentration (CT) of each PLFA or AS was detected by gas chromatography
89
(GC) (Table S1). The
90
ASs was determined by GC-mass spectrometry (GC/MS) and the concentration of
91
13
13
13
C-labeled glucose (99 atom%) and
. A set of incubation without glucose addition was 13
C abundance from the background soil. All the
C-enrichment (Atom Percentage Excess, APE) of PLFAs or
C-PLFAs or ASs (CL) was calculated according to the following equation: CL = CT
13
92
× APE/100 (He et al., 2006). The
93
isotope ratio mass spectrometer coupled to an elemental analyzer. Repeated measures
94
one-way ANOVA and post hoc Duncan’s tests were employed to identify the effects
95
of substrate addition on
96
Student’s t test was used to detect the significantly different between two soils
97
(Mollisol and Ultisol).
13
C-SOC,
C enrichment of SOC was quantified using an
13
C-PLFAs, and
13
C-ASs during time dynamics.
98
During the whole incubation period, the 13C-PLFA concentrations increased in both
99
soils (Fig. S1), reflecting the stimulation of microbial proliferation by glucose (Hoyle
100
et al., 2007; Meidute et al., 2008). However, the ratios of 13C-PLFA concentrations to
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initial values, an index of the activation of microorganisms (dominantly including
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fungi and bacteria), were higher in Ultisol than in Mollisol, (Fig. 1a, c, d; Table S2).
103
Consequently, the
104
larger than those in Mollisol (Figs. 1b, S2; Table S2), suggesting that more microbial
105
biomass was actively involved in labile C immobilization in Ultisol than in Mollisol.
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Considering the lower contents of SOC in Ultisol than in Mollisol (Table 1), our
107
results clearly indicated that microorganisms constrained by starvation in C-poor soil
108
could maintain higher metabolic readiness and thus exhibited greater activity to
109
assimilate exogenous labile substrates, compared with the weaker response of
110
microorganisms in C-rich soil (Kong et al., 2018; Zhang et al., 2019).
111
13
C-PLFA concentrations per unit
With sustained glucose addition, the ratios of
13
13
C-SOC in Ultisol remained
C-fungal- to
13
C-bacterial-PLFAs
112
remained time-independent in Mollisol but increased sharply in Ultisol, despite the
113
initially rapid incorporation of
13
C into bacterial PLFAs (Fig. 1e). Under starvation
114
conditions in Ultisol, such shift in the microbial community involved in glucose
115
assimilation was mainly attributed to the intensive competition between fungi and
116
bacteria following the initial preference for glucose by bacteria (Neumann et al.,
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2014). The reduced competition between microbial groups in Mollisol, attributed to
118
the decreased dependence on exogenous C, could explain the identical responses of
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fungi and bacteria to glucose. Furthermore, the soil pH decreased significantly during
120
our incubation by approximately 0.8 units in both soil microcosms (Fig. S3), and this
121
could also contribute to the succession of fungi to bacteria in Ultisol, together with
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native C availability. Thus, as expected, an increase in fungal biomass production in
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Ultisol would favor effective exogenous C immobilization (de Graaff et al. 2010;
124
Zhang et al. 2013).
125
Along with the activation of microbial groups, microbial residues accumulated in 13
C-ASs to
13
126
soil microcosms (Fig. S4). The ratio of
C-PLFAs could be used to
127
compare the retention efficiency of glucose-C from microbial biomass to necromass
128
between different soils. During our incubation, 13C-ASs/13C-PLFAs ratios in Mollisol,
129
both of the total and heterogeneous (derived from fungi or bacteria), remained higher
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than those in Ultisol (Fig. 2; Table S2), suggesting that microbial residue
131
accumulation was greater in C-rich soil despite the lower rate of proliferation in
132
response to labile C. In contrast, the addition of labile C favored the maintenance of
133
the activated microbial biomass in C-poor Ultisol and thereby led to extended
134
microbial biomass turnover (Liu et al., 2018). During the accumulation of
135
heterogeneous microbial necromass, similar to the entombing of microbial biomass,
13
C-GluN/13C-fungal PLFAs ratios increased and/or remained steady while
136
the
137
13
138
the higher retention of fungal residues than bacterial counterparts (Guggenberger et al.,
139
1999; Six et al., 2006; He et al., 2011; Ding et al., 2013). Until the end of the
140
incubation, the
141
while the
142
suggesting that immobilized C in C-poor soil may be more stable than that in C-rich
143
soil due to the enhanced fungal residue retention (Soares and Rousk, 2019). Our
144
finding indicated an importance of slow fungal necromass turnover in soil C retention,
145
as in line with the role of extramatrical mycelia necromass in building up forest
146
humus (Ekblad et al., 2016).
