The Combined Use of 137 Cs Measurements and Zr-Methods for Estimating Soil Erosion and Weathering in Karst Areas of Southwestern China

Soil physical erosion and chemical weathering quantification at the slope scale are important to reveal the material cycle of the ecosystem in the karst region, because of the high heterogeneity due to the complex habitats. The Zr-based mass balance method has been widely used to quantify long-term physical erosion and chemical weathering at the slope scale, but the method is still in the exploratory research stage for quantifying short-term physical erosion and chemical weathering. In this paper, sloping fields (nine sampling sites and two sloping fields) in Zunyi, within the Guizhou karst region, were studied. We attempted to quantify the short-term physical erosion and chemical weathering rates by 137 Cs combined with the Zr-based mass balance method, and an ideal distribution curve of the Zr concentration in the soil surface layer of a karst region is proposed. The results showed the following: (1) The average soil erosion rate on the slope of the study area is 580 t/(km 2 ·a), which is equivalent to 14% of the average value of the Wujiang River basin in which it is located. This shows that the spatial distribution of soil erosion in this area varies significantly. (2) The Zr concentration in the erosion profile (EUC (1)) corresponds to 48% of that in the deposition profile (DUC (3)). This indicates that physical erosion in the study area survives chemical weathering, which is also consistent with the relationship model hypothesis. In addition, the vertical distribution characteristics of Zr concentration in all profiles are basically consistent with the ideal hypothesis curve. (3) The chemical weathering rate of the topsoil has been preliminarily estimated to be around 30 t/(km 2 ·a), and the ratio between the physical soil erosion and chemical weathering is 20:1. The results show that the physical erosion is dominant over the chemical weathering of topsoil, and the chemical weathering rate was proportional to the physical erosion. This study provides a new method for quantifying short-term soil erosion and weathering erosion at the slope scale in karst regions, which is important for regional ecological restoration and sustainable development.