Arsenic in Karstic Paddy Soil with High Geochemical Background in Guangxi, China: Its Bioavailability and Controlling Factors

Abstract

Arsenic (As) is widely concerned due to its serious harmfulness to ecological environment and human health. Soils from karst regions are characterized by high geochemical background of As. However, the bioavailability of paddy soil As and whether there is high transfer risk of Aa form soil to rice is still unclear. In this study, 305 paired soil–rice samples were collected from karst regions in Guangxi, China, to study the As bioavailability and controlling factors in soil–rice system. It was found that As concentrations in the paddy soil of the karst region were significantly higher than those in the non-karst region, while As concentration in rice grains was low and there were only 0.62% of rice samples over the permissible value of inorganic arsenic (0.2 mg/kg). As in karstic paddy soil was mostly present in the residual fraction, and the water-soluble and exchangeable fractions of As readily absorbed by rice were extremely rare. The high content but low bioavailability of As in karstic paddy soil was ascribed mostly to the abundant Fe–Mn concretions which contributed 64.45% As to host soil. In the interior of Fe–Mn concretions, As was primarily bound to Fe(oxyhydr)oxides, which could be released into paddy soil under some reduction conditions through reductive dissolution of Fe (oxyhydr)oxides. In nature pH condition of karstic paddy soil (pH 4.97–8.38), the leaching of As was almost negligible and As can be steadily adhered within Fe–Mn concretions. However, extremely acidic or alkaline conditions fostered the tendency of As release from Fe–Mn concretions.

Keywords: arsenic, paddy soil, bioavailability, Fe-Mn concretions, karst regions