C-MurN/13C-bacterial PLFAs ratios decreased in both soils (Fig. 2b, c), confirming
13
13
C-GluN/13C-SOC ratio in Ultisol was higher than that in Mollisol
C-MurN/13C-SOC ratio showed the opposite pattern (Fig. 3; Table S2),
147
Therefore, by combining PLFA-SIP and AS-SIP analyses, we found that distinct
148
microbial processes regulated the immobilization of labile C in different soils, with a
149
strong effect of microbial starvation. Mollisol with a high organic C content exhibited
150
more efficient microbial residue accumulation, even though microorganisms were less
151
sensitive to labile C. Microorganisms in C-poor Ultisol retained higher microbial
152
response and prolonged microbial biomass turnover. The greater extraneous C
153
immobilization in Ultisol was mainly mediated by the succession pattern from
154
bacteria to fungi as well as the preferential C accrual in fungal residues. Our findings
155
exhibited important implications on exploring the sequestration and stabilization
156
mechanisms of SOC driven by microorganism process.
157
158
Acknowledgements
159
We thank all individuals who helped collect and process the soil samples in the
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Ecological Experimental Station of Red Soil, Chinese Academy of Sciences. This
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work was supported by The Natural Science Foundation of China (41630862,
162
41977025) and National Key Research & Development Program (2017YFD0200100).
163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185
186
Figure captions
187 188
Fig. 1 Ratios of 13C-phospholipid fatty acids (PLFAs) to initial PLFAs (a, c, d), ratios
189
of total 13C-PLFAs to
190
PLFAs (e) during 8-week incubation in different soils. Error bars represent standard
191
error (n=3). For each soil, different letters denote significant differences between
192
sampling intervals (P < 0.05).
13
C-SOC (b) and ratios of
13
C-Fungal PLFAs to
13
C-Bacterial
193 194
Fig. 2 Ratios of 13C-labeled amino sugars (13C-ASs) to 13C-labeled phospholipid fatty
195
acids (13C-PLFAs) during 8-week incubation in different soils. The
196
sum of
197
(MurN). Error bars represent standard error (n=3). For each soil, different letters
198
denote significant differences between sampling intervals (P < 0.05).
13
13
C-ASs was the
C-labeled glucosamine (GluN), galactosamine (GalN) and muramic acid
199 13
C-labeled amino sugars (13C-ASs) in
13
200
Fig. 3 The proportions of
201
8-week incubation in different soils. Error bars represent standard error (n=3). For
202
each soil, different letters denote significant differences between sampling intervals (P
203
< 0.05).
204 205 206 207 208 209 210 211 212
C-SOC during
213
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Table 1 Some properties of the studied soil samples (0-20 cm) and the initial concentrations of PLFAs and amino sugars in soil
Samples
Mollisol
Ultisol
Sample plot
Jilin Academy of Agricultural Sciences, Gongzhuling, Jilin Province, China (124°48´E, 43°30´N) Ecological Experimental Station of Red soil, Chinese Academy of Sciences, Yingtan, Jiangxi Province, China (116°55´ E, 28°15´N)
Soil Organic C1 (g kg-1)
Total N (g kg-1)
C/N1
15.35
1.37
11.2
5.67
0.67
8.46
1
pH (soil: water=1:2.5)
PLFAs2 (mg kg-1 soil)
Amino Sugars (mg kg-1 soil)
Bacterial PLFA
Fungal PLFA
MurN3
GluN3
6.00
7.19
0.79
64.4
612.67
4.46
4.67
0.59
39.73
319.72
Note: 1. Soil Organic C was the soil organic carbon. Total N means the total soil nitrogen. C/N represents the ratio of soil organic carbon and soil total nitrogen. 2. PLFAs: Phospholipid fatty acids 3. MurN: muramic acid and GluN: glucosamine
Highlights
•
PLFA/AS-SIP is powerful for exploring microbial process in mediating SOC accrual.
