| Soil is an important place for material circulation, energy conversion, the interactions between biotic and abiotic environment. Soil erosion could lead to soil degradation, soil fertility decline, nutrient loss and a series of problems, which seriously impact the soil status of ecological environment. Therefore, studies of soil erosion are conducive to the healthy and stable development of soil ecological environment.137Cs radioactive isotope tracer technique is a kind of emerging in the field of environment. As it is simple, rapid and high accuracy,137Cs is considered to be effective evaluation of soil erosion and soil conservation measures. The Yunnan-Guizhou Plateau located in Southwest China, is one of the most serious soil erosion areas in China. The Chaihe reservoir in Dianchi watershed was selected as the target region.137Cs technique was used to determine the soil erosion status; Conventional chemical analysys method was used to detect the concentrations of soil organic matter, total nitrogen and total phosphorus; The heavy metals concentrations including Cu, Pb and Cd were analyzed by atomic absorption spectrometry (PE AA800) to study the geochemical indicators. The single factor index method and Nemerow index method were used to evaluate the status of soil heavy metal pollution in the basin. The main results obtained are as follows:(1)Different types of soil samples within the catchment,137Cs contents ranged from217.81bq/m2-1010.01bq/m2, and the erosion rate ranged from12.09t/km2·a-7832.46t/km2·a. The rates of soil erosion of different types were:farmland grassland> forest land> abandoned land> stem watershed, reflecting that the changes in land uses would affect the soil erosion rate. Due to the impact of human activities, the soil erosion rates of farmland were higher than those of cultivated soil. In the non-arable soils, due to forest and grassland, woodland and grassland vegetation with mutual combination of vegetation cover types. The rainwater barrier and surface runoff interception capability were stronger, which could effectively reduce soil gully and rill conditions occurrences. The erosion rate was less than the grass of the forest and grassland, abandoned land. Compared137Cs content distributions in the grass sample profiles of137Cs content distribution, due to surface runoff and natural action such as infiltration, erosion situation was more serious. According to the guidelines of the National Ministry of Water Resources division, the study of agricultural land in the region as a whole was in the intensity of erosion level, non-cultivated soil in the majority was mild to slight erosion level.(2)The percent contents of SOM, TN and TP in the soil of the basin ranged from0.59%-4.37%,0.17%-0.75%,0.016%-0.074%, respectively, three of the contents were low. Statistical analysis results showed that the SOM, TN and TP coefficient of variation were relatively close, as0.477,0.446and0.457, which were regarded as medium degree of variation. Vertical distribution of SOM and TN contents in the samples were similar, while different with the vertical variation of TP. The C/N ratio in the soil profiles was less than10, indicating high degree of humification of soil organic matter. And plants could get more nitrogen from organic nitrogen mineralization process. The C/P ratios in the two sectional samples were27.36and24.97, respectively, which was the result of lower TP content in the soil.(3)Soil erosion significantly affected the soil SOM contents. Different types of land, soil erosion modulus were of different sizes. The content of soil organic matter was also quite different. Comparison of full and profile soil samples showed that the lower the soil erosion rate, the greater the SOM contents. Correlation analysis between the results also showed that the rate of soil erosion and soil SOM content was negatively correlated (Spearman coefficient-0.34).(4) On the basis of the study of isotope tracer for soil erosion rate, Cu, Pb and Cd distributions were analyzed in surface soil. Results showed that Pb, Cu and Cd contents ranged from5.73mg/kg-86.06mg/kg,14.45mg/kg-63.3mg/kg and0.073mg/kg-0.713mg/kg, respectively. Results of Ordinary Kriging spatial difference showed that the distribution of three kinds of heavy metal levels were in strip ribbon and patchy pattern. Single factor index analysis showed that the heavy metal pollution in the area was Cu> Pb> Cd, indicating relatively clean pollution levels. Nemerow integrated pollution index analysis showed that the comprehensive pollution index ranged from0.45-2.54, an average of0.91. The soil pollution was graded II alert level. |
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