137Cs and 210Pbex tracing of soil erosions on cultivated and reforested slope lands in Three North-Shelter Forest Region

Abstract: 210Pbex and 137Cs measurements were carried out to trace the variations in soil depth and erosion rates in the cultivated and reforested land, aiming to explore the changes of soil retention due to the construction of the "Three North" shelterbelt program in the north of China. A typical shelterbelt and wind dominant region was selected, particularly on the Bashang Region of Zhangjiakou City, Hebei province, China. The results showed that in the cultivated land, both 137Cs and 210Pbex in the soil profile were uniformly distributed, due mainly to the mixing effect of tillage. In the reforested land, the 137Cs and 210Pbex concentrations in the 0-0.05 m soil layers were obviously higher than those in the other depth soil layers. There were roughly uniform concentrations of 137Cs and 210Pbex from 0.05 m to 0.15 m of the upper 0.15 m soil layer. It infers that the 137Cs and 210Pbex profile of soil can be changed after reforestation, thereby posing a great challenge on the use of 137Cs and 210Pbex to estimate the variation in soil erosion rates under the conditions of reforestation. A revised 210Pbex model of soil erosion in the reforested land was proposed using the 210Pbex mass balance equation in the temporal change trends of 137Cs and 210Pbex profiles. In the 137Cs proportional model, the mean soil erosion rates of reforested and cultivated land were 27.94?11.92 t/(hm2·a) and 29.11?14.42 t/(hm2·a). In the revised 210Pbex conversion model, the soil erosion rates of reforested and cultivated land were 82.16?14.36 t/(hm2·a) and 68.55?22.11 t/(hm2·a) before the construction of Three-North shelter forest, whereas, those were -41.28?33.91 t/(hm2·a) and -8.52?47.32 t/(hm2·a) after the constructed. It indicates that the 137Cs tracing can be used to characterize the average soil erosion rates and deposition rates on the slopes since 1963. Nevertheless, the 210Pbex tracer technology can be used to trace the variations in soil erosion rates before and after the shelter forest was constructed. In addition, the wind erosion rates of cultivated and reforested land obviously declined, due to the construction of Three-North Shelter Forest, where the cultivated and reforested land have been transformed from erosion zone to deposition zone in the study area.