•
The activation of microbial responsiveness was controlled by starving effect.
•
Successional fungal growth in C-poor Ultisol favored efficient C immobilization.
•
The conversion of labile C to microbial residues was rapid in C-rich Mollisol.
Conflicts of interest The authors declare no conflict of interest.
This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal’s policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.
S0038-0717(20)30020-1
DOI:
https://doi.org/10.1016/j.soilbio.2020.107723
Reference:
SBB 107723
To appear in:
Soil Biology and Biochemistry
Received Date: 10 October 2019 Revised Date:
16 December 2019
Accepted Date: 14 January 2020
Please cite this article as: Wang, X., Zhang, W., Zhou, F., Liu, Y., He, H., Zhang, X., Distinct regulation of microbial processes in the immobilization of labile carbon in different soils, Soil Biology and Biochemistry (2020), doi: https://doi.org/10.1016/j.soilbio.2020.107723. 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 Ltd.
1
Type of contribution: Short Communication
2
Date of resubmission: December 16, 2019
3
Total text pages: 12
4
Total Figures and tables: 3 figures and 1 table
5 6
Distinct regulation of microbial processes in the immobilization of labile carbon
7
in different soils
8
Xinxin Wanga,b, Wei Zhanga, Feng Zhoua,b, Yan Liua,b, Hongbo Hea,c*, Xudong
9
Zhanga**
10 11
a
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
12
b
University of Chinese Academy of Sciences, Beijing 100049, China
13
c
National Field Research Station of Shenyang Agroecosystems, Chinese Academy of
14
Sciences, Shenyang 110016, China
15 16
* Corresponding author: Hongbo He
17
** Co-corresponding author: Xudong Zhang
18
E-mail address: [email protected] (Hongbo He), [email protected] (Xudong Zhang).
19 20 21 22 23 24
25
Abstract
26
A process-based understanding of soil carbon (C) sequestration and stabilization
27
has not been precisely characterized due to the lacking of linkage between microbial
28
proliferation and mortality. In this study, stable isotope probing of phospholipid fatty
29
acids and amino sugars were used to determine the microbial responses and microbial
30
residue retention in two soils (Mollisol and Ultisol) with 13C-labeled glucose addition.
31
The microbial responses stimulated by glucose were greater in C-poor Ultisol than in
32
C-rich Mollisol. However, the transformation of labile C to microbial residues in
33
Mollisol was more rapid. Therefore, the starvation effect may control microbial
34
growth and microbial residue production, and thus resulting in distinct sequestration
35
and stabilization process of labile C in different soils.
36 37
Keywords: Phospholipid fatty acids; Amino sugars;
38
response; Microbial residue; Soil C sequestration
13
C-labeled glucose; Microbial
39 40
Microorganisms are essential for soil organic carbon (SOC) turnover (Miltner et al.,
41
2012; Bastida et al., 2013; Schaeffer et al., 2015). However, the linkage between
42
microbial responses and microbial necromass formation is unclear, even though the
43
microbial carbon (C) pump model predicts that microbial turnover controls SOC
44
sequestration and stabilization (Liang et al., 2017). The key obstacle to characterizing
45
this linkage is the inability to accurately estimate both microbial biomass and
46
necromass. Methods for tracking microbial biomarker dynamics can resolve this issue
47
(Liang et al., 2016; Gunina et al., 2017). Microbial responses to extraneous C
48
immobilization could be characterized by stable isotope probing of phospholipid fatty
49
acids (13C-PLFA-SIP) (Ma et al., 2018). Subsequently, accrued C can be stabilized in
50
microbial residues, as determined by stable isotope probing of amino sugars
51
(13C-AS-SIP) (Liang et al., 2010). Therefore, linking snapshot variation in
52
phospholipid fatty acids (PLFAs) with the legacy effects of amino sugars (ASs) could
53
reflect the intrinsic biological balance in microbial growth, metabolism and mortality,
54
and provide a process-based understanding of microbial functions in mediating SOC
55
dynamics (Shao et al., 2019).
