Influence of rice cultivation on the distribution and movement of acids in profiles of the acid sulfate soil

【Objectives】 Acid sulphate soil(ASS)has high acid content and high mobility of Fe, Al, Mn, and other potentially toxic elements. Cultivation models significantly affect the oxidation of the main parent material of ASS(pyrite)and bring ecological risks. Rice cultivation has been considered as a model with low ecological risk. Therefore, this study on the acid content of ASS was conducted in paddy fields and abandoned land respectively, in order to examine the effects that rice cultivation has on the acid content of ASS and to confirm this conclusion. 【Methods】 Acid sulfate soil samples were collected from paddy fields and fallow land in the coastal area of Pearl River Delta in Taishan City, Guangdong Province in August, 2013. The soil samples were collected at 20 cm intervals in soil profiles of 300 cm in depth at five locations. The pH value, soluble acidity, exchangeable acidity, retainable acidity and soluble sulphate were determined. 【Results】 The acid content varied according to soil depth. The total acidity from 0 to 80 cm deep increased by 62% every 20 cm deeper. From 80 to 220 cm deep, the total acidity decreased by 62% every 20 cm in depth from 80 to 180 cm. The acidity in soil profiles between 220 and 300 cm remained stable and very low with a pH value of approximately 6.0. Rice cultivation influenced the distribution and movement of acid in ASS profiles significantly. Compared to abandoned land, the total amount of acidity present in soil from 0 to 80 cm in depth in paddy fields decreased significantly, while soluble acidity, exchangeable acidity and retainable acidity decreased by 77%, 37% and 28% respectively, and the difference in soluble acidity and exchangeable acidity was at a significant level whereas retainable acidity decreased significantly only in soil layers from 0-20 cm and 60-80 cm in the paddy fields. However, the total amount of acidity existing in soil from 100 to 120 cm in depth in paddy fields was significantly higher than in the abandoned land, while soluble acidity, exchangeable acidity and retainable acidity increased by 128%, 55% and 155%, respectively. The exchangeable acidity and retainable acidity in soil from 120 to 240 cm in depth in paddy fields were slightly higher but not significantly than those in the abandoned land. The acid content in soil from 240 to 300 cm in depth in paddy fields was similar to that in the abandoned land. Basically, rice cultivation affected the vertical distribution of acid content in the ASS profile, while the acidity of soils from 0 to 80 cm in depth decreased significantly, and the acidity of soil from 100 to 120 cm in depth increased significantly, and was absorbed by jarosite and accumulated as retainable acidity. The soluble sulphate of soil in the paddy fields was significantly lower than that in the abandoned land from 0 to 80 cm in depth, but was significantly higher than that in the abandoned land from 100 to 120 cm in depth. Soluble sulphate was correlated positively with soluble acidity, exchangeable acidity and retainable acidity, implying that rice cultivation might affect the distribution and movement of acid in ASS profiles by causing the transformation of sulphate minerals. 【Conclusions】 Rice cultivation significantly reduced the acid content in the upper layer of soil, increased the acid leaching leading to higher rates of acidity in soil from 100 to 120 cm in depth, which was temporarily retained in hydroxy-sulphate secondary minerals(such as jarosite). Therefore, rice cultivation can effectively lower the acid content of ASS, hence reduce its harm on plants in situ. On the other hand, rice cultivation may increase groundwater pollution due to higher leaching.