Long-term light grazing does not change soil organic carbon stability and stock in biocrust layer in the hilly regions of drylands
MA Xinxin1,2,3, ZHAO Yunge1,2, YANG Kai4, MING Jiao1,2,3, QIAO Yu4, XU Mingxiang1,2,3,*(), PAN Xinghui5
1Research Center of Soil and Water Conservation and Ecological Environment, Chinese Academy of Sciences and Ministry of Education, Yangling 712100, China 2Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China 3University of Chinese Academy of Sciences, Beijing 100049, China 4College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China 5College of Life Sciences, Northwest A&F University, Yangling 712100, China
Livestock grazing is the most extensive land use in global drylands and one of the most extensive stressors of biological soil crusts (biocrusts). Despite widespread concern about the importance of biocrusts for global carbon (C) cycling, little is known about whether and how long-term grazing alters soil organic carbon (SOC) stability and stock in the biocrust layer. To assess the responses of SOC stability and stock in the biocrust layer to grazing, from June to September 2020, we carried out a large scale field survey in the restored grasslands under long-term grazing with different grazing intensities (represented by the number of goat dung per square meter) and in the grasslands strictly excluded from grazing in four regions (Dingbian County, Shenmu City, Guyuan City and Ansai District) along precipitation gradient in the hilly Loess Plateau, China. In total, 51 representative grassland sites were identified as the study sampling sites in this study, including 11 sites in Guyuan City, 16 sites in Dingbian County, 15 sites in Shenmu City and 9 sites in Ansai District. Combined with extensive laboratory analysis and statistical analysis, at each sampling site, we obtained data on biocrust attributes (cover, community structure, biomass and thickness), soil physical-chemical properties (soil porosity and soil carbon-to-nitrogen ratio (C/N ratio)), and environmental factors (mean annual precipitation, mean annual temperature, altitude, plant cover, litter cover, soil particle-size distribution (the ratio of soil clay and silt content to sand content)), SOC stability index (SI) and SOC stock (SOCS) in the biocrust layer, to conduct this study. Our results revealed that grazing did not change total biocrust cover but markedly altered biocrust community structure by reducing plant cover, with a considerable increase in the relative cover of cyanobacteria (23.1%) while a decrease in the relative cover of mosses (42.2%). Soil porosity and soil C/N ratio in the biocrust layer under grazing decreased significantly by 4.1%-7.2% and 7.2%-13.3%, respectively, compared with those under grazing exclusion. The shifted biocrust community structure ultimately resulted in an average reduction of 15.5% in SOCS in the biocrust layer under grazing. However, compared with higher grazing (intensity of more than 10.00 goat dung/m2), light grazing (intensity of 0.00-10.00 goat dung/m2 or approximately 1.20-2.60 goat/(hm2·a)) had no adverse effect on SOCS. SOC stability in the biocrust layer remained unchanged under long-term grazing due to the offset between the positive effect of the decreased soil porosity and the negative effect of the decreased soil C/N ratio on the SOC resistance to decomposition. Mean annual precipitation and soil particle-size distribution also regulated SOC stability indirectly by influencing soil porosity through plant cover and biocrust community structure. These findings suggest that proper grazing might not increase the CO2 release potential or adversely affect SOCS in the biocrust layer. This research provides some guidance for proper grazing management in the sustainable utilization of grassland resources and C sequestration in biocrusts in the hilly regions of drylands.
MA Xinxin, ZHAO Yunge, YANG Kai, MING Jiao, QIAO Yu, XU Mingxiang, PAN Xinghui. Long-term light grazing does not change soil organic carbon stability and stock in biocrust layer in the hilly regions of drylands. Journal of Arid Land, 2023, 15(8): 940-959.
Fig. 1Overview of the Loess Plateau and distribution of sampling sites on the Loess Plateau (a), the typical landscape of grazing in the hilly regions (b1 and b2), and the land surface characteristics of grasslands along grazing intensity gradient (from grazing exclusion to heavy grazing) (c1-c3).
Variable
Dingbian County
Shenmu City
Guyuan City
Ansai District
MAP (mm)
374
427
468
571
MAT (°C)
8.0
9.7
7.8
9.9
Soil type (IUSS Working Group WRB, 2015)
Regosols
Regosols
Regosols and Umbrisols
Regosols
Clay content (≤0.002 mm) (%)
12.5±0.4b
10.4±0.8c
18.5±0.9a
14.3±0.8b
Silt content (0.002-0.050 mm) (%)
20.3±0.6c
16.0±1.5d
29.7±0.7a
24.7±1.1b
Sand content (0.050-2.000 mm) (%)
67.2±0.9b
73.6±2.3a
51.8±1.5d
61.0±1.8c
Soil particle-size distribution
0.49±0.02c
0.38±0.04d
0.95±0.06a
0.65±0.05b
Table 1 Variations in meteorological condition and soil particle-size distribution in the biocrust layer in the four regions
Region
Number of sampling sites
Number of goat dung per square meter (goat dung/m2)
Elevation(m)
Slope gradient (°)
Main plant species
Plant cover (%)
Litter cover (%)
Dingbian County
16
Grazing exclusion
3
0.00
1657-1662
10-15
Stipa bungeana, Stipa capillata and Potentilla reptans
35.0-55.0
35.8-41.1
Grazing
13
4.15-29.97
1526-1701
15-30
Thymus mongolicus, S. bungeana, Oxytropis racemosa and Artemisia sacrorum
16.3-61.7
14.3-41.0
Shenmu City
15
Grazing exclusion
3
0.00
1209-1214
5
S. bungeana, Lespedeza bicolor and Poa sphondylodes
32.0-63.0
21.2-37.4
Grazing
12
5.03-73.20
1192-1522
10-20
Artemisia desertorum, S. bungeana, L. bicolor and P. sphondylodes
5.0-54.0
2.4-43.1
Guyuan City
11
Grazing exclusion
0
0.00
1952-2079
6-11
Carex dahurica and A. sacrorum
90.0-95.0
5.0-10.0
Grazing
11
0.88-26.16
1675-2010
5-25
S. bungeana, A. sacrorum, P. reptans, Potentilla tanacetifolia and T. mongolicus
41.7-78.3
8.6-37.9
Ansai District
9
Grazing exclusion
3
0.00
1199-1215
18-23
A. sacrorum
48.3-56.7
13.2-20.4
Grazing
6
0.28-19.18
1178-1208
10-19
Bothriochloa ischcemum, L. bicolor and S. bungeana
31.7-86.0
14.3-36.4
Table 2 Characteristics of sampling sites (under grazing and grazing exclusion) in the four regions
Fig. 2Variations in biocrust attributes of cover (a and d), biomass (b and e), relative cover (c and f) and thickness (g), as well as soil physical-chemical properties of soil porosity (h) and soil C/N ratio (i) in the biocrust layer with increasing grazing intensity. Soil C/N ratio, soil carbon-to-nitrogen ratio. The linear or nonlinear regression equations are shown at P<0.05 level but omitted at P>0.05 level. The same below.