56
The control of microbial processes by interactions between substrate availability
57
and potential decomposers are highly dependent on soil types (Hicks et al., 2019).
58
Even for substrates with high availability (i.e. glucose), the amounts of
59
and bacterial-PLFAs in soil differ depending on ecosystems and management
60
strategies (Dungait et al., 2011; Lemanski and Scheu 2014; Zhang et al., 2016).
61
However, distinct microbial responses in different habitats have not been
62
characterized. Soils with higher organic C can generally maintain higher ASs contents
63
(Liang et al., 2006), but it is not clear how labile substrates promote the accumulation
64
of fungal or bacterial residues. This requires an understanding of both intrinsic
65
stability and the transformation from microbial proliferation to mortality.
13
C-fungal-
66
Therefore, we added 13C-glucose into two types of soils to explore the dynamics of
67
labile C flow from living biomass to necromass of microorganisms by PLFA-SIP and
68
AS-SIP. We hypothesized that the response of soil microorganisms to labile C is
69
controlled by the intrinsic microbial status and thus the sequestration of extraneous C
70
is driven by distinct microbial responses and the accumulation of bacterial and fungal
71
debris.
72
The two soils (Mollisol and Ultisol) were collected from long-term fertilization
73
field experiment of Jilin Academy of Agricultural Sciences (Gongzhuling, Jilin) and
74
Ecological Experimental Station of Red soil (Yingtan, Jiangxi), respectively (Table 1).
75
Maize (Zea mays L.) was annually sowed in Mollisol and total amounts of 150 kg N
76
ha-1, 33 kg P ha-1 and 63 kg K ha-1 was applied annually. Peanut (Arachis hypogaea)
77
was annually sowed in Ultisol and 121 kg N ha-1, 40 kg P ha-1 and 112 kg K ha-1 were
78
applied annually. The top 0-20 cm of these soils was sampled in 2014 using a
79
stainless-steel hand auger. Ten cores were collected and sieved through a 2 mm sieve
80
and the composite soil samples were air-dried and stored till 2018. After the soils
81
were pre-incubated for 7 days with K2HPO4,
82
NH4NO3 were added weekly at rates of 1.0 g C kg-1 and 0.1 g N kg-1 soil during
83
8-week incubation at 25
84
conducted to obtain natural
85
incubated soils were destructively sampled weekly with three replicates. Portions of
86
the sampled soil were lyophilized for PLFAs analyses (Bligh and Dyer, 1959;
87
Frostegård et al., 1991) or air-dried for ASs analyses (Zhang and Amelung, 1996).
88
The concentration (CT) of each PLFA or AS was detected by gas chromatography
89
(GC) (Table S1). The
90
ASs was determined by GC-mass spectrometry (GC/MS) and the concentration of
91
13
13
13
C-labeled glucose (99 atom%) and
. A set of incubation without glucose addition was 13
C abundance from the background soil. All the
C-enrichment (Atom Percentage Excess, APE) of PLFAs or
C-PLFAs or ASs (CL) was calculated according to the following equation: CL = CT
13
92
× APE/100 (He et al., 2006). The
93
isotope ratio mass spectrometer coupled to an elemental analyzer. Repeated measures
94
one-way ANOVA and post hoc Duncan’s tests were employed to identify the effects
95
of substrate addition on
96
Student’s t test was used to detect the significantly different between two soils
97
(Mollisol and Ultisol).
13
C-SOC,
C enrichment of SOC was quantified using an
13
C-PLFAs, and
13
C-ASs during time dynamics.
