Profile Variations of ~(137)Cs, ~(210)Pb_(ex) and SOM as Affected by Intensive Tillage on Steep Slop

Few direct measurements have conducted to investigate the dynamic process of soil degradation as affected by intensive tillage at the field level. Our objective is to test the potential of 137Cs and 210Pbexfor directly assessing the movement of SOM on the slope land as affected by tillage operations. We conducted 50 plowing operations over a 5day period using a donkeydrawn moldbordplow on steep backslope of the Chinese Loess Plateau. Profile variations of 137Cs, 210Pbex and SOM contents were measured at upper, mid, and lower portion of the slope after 50 plowing operations. Intensive tillage effects on 137Cs, 210Pbex and SOM contents were determined from a control slope adjacent to experimental slope. 137Cs concentration was uniformly mixed in the upper 0~30 cm of soil whereas 210Pbex showed a linear decrease at upper and mid portion, and an exponential decrease with depth on the lower portion of the control slope. SOM contents of 0~30 cm layers were much higher than the soil layers below 30 cm on the control slope, and showed a similar decrease pattern to profile pattern of 210Pbex on the mid and lower portion of the control slope. Fifty 50plowing operations resulted in a decrease of SOM content (g/kg) by 38% and by 47% for the soil layers of 0~45 cm at upper portion and mid portion, respectively. However, the lower position showed an increase of SOM content by 18% in the soil layers of 0~100 cm after 50 plowing operations. Weighed mean values of 137Cs concentrations decreased from 1.48 Bq/kg to 0.29 Bq/kg at upper position, from 2.53 Bq/kg to 0.33 Bq/kg at mid position, and increased from 1.48 Bq/kg to 2.81 Bq/kg at lower position. Weighed mean values of 210Pbex concentrations decreased from 27.71 Bq/kg to 6.15 Bq/kg at upper position, from 35.46 Bq/kg to 1.57 Bq/kg at mid position, and from 25.53 Bq/kg to 19.40 Bq/kg at lower position. Profile concentrations of 137Cs and 210Pbex are significantly correlated with SOM contents with R2 values of 0.81~0.86 for control slope, and 0.86~0.91 for experimental slope. Our results suggested that fallout 137Cs and 210Pbex could be used directly for quantifying soil qualitysoil redistribution relationship as affected by tillage erosion. 