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Effects of desertification on temporal and spatial distribution of soil macro-arthropods in Horqin sandy grassland, Inner Mongolia
Geoderma 223–225 (2014) 62–67
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Effects of desertification on temporal and spatial distribution of soil macro-arthropods in Horqin sandy grassland, Inner Mongolia Ha-Lin Zhao a,⁎, Jin Li a, Ren-Tao Liu b, Rui-Lian Zhou c, Hao Qu a, Cheng-Chen Pan a a b c
Cold and Arid Regions Environment and Engineering Institute, CAS, Lanzhou, Gansu 730000, China Ningxia University, Yinchuan, Ningxia 750021, China Faculty of Life Sciences, Ludong University, Yantai, Shandong 264025, China
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 18 January 2014 Accepted 26 January 2014 Available online 5 March 2014 Keywords: Grassland desertification Soil macro-arthropod Richness Density Temporal and spatial changes
a b s t r a c t The richness and density of soil macro-arthropods in 0–30 cm soil were investigated by a method of space-for-time substitution from spring to autumn of 2008–2009 in Horqin Sand Land, Inner Mongolia, to understand the effects of grassland desertification on the temporal and spatial distribution of the soil macro-arthropod community. The results showed that: 1) grassland desertification had a serious damage to the soil macro-arthropod community, more serious in the summer than in the spring and autumn, and resulting in obvious change on seasonal distribution pattern of the soil macro-arthropod community; 2) the effects of desertification on trophic groups differed among seasons, with greater effects on predators in spring, phytozoa in summer, and omnivores in autumn; 3) from large to small, the order of richness and density was 0–10 cm soil N 10–20 cm soil N 20–30 cm soil and 10–20 cm N 0–10 cm soil N 20–30 cm in non-desertified grassland, respectively, and both changed gradually to 0–10 cm soil ≥ 20–30 cm soil N 10–20 cm soil with desertification development; 4) with desertification development, the richness and density of phytozoa, predators, and omnivores all decreased significantly in different soil layers, with the magnitude of the decrease being lower for predators in 0–10 cm soil and omnivores in 10–20 cm and 20–30 cm soil and greatest for phytozoa in all three soil layers; 5) with desertification development, the dominant family in the community changed gradually and the dominance of the dominant family intensified significantly; and 6) changes in the soil macro-arthropod community were attributed to the degradation of vegetation and deterioration of the soil environment in the desertification process, including notable decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There are about 45.6 million km2 of desertified land in the world, accounting for 35% of the terrestrial parts of the earth, occurring in more than 100 countries, and affecting 8.5 × 108 people (Zhao, 2012; Zhu and Chen, 1994). As desertification can not only result in soil degradation and severe decreases in land potential productivity (Zhao et al., 2009; Zhou et al., 2008) but also damage the eco-environment and economic development, it is one of the most serious environmental and socioeconomic problems in many arid and semiarid regions of the world (Gomes et al., 2003; Zhu and Chen, 1994). Attention to desertification has increased in recent years, as the international community places increasing importance on environmental conservation (Zhao, 2012; Zhao et al., 2009). There is a great body of literature on the causes, processes, and mechanisms of desertification and its control. Research has confirmed that desertification is an important process that affects both the surface features and the biological potential of grassland soils (Lal, 1998; Zhu and Chen, 1994). Through wind erosion and the accumulation of sand, ⁎ Corresponding author. Tel.: +86 931 4967201; fax: +86 931 4967219. E-mail address: [email protected] (H.-L. Zhao). 0016-7061/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2014.01.026
desertification can lead to the loss of fine particles from soil, producing a more sandy texture (Zhao, 2012; Zhou et al., 2008). Such losses of fine particles can result in a noticeable decline in soil organic matter and finally lead to partial or even complete destruction of soil productivity by increasing bulk density, reducing porosity and water infiltration rates, and damaging the soil structure and aggregate stability as well as the storage and availability of nutrients (Zhao et al., 2003; Zhou et al., 2008). Along with soil coarsening and impoverishment, the cover and height decrease and the species diversity and biomass are reduced in grassland. Serious desertification can even lead to collapse of the grassland ecosystem. However, studies have rarely considered the effect of grassland desertification on the soil macro-arthropod community (Wang et al., 2010; Zhao, 2012). In China, there are 3.9 × 105 km2 of sandy desertified land, distributed mainly in arid and semiarid regions of Inner Mongolia (Wang and Zhao, 2005). Some researchers have investigated the types of desertification and the causes and distribution of grasslands in this area (Zhao et al., 2003; Zhu and Chen, 1994). Others have studied wind erosion and airborne dust deposition in grassland and the characteristics of soil degradation and grassland vegetation affected by desertification (Li et al., 2004; Su et al., 2002). However, there are few published studies on the effects of grassland desertification on the temporal and spatial
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2. Materials and methods
by the pipette method in a sedimentation cylinder using Nahexametha phosphate as the dispersing agent (Bao, 2000). Soil pH and electrolytic conductivity were determined with a combination pH electrode (Multiline F/SET-3, Thomas Scientific, Germany) in 1:1 soil–water slurry and 1:5 soil–water aqueous extract, respectively. Soil organic matter was measured using the K2Cr2O7–H2SO4 oxidation method, total N was analysed by the Kjeldahl procedure (UDK 140 Automatic Steam Distilling Unit, Automatic Titroline 96, Velp Scientifica, Italy), and total P was measured by a UV-1601 Spectrophotometer (Shimadzu, Japan) after H2SO4–HCIO4 digestion (Bao, 2000).
2.1. Study site
2.3. Data analysis
The study area is located in Naiman County (42° 15′ N, 120° 42′ E, 345 m a.s.l.) within the Horqin Sandy Land in the eastern part of Inner Mongolia, China. This region is characterised by a temperate continental semiarid monsoon climate regime. The mean annual precipitation is 364 mm, the mean annual potential evaporation is 1920 mm, and the mean annual temperature is 6.3 °C. The annual frost-free period is approximately 141 days, and the average annual wind speed is 3.4–4.5 m s−1. The landscape in this region is characterised by farmlands alternating with natural grassland. The soils have been identified as degraded sandy chestnut soils. Natural grasslands are in different stages of desertification because of the effects of climatic change and human activities including extensive heavy grazing, land conversion, and deforestation. The dominant plant species in natural grasslands include Artemisia halodendron, Artemisia frigida, Pennisetum centrasiaticum, Setaria viridis, and Chloris virgata (Zhou et al., 2008).
All data were analysed using the SPSS programme for Windows Version 11.5 (SPSS Inc., USA). Multiple-comparison and one-way analysis of variance (ANOVA) procedures were used to compare the differences among the treatments. Least significant difference (LSD) tests were performed to determine significant differences among treatment means at P b 0.05. A Pearson correlation analysis was carried out to determine the strength of the relationship between two sequences of average values.
distribution of the soil macro-arthropod community in this area. The objectives of this paper are: 1) to compare the differences in the soil macro-arthropod community between different seasons and soil depths along the grassland desertification gradient; 2) to analyse changes in the temporal and spatial distribution patterns of soil macroarthropods affected by grassland desertification; and 3) to discuss the relationship between changes in soil macro-arthropod properties and changes in vegetation properties and soil environments.
