Quantitative Study Of Aggregate Turnover Path During Slope Erosion Using Rare Earth Elements As Tracers

Soil aggregate is the basic unit of soil structure.The change of particle size distribution of surface soil aggregate is of great significance to the development of soil structure and erosion process.However,most of the previous studies focused on the relationship between aggregates and erosion process from the scale of single particle size,ignoring the mutual transformation process of soil aggregates.In view of this,this paper takes three different types of soil（forest land,shrub land and grassland）as the studied soils,through the tracing method of rare earth oxides,splash erosion experiment with different rainfall kinetic energy(501.95 J m^-2 h^-1,326.00 J m^-2 h^-1,252.50 J m^-2 h^-1 and 153.39 J m^-2 h^-1)and interrill erosion with different rainfall duration controlled were separately conducted to quantitatively trace the transformation process of different soil aggregates during erosion,and to quantify the specific sources of erosion sediment.The quantitative relationship between the dynamic change process of aggregates during erosion and erosion characteristics,organic carbon loss was discussed,which could enrich mechanism of soil erosion.The main conclusions are as follows:（1）The source and characteristics of splash sediment are affected by rainfall kinetic energy and soil characteristics.With the increase of rainfall kinetic energy or the content of water-stable aggregates in soil,the contribution rate gap of aggregates in each particle size increased.On the whole,the erosion sediment was mainly derived from that the aggregates of this size fraction hit by raindrops,such as the proportion of 2-5 mm in the>2 mm sediment achieved 87%at most,followed by the 0.25-2 mm aggregate with the highest proportion in the soil and the<0.053 mm aggregate that acted as the smallest particle size in studied soil.And the contribution of clay&silt fraction increased with the decreasing size of splash erosion particles.（2）The turnover process of aggregates during splash erosion was closely related to rainfall kinetic energy and soil aggregate size.In general,the cumulative fragmentation rate of 0.053-2 mm aggregates was higher（24.51%-37.37%）,and the cumulative fragmentation rate of 0.25-2 mm aggregates was lower（16.05%-26.49%）due to their highest content.At the same time,<0.25 mm aggregates have stronger cohesion with greater specific surface area,and the whole soil was transformed in the direction of breakdown;increasing the rainfall kinetic energy within a certain range,the degree of breakdown rate of large aggregates could increase,but the dispersed small pieces of aggregates also increased,so that the turnover rate of<0.053 mm aggregates was improved.The SOC content within the aggregates decreased to varying degrees in the splash test,affected by different transfer paths.The relative change of SOC in the eroded soil was negatively correlated with the cumulative breakdown rate of all aggregates（Y=-2.41*BD（TBS）+1.48,R²=0.635）.（3）In the process of interrill erosion,the eroded sediment was mainly formed by the aggregates of this size fraction transported by runoff（the content of 0.25-2 mm aggregates in 0.25-2 mm sediment reached 28%-42%）,and the content decreased with the increase of rainfall time,followed by SCP and SMA.At the same time,the residual aggregates also showed obvious turnover process.The>0.25 mm aggregates tended to be broke up（the cumulative breakdown rate reached 45.3%-68.2%and 33.2%-48.5%,the cumulative formation rate was only 8.7%-9.9%and 16.3%-23.4%）,and the overall degree of soil breakdown rate was increasing.The characteristics of runoff and sediment yield were closely related to the changes of topsoil structure and sediment migration.Based on correlation analysis and partial least squares analysis,the sediment content of<0.053 mm,0.25-2 mm and the contribution rate of corresponding aggregate size-fractions in sediment played important role in runoff coefficient and fractal dimension of sediment.However,from the perspective of aggregate turnover,there was a power function relationship between the conversion amount of transfer ratio from microaggregate to large macroaggregate and the sediment yield rate,between the turnover rate of silt&clay fraction and the runoff coefficient,and between the net formation rate of soil and the relative change of soil particle size fractal dimension.