Variation of Soil Carbon, Nitrogen and Phosphorus and Ecological Stoichiometric Characteristics During Land Degradation in Coal Mining Subsidence Area

: In order to explore the effects of land degradation on soil carbon, nitrogen and phosphorus and ecological stoichiometry in semi-arid coal mining subsidence area, the spatial pattern of soil carbon, nitrogen and phosphorus content, reserves and ecological stoichiometric ratio in the process of land degradation was studied by space-time substitution method with the soil in three stages of non-degradation, mild degradation and severe degradation as the research object. The results showed that :(1)Land degradation caused an average decrease of soil moisture content and porosity by 50.8% and 22.4%, respectively, resulting in an increase of soil pH and bulk density by 16.0% and 23.8%, respectively.(2)The soil organic carbon content and storage decreased by 55.6% and 50.7%, respectively, due to land degradation. The carbon loss in the surface soil was significant, and the nitrogen content and storage decreased by 21.8% and 13.9%, respectively. The soil nitrogen damage in the middle layer was the largest, and the phosphorus content and storage decreased by 37.9% and 31.8%, respectively.(3)Soil C/N decreased by 18.9%, C/P and N/P increased by 7.9% and 35.4%, respectively, after mild land degradation. Soil C/N and C/P decreased by 68.2% and 64.4%, respectively, while N/P increased by 16.1% after severe land degradation. (4) Soil water content and other environmental factors have different degrees of influence on soil carbon, nitrogen and phosphorus content, C/N and C/P, but no significant correlation with soil N/P. In summary, land degradation caused by multi-dynamic erosion damage in coal mining subsidence areas will significantly reduce soil environmental quality, cause soil drought, barrenness and salinization, and reduce the storage and supply capacity of ecosystem carbon, nitrogen and


Overview of the study area
The research area is located in Ejin Horo Banner, Ordos City, Inner Mongolia, which belongs to the coal mining subsidence area of Shendong mining area. The study area belongs to the temperate continental climate, with the characteristics of the same period of rain and heat. The rainfall is concentrated in summer and autumn, and the spring and winter are dry and windy. The average annual temperature is 8.9°C, the average annual precipitation is 396mm, the average annual evaporation is 1790mm, and the average annual wind speed is 3.5 m/s. The soil in this area is mainly sandy soil, with poor structure and low organic matter content. Common vegetations include Corethrodendron fruticosum, Hedysarum mongolicum, Artemisia ordosica, Hippophae rhamnoides, Setaria viridis and Stipa bungeana.

Experimental design
In this study, a soil survey was carried out in the typical coal mining subsidence area in Shendong mining area in July 2022. According to the vegetation growth and coverage in the erosion area, the types of sample plots are divided. (1) Severe degradation. The area is the core area where block subsidence and ground fissures occur intensively. The deformation and displacement of the soil are very obvious. The roots of the vegetation are exposed in large quantities, or the fracture and pull-out failure occur under the action of soil stress. The phenomenon of plant death is more, the trend of community degradation is more obvious, and the. (2) Mild degradation. The area is far away from the land subsidence and ground fissures, and is not disturbed by direct gravity erosion, but it is still affected. Some plants die due to the overall dislocation of the land. The surface appears sporadically exposed patches, the internal structure of the soil is deformed, and the degradation is still severe under the action of wind erosion.(3) It is not degraded. The adjacent non-subsidence area in this area has relatively stable ecological environment, good vegetation growth, excellent natural regeneration ability and complete succession sequence, and the surface biological crust develops well. Three 10 m × 10 m rectangular plots were randomly set up at different land degradation stages in the study area to investigate the growth status of shrubs in the plots, and three 1 m × 1 m herbaceous plots were set up along the diagonal in each rectangular plot to investigate the growth status of herbaceous plants. Three soil profiles were randomly excavated in each rectangular plot. The basic situation of each plot is shown in table 1. Note:Different lowercase letters indicate significant differences in indicators between plots ( P < 0.05 ), the same below.

Indicator determination
Three soil profiles were dug in each rectangular plot.
Considering the soil development characteristics in the study area, the stratified sampling depths were 0-10 cm, 10-20 cm, 20-40 cm and 40-60 cm. The physical properties of undisturbed soil such as bulk density, porosity and soil water content were measured by cutting ring method, and the soil quality per unit area of each layer was calculated. The undisturbed soil samples were collected. During the sampling process, the soil block should be kept from being squeezed and deformed, and the deformed part of the soil block directly in contact with the soil shovel outside the soil block should be stripped. Then the samples were placed in an aluminum box (Φ100×50 mm) and stored, and brought back to the room for processing. The soil samples were determined by air drying, impurity removal, grinding, sieving, mixing and sampling. The particle size composition of surface soil (0-10cm) was analyzed by Bettersize3000Plus laser particle size analyzer. Soil pH was measured by HI99121 portable soil pH analyzer. Soil organic carbon was determined by potassium dichromate-concentrated sulfuric acid oxidation external heating method. Total nitrogen was determined by Kjeldahl method. Total phosphorus was determined by NaOH melting-molybdenum antimony colorimetric method.