2.2. Methods The study was conducted in early May (spring), August (summer), and September (autumn) in 2008 and 2009. The field sampling sites, which had a total area of 1500 ha, were located near Yaoledianzi village, Naiman county, which belongs to an area that is monitored over the long term by the Naiman Desertification Research Station (NDRS) of the Chinese Ecosystem Research Network. For the present work, we carried out a space-for-time substitution approach in order to establish a desertification-severity gradient (Pickett, 1989). According to the classification of desertification types and degrees provided by Zhu and Chen (1994), the experimental grassland was divided into five desertification degrees or intensities: control grassland with no desertification (NDG), lightly desertified grassland (LDG), moderately desertified grassland (MDG), heavily desertified grassland (HDG), and severely desertified grassland (SDG). Three adjacent sites (40–50 ha in size) with similar topographies were selected for each desertification level. Within each site, three replicate 30 m × 30 m plots were established. Three 1 m × 1 m vegetation quadrats (n = 27) and three 25 cm × 25 cm × 30 cm soil quadrats (n = 27) were placed in each plot for the determination of vegetation and soil macro-arthropod community properties, respectively. The vegetation cover, aboveground biomass, and litter were measured in each vegetation quadrat in early August. In each soil quadrat, all macro-arthropods (2–20 mm, visible to the naked eye) at three depths (0–10, 10–20, and 20–30 cm) were recovered in early May (spring), August (summer), and September (autumn) by a standard hand-sorting method and preserved in a 75% alcohol solution before being brought back to the laboratory for identification and classification (Chen, 1983). Classification of the animal's trophic group is based on its main source of food, including predators, phytozoa, omnivores, scavengers, and parasites (Liu et al., 1996; Lv et al., 2007). Trophic group structure is a proportional relation of predators, phytozoa, omnivores, scavengers, and parasites in the community. Soil temperature was measured using a portable thermometer with conductivity wires (Sato Keiryoki MFG Co. Ltd., Japan). Soil samples were collected at 0–30 cm depth in each quadrat for soil physicochemical analyses. Soil particle size distribution was determined
3. Results and analysis 3.1. Changes in vegetation and soil physical–chemical properties As shown in Table 1, vegetation cover, aboveground biomass, litter, soil clay and silt content, soil organic carbon, and total soil N and P decreased significantly with desertification development, while soil sand content increased significantly (P b 0.05). Vegetation cover, aboveground biomass, litter, soil clay and silt content, organic carbon, total N and P, soil temperature, and pH decreased by 83.1, 73.9, 97.5, 49.1, 95.3, 91.3, 79.4, 4.3, and 0.4%, respectively, in the SDG compared to the NDG, while sand content and soil moisture increased by 4.0 and 9.2%, respectively. Soil temperature, moisture and soil pH had fluctuating trends with increasing desertification, the differences were not significant in the SDG compared to the NDG (P N 0.05). 3.2. Seasonal changes in the richness and density The richness in the NDG was 18, 34, and 17 families in spring, summer, and autumn (Fig. 1), respectively. The richness decreased significantly in all three seasons with desertification development (P b 0.05). Compared to the NDG, LDG, MDG, and SDG the richness decreased by 47.6, 42.9, 52.4, and 57.1% in the spring, by 47.1, 50.0, 50.0 and 76.5% in the summer, and by 55.0, 50.0, 35.0, and 50.0% in the autumn, respectively. The change in density differed between seasons with desertification development. The density decreased by 44.7 and 89.4% in spring and summer, respectively, and increased by 35.3% in the autumn in the SDG compared to the NDG. 3.3. Changes in richness and density in 0–30 cm soil profiles As shown in Table 2, the richness and density decreased significantly in the three soil layers with desertification development (P b 0.05), with the greatest magnitudes of the decrease in the LDG. The richness and density decreased by 86.7 and 88.2% in the 0–10 cm soil layer, 96.3 and 95.6% in the 10–20 cm soil layer, and 70.6 and 79.4% in the 20–30 cm soil layer, respectively, in the SDG compared to the NDG. 3.4. Seasonal changes in trophic group structure of soil macro-arthropods In the spring, the density of predators and phytozoa decreased significantly (P b 0.05) with desertification development, but the density of omnivores fluctuated and increased (Table 3). From large to small, the order of density proportion in the community was phytozoa N predators N omnivores in the NDG, but changed
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
8.44 7.88 8.32 8.23 8.41 0.11a 0.09b 0.02c 0.03d 0.03d ± ± ± ± ±
TP (g/kg)
0.39 0.23 0.18 0.09 0.08 0.12a 0.02ab 0.01bc 0.01c 0.01c ± ± ± ± ±
TN (g/kg)
0.46 0.31 0.15 0.06 0.04 ± ± ± ± ±
1.2 ac 2.3b 1.2b 2.3bc 2.4a
± ± ± ± ±
SOC (g/kg)
4.30 3.22 1.22 0.70 0.20
Temp. (°C)
30.3 29.0 28.2 28.8 29.0 0.22 ac 0.24b 0.25b 0.22ab 0.18c ± ± ± ± ±
Moisture (%)
1.96 1.55 1.39 1.67 2.14 0.76a 0.22b 0.21b 0.11c 0.99c ± ± ± ± ±
Clay–silt (%)
7.48 6.78 5.14 3.65 3.81 2.31a 1.12b 3.45bc 3.21c 2.23c ± ± ± ± ±
Sand (%)
92.50 91.25 94.87 96.3 96.2 20.4a 18.2a 9.4b 1.1c 0.8d ± ± ± ± ± 151.5 142.4 66.0 47.7 3.8 10.2a 10.0ab 8.1b 8.0c 2.3c ± ± ± ± ±
3.5a 2.9b 1.9c 3.2d 3.1e
± ± ± ± ± 184.2 167.6 152.8 60.4 48.1 63.5 51.6 45.0 28.3 10.7
Litter (g/m2) Biomass (g/m2) Cover (%)
to omnivores N phytozoa N predators from the MDG to the SDG. In the summer, the density of all trophic groups tended to decrease with desertification development (P b 0.05), but the magnitude of the decrease was greater for phytozoa than for omnivores and predators, resulting in a decrease in the proportion of phytozoa and an increase in the proportion of omnivores and predators in the community. In the autumn, the density of predators and phytozoa first decreased and then increased with desertification development, but the density of omnivores first increased and then decreased. 3.5. Changes in trophic structure in 0–30 cm soil profile In three soil layers, the density of phytozoa, predators, and omnivores decreased significantly with the development of grassland desertification (Table 4). The density of phytozoa, predators, and omnivores decreased by 75.6, 93.9, and 92.5% in the 0–10 cm soil layer, 100, 100, and 90.4% in the 10–20 cm soil layer, and 75.7, 86.4, and 72.9% in the 20–30 cm soil layer, respectively, in the SDG compared to the NDG. From the NDG to the SDG, the dominant trophic group in the community changed from phytozoa to predators in the 0–10 cm soil layer, from phytozoa to omnivores in the 10–20 cm soil layer, and from predators and phytozoa to predators and omnivores in the 20–30 cm soil layer, respectively. 3.6. Temporal and spatial changes in dominant family With desertification development, the dominant family in the community changed significantly and dominance index of the dominant family tended to increase in all three seasons (Table 5). The dominant families were Anthicidae in spring and Melolonthidae in autumn in the NDG, they were replaced by Tenebrionidae in spring and Anthicidae and Tenebrionidae in autumn in the SDG. The dominant family in summer was Anthicidae in the NDG, but there was no dominant family in the SDG. With desertification development, the dominant family changed significantly in the three soil layers. In the 0–10 cm soil layer, the dominant family changed from Anthicidae in the NDG to Myrmeleontidae in the SDG. In the 10–20 cm soil layer, the dominant family changed from Staphylinidae in the NDG to Labiduridae in the SDG. In the 20–30 cm soil layer, the dominant family was Anthicidae in the NDG, while there was no dominant family in the MDG and SDG. The dominance index of the dominant family tended to increase in the three soil layers with desertification development. 3.7. Analysis of the relationship between corresponding community properties and soil/vegetation factors To understand the mechanism of soil macro-arthropod community changes relative to the effects of desertification on habitat, we analysed the correlation between soil macro-arthropod community properties and vegetation and soil properties in the summer (Table 6). The results showed that the richness and density had a positive correlation with vegetation cover, aboveground biomass, litter, clay and silt content, soil temperature, soil organic carbon, total N, total P, and pH and a negative correlation with sand content and soil moisture. Most correlation coefficients were not significant (P N 0.05) except for the correlations between richness and density and vegetation cover, litter, clay + silt, organic carbon, total N, and total P (P b 0.05). 4. Discussion 4.1. Effects of desertification on seasonal changes of soil macro-arthropods
NDG LDG MDG HDG SDG
Desertification level
Table 1 Changes in vegetation and soil properties in 0–30 cm soil with desertification development. Values are mean ± SD. Values with the same letter within a column are not significantly different at P b 0.05.