98
During the whole incubation period, the 13C-PLFA concentrations increased in both
99
soils (Fig. S1), reflecting the stimulation of microbial proliferation by glucose (Hoyle
100
et al., 2007; Meidute et al., 2008). However, the ratios of 13C-PLFA concentrations to
101
initial values, an index of the activation of microorganisms (dominantly including
102
fungi and bacteria), were higher in Ultisol than in Mollisol, (Fig. 1a, c, d; Table S2).
103
Consequently, the
104
larger than those in Mollisol (Figs. 1b, S2; Table S2), suggesting that more microbial
105
biomass was actively involved in labile C immobilization in Ultisol than in Mollisol.
106
Considering the lower contents of SOC in Ultisol than in Mollisol (Table 1), our
107
results clearly indicated that microorganisms constrained by starvation in C-poor soil
108
could maintain higher metabolic readiness and thus exhibited greater activity to
109
assimilate exogenous labile substrates, compared with the weaker response of
110
microorganisms in C-rich soil (Kong et al., 2018; Zhang et al., 2019).
111
13
C-PLFA concentrations per unit
With sustained glucose addition, the ratios of
13
13
C-SOC in Ultisol remained
C-fungal- to
13
C-bacterial-PLFAs
112
remained time-independent in Mollisol but increased sharply in Ultisol, despite the
113
initially rapid incorporation of
13
C into bacterial PLFAs (Fig. 1e). Under starvation
114
conditions in Ultisol, such shift in the microbial community involved in glucose
115
assimilation was mainly attributed to the intensive competition between fungi and
116
bacteria following the initial preference for glucose by bacteria (Neumann et al.,
117
2014). The reduced competition between microbial groups in Mollisol, attributed to
118
the decreased dependence on exogenous C, could explain the identical responses of
119
fungi and bacteria to glucose. Furthermore, the soil pH decreased significantly during
120
our incubation by approximately 0.8 units in both soil microcosms (Fig. S3), and this
121
could also contribute to the succession of fungi to bacteria in Ultisol, together with
122
native C availability. Thus, as expected, an increase in fungal biomass production in
123
Ultisol would favor effective exogenous C immobilization (de Graaff et al. 2010;
124
Zhang et al. 2013).
125
Along with the activation of microbial groups, microbial residues accumulated in 13
C-ASs to
13
126
soil microcosms (Fig. S4). The ratio of
C-PLFAs could be used to
127
compare the retention efficiency of glucose-C from microbial biomass to necromass
128
between different soils. During our incubation, 13C-ASs/13C-PLFAs ratios in Mollisol,
129
both of the total and heterogeneous (derived from fungi or bacteria), remained higher
130
than those in Ultisol (Fig. 2; Table S2), suggesting that microbial residue
131
accumulation was greater in C-rich soil despite the lower rate of proliferation in
132
response to labile C. In contrast, the addition of labile C favored the maintenance of
133
the activated microbial biomass in C-poor Ultisol and thereby led to extended
134
microbial biomass turnover (Liu et al., 2018). During the accumulation of
135
heterogeneous microbial necromass, similar to the entombing of microbial biomass,
13
C-GluN/13C-fungal PLFAs ratios increased and/or remained steady while
136
the
137
13
138
the higher retention of fungal residues than bacterial counterparts (Guggenberger et al.,
139
1999; Six et al., 2006; He et al., 2011; Ding et al., 2013). Until the end of the
140
incubation, the
141
while the
142
suggesting that immobilized C in C-poor soil may be more stable than that in C-rich
143
soil due to the enhanced fungal residue retention (Soares and Rousk, 2019). Our
144
finding indicated an importance of slow fungal necromass turnover in soil C retention,
145
as in line with the role of extramatrical mycelia necromass in building up forest
146
humus (Ekblad et al., 2016).
