Using 137 Cs and 210 Pb ex to Investigate the Soil Erosion Moduli of the Sandy Area of Typical Grasslands in Northern China

Soil erosion results in land degradation and desertification in northern China. The Xilingol League of Inner Mongolia is an important part of the “Two Barriers and Three Belts”, and has been given the main function of “a windbreak and sand-fixing belt of northern China”. Accurate measuring of soil erosion moduli, analyzing the differences in soil erosion moduli across different periods and regions, are the basis for carrying out soil conservation and evaluating the effectiveness of ecological governance. Some radioisotopes are good environmental tracers because they are closely combined with the fine particles of the surface soil and are only affected by the mechanical movement of soil particles. In this paper, Taipusi Banner and Zhengxiangbai Banner, which are in the farming–pastoral ecotone in northern China, were selected as the study area. A regional reference inventory, that is, the activity of 137 Cs and 210 Pb ex in the sample without any soil erosion, accumulation/deposition, or any kind of manual disturbances, as well as the soil erosion moduli, were determined by 137 Cs and 210 Pb ex composite tracing technology and multiple lines of evidence. The results are as follows: (1) The regional 137 Cs reference inventory was 1928 Bq∙m −2 , and the regional 210 Pb ex reference inventory was 10,041 Bq∙m −2 . (2) On a 50-year time scale, the soil erosion moduli in the study area ranged from 140 t∙km −2 ∙a −1 to 1030 t∙km −2 ∙a −1 ; on a 100-year scale, the soil erosion moduli in the study area ranged from 35 t∙km −2 ∙a −1 to 2637 t∙km −2 ∙a −1 ; the entire study area was in a lightly eroded state. (3) Compared with two periods before and after the 1970s, the southern parts (cultivated land and grassland) experienced an increasing trend in soil erosion moduli due to land reclamation, grassland grazing, and other activities. Due to weakening wind and increasing precipitation, soil erosion moduli in the northern parts (southern margin of the Hunshandake Sandy Land) slowed down. The study also discussed the uncertainty and application potential of isotope-tracing technology in sandy land of typical grasslands in northern China.