Using 137Cs and 210Pbex for quantifying soil organic carbon redistribution affected by intensive tillage on steep slopes

Studying on spatial and temporal variation in soil organic carbon (SOC) is of great importance because of global environmental concerns. Tillage-induced soil erosion is one of the major processes affecting the redistribution of SOC in fields. However, few direct measurements have been made to investigate the dynamic process of SOC under intensive tillage in the field. Our objective was to test the potential of 137Cs and 210Pbex for directly assessing SOC redistribution on sloping land as affected by tillage. Fifty plowing operations were conducted over a 5-day period using a donkey-drawn moldboard plow on a steep backslope of the Chinese Loess Plateau. Profile variations of SOC, 137Cs and 210Pbex concentrations were measured in the upper, middle and lower positions of the control plot and the plot plowed 50 times. 137Cs concentration did not show variations in the upper 0–30 cm of soil whereas 210Pbex showed a linear decrease (P < 0.05) with soil depth in the upper and middle positions, and an exponential decrease (P < 0.01) at the lower position of the control plot. The amounts of SOC, 137Cs and 210Pbex of sampling soil profiles increased in the following order: lower > middle > upper positions on the control plot. Intensive tillage resulted in a decrease of SOC amounts by 35% in the upper and by 44% in the middle positions for the soil layers of 0–45 cm, and an increase by 21% in the complete soil profile (0–100 cm) at the lower position as compared with control plot. Coefficients of variation (CVs) of SOC in soil profile decreased by 18.2% in the upper, 12.8% in the middle, and 30.9% in the lower slope positions whereas CVs of 137Cs and 210Pbex decreased more than 31% for all slope positions after 50 tillage events. 137Cs and 210Pbex in soil profile were significantly linearly correlated with SOC with R2 of 0.81 and 0.86 (P < 0.01) on the control plot, and with R2 of 0.90 and 0.86 (P < 0.01) on the treatment plot. Our results evidenced that 37Cs and 210Pbex, and SOC moved on the sloping land by the same physical mechanism during tillage operations, indicating that fallout 137Cs and 210Pbex could be used directly for quantifying dynamic SOC redistribution as affected by tillage erosion.