Modeling the erosion-induced fractionation of soil organic carbon aggregates on cultivated hill slopes through positive matrix factorization

Abstract

The lack of information concerning the compositions and erosion-induced redistributions of soil aggregates will limit our understanding regarding the stability and the fate of eroded soil organic carbon (SOC). In this work, we develop a tool for modeling the erosion-induced fractionation of soil aggregates and characterizing their compositions and redistribution pathways in the landscapes. Erosion-induced redistributions of soil organic carbon (SOC) on cultivated hill slopes with contour hedgerows planted to reduce soil and nutrient losses were investigated by the 137Cs tracing technique combined with soil composition characterization. The soil composition in 56 samples taken across cultivated plots was treated using the positive matrix factorization (PMF) method of receptor modeling. Five factors driving the interrelationships between the soil parameters and 137Cs in the plots were interpreted as representing the five types of soil aggregates, which were fractionated in situ according to their compositions, specific densities, and redistribution pathways under erosion forces combined with tillage and conservation measures. According to the PMF model, approximately three quarters of the 137Cs activity was attributed to the SOC aggregated with sand particles, which accounted for 33% of the SOC concentration and was identified as the light, labile SOC fraction. There was another labile SOC-sand aggregates, which did not contain 137Cs and accounted for 13% of the SOC concentration. The heavy, stable fraction that accounted for 44% of SOC concentration was in the SOC-clay aggregates, which contain the largest amounts of N, P, K, Kdis, and the highest CEC. The labile SOC-sand aggregates were concentrated in the depositional areas above the hedgerows whereas the stable SOC-clay aggregates were found in the eroding areas between the hedgerows. Labile SOC was depleted in the cultivated plot without conservation measures, but it was retained appreciably in the four cultivated plots with hedgerows.