Effects of soil erosion on organic carbon fractions in black soils in sloping farmland of Northeast China by using137 Cs tracer measurements

Abstract: Soil Organic Carbon (SOC) in agriculture is an important part of soil fertility and terrestrial ecosystems, and its variations affect food production and global carbon cycle, while soil erosion is the main cause of SOC migration and loss. However, the change mechanisms of SOC in agricultural fields and the quantitative relationship between soil erosion and these nutrients are still poorly understood. Studying the effects of soil erosion on organic carbon loss in the different components can provide a theoretical basis for scientifically assessing the role of soil erosion in the carbon cycle and exploring the mechanism of organic carbon turnover in the farmland. In the study, soil erosion characteristics and spatial distribution of its intensity on the convex slopes located in typical black soil areas of Northeast China were investigated based on the 137Cs tracing technique. The relationship between soil erosion intensity and organic carbon components was quantified. Moreover, the differences in organic carbon fractions between uncultivated and cultivated land was also compared. The results showed that the average annual erosion rate from the slope studied was 3801.71 t/(km2×a), thus the soil erosion intensity was categorised as moderate erosion. Among the all the sampling points, the 33.33% of them belonged to the severe erosion and 11.11% of them was extremely severe, which were mainly located in the middle of the steeply convex slope, and 26.67% of the points was in the deposition area, which were mainly distributed in the west area at the bottom of the slope. Compared to soils from the uncultivated slope, the average SOC content had decreased by 13.58%, of which Mineral Organic Carbon (MOC) and Particulate Organic Carbon (POC) had decreased by 7.52% and 40.49%, respectively, and the Coarse Particulate Organic Carbon (CPOC) had decreased by 73.24%, while the Fine Particulate Organic Carbon (FPOC) was not significantly different. The SOC, MOC, and FPOC at the deposition sites on the slope were significantly higher than those at the sites eroded (P<0.01); SOC and MOC at the deposition sites and the mild erosion sites were significantly higher than those of above- mild erosion sites (P<0.01); the higher decrease of both SOC and components of MOC and FPOC was observed in the sites where the moderate erosion occurred and changed slightly thereafter. The MOC and FPOC in organic carbon fractions decreased with the increasing erosion intensity, but there was no significant correlation between CPOC and the erosion intensity, and no significant difference between deposition sites and different erosion intensities was also observed(P>0.05). The result indicated that the difference of driving mechanisms of CPOC and MOC reduction may exist on the slope cropland. Overall, the severity of soil erosion in some black soil areas in Northeast China has not been greatly improved. The SOC content has reduced considerably as land use types have changed (reclamation), posing a serious threat to the sustainable development of agriculture in the black soil area. Erosion control and soil fertility replenishment may be the key to achieving sustainable development in the region.