Characterization And Mechanism Of Red Soil Aggregate Breakdown And Organic Carbon Release In Erosion Transport System

The southern red soil hilly area is one of the most important hydraulic erosion areas in China.The ecological and environmental problems such as land degradation and soil erosion caused by hydraulic erosion are of significant concern.The fragmentation characteristics of soil aggregates under different erosion transport intensities and their effects on the lateral transport of soil organic carbon(SOC)are still unclear.In this study,we selected the red soil hilly area of southern China as the research site and conducted the indoor simulation experiment of the erosion and transportation process using a self-developed physical model.In this study,we conducted an indoor simulation experiment of erosion transport process using soil in southern red soil hilly area,designed four erosion transport dynamic conditions(no erosion transport intensity,CK;low erosion transport intensity,L;moderate erosion transport intensity,M;high erosion transport intensity,H)and six observation times(1,2,4,12,48,96 h)to analyze the effects of different erosion transport intensity on soil aggregates,SOC and related reactive organic carbon.To investigate the influence of erosion transport process on the change characteristics of soil aggregates with SOC and related components,and to reveal the correlation change pattern between them.The results of the study showed that:1.The erosion transport processes significantly altered the percentage content of each particle size of the soil aggregates and the stability of the aggregates.(1)>5 mm and <0.25 mm were the most significant changes in particle size during erosion transport,which significantly reduced the content of >5 mm particle size and increased<0.25 mm particle size.Compared with CK,low erosion transport intensity did not significantly change the agglomerate size(p>0.05);moderate and high erosion transport intensity significantly reduced >5mm grain size by 16.9% and 18.3%,respectively(p<0.05);and significantly increased <0.25 mm grain size by 149.4% and170.2%(p<0.05).The longer the erosion time,the more obvious the change magnitude of each particle size.(2)Compared with CK,moderate and high erosion transport intensity significantly reduced the mean weight diameter(MWD)of soil aggregates by14.5% and 16.3%,respectively;and significantly reduced the geometric mean diameter(GMD)by 35.3% and 37.3%,respectively(p<0.05).The reduction was not significant under low erosion transport dynamics conditions.Under the same erosion time,MWD and GMD showed the characteristics of CK>low erosion transport intensity(L)>moderate erosion transport intensity(M)>high erosion transport intensity(H).Moreover,the trend of increasing and then decreasing with the increase of erosion time is fluctuating.(3)The effects of moderate and high erosive transport intensity on fractal dimension(D)correspond with the power function distribution,showing the characteristics of rapid decrease followed by smooth change.(4)Correlation analysis shows that the fragmentation of >5 mm particle size aggregates during erosion transport may be the most important reason for the generation of <0.25 mm fine particles.(5)Principal component analysis(PCA)showed that the effect of erosion transport intensity on soil aggregates was mainly reflected in the late erosion period of moderate erosion transport intensity and high erosion transport intensity.2.The characteristics of SOC content changes in runoff-sediment phases and soluble organic carbon(DOC)and light fraction organic carbon(LFOC)content changes in sediment under different erosion and transport dynamics were elucidated.The following conclusions were obtained:(1)Compared with CK,the SOC content of runoff was significantly increased at low,medium and high erosion transport intensities,with increases of 78.4%,120.6% and 215.1%(p<0.05);medium and high erosion transport dynamics significantly reduced the sediment SOC content by 15.8% and18.1%,respectively(p<0.05),and the changes were not significantly different at low erosion transport intensities.It shows that the increase of erosion transport intensity significantly increases the SOC content in runoff and decreases the sediment SOC content.The sediment-bound SOC was the most dominant mode of lateral transport of organic carbon.(2)Compared with CK,DOC content in sediment was significantly reduced at moderate and high erosion transport intensity by 23.9% and 32.7%,respectively(p<0.05);while the difference was not significant at low erosion transport intensity.Erosion transport intensity on LFOC content increased with erosion time,but the difference was not significant.It indicates that the erosion transport process has a stronger effect on DOC,while a lower effect on LFOC.3.The characteristics of the change of agglomerate parameters during erosion transport were both related to the SOC content.Pearson correlation analysis and redundancy analysis showed that agglomerate fragmentation led to a decrease in SOC and DOC content in sediment and increased runoff SOC content during erosion transport,but had little effect on LFOC content.The >5 mm particle size and <0.25 mm particle size are the key factors affecting SOC and its components during erosion transport.This study is of great importance to understand the fragmentation characteristics of agglomerates by hydraulic erosion process,the lateral migration and release of soil organic carbon by water erosion.And it is important for mitigating global climate change and providing scientific and theoretical basis for achieving the double carbon goal.