Response Of Soil Carbon And Nitrogen Changes To Erosion Intensity On Sloping Cropland In The Black Soil Region Of Northeast China

The changes of soil organic carbon（SOC）and total nitrogen（TN）caused by soil erosion reduce soil function and play a decisive role in climate regulation.Although many studies have extensively studied the erosion response of SOC and TN,few studies have attempted to explore the impact of soil erosion on various organic carbon and nitrogen components,namely carbon or nitrogen in coarse-grained,fine-grained and mineral organic matter（CPOM,FPOM and MAOM）.In this study,two black soil farmland with different slope types（convex and straight）and reclamation years（50 and≥100 years）in Northeast China were selected.Nuclide ¹³⁷Cs tracer technology was used to quantitatively evaluate erosion.Combined with traditional statistical and geostatistical methods,the spatial variation characteristics of soil erosion intensity,soil organic carbon and nitrogen components,particle composition and parameters,as well as the response of erosion to soil organic carbon and nitrogen components and particle parameters were analyzed.The research is expected to provide a scientific basis for the response mechanism of soil carbon and nitrogen to erosion and the evaluation and management of the relationship between soil erosion and carbon and nitrogen sources and sinks in the black soil area of Northeast China.The main conclusions are as follows:（1）The net annual average erosion rates of convex and straight cultivated land are 3801.71and 2810.09t km^-2a^-1 respectively,which belong to the level of moderate erosion.In the sampling points of convex cultivated land,the erosion points accounted for 73.33%,of which extremely strong and severe accounted for 11.11%,mainly concentrated in the middle of the slope;The erosion points of straight cultivated land accounted for 89.58%,of which the strong accounted for 16.67%,mainly concentrated in the east of the slope and showed a fluctuating trend.The sedimentary areas of the two cultivated lands are mainly concentrated at the foot of the slope and scattered on the west side of the slope top.The fitting curve of the semi variance function of the annual average erosion rate of convex and straight cultivated land conforms to the change trend of Gaussian and Spherical models respectively,and has strong spatial autocorrelation in the range of 66.51 and 12.90 m respectively,which is mainly affected by structural factors.（2）The total amount of organic carbon and nitrogen in the two cultivated landscan reach the first level,and the content and proportion of components are MAOM>FPOM>CPOM,which belong to medium variation.Compared with uncultivated land,SOC of convex and straight cultivated land decreased by 13.58%(0.089 g kg^-1a^-1)and 35.18%(0.116g kg^-1a^-1)respectively,and TN decreased by 18.07%(0.010 g kg^-1a^-1)and 22.48%(0.006g kg^-1a^-1)respectively.The relative decline of CPOM in components is greater,but the absolute decline of MAOM is more significant.The corresponding SOC components of cultivated land are highly similar to TN fractions in spatial distribution.They are enriched in low-lying areas（i.e.depositional area）,and show obvious low-value distribution characteristics in erosion areas.The overall organic carbon and nitrogen fractions showed patchy and massive distribution patterns in spatial distribution.The curve fits of the semi-covariance functions of total SOC and TN in the cropland were consistent,and the trends of the Guassian and Linear models were consistent in the convex and straight cropland,respectively.The curve fits of the semi-covariance functions of total SOC and TN in the cropland were consistent,and conformed to the trends of Guassian and Linear models in convex and straight cropland,respectively,and the component CPOM-C was mainly influenced by structural factors,while the performance of other carbon and nitrogen components were all different.Stoichiometric characteristics showed that SOC/TN of uncultivated land and convex cultivated land was significantly higher than that of straight cultivated land.The C/N of POM was significantly higher than that of MAOM,while the C/N of FPOM decreased significantly in uncultivated,convex and straight cultivated land.（3）According to the final particle size distribution of soil particles,the soil texture of the two cultivated lands belongs to silty clay loam.Compared with the uncultivated land,the effective particle size of the two cultivated lands decreased significantly,and the contents of silt and clay increased significantly.The median particle size(D₅₀)decreased significantly,and the specific surface area（SSA）and fractal dimension（D）increased significantly.Compared with the final grain size of uncultivated land,sand（convex cultivated land）and silt（straight cultivated land）decreased significantly,and clay（two cultivated land）increased significantly.D₅₀ was only significantly greater than convex cultivated land,SSA was only significantly greater than straight cultivated land,and D had no significant change.The fractal dimension（D）of the two cultivated lands is the final particle size>effective particle size,which is at a high level.There was no significant difference in sand agglomeration rate(AR_sand)between uncultivated land and cultivated land,but the silt agglomeration rate(AR_silt)and clay agglomeration rate(AR_clay)in cultivated land increased significantly.The spatial distribution of soil particles and parameters changed significantly before and after dispersion.After dispersion,the spatial distribution of particle parameters was relatively more uniform and still had certain heterogeneity.In general,the spatial distribution was massive and patchy.The optimal semi variance function models of soil particles and parameters before and after soil dispersion in the two cultivated lands changed（except clay particles）.All parameters before dispersion in straight cultivated land conform to the variation trend of Spherical model,and the block base ratio is less than 25%,which is mainly affected by structural factors.（4）In the SOC loss caused by erosion in convex and straight cultivated land,the contribution of erosion accounted for 69.54%and 13.09%respectively.The effect of soil erosion on relatively fine components such as FPOM and MAOM in convex cultivated land is greater than that in straight cultivated land.Compared with the uncultivated soil,most of the SOC and TN components in the erosion and sedimentary areas showed a trend of loss.With the increase of erosion,the proportion of in-situ MAOM soil gradually increased with the increase of total erosion,while the ratio of FPOM and CPOM decreased.The relationship between erosion rate of convex cultivated land and all particle parameters did not reach a significant level,while the relationship between erosion rate of straight cultivated land and particle parameters before and after dispersion was more significant.The particle parameters of the two cultivated lands change little under different erosion levels,and there is no significant difference in terms of erosion and deposition areas.The total effect of soil erosion on the reduction of organic carbon fraction was-0.4892,which was lower than the direct effect of pre-dispersed particles（-0.5948）,indicating that pre-dispersed particles were the main factor affecting the reduction of organic carbon fraction,but the effect of soil erosion should not be neglected.The direct negative effects of soil erosion（-0.222）,pre-dispersed particles（-0.3375）and agglomeration rate（-0.673）on the total nitrogen fraction indicated that the agglomeration rate was the direct main factor affecting the reduction of total nitrogen fraction.This study clarified the spatial variation law of soil erosion,organic carbon and nitrogen components and particle parameters of different slope cultivated land in Northeast black soil area,and revealed the influence mechanism of soil erosion on organic carbon and nitrogen components and particle parameters.The soil erosion of the two cultivated lands is still very serious.For the straight cultivated land with long reclamation years but less annual erosion,the decline of organic carbon and nitrogen is greater,but the contribution of erosion is smaller;For the convex cultivated land with short reclamation years but high annual erosion rate,the decline of organic carbon and nitrogen is smaller,but the contribution of erosion is larger.For components,erosion has a greater impact on finer particles,such as fine particles and mineral organic carbon and nitrogen.Due to the existence of tillage,erosion has little effect on particles and weak sorting effect.In general,organic carbon and nitrogen components are affected by soil erosion to a certain extent,and the degree of influence is related to erosion intensity and tillage.Soil erosion plays a great role in relatively serious erosion conditions,but farming has a greater impact in relatively low erosion conditions.Therefore,while doing a good job in the prevention and control of soil and water loss in the black soil area of Northeast China,we should strengthen the supplement of fertility and realize the sustainable development of black soil resources.