0.87a 0.34b 0.11c 0.09 cd 0.12d
pH
± ± ± ± ±
0.04a 0.03a 0.08a 0.01a 0.03a
64
Normally, the soil macro-arthropod community changes regularly with seasonal changes (Frouz et al., 2004; Liu and Zhao, 2009). The
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
40
NDG
LDG
MDG
HDG
SDG
65
NDG
200
LDG
MDG
HDG
SDG
35
Density (Ind /.m2)
Richness
30 25 20 15 10
160 120 80 40
5 0
0 Spring
Summer Seasons
Autumn
Spring
Summer Seasons
Autumn
Fig. 1. Seasonal changes in richness and density. Bars represent means ± SD.
structure. These changes not only mean degradation succession of the soil macro-arthropod community in grassland, but also mean a food shortage of secondary consumers in summer and an increase in insect pests in autumn because phytozoa are a primary consumer in the grassland soil ecosystem (Liu et al., 1996; Marasas et al., 2001).
present results showed that the richness and density of soil macroarthropods in non-desertified grassland was summer N spring N autumn. This result is in agreement with the results of Liu et al. (1996) for the Inner Mongolia Steppe. The richness and density in all three seasons decreased significantly with grassland desertification development except for an increase in density in autumn, and the magnitude of the decrease was summer N spring N autumn. So the order of the richness and density from large to small changed to autumn N spring N summer in the SDG. This is in agreement with the findings of Guan et al. (1999) for severely desertified grassland in Horqin Sand Land. The results suggested that grassland desertification had a serious damage to the soil macroarthropod community, more serious in the summer than in the spring and autumn, and resulting in obvious change on seasonal distribution pattern of the soil macro-arthropod community. In addition, the present results also show that the effects of desertification on the soil macroarthropod community differed among desertification stages, with larger effects in the light and severe desertification stages than in the moderate and heavy desertification stages. These results are consistent with the findings of Zhao et al. (2003) on the effects of desertification on natural vegetation in Horqin Sand Land. The dominant families and trophic functional group structure of the soil macro-arthropod community are important indicators of healthy ecosystem function (Liu and Zhao, 2009; Lv et al., 2007). The present results also showed that grassland desertification had significant effects on the dominant families and trophic functional group structure of the soil macro-arthropod community. The dominant families in the NDG were Anthicidae in spring and summer and Melolonthidae in autumn. In the SDG, they were gradually replaced by Carabidae and Tenebrionidae in the spring and Elateridae and Tenebrionidae in the autumn, while there was no dominant family in summer. The density of phytozoa and predators decreased sharply and the density of omnivores increased slightly in spring. The density of predators, phytozoa, and omnivores decreased significantly in the summer, while the complete opposite occurred in autumn. These results suggested that the effects of desertification on the dominant families and trophic groups were greater in the summer than in the spring and autumn, resulting in a significant change in the seasonal distribution of dominant families and the trophic group
4.2. Effects of desertification on distribution of soil-arthropods in the soil profile The present results showed that the richness and density of soil macro-arthropods presented a declining trend from the topsoil to the subsoil in the NDG. This was consistent with the findings in other parts of China (Lin et al., 2004; Liu et al., 1996; Lv et al., 2007). The richness and density decreased significantly in the whole soil profile with desertification development, but the magnitude of the decrease differed among different soil layers. The magnitude of the decrease was the largest in the 10–20 cm soil layer and smallest in the 0–10 cm soil layer. Particularly, the dominant families in the community also changed in different soil layers with grassland desertification development. The results suggested that, on the one hand, grassland desertification could not only result in a significant decrease of the richness and density in the soil profile but also change the distribution pattern of soil macroarthropods in the soil profile, resulting in an abnormal distribution of soil macro-arthropods (Liu and Zhao, 2009; Lv et al., 2007); on the other hand, grassland desertification could result in replacement of the dominant families in the different soil layers and even disappearance in the subsoil. These results are in agreement with the findings of Liu et al. (1996) on the effects of grazing and fire on the vertical distribution of soil fauna in the Inner Mongolia Steppe. The distribution of different trophic groups differed among soil layers because there are significant differences in food resources and environmental conditions in the soil profile (Marasas et al., 2001; Zhao, 2012). The present results showed that although the density of predators, phytozoa, and omnivores decreased significantly in the three soil layers with the development of grassland desertification, the trophic structure in the community changed significantly because the magnitude of the decrease differed among different trophic groups. Of these,
Table 2 Changes in richness and density in the 0–30 cm soil profile. Values are means ± SD. Values with the same letter within a column are not significantly different at P b 0.05. Item
Depth 2
Richness (family/m )
Density (ind. m−2)
0–10 10–20 20–30 0–10 10–20 20–30
NDG cm cm cm cm cm cm
30 27 17 67.8 73.3 38.8
LDG ± ± ± ± ± ±
5.0aA 3.5aA 3.0aA 19.2aA 40.8aA 39.4aB
11 10 4 51.2 33.6 9.6
MDG ± ± ± ± ± ±
3bA 1bA 1bB 4.8bA 1.6bB 3.2bcC
15 8 4 41.6 22.4 6.4
± ± ± ± ± ±
HDG 1bA 3bB 1bC 3.2bA 3.2bB 3.2cC
7 4 6 27.2 8.0 17.6
SDG ± ± ± ± ± ±
2cA 1cA 2bA 3.2cA 4.8cC 14.4bBC
4 1 5 8.0 3.2 8.0
± ± ± ± ± ±
0dA 0 dB 2bA 1.6dA 1.6cA 4.8cA
66
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
Table 3 Seasonal changes in density (ind./m2)/density proportion (%) of trophic group with desertification development.
Spring
Summer
Autumn
Trophic group
NDG
LDG
MDG
HDG
SDG
Predators Phytozoa Omnivores Others Predators Phytozoa Omnivores Others Predators Phytozoa Omnivores Others
40.6/31.9 51.4/40.4 35.2/27.7 0/0 69.8/38.6 58.5/32.4 49.6/27.5 2.8/1.5 19.4/28.3 25.8/37.6 21.3/31.0 2.1/3.1
35.2/45.8 19.2/25.0 22.4/29.2 0/0 25.6/26.7 27.2/28.3 43.2/45.0 0/0 12.8/21.1 19.2/31.6 28.8/47.4 0/0
16.0/12.3 22.4/17.2 82.2/63.1 9.6/7.4 32.0/45.5 14.4/20.5 24.0/34.1 0/0 16.0/20.8 22.4/29.2 38.4/50.0 0/0
6.4/6.0 28.8/27.2 51.4/48.6 19.2/18.1 14.4/22.5 12.8/20.0 36.8/57.5 0/0 28.8/20.5 54.4/38.6 57.6/40.9 0/0
3.2/4.5 28.8/40.9 38.4/54.5 0/0 8.0/41.7 4.8/25.0 6.4/33.3 0/0 28.8/31.0 41.6/44.8 22.4/24.1 0/0
the dominant trophic group changed from phytozoa to predators in the 0–10 cm soil layer, from predators to omnivores in the 10–20 cm soil layer, and from phytozoa to omnivores in the 20–30 cm soil layer. The results suggested that grassland desertification caused more serious damage to phytozoa than to predators and omnivores, as soil macroarthropods turned away from a single food resource to a variety of food resources because of a shortage of plant food resources (DoblasMiranda et al., 2007; Frouz et al., 2004). 4.3. Mechanisms involved in the effects of desertification on soil arthropods As is well known, desertification is a land degradation process characterised mainly by blowing sand and aeolian erosion processes and is one of the most serious types of grassland degradation (Wang and Zhao, 2005; Zhu and Chen, 1994). Soil wind erosion and aeolian deposition result not only in decreases in the vegetation cover, biomass, and litter but also in decreases in the fine particle soil elements, organic matter, and soil nutrients with a corresponding increase of sand content, causing significant changes in the soil hydrothermal conditions (Liu et al., 2004; Zhao et al., 2003). In the present study, vegetation cover, biomass, litter, soil clay and silt, organic carbon, and soil total N and total P decreased while sand content increased significantly with desertification development. Soil macro-arthropods have relatively weak migration ability and small ranges of activity because their movements are restricted by soil particles (Calkins and Kirk, 1975; Lv et al., 2007). As a result, they are highly sensitive to the amount of food resources in soil and changes in the soil environment, and any shortage of food resources or deterioration of the environmental conditions will cause harm to them (Doblas-Miranda et al., 2007; Guan et al., 1999). So the species composition, abundance, biomass, and distribution of the soil macro-arthropod community have a close relationship with soil physical and chemical properties and the amount of food resources in the soil (Calkins and Kirk, 1975; Zhao, 2012). Usually, the richness and density of soil macro-arthropods have been shown to have a positive relationship with soil organic matter and soil nutrients and a negative relationship with soil temperature and soil pH (Lin et al., 2004; Vreeken-Buijs et al., 1998). In the present study, changes in the richness