Data processing
Excel and SPSS 20.0 software were used for experimental data processing and statistical analysis.
Sigmaplot 14.0 software was used for text interpolation. The least significant difference (LSD) method was used to test the difference significance of soil carbon, nitrogen and phosphorus content and ecological stoichiometric characteristics at different stages of land degradation and different soil depths (α = 0.05). Pearson test was used to test the correlation between soil carbon, nitrogen and phosphorus, ecological stoichiometric ratio and environmental factors. Two-Way ANOVA was used to test the effects of land degradation, soil depth and their interactions on soil carbon, nitrogen and phosphorus.

Changes of soil environment during land degradation
As shown in table 2, land degradation significantly changed the physical and chemical properties of soil in coal mining subsidence area (P<0.05). The soil moisture content of each soil layer decreased gradually during the land degradation process. Compared with the non-degraded land, the soil moisture content of the lightly degraded land decreased by 31.9% on average, and the severe degradation decreased by 69.6%. The middle layer soil (20-40 cm) was the most serious. The soil pH value of each soil layer showed a significant upward trend during the land degradation process, and the soil gradually transitioned from neutral to alkaline. Compared with the non-degraded land, the pH value of the lightly degraded soil increased by 15.2% on average, and the heavily degraded soil increased by 16.8%. The salinization rate of the surface soil (0-20 cm) was the fastest ; in the process of land degradation, the soil bulk density of each soil layer increased significantly, the porosity decreased, and the soil structure and permeability became worse. Compared with the non-degraded land, the soil bulk density of mildly degraded land increased by 15.9% on average, the porosity decreased by 15.2%, and the bulk density of severely degraded land increased by 31.7%. The porosity decreased by 29.5%, and the surface soil (0-20 cm) had the most obvious process of changing from loose to compact. Note: Different lowercase letters indicate that there are significant differences between different soil layers of the same degradation degree, and different uppercase letters indicate that there are significant differences between different degradation degrees of the same soil layer ( P < 0.05 ).

Changes of soil carbon, nitrogen and phosphorus contents during land degradation
As shown in Fig.1, land degradation significantly changed the spatial distribution of soil carbon, nitrogen and phosphorus in coal mining subsidence area (P<0.05), and soil impoverishment was obvious. The soil organic carbon content of each soil layer decreased gradually during the process of land degradation. Compared with the non-degraded land, the organic carbon content of the lightly degraded soil decreased by 31.8% on average, and the severe degradation decreased by 79.3%. The surface soil (0-20 cm) lost the most organic carbon, with an average loss rate of 63.7%. The soil nitrogen content in each soil layer decreased gradually during the process of land degradation. Compared with the non-degraded land, the nitrogen content in the lightly degraded soil decreased by 13.8 % on average, and the heavy degradation decreased by 29.7%. The nitrogen loss in the middle layer (20-40 cm) was the most, with an average loss rate of 23.1%. The soil phosphorus content also decreased significantly during the process of land degradation. Compared with the non-degraded land, the phosphorus content of the lightly degraded soil decreased by 36.3% on average, and the heavy degradation decreased by 39.6%. The middle layer soil (20-40 cm) lost the most phosphorus, with an average loss rate of 39.1%. Land degradation caused a significant decrease in soil carbon, nitrogen and phosphorus storage (P mild degradation (241.0 tꞏhm -2 ) > severe degradation (81.4 tꞏhm -2 ), soil nitrogen storage was non-degradation (20.7 tꞏhm -2 ) > mild degradation (19.0 tꞏhm -2 ) > severe degradation (16.8 tꞏhm -2 ), and soil phosphorus storage was slightly different, showing non-degradation (6.6 t ꞏ hm -2 ) > severe degradation (4.6 tꞏhm -2 ) > mild degradation (4.44 tꞏhm -2 ). Under the influence of land degradation, the loss of carbon storage in surface soil (0-20 cm) was the largest, while the loss of nitrogen and phosphorus storage in middle soil (20-40 cm) was the most significant.

Conclusion
In this study, soil erosion in the coal mining subsidence area caused serious land degradation, and the basic physical and chemical properties of the soil changed significantly. One is that the soil moisture content decreased significantly due to land degradation, and there was a significant drought phenomenon, which is consistent with the results of scholars ' research on sites with difficult erosion [1] . The first reason is that the surface is exposed due to the decrease of vegetation coverage, resulting in the increase of soil moisture evaporation plastice [2] , followed by the decrease of soil clay content and the increase of sand content in the process of land degradation, and the decrease of soil water retention and water holding capacity [3] . The data also shows that the middle layer soil (20-40 cm) is the most serious, which may be because the study area is a semi-arid area with scarce precipitation. The surface soil can infiltrate a small amount of water after rainfall, and the limited rainfall cannot form an effective recharge to the middle layer soil. [4] The water shortage caused by subsidence erosion is more obvious [5] . The second is that the soil pH value increased significantly due to land degradation, and there is a clear trend of salinization, especially the surface soil (0-20 cm) salinization rate is the fastest, from neutral (7.07) to strong alkaline (8.60), which is similar to the research results of many scholars, indicating that land degradation has the harm of secondary salinization..