Monitoring of soil erosion and nutrient loss on the mountain slope in karst valley region based on isotope

Abstract: This paper describes research results based on soil samples that were collected in different layers of 5 cm thick each from a vegetable plot、abandoned land、forest land, and shrubs located at different altitudes on the same slope side of Zhongliang Mountain in the Karst Valley in Chongqing. The soil erosion on the slope in the Karst area is characterized by using combined indicators of 137Cs、soil nutrient and surface accumulation coefficient, which provides a scientific basis of understanding soil erosion. The soil samples collected from each layer of 5 cm thick were analyzed for their 137Cs and soil nutrient contents. The coefficient of enrichment of soil nutrient near surface was defined as a ratio of soil nutrient content within a 10cm(20cm) soil layer to that within a 40cm soil layer in the same soil profile. If the coefficient of enrichment of the soil nutrient from the layer of 0-10cm (0-20cm) thick was bigger than 0.25(0.5), it indicates that a superficial accumulation phenomena prevails. The soil erosion formula calculated with 137Cs for the Karst region was duly revised by taking into account a soil erosion formula with 137Cs in the non-Karst region and measured data obtained from a field runoff plot. It is found that the spatial distribution of nutrient content in both soil profiles and slopes can be presented in the form of a dual structure of the soil profile. The established behavior that the soil nutrient content in the surface soil layer decreases with lowering of the slope position does not equally apply to the Karst area. The ratio of soil nutrient content of the 0~10cm soil layer to the 0~40cm soil layer is very obvious (the ratio>0.25), while that of soil nutrient content within the 0~20 cm soil layer to the 0~40cm soil layer is unobvious (the ratio<0.5). Analysis of the Karst soil profile using the ratio value indicates that there exists erosion and leakage at various depths. The existing soil erosion formula with 137Cs was revised by calibration with the runoff plots through incorporating two factors: both the size of the rock that crops out on the surface and the fissure porosity of the fractured rock. By comparison, the amount of soil erosion estimated with 137Cs is bigger than that with the measured results on the runoff plot, because the former is the averaged soil erosion amount with 137Cs since 1963, while the latter is the average value between 2009 and 2010 with the runoff plot. The result show that the past exploded soil erosion was more severe than that in the present. The amount of soil erosion is getting smaller now because the soil layer is thinner than before, the amount of allowed soil erosion amount is smaller, and the environment is protected better now. The soil erosion intensity values in different land use types were calculated based on formula 1-4. It was reflected in the order of increase :farmland>range land>abandoned land. This research sheds light into the soil erosion characteristics in the Karst Valley, and hence consolidates the scientific basis for managing soil erosion and preventing rocky desertification.