Table 4 Changes in density (ind./m2)/density proportion (%) of main trophic groups in soil profile. Soil depth
Trophic group
NDG
LDG
MDG
HDG
SDG
0–10 cm
Predators Phytozoa Omnivores Predators Phytozoa Omnivores Predators Phytozoa Omnivores
19.7/29.1 26.3/38.8 21.3/31.4 36.7/50.1 18.4/25.1 16.6/22.7 13.2/34.0 13.3/34.3 11.8/30.4
17.6/34.4 14.4/28.1 19.2/37.5 3.2/9.5 11.2/33.3 19.2/57.1 1.6/16.7 1.6/16.7 6.4/66.7
24.0/57.7 8.0/19.2 9.6/23.1 8.0/35.7 3.2/14.3 11.2/50.0 0.0/0.0 3.2/50.0 3.2/50.0
4.8/17.7 6.4/23.5 16.0/58.8 3.2/40.0 1.6/20.0 3.2/40.0 3.2/18.2 4.8/27.3 9.6/54.5
4.8/60.0 1.6/20.0 1.6/20.0 0/0 0/0 1.6/100 3.2/40.0 1.6/20.0 3.2/40.0
10–20 cm
20–30 cm
and density of soil macro-arthropods were positively correlated with changes in vegetation cover, above-ground biomass, litter, soil clay and silt, soil temperature, soil organic carbon, total N and P, and pH and negatively correlated with soil sand content and soil moisture. The correlation between the richness and density and vegetation cover, litter, clay and silt, soil organic carbon, and total N and P reached a significant level. These results were in agreement with those of Vreeken-Buijs et al. (1998) and Bird et al. (2000). These results suggest that decreases in the richness and density were the main attribution of decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P and the increase in soil sand content. Although the decrease in biomass and increase in the sand and water contents were detrimental to soil macro-arthropods, they were not the main factors affecting the richness and density of soil macro-arthropods. The changes in soil macro-arthropod communities were attributed to vegetation degradation and deterioration of the soil environment in the grassland desertification process (Guan et al., 1999; Zhao, 2012). A large number of studies showed that the effects of grassland desertification on soil environment and vegetation differed between different seasons and soil depths (Lin et al., 2004; Vreeken-Buijs et al., 1998). Usually, grassland desertification can accelerate soil warming in spring and cooling of the topsoil temperature in autumn, and also increase the extreme soil temperature in summer as well as the surface stratification of plant root biomass in the soil profile. Especially, severe desertification may also result in a dry sand interlayer in the soil profile because the intensity of individual rainfall events is lower in summer in Horqin Sand Land (Zhao, 2012; Zhao et al., 2003). In the SDG, most soil arthropod families selected a favourable opportunity to breed and expand their populations in the autumn to escape extreme soil environments in the summer, thus causing a change in the seasonal distribution pattern of the soil arthropod community (Frouz et al., 2004; Guan et al., 1999). Similarly, in order to escape a dry sand interlayer and reach more food sources, most soil arthropods chose to live in the 0–10 or 20–30 cm soil layers. Thus the distribution pattern of soil arthropods changed (Liu and Zhao, 2009; Lv et al., 2007). 5. Conclusion We conclude that: 1) desertification results in a significant decrease in the richness and density of soil macro-arthropods and changes the seasonal distribution of the soil arthropod community; 2) desertification has greater effects on phytozoa than on omnivores in the growth season and resulted in a significant change in the seasonal pattern of the trophic structure; 3) desertification caused obvious damage to soil macro-arthropods in all three soil layers, with the most damage occurring for soil arthropods in the 10–20 cm soil layer, and resulted in a significant change in the distribution pattern of soil arthropods in the soil profile; 4) desertification has a smaller effect on predators in 0–10 cm soil and omnivores in 10–30 cm soil and a more serious effect on phytozoa in all three soil layers; 5) with desertification development, the dominant family changed from Anthicidae in the spring and Melolonthidae in autumn to Tenebrionidae, from Anthicidae to Myrmeleontidae in the 0–10 cm soil layer, and from Staphylinidae to Labiduridae in the 10–20 cm soil layer; and 6) changes in the soil macro-arthropod community were attributed to the degradation of vegetation and deterioration of the soil environment during the desertification process, with notable decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P. Acknowledgements The authors are grateful to the anonymous reviewers for their critical review and comments on drafts of this manuscript. This research was funded by one of the Chinese National Fund Projects (31270752, 30972422) and one of the Chinese National Support Projects of Science and Technology (2011BAC07B02-06).
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Table 5 Temporal and spatial changes of dominant families (dominance index). Items
Season
NDG
LDG
MDG
HDG
SDG
Seasonal changes
Spring
Anthicidae (19.3)
Carabidae (20.8)
Tenebrionidae (50.8)
Tenebrionidae (50.0)
Summer Autumn
Anthicidae (20.7) Melolonthidae (24.9)
Anthicidae (28.3) Anthicidae (36.8)
Anthicidae (25.0) Anthicidae (29.2)
Tenebrionidae (24.4), Anthicidae (24.2) Anthicidae (35.0) Elateridae (31.8)
0–10 cm
Anthicidae (18.9)
Anthicidae (28.1)
10–20 cm
Staphylinidae (32.7)
Anthicidae (29.4), Tenebrionidae (29.4) Tenebrionidae (40.0)
20–30 cm
Anthicidae (27.6)
Anthicidae (28.6) Tenebrionidae (28.6) Anthicidae (50.0)
Anthicidae (15.4) Araneidae (15.4) Anthicidae (42.9)
Depth changes
Tenebrionidae (24.1) Melolonthidae (24.1) Myrmeleontidae (40.0) Labiduridae (100)
Anthicidae (54.0)
Table 6 Correlation analysis between the community properties and soil factors. Item
Vegetation factors Cover
Richness Density
0.876⁎ 0.897⁎
Soil factors
Biomass
Litter
0.547 0.644
0.821⁎ 0.866⁎
Sand −0.591 −0.673
Clay + silt 0.791 0.856⁎
Water −0.026 0.002
Temp.
SOC
TN
TP
pH
0.735 0.767
0.867⁎ 0.918⁎
0.882⁎ 0.933⁎
0.922⁎ 0.960⁎⁎
0.145 0.097
⁎ P b 0.05. ⁎⁎ P b 0.01.
References Bao, S.D., 2000. Analysis Method for Soil Agro-chemistry. China Agriculture Press, Beijing. Bird, S., Coulson, R.N., Crossley Jr., D.A., 2000. Impacts of silvicultural practices on soil and litter arthropod diversity in a Texas pine plantation. For. Ecol. Manag. 131, 65–80. Calkins, C.O., Kirk, V.M., 1975. Distribution of false wireworms (Coleoptera: Tenebrionidae) in relation to soil texture. Environ. Entomol. 4, 373–374. Chen, P., 1983. Sampling methods of soil animals. Chin. J. Ecol. 2 (2), 46–51. Doblas-Miranda, E., Sánchez-Pińero, F., González-Megías, A., 2007. Soil macro-invertebrate fauna of a Mediterranean arid system: composition and temporal changes in the assemblage. Soil Biol. Biochem. 39, 1916–1925. Frouz, J.A.A., Frouzova, J., Lobinske, R.J., 2004. Horizontal and vertical distribution of soil macro-arthropods along a spatial–temporal moisture gradient in subtropical Central Florida. Environ. Entomol. 33 (5), 1282–1295. Gomes, L., Arrue, J.L., Lopez, M.V., Sterk, G., Richard, D., Gracia, R., Sabre, M., Gaudichet, A., Frangi, J.P., 2003. Wind erosion in a semiarid agricultural area of Spain: the WELSONS project. Catena 52, 235–256. Guan, H.B., Guo, L., Liu, Y.J., 1999. The vertical distribution, seasonal dynamics and community variety of soil animal in Horqin Sandy Land. J. Desert Res. 19 (S1), 110–114. Lal, R., 1998. Soil erosion impact on agronomic productivity and environment quality. Crit. Rev. Plant Sci. 17, 319–464. Li, F.R., Zhao, L.Y., Zhang, T.H., 2004. Wind erosion and airborne dust deposition in farmland during spring in the Horqin Sandy Land of eastern Inner Mongolia, China. Soil Tillage Res. 75, 121–130. Lin, Y.H., Zhang, F.D., Yang, X.Y., 2004. Study on the relationship between agricultural soil fauna and soil physicochemical properties. Sci. Agric. Sin. 37 (6), 871–877. Liu, R.T., Zhao, H.L., 2009. Research progress and suggestion for study on soil animal in sandy grassland. J. Desert Res. 29 (4), 656–662.