C-MurN/13C-bacterial PLFAs ratios decreased in both soils (Fig. 2b, c), confirming
13
13
C-GluN/13C-SOC ratio in Ultisol was higher than that in Mollisol
C-MurN/13C-SOC ratio showed the opposite pattern (Fig. 3; Table S2),
147
Therefore, by combining PLFA-SIP and AS-SIP analyses, we found that distinct
148
microbial processes regulated the immobilization of labile C in different soils, with a
149
strong effect of microbial starvation. Mollisol with a high organic C content exhibited
150
more efficient microbial residue accumulation, even though microorganisms were less
151
sensitive to labile C. Microorganisms in C-poor Ultisol retained higher microbial
152
response and prolonged microbial biomass turnover. The greater extraneous C
153
immobilization in Ultisol was mainly mediated by the succession pattern from
154
bacteria to fungi as well as the preferential C accrual in fungal residues. Our findings
155
exhibited important implications on exploring the sequestration and stabilization
156
mechanisms of SOC driven by microorganism process.
157
158
Acknowledgements
159
We thank all individuals who helped collect and process the soil samples in the
160
Ecological Experimental Station of Red Soil, Chinese Academy of Sciences. This
161
work was supported by The Natural Science Foundation of China (41630862,
162
41977025) and National Key Research & Development Program (2017YFD0200100).
163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185
186
Figure captions
187 188
Fig. 1 Ratios of 13C-phospholipid fatty acids (PLFAs) to initial PLFAs (a, c, d), ratios
189
of total 13C-PLFAs to
190
PLFAs (e) during 8-week incubation in different soils. Error bars represent standard
191
error (n=3). For each soil, different letters denote significant differences between
192
sampling intervals (P < 0.05).
13
C-SOC (b) and ratios of
13
C-Fungal PLFAs to
13
C-Bacterial
193 194
Fig. 2 Ratios of 13C-labeled amino sugars (13C-ASs) to 13C-labeled phospholipid fatty
195
acids (13C-PLFAs) during 8-week incubation in different soils. The
196
sum of
197
(MurN). Error bars represent standard error (n=3). For each soil, different letters
198
denote significant differences between sampling intervals (P < 0.05).
13
13
C-ASs was the
C-labeled glucosamine (GluN), galactosamine (GalN) and muramic acid
199 13
C-labeled amino sugars (13C-ASs) in
13
200
Fig. 3 The proportions of
201
8-week incubation in different soils. Error bars represent standard error (n=3). For
202
each soil, different letters denote significant differences between sampling intervals (P
203
< 0.05).
204 205 206 207 208 209 210 211 212
C-SOC during
213
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Table 1 Some properties of the studied soil samples (0-20 cm) and the initial concentrations of PLFAs and amino sugars in soil
Samples
Mollisol
Ultisol
Sample plot
Jilin Academy of Agricultural Sciences, Gongzhuling, Jilin Province, China (124°48´E, 43°30´N) Ecological Experimental Station of Red soil, Chinese Academy of Sciences, Yingtan, Jiangxi Province, China (116°55´ E, 28°15´N)
Soil Organic C1 (g kg-1)
Total N (g kg-1)
C/N1
15.35
1.37
11.2
5.67
0.67
8.46
1
pH (soil: water=1:2.5)
PLFAs2 (mg kg-1 soil)
Amino Sugars (mg kg-1 soil)
Bacterial PLFA
Fungal PLFA
MurN3
GluN3
6.00
7.19
0.79
64.4
612.67
4.46
4.67
0.59
39.73
319.72
Note: 1. Soil Organic C was the soil organic carbon. Total N means the total soil nitrogen. C/N represents the ratio of soil organic carbon and soil total nitrogen. 2. PLFAs: Phospholipid fatty acids 3. MurN: muramic acid and GluN: glucosamine
Highlights
•
PLFA/AS-SIP is powerful for exploring microbial process in mediating SOC accrual.
•
The activation of microbial responsiveness was controlled by starving effect.
•
Successional fungal growth in C-poor Ultisol favored efficient C immobilization.
•
The conversion of labile C to microbial residues was rapid in C-rich Mollisol.
Conflicts of interest The authors declare no conflict of interest.
This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal’s policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.