Arsenic fractionation and mobilization in agricultural soils of NE Punjab, India

Bioavailability of arsenic (As) is strongly controlled by its partitioning in different soil constituents which alternatively determines its toxicity extent even though total As contents of soils might be comparable. Five soil samples with contrasting crop practices and physico-chemical nature were examined for As contents in different constituents by sequential extraction process and its mobility under oxic and anoxic conditions in presence of competing anions. Soils are of alkaline nature (pH 8.2-8.7) with high carbon content (C/N = 10.3-11.9). Mobilization of As increases with time and enhanced by presence of competitive anions (NO3- + PO43-) with a greater mobility under anoxic condition compared to that in oxic condition reaching up to similar to 19% of soil As. Overall decrease in pH was observed that was better pronounced under oxic conditions and drop in pH was relatively more in NO3- + PO43- containing reactors. Fluoride, Cl- and SO42- release were negligible within 21 days where lesser SO42- was measured in anoxic condition contrary to F- and Cl-. Almost complete NO3- and PO43- retention under anoxic reactors together with greater As release imply an ion exchange mechanism of As mobilization. Poor bioavailability of soil As was envisaged because of negligible water exchangeable As contents. However, a ligand exchange mechanism is proposed between PO43- and HAsO42- resulting greatest As extraction through phosphate extraction process where significant As contents remained intact onto the crystalline phases.