Liu, X.M., Qian, D.M., Liu, Y.J., 1996. Effects of grazing intensity and fire on vertical distribution in main group of soil fauna, Inner Mongolia Steep. J. Inn. Mong. Coll. Educ. 2, 39–45. Liu, Y.R., Yang, C., Zhu, Z.M., 2004. Soil C and N dynamics during desertification of grassland in Northern China. Chin. J. Appl. Ecol. 15 (9), 1604–1606. Lv, S.H., Lu, X.S., Gao, J., 2007. Responses of soil fauna to environment degeneration in the process of wind erosion desertification of Hulunbeir steppe. Chin. J. Appl. Ecol. 18 (9), 2055–2060. Marasas, M.E., Sarandón, S.J., Cicchino, A.C., 2001. Changes in soil macro-arthropod functional group in a wheat crop under conventional and no tillage systems in Argentina. Appl. Soil Ecol. 18, 61–68. Pickett, S.T.A., 1989. Space-for-time substitution as an alternative to long-term studies. In: Likens, G.E. (Ed.), Long-term Studies in Ecology: Approaches and Alternatives. Springer-Verlag, New York, Berlin, pp. 110–135. Su, Y.Z., Zhao, H.L., Zhang, T.H., Li, Y.L., 2002. Processes and characteristics of soil degradation in rainfed farmland in the Horqin sandy land. J. Soil Water Conserv. 16, 25–28. Vreeken-Buijs, M.J., Hassink, J., Brussaard, L., 1998. Relationships of soil micro-arthropod biomass with organic matter and pore size distribution in soils under different land use. Soil Biol. Biochem. 30, 97–106. Wang, T., Zhao, H.L., 2005. Fifty-year history of China desert sciences. J. Desert Res. 25 (2), 145–165. Wang, Y., Wei, W., Yang, X.Z., 2010. Interrelationships between soil fauna and soil environmental factors in China. Chin. J. Appl. Ecol. 21 (9), 2441–2448. Zhao, H.L., 2012. Desert Ecology. China Science Press, Beijing. Zhao, H.L., Zhao, X.Y., Zhang, T.H., Wu, W., 2003. Desertification Process and Its Restoration Mechanism in Horqin Sand Land. China Ocean Press, Beijing. Zhao, H.L., He, Y.H., Zhou, R.L., 2009. Effects of desertification on soil organic C and N content in sandy farmland and grassland of Inner Mongolia. Catena 77, 187–191. Zhou, R.L., Li, Y.Q., Zhao, H.L., Drake, S., 2008. Desertification effects on C and N content of sandy soils under grassland in Horqin Sand Land, China. Geoderma 145, 370–375. Zhu, Z.D., Chen, G.T., 1994. Sandy Desertification in China. Science Press, Beijing.
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Effects of desertification on temporal and spatial distribution of soil macro-arthropods in Horqin sandy grassland, Inner Mongolia Ha-Lin Zhao a,⁎, Jin Li a, Ren-Tao Liu b, Rui-Lian Zhou c, Hao Qu a, Cheng-Chen Pan a a b c
Cold and Arid Regions Environment and Engineering Institute, CAS, Lanzhou, Gansu 730000, China Ningxia University, Yinchuan, Ningxia 750021, China Faculty of Life Sciences, Ludong University, Yantai, Shandong 264025, China
a r t i c l e
i n f o
Article history: Received 5 August 2013 Received in revised form 18 January 2014 Accepted 26 January 2014 Available online 5 March 2014 Keywords: Grassland desertification Soil macro-arthropod Richness Density Temporal and spatial changes
a b s t r a c t The richness and density of soil macro-arthropods in 0–30 cm soil were investigated by a method of space-for-time substitution from spring to autumn of 2008–2009 in Horqin Sand Land, Inner Mongolia, to understand the effects of grassland desertification on the temporal and spatial distribution of the soil macro-arthropod community. The results showed that: 1) grassland desertification had a serious damage to the soil macro-arthropod community, more serious in the summer than in the spring and autumn, and resulting in obvious change on seasonal distribution pattern of the soil macro-arthropod community; 2) the effects of desertification on trophic groups differed among seasons, with greater effects on predators in spring, phytozoa in summer, and omnivores in autumn; 3) from large to small, the order of richness and density was 0–10 cm soil N 10–20 cm soil N 20–30 cm soil and 10–20 cm N 0–10 cm soil N 20–30 cm in non-desertified grassland, respectively, and both changed gradually to 0–10 cm soil ≥ 20–30 cm soil N 10–20 cm soil with desertification development; 4) with desertification development, the richness and density of phytozoa, predators, and omnivores all decreased significantly in different soil layers, with the magnitude of the decrease being lower for predators in 0–10 cm soil and omnivores in 10–20 cm and 20–30 cm soil and greatest for phytozoa in all three soil layers; 5) with desertification development, the dominant family in the community changed gradually and the dominance of the dominant family intensified significantly; and 6) changes in the soil macro-arthropod community were attributed to the degradation of vegetation and deterioration of the soil environment in the desertification process, including notable decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There are about 45.6 million km2 of desertified land in the world, accounting for 35% of the terrestrial parts of the earth, occurring in more than 100 countries, and affecting 8.5 × 108 people (Zhao, 2012; Zhu and Chen, 1994). As desertification can not only result in soil degradation and severe decreases in land potential productivity (Zhao et al., 2009; Zhou et al., 2008) but also damage the eco-environment and economic development, it is one of the most serious environmental and socioeconomic problems in many arid and semiarid regions of the world (Gomes et al., 2003; Zhu and Chen, 1994). Attention to desertification has increased in recent years, as the international community places increasing importance on environmental conservation (Zhao, 2012; Zhao et al., 2009). There is a great body of literature on the causes, processes, and mechanisms of desertification and its control. Research has confirmed that desertification is an important process that affects both the surface features and the biological potential of grassland soils (Lal, 1998; Zhu and Chen, 1994). Through wind erosion and the accumulation of sand, ⁎ Corresponding author. Tel.: +86 931 4967201; fax: +86 931 4967219. E-mail address: [email protected] (H.-L. Zhao). 0016-7061/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2014.01.026
desertification can lead to the loss of fine particles from soil, producing a more sandy texture (Zhao, 2012; Zhou et al., 2008). Such losses of fine particles can result in a noticeable decline in soil organic matter and finally lead to partial or even complete destruction of soil productivity by increasing bulk density, reducing porosity and water infiltration rates, and damaging the soil structure and aggregate stability as well as the storage and availability of nutrients (Zhao et al., 2003; Zhou et al., 2008). Along with soil coarsening and impoverishment, the cover and height decrease and the species diversity and biomass are reduced in grassland. Serious desertification can even lead to collapse of the grassland ecosystem. However, studies have rarely considered the effect of grassland desertification on the soil macro-arthropod community (Wang et al., 2010; Zhao, 2012). In China, there are 3.9 × 105 km2 of sandy desertified land, distributed mainly in arid and semiarid regions of Inner Mongolia (Wang and Zhao, 2005). Some researchers have investigated the types of desertification and the causes and distribution of grasslands in this area (Zhao et al., 2003; Zhu and Chen, 1994). Others have studied wind erosion and airborne dust deposition in grassland and the characteristics of soil degradation and grassland vegetation affected by desertification (Li et al., 2004; Su et al., 2002). However, there are few published studies on the effects of grassland desertification on the temporal and spatial
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2. Materials and methods
by the pipette method in a sedimentation cylinder using Nahexametha phosphate as the dispersing agent (Bao, 2000). Soil pH and electrolytic conductivity were determined with a combination pH electrode (Multiline F/SET-3, Thomas Scientific, Germany) in 1:1 soil–water slurry and 1:5 soil–water aqueous extract, respectively. Soil organic matter was measured using the K2Cr2O7–H2SO4 oxidation method, total N was analysed by the Kjeldahl procedure (UDK 140 Automatic Steam Distilling Unit, Automatic Titroline 96, Velp Scientifica, Italy), and total P was measured by a UV-1601 Spectrophotometer (Shimadzu, Japan) after H2SO4–HCIO4 digestion (Bao, 2000).
2.1. Study site
2.3. Data analysis
The study area is located in Naiman County (42° 15′ N, 120° 42′ E, 345 m a.s.l.) within the Horqin Sandy Land in the eastern part of Inner Mongolia, China. This region is characterised by a temperate continental semiarid monsoon climate regime. The mean annual precipitation is 364 mm, the mean annual potential evaporation is 1920 mm, and the mean annual temperature is 6.3 °C. The annual frost-free period is approximately 141 days, and the average annual wind speed is 3.4–4.5 m s−1. The landscape in this region is characterised by farmlands alternating with natural grassland. The soils have been identified as degraded sandy chestnut soils. Natural grasslands are in different stages of desertification because of the effects of climatic change and human activities including extensive heavy grazing, land conversion, and deforestation. The dominant plant species in natural grasslands include Artemisia halodendron, Artemisia frigida, Pennisetum centrasiaticum, Setaria viridis, and Chloris virgata (Zhou et al., 2008).
