137Cs tracing dynamics of soil erosion, organic carbon and nitrogen in sloping farmland converted from original grassland in Tibetan plateau

There is a shortage of research concerning the relationships between land-use change, soil erosion, and soil organic carbon (SOC) and nitrogen (N) dynamics in alpine environments such as those found in the Tibetan plateau. In this paper, typical sloping farmlands converted from grassland 50 years ago in eastern Tibet were selected to determine dynamics of soil erosion, SOC, and total N associated with land-use change. Soil samples were collected from sloping farmland and control fields (grassland). The 137Cs, SOC, total N contents, and soil particle size fractions were analyzed in these samples. As compared with the control fields, 137Cs, SOC, and total N inventories in the sloping farmlands decreased by 30%, 27%, and 33%, respectively. Meanwhile variations in the three parameters were enhanced in the sloping farmlands, with coefficients of variation (CVs) of 38%, 23%, and 20%, respectively, for 37Cs, SOC, and total N. In addition, SOC and total N inventories significantly decreased with increasing soil erosion in the sloping farmland. In a sloping farmland with a steep 24° gradient, the 137Cs inventory gradually increased along a downslope transect with its lowest value at 0 Bq m−2 in the top-slope position (0 m). The soil clay (<0.002 mm) content in such an area increased with decreasing elevation (r=−0.95, p=0.001). Significant correlations between 137Cs and clay (r=0.92, p=0.003), SOC (r=0.96, p=0.001), or total N (r=0.95, p=0.001) were also found in the farmland. These results showed that converting alpine grassland to sloping farmland accelerates soil erosion, losses in SOC and N, and increases the soil’s spatial variability. The combined impacts of tillage and water erosion contributed a significant decrease in the soil’s organic carbon and N storages. Particularly in steep sloping farmlands, tillage erosion contributed for severe soil loss, but the soil redistribution pattern was dominated by water erosion, not tillage erosion, due to the lack of boundaries across the field patches. It was also found that 137Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net C and N losses during soil redistribution. The negative influences of land-use conversion from grassland to farmland in sloping areas in the Tibetan plateau should be addressed.