Fig. 3Variations in the SOC content (a), SOC fractions of different lability contents (b-e) and SI (f) in the biocrust layer with increasing grazing intensity. SOC, soil organic carbon; C1, very labile C; C2, labile C; C3, less labile C; C4, nonlabile C; SI, SOC stability index.
Fig. 4Variations in SOC stock (SOCS) in the biocrust layer with increasing grazing intensity (a) and under different grazing intensity grades (b). Different lowercase letters indicate significant differences among different grazing intensity grades (P<0.05). The lower and upper boundaries of the box indicate the 25th and 75th percentiles, respectively; whiskers below and above the box indicate the 10th and 90th percentiles, respectively. The black horizontal line within each box indicates the median value, and the red dot indicates the mean value.
Impact factor
Mean square
F
P
MAP
0.105
1.963
0.169
MAT
0.123
2.314
0.136
Altitude
0.408
7.657
0.008**
Plant cover
0.252
4.715
0.036*
Litter cover
0.171
3.208
0.081
Soil particle-size distribution
0.001
0.024
0.876
Grazing intensity grades
0.083
1.553
0.215
Table 3 Results of the multi-way analysis of covariance (ANCOVA) showing the impacts of environmental conditions and grazing intensity grades on SOC stock (SOCS) in the biocrust layer
Variable
Total biocrust cover
Cyanobacteria cover
Moss cover
Cyanobacteria biomass
Moss biomass
Relative cover of cyanobacteria
Relative cover of mosses
Biocrust thickness
Grazing intensity
-0.138
0.256
-0.342*
0.317*
-0.289*
0.366**
-0.363**
-0.369**
MAP
0.043
-0.237
0.240
-0.204
0.023
-0.248
0.242
0.135
Plant cover
0.006
-0.448**
0.376**
-0.296*
0.231
-0.482**
0.479**
0.018
Soil particle-size distribution
0.015
-0.253
0.200
-0.155
0.081
-0.298*
0.300*
-0.105
Soil porosity
0.130
-0.367**
0.445**
-0.451**
0.429**
-0.524**
0.524**
0.355*
Soil C/N ratio
-0.100
-0.140
0.047
-0.177
0.227
-0.023
0.025
-0.014
SI
-0.035
0.236
-0.234
0.238
-0.239
0.277*
-0.276*
-0.173
Table 4 Pearson correlation coefficients of biocrust attributes with grazing intensity, environmental conditions, soil physical-chemical properties and SI
Fig. 5Structural equation model (SEM) showing the mechanisms by which grazing affected SOC stability in the biocrust layer under the influences of environmental factors. The arrow direction represents hypothetical causality. Standardized path coefficient (β) close to the arrow indicates the effect size of the relationship. The red and blue arrows indicate positive and negative relationships, respectively. The width of the arrows is proportional to the coefficient. Significant coefficients are represented by values with asterisks, while nonsignificant coefficients (P>0.05) are represented by values without asterisks. *, P<0.05 level; **, P<0.01 level; ***, P<0.001 level.
Fig. S1Variations in total biocrust cover (a), soil bulk density (b) and soil particle-size distribution (c) in the biocrust layer with increasing grazing intensity. Soil particle-size distribution is the ratio of soil clay and silt content to sand content.
Fig. S2Variations in the relative contents of C1, C2, C3 and C4 to SOC in the biocrust layer with grazing intensity. (a), C1/SOC; (b), C2/SOC; (c), C3/SOC; (d), C4/SOC. C1, very labile C; C2, labile C; C3, less labile C; C4, nonlabile C.
Fig. S3Relationships of the SOC content of the biocrust layer with altitude (a) and soil particle-size distribution (b)
Fig. S4Relationships of the relative cover of cyanobacteria with the relative contents of C1, C2, C3 and C4 to SOC in the biocrust layer. (a), C1/SOC; (b), C2/SOC; (c), C3/SOC; (d), C4/SOC.
Fig. S5Quantitative relationship between grazing intensity and the number of goat dung per square meter
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