All data were analysed using the SPSS programme for Windows Version 11.5 (SPSS Inc., USA). Multiple-comparison and one-way analysis of variance (ANOVA) procedures were used to compare the differences among the treatments. Least significant difference (LSD) tests were performed to determine significant differences among treatment means at P b 0.05. A Pearson correlation analysis was carried out to determine the strength of the relationship between two sequences of average values.
distribution of the soil macro-arthropod community in this area. The objectives of this paper are: 1) to compare the differences in the soil macro-arthropod community between different seasons and soil depths along the grassland desertification gradient; 2) to analyse changes in the temporal and spatial distribution patterns of soil macroarthropods affected by grassland desertification; and 3) to discuss the relationship between changes in soil macro-arthropod properties and changes in vegetation properties and soil environments.
2.2. Methods The study was conducted in early May (spring), August (summer), and September (autumn) in 2008 and 2009. The field sampling sites, which had a total area of 1500 ha, were located near Yaoledianzi village, Naiman county, which belongs to an area that is monitored over the long term by the Naiman Desertification Research Station (NDRS) of the Chinese Ecosystem Research Network. For the present work, we carried out a space-for-time substitution approach in order to establish a desertification-severity gradient (Pickett, 1989). According to the classification of desertification types and degrees provided by Zhu and Chen (1994), the experimental grassland was divided into five desertification degrees or intensities: control grassland with no desertification (NDG), lightly desertified grassland (LDG), moderately desertified grassland (MDG), heavily desertified grassland (HDG), and severely desertified grassland (SDG). Three adjacent sites (40–50 ha in size) with similar topographies were selected for each desertification level. Within each site, three replicate 30 m × 30 m plots were established. Three 1 m × 1 m vegetation quadrats (n = 27) and three 25 cm × 25 cm × 30 cm soil quadrats (n = 27) were placed in each plot for the determination of vegetation and soil macro-arthropod community properties, respectively. The vegetation cover, aboveground biomass, and litter were measured in each vegetation quadrat in early August. In each soil quadrat, all macro-arthropods (2–20 mm, visible to the naked eye) at three depths (0–10, 10–20, and 20–30 cm) were recovered in early May (spring), August (summer), and September (autumn) by a standard hand-sorting method and preserved in a 75% alcohol solution before being brought back to the laboratory for identification and classification (Chen, 1983). Classification of the animal's trophic group is based on its main source of food, including predators, phytozoa, omnivores, scavengers, and parasites (Liu et al., 1996; Lv et al., 2007). Trophic group structure is a proportional relation of predators, phytozoa, omnivores, scavengers, and parasites in the community. Soil temperature was measured using a portable thermometer with conductivity wires (Sato Keiryoki MFG Co. Ltd., Japan). Soil samples were collected at 0–30 cm depth in each quadrat for soil physicochemical analyses. Soil particle size distribution was determined
3. Results and analysis 3.1. Changes in vegetation and soil physical–chemical properties As shown in Table 1, vegetation cover, aboveground biomass, litter, soil clay and silt content, soil organic carbon, and total soil N and P decreased significantly with desertification development, while soil sand content increased significantly (P b 0.05). Vegetation cover, aboveground biomass, litter, soil clay and silt content, organic carbon, total N and P, soil temperature, and pH decreased by 83.1, 73.9, 97.5, 49.1, 95.3, 91.3, 79.4, 4.3, and 0.4%, respectively, in the SDG compared to the NDG, while sand content and soil moisture increased by 4.0 and 9.2%, respectively. Soil temperature, moisture and soil pH had fluctuating trends with increasing desertification, the differences were not significant in the SDG compared to the NDG (P N 0.05). 3.2. Seasonal changes in the richness and density The richness in the NDG was 18, 34, and 17 families in spring, summer, and autumn (Fig. 1), respectively. The richness decreased significantly in all three seasons with desertification development (P b 0.05). Compared to the NDG, LDG, MDG, and SDG the richness decreased by 47.6, 42.9, 52.4, and 57.1% in the spring, by 47.1, 50.0, 50.0 and 76.5% in the summer, and by 55.0, 50.0, 35.0, and 50.0% in the autumn, respectively. The change in density differed between seasons with desertification development. The density decreased by 44.7 and 89.4% in spring and summer, respectively, and increased by 35.3% in the autumn in the SDG compared to the NDG. 3.3. Changes in richness and density in 0–30 cm soil profiles As shown in Table 2, the richness and density decreased significantly in the three soil layers with desertification development (P b 0.05), with the greatest magnitudes of the decrease in the LDG. The richness and density decreased by 86.7 and 88.2% in the 0–10 cm soil layer, 96.3 and 95.6% in the 10–20 cm soil layer, and 70.6 and 79.4% in the 20–30 cm soil layer, respectively, in the SDG compared to the NDG. 3.4. Seasonal changes in trophic group structure of soil macro-arthropods In the spring, the density of predators and phytozoa decreased significantly (P b 0.05) with desertification development, but the density of omnivores fluctuated and increased (Table 3). From large to small, the order of density proportion in the community was phytozoa N predators N omnivores in the NDG, but changed
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8.44 7.88 8.32 8.23 8.41 0.11a 0.09b 0.02c 0.03d 0.03d ± ± ± ± ±
TP (g/kg)
0.39 0.23 0.18 0.09 0.08 0.12a 0.02ab 0.01bc 0.01c 0.01c ± ± ± ± ±
TN (g/kg)
0.46 0.31 0.15 0.06 0.04 ± ± ± ± ±
1.2 ac 2.3b 1.2b 2.3bc 2.4a
± ± ± ± ±
SOC (g/kg)
4.30 3.22 1.22 0.70 0.20
Temp. (°C)
30.3 29.0 28.2 28.8 29.0 0.22 ac 0.24b 0.25b 0.22ab 0.18c ± ± ± ± ±
Moisture (%)
1.96 1.55 1.39 1.67 2.14 0.76a 0.22b 0.21b 0.11c 0.99c ± ± ± ± ±
Clay–silt (%)
7.48 6.78 5.14 3.65 3.81 2.31a 1.12b 3.45bc 3.21c 2.23c ± ± ± ± ±
Sand (%)
92.50 91.25 94.87 96.3 96.2 20.4a 18.2a 9.4b 1.1c 0.8d ± ± ± ± ± 151.5 142.4 66.0 47.7 3.8 10.2a 10.0ab 8.1b 8.0c 2.3c ± ± ± ± ±
3.5a 2.9b 1.9c 3.2d 3.1e
± ± ± ± ± 184.2 167.6 152.8 60.4 48.1 63.5 51.6 45.0 28.3 10.7
Litter (g/m2) Biomass (g/m2) Cover (%)
to omnivores N phytozoa N predators from the MDG to the SDG. In the summer, the density of all trophic groups tended to decrease with desertification development (P b 0.05), but the magnitude of the decrease was greater for phytozoa than for omnivores and predators, resulting in a decrease in the proportion of phytozoa and an increase in the proportion of omnivores and predators in the community. In the autumn, the density of predators and phytozoa first decreased and then increased with desertification development, but the density of omnivores first increased and then decreased. 3.5. Changes in trophic structure in 0–30 cm soil profile In three soil layers, the density of phytozoa, predators, and omnivores decreased significantly with the development of grassland desertification (Table 4). The density of phytozoa, predators, and omnivores decreased by 75.6, 93.9, and 92.5% in the 0–10 cm soil layer, 100, 100, and 90.4% in the 10–20 cm soil layer, and 75.7, 86.4, and 72.9% in the 20–30 cm soil layer, respectively, in the SDG compared to the NDG. From the NDG to the SDG, the dominant trophic group in the community changed from phytozoa to predators in the 0–10 cm soil layer, from phytozoa to omnivores in the 10–20 cm soil layer, and from predators and phytozoa to predators and omnivores in the 20–30 cm soil layer, respectively. 3.6. Temporal and spatial changes in dominant family With desertification development, the dominant family in the community changed significantly and dominance index of the dominant family tended to increase in all three seasons (Table 5). The dominant families were Anthicidae in spring and Melolonthidae in autumn in the NDG, they were replaced by Tenebrionidae in spring and Anthicidae and Tenebrionidae in autumn in the SDG. The dominant family in summer was Anthicidae in the NDG, but there was no dominant family in the SDG. With desertification development, the dominant family changed significantly in the three soil layers. In the 0–10 cm soil layer, the dominant family changed from Anthicidae in the NDG to Myrmeleontidae in the SDG. In the 10–20 cm soil layer, the dominant family changed from Staphylinidae in the NDG to Labiduridae in the SDG. In the 20–30 cm soil layer, the dominant family was Anthicidae in the NDG, while there was no dominant family in the MDG and SDG. The dominance index of the dominant family tended to increase in the three soil layers with desertification development. 3.7. Analysis of the relationship between corresponding community properties and soil/vegetation factors To understand the mechanism of soil macro-arthropod community changes relative to the effects of desertification on habitat, we analysed the correlation between soil macro-arthropod community properties and vegetation and soil properties in the summer (Table 6). The results showed that the richness and density had a positive correlation with vegetation cover, aboveground biomass, litter, clay and silt content, soil temperature, soil organic carbon, total N, total P, and pH and a negative correlation with sand content and soil moisture. Most correlation coefficients were not significant (P N 0.05) except for the correlations between richness and density and vegetation cover, litter, clay + silt, organic carbon, total N, and total P (P b 0.05). 4. Discussion 4.1. Effects of desertification on seasonal changes of soil macro-arthropods
NDG LDG MDG HDG SDG
Desertification level
Table 1 Changes in vegetation and soil properties in 0–30 cm soil with desertification development. Values are mean ± SD. Values with the same letter within a column are not significantly different at P b 0.05.
