桂西北典型岩溶区水稻田成土剖面砷的垂向分布特征及其影响因素

调查分析桂西北典型岩溶矿区及其周边水稻田成土剖面砷含量及相关土壤参数垂向分布特征，研究了岩溶区土壤砷的含量垂向分布及迁移规律，并探讨了岩溶区土壤砷垂向迁移的主要影响因素。结果表明:研究区三个典型土壤剖面的主要土壤类型为石灰岩土，化学风化程度较高，土壤砷自然来源主要为土壤母质；土壤剖面砷含量的垂向分布均为表层相对富集、深层相对稳定，而土壤有机碳是制约岩溶区水稻田土壤砷垂向迁移的关键因素；矿区外围的两个土壤剖面砷含量主要富集在10～20 cm层，而在0～10 cm层砷含量相对较低，土壤砷含量平均值分别为5.5 mg·kg-1 和5.0 mg·kg-1 ；而临近矿区的土壤剖面具有土壤砷高背景值的特点，土壤砷含量平均值为46.2 mg·kg-1 ，且砷主要富集在20～40 cm的次表层，而在10~20 cm层砷含量则相对较低，但在0～10 cm层土壤砷含量又显著高于10～20 cm层，这表明外来源对该剖面表层土壤也具有显著影响。 

Arsenic vertical distribution and its controlling factors in paddy soil profiles in a typical karst area, northwest of Guangxi, China

The characteristics of topsoil soil arsenic (As) accumulation and vertical migration in karst areas is of great significance to understand the migration and transformation of soil As and its geochemical behavior in soil environment. To explore the concentrations and migration characters of As in karst region, three paddy soil profiles in a typical karst mining area, northwest of Guangxi, were investigated and analyzed. This study area is located in the Dachang mining area in the southwestern part of Nandan district, characterized by the south-middle subtropical monsoon climate. The contents of the soil samples collected from the soil profiles were analyzed in a geochemistry laboratory,where the soil As concentrations were analyzed using the atomic fluorescence spectrometric method; the concentrations of MgO, K2O, CaO and Na2O were analyzed using ICP spectrometric method; the soil organic carbon concentrations were analyzed using redox volumetric method; and the soil pH was analyzed using ion selective electrode method. Afterward, the statistical software SPSS was applied to perform the data analyses, combined with the background of study area. Results show that the soil profiles consists of limestone soils with high degree of chemical weathering originated from soil parent materials. The soil As concentration in all the profiles studied has similar character, with relatively high As levels near the surface which becomes stable at the deeper portions of the soil. The dominant controlling factor of As migration and transformation was considered to be the soil organic carbon. In the periphery of the mine, the analytical result of two soil samples shows that the soil As mainly concentrated at the depth of 10-20 cm, rather than 0-10 cm . The mean concentrations of soil As in these soil profiles are 5.5 mg·kg-1 and 5.0 mg·kg-1 , respectively, with the ranges of 2.7 mg·kg-1 -9.9 mg·kg-1 and 2.5 mg·kg-1 -8.5 mg·kg-1 . The soil As concentrations of the profile closer to the mining area are characterized by high As background value, and the mean concentration of soil As is 46.2 mg·kg-1 ranging from 34.2 mg·kg-1 to 84.5 mg·kg-1 . The higher soil As concentrations in this soil profile gathers at the depth of 20-40 cm，while it is relatively lower in 10-20 cm deep. Notably, the soil As concentrations in this soil profile at 0-10 cm is also significantly higher than that of 10-20 cm deep, indicating that exogenous sources had significant effects on the surface soil of this profile. The ecological environment of karst mining area is extremely vulnerable, and the issue of agricultural soil As pollution in the surrounding areas should be paid sufficient attentions.