0.87a 0.34b 0.11c 0.09 cd 0.12d
pH
± ± ± ± ±
0.04a 0.03a 0.08a 0.01a 0.03a
64
Normally, the soil macro-arthropod community changes regularly with seasonal changes (Frouz et al., 2004; Liu and Zhao, 2009). The
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
40
NDG
LDG
MDG
HDG
SDG
65
NDG
200
LDG
MDG
HDG
SDG
35
Density (Ind /.m2)
Richness
30 25 20 15 10
160 120 80 40
5 0
0 Spring
Summer Seasons
Autumn
Spring
Summer Seasons
Autumn
Fig. 1. Seasonal changes in richness and density. Bars represent means ± SD.
structure. These changes not only mean degradation succession of the soil macro-arthropod community in grassland, but also mean a food shortage of secondary consumers in summer and an increase in insect pests in autumn because phytozoa are a primary consumer in the grassland soil ecosystem (Liu et al., 1996; Marasas et al., 2001).
present results showed that the richness and density of soil macroarthropods in non-desertified grassland was summer N spring N autumn. This result is in agreement with the results of Liu et al. (1996) for the Inner Mongolia Steppe. The richness and density in all three seasons decreased significantly with grassland desertification development except for an increase in density in autumn, and the magnitude of the decrease was summer N spring N autumn. So the order of the richness and density from large to small changed to autumn N spring N summer in the SDG. This is in agreement with the findings of Guan et al. (1999) for severely desertified grassland in Horqin Sand Land. The results suggested that grassland desertification had a serious damage to the soil macroarthropod community, more serious in the summer than in the spring and autumn, and resulting in obvious change on seasonal distribution pattern of the soil macro-arthropod community. In addition, the present results also show that the effects of desertification on the soil macroarthropod community differed among desertification stages, with larger effects in the light and severe desertification stages than in the moderate and heavy desertification stages. These results are consistent with the findings of Zhao et al. (2003) on the effects of desertification on natural vegetation in Horqin Sand Land. The dominant families and trophic functional group structure of the soil macro-arthropod community are important indicators of healthy ecosystem function (Liu and Zhao, 2009; Lv et al., 2007). The present results also showed that grassland desertification had significant effects on the dominant families and trophic functional group structure of the soil macro-arthropod community. The dominant families in the NDG were Anthicidae in spring and summer and Melolonthidae in autumn. In the SDG, they were gradually replaced by Carabidae and Tenebrionidae in the spring and Elateridae and Tenebrionidae in the autumn, while there was no dominant family in summer. The density of phytozoa and predators decreased sharply and the density of omnivores increased slightly in spring. The density of predators, phytozoa, and omnivores decreased significantly in the summer, while the complete opposite occurred in autumn. These results suggested that the effects of desertification on the dominant families and trophic groups were greater in the summer than in the spring and autumn, resulting in a significant change in the seasonal distribution of dominant families and the trophic group
4.2. Effects of desertification on distribution of soil-arthropods in the soil profile The present results showed that the richness and density of soil macro-arthropods presented a declining trend from the topsoil to the subsoil in the NDG. This was consistent with the findings in other parts of China (Lin et al., 2004; Liu et al., 1996; Lv et al., 2007). The richness and density decreased significantly in the whole soil profile with desertification development, but the magnitude of the decrease differed among different soil layers. The magnitude of the decrease was the largest in the 10–20 cm soil layer and smallest in the 0–10 cm soil layer. Particularly, the dominant families in the community also changed in different soil layers with grassland desertification development. The results suggested that, on the one hand, grassland desertification could not only result in a significant decrease of the richness and density in the soil profile but also change the distribution pattern of soil macroarthropods in the soil profile, resulting in an abnormal distribution of soil macro-arthropods (Liu and Zhao, 2009; Lv et al., 2007); on the other hand, grassland desertification could result in replacement of the dominant families in the different soil layers and even disappearance in the subsoil. These results are in agreement with the findings of Liu et al. (1996) on the effects of grazing and fire on the vertical distribution of soil fauna in the Inner Mongolia Steppe. The distribution of different trophic groups differed among soil layers because there are significant differences in food resources and environmental conditions in the soil profile (Marasas et al., 2001; Zhao, 2012). The present results showed that although the density of predators, phytozoa, and omnivores decreased significantly in the three soil layers with the development of grassland desertification, the trophic structure in the community changed significantly because the magnitude of the decrease differed among different trophic groups. Of these,
Table 2 Changes in richness and density in the 0–30 cm soil profile. Values are means ± SD. Values with the same letter within a column are not significantly different at P b 0.05. Item
Depth 2
Richness (family/m )
Density (ind. m−2)
0–10 10–20 20–30 0–10 10–20 20–30
NDG cm cm cm cm cm cm
30 27 17 67.8 73.3 38.8
LDG ± ± ± ± ± ±
5.0aA 3.5aA 3.0aA 19.2aA 40.8aA 39.4aB
11 10 4 51.2 33.6 9.6
MDG ± ± ± ± ± ±
3bA 1bA 1bB 4.8bA 1.6bB 3.2bcC
15 8 4 41.6 22.4 6.4
± ± ± ± ± ±
HDG 1bA 3bB 1bC 3.2bA 3.2bB 3.2cC
7 4 6 27.2 8.0 17.6
SDG ± ± ± ± ± ±
2cA 1cA 2bA 3.2cA 4.8cC 14.4bBC
4 1 5 8.0 3.2 8.0
± ± ± ± ± ±
0dA 0 dB 2bA 1.6dA 1.6cA 4.8cA
66
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
Table 3 Seasonal changes in density (ind./m2)/density proportion (%) of trophic group with desertification development.
Spring
Summer
Autumn
Trophic group
NDG
LDG
MDG
HDG
SDG
Predators Phytozoa Omnivores Others Predators Phytozoa Omnivores Others Predators Phytozoa Omnivores Others
40.6/31.9 51.4/40.4 35.2/27.7 0/0 69.8/38.6 58.5/32.4 49.6/27.5 2.8/1.5 19.4/28.3 25.8/37.6 21.3/31.0 2.1/3.1
35.2/45.8 19.2/25.0 22.4/29.2 0/0 25.6/26.7 27.2/28.3 43.2/45.0 0/0 12.8/21.1 19.2/31.6 28.8/47.4 0/0
16.0/12.3 22.4/17.2 82.2/63.1 9.6/7.4 32.0/45.5 14.4/20.5 24.0/34.1 0/0 16.0/20.8 22.4/29.2 38.4/50.0 0/0
6.4/6.0 28.8/27.2 51.4/48.6 19.2/18.1 14.4/22.5 12.8/20.0 36.8/57.5 0/0 28.8/20.5 54.4/38.6 57.6/40.9 0/0
3.2/4.5 28.8/40.9 38.4/54.5 0/0 8.0/41.7 4.8/25.0 6.4/33.3 0/0 28.8/31.0 41.6/44.8 22.4/24.1 0/0
the dominant trophic group changed from phytozoa to predators in the 0–10 cm soil layer, from predators to omnivores in the 10–20 cm soil layer, and from phytozoa to omnivores in the 20–30 cm soil layer. The results suggested that grassland desertification caused more serious damage to phytozoa than to predators and omnivores, as soil macroarthropods turned away from a single food resource to a variety of food resources because of a shortage of plant food resources (DoblasMiranda et al., 2007; Frouz et al., 2004). 4.3. Mechanisms involved in the effects of desertification on soil arthropods As is well known, desertification is a land degradation process characterised mainly by blowing sand and aeolian erosion processes and is one of the most serious types of grassland degradation (Wang and Zhao, 2005; Zhu and Chen, 1994). Soil wind erosion and aeolian deposition result not only in decreases in the vegetation cover, biomass, and litter but also in decreases in the fine particle soil elements, organic matter, and soil nutrients with a corresponding increase of sand content, causing significant changes in the soil hydrothermal conditions (Liu et al., 2004; Zhao et al., 2003). In the present study, vegetation cover, biomass, litter, soil clay and silt, organic carbon, and soil total N and total P decreased while sand content increased significantly with desertification development. Soil macro-arthropods have relatively weak migration ability and small ranges of activity because their movements are restricted by soil particles (Calkins and Kirk, 1975; Lv et al., 2007). As a result, they are highly sensitive to the amount of food resources in soil and changes in the soil environment, and any shortage of food resources or deterioration of the environmental conditions will cause harm to them (Doblas-Miranda et al., 2007; Guan et al., 1999). So the species composition, abundance, biomass, and distribution of the soil macro-arthropod community have a close relationship with soil physical and chemical properties and the amount of food resources in the soil (Calkins and Kirk, 1975; Zhao, 2012). Usually, the richness and density of soil macro-arthropods have been shown to have a positive relationship with soil organic matter and soil nutrients and a negative relationship with soil temperature and soil pH (Lin et al., 2004; Vreeken-Buijs et al., 1998). In the present study, changes in the richness
Table 4 Changes in density (ind./m2)/density proportion (%) of main trophic groups in soil profile. Soil depth
Trophic group
NDG
LDG
MDG
HDG
SDG
0–10 cm
Predators Phytozoa Omnivores Predators Phytozoa Omnivores Predators Phytozoa Omnivores
19.7/29.1 26.3/38.8 21.3/31.4 36.7/50.1 18.4/25.1 16.6/22.7 13.2/34.0 13.3/34.3 11.8/30.4
17.6/34.4 14.4/28.1 19.2/37.5 3.2/9.5 11.2/33.3 19.2/57.1 1.6/16.7 1.6/16.7 6.4/66.7
24.0/57.7 8.0/19.2 9.6/23.1 8.0/35.7 3.2/14.3 11.2/50.0 0.0/0.0 3.2/50.0 3.2/50.0
4.8/17.7 6.4/23.5 16.0/58.8 3.2/40.0 1.6/20.0 3.2/40.0 3.2/18.2 4.8/27.3 9.6/54.5
4.8/60.0 1.6/20.0 1.6/20.0 0/0 0/0 1.6/100 3.2/40.0 1.6/20.0 3.2/40.0
10–20 cm
20–30 cm
and density of soil macro-arthropods were positively correlated with changes in vegetation cover, above-ground biomass, litter, soil clay and silt, soil temperature, soil organic carbon, total N and P, and pH and negatively correlated with soil sand content and soil moisture. The correlation between the richness and density and vegetation cover, litter, clay and silt, soil organic carbon, and total N and P reached a significant level. These results were in agreement with those of Vreeken-Buijs et al. (1998) and Bird et al. (2000). These results suggest that decreases in the richness and density were the main attribution of decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P and the increase in soil sand content. Although the decrease in biomass and increase in the sand and water contents were detrimental to soil macro-arthropods, they were not the main factors affecting the richness and density of soil macro-arthropods. The changes in soil macro-arthropod communities were attributed to vegetation degradation and deterioration of the soil environment in the grassland desertification process (Guan et al., 1999; Zhao, 2012). A large number of studies showed that the effects of grassland desertification on soil environment and vegetation differed between different seasons and soil depths (Lin et al., 2004; Vreeken-Buijs et al., 1998). Usually, grassland desertification can accelerate soil warming in spring and cooling of the topsoil temperature in autumn, and also increase the extreme soil temperature in summer as well as the surface stratification of plant root biomass in the soil profile. Especially, severe desertification may also result in a dry sand interlayer in the soil profile because the intensity of individual rainfall events is lower in summer in Horqin Sand Land (Zhao, 2012; Zhao et al., 2003). In the SDG, most soil arthropod families selected a favourable opportunity to breed and expand their populations in the autumn to escape extreme soil environments in the summer, thus causing a change in the seasonal distribution pattern of the soil arthropod community (Frouz et al., 2004; Guan et al., 1999). Similarly, in order to escape a dry sand interlayer and reach more food sources, most soil arthropods chose to live in the 0–10 or 20–30 cm soil layers. Thus the distribution pattern of soil arthropods changed (Liu and Zhao, 2009; Lv et al., 2007). 5. Conclusion We conclude that: 1) desertification results in a significant decrease in the richness and density of soil macro-arthropods and changes the seasonal distribution of the soil arthropod community; 2) desertification has greater effects on phytozoa than on omnivores in the growth season and resulted in a significant change in the seasonal pattern of the trophic structure; 3) desertification caused obvious damage to soil macro-arthropods in all three soil layers, with the most damage occurring for soil arthropods in the 10–20 cm soil layer, and resulted in a significant change in the distribution pattern of soil arthropods in the soil profile; 4) desertification has a smaller effect on predators in 0–10 cm soil and omnivores in 10–30 cm soil and a more serious effect on phytozoa in all three soil layers; 5) with desertification development, the dominant family changed from Anthicidae in the spring and Melolonthidae in autumn to Tenebrionidae, from Anthicidae to Myrmeleontidae in the 0–10 cm soil layer, and from Staphylinidae to Labiduridae in the 10–20 cm soil layer; and 6) changes in the soil macro-arthropod community were attributed to the degradation of vegetation and deterioration of the soil environment during the desertification process, with notable decreases in vegetation cover, litter, soil clay and silt, soil organic carbon, and soil N and P. Acknowledgements The authors are grateful to the anonymous reviewers for their critical review and comments on drafts of this manuscript. This research was funded by one of the Chinese National Fund Projects (31270752, 30972422) and one of the Chinese National Support Projects of Science and Technology (2011BAC07B02-06).
H.-L. Zhao et al. / Geoderma 223–225 (2014) 62–67
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Table 5 Temporal and spatial changes of dominant families (dominance index). Items
Season
NDG
LDG
MDG
HDG
SDG
Seasonal changes
Spring
Anthicidae (19.3)
Carabidae (20.8)
Tenebrionidae (50.8)
Tenebrionidae (50.0)
Summer Autumn
Anthicidae (20.7) Melolonthidae (24.9)
Anthicidae (28.3) Anthicidae (36.8)
Anthicidae (25.0) Anthicidae (29.2)
Tenebrionidae (24.4), Anthicidae (24.2) Anthicidae (35.0) Elateridae (31.8)
0–10 cm
Anthicidae (18.9)
Anthicidae (28.1)
10–20 cm
Staphylinidae (32.7)
Anthicidae (29.4), Tenebrionidae (29.4) Tenebrionidae (40.0)
20–30 cm
Anthicidae (27.6)
Anthicidae (28.6) Tenebrionidae (28.6) Anthicidae (50.0)
Anthicidae (15.4) Araneidae (15.4) Anthicidae (42.9)
Depth changes
Tenebrionidae (24.1) Melolonthidae (24.1) Myrmeleontidae (40.0) Labiduridae (100)
Anthicidae (54.0)
Table 6 Correlation analysis between the community properties and soil factors. Item
Vegetation factors Cover
Richness Density
0.876⁎ 0.897⁎
Soil factors
Biomass
Litter
0.547 0.644
0.821⁎ 0.866⁎
Sand −0.591 −0.673
Clay + silt 0.791 0.856⁎
Water −0.026 0.002
Temp.
SOC
TN
TP
pH
0.735 0.767
0.867⁎ 0.918⁎
0.882⁎ 0.933⁎
0.922⁎ 0.960⁎⁎
0.145 0.097
⁎ P b 0.05. ⁎⁎ P b 0.01.
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