Studies on Fractionation of Arsenic in Soil in Relation to Crop Uptake

Widespread arsenic (As) contamination in West Bengal and Bangladesh is of great concern as it affects millions of people due to its toxicity. Groundwater, when used for irrigation, helps entry of arsenic into the food chain via a soil-plant-animal continuum. In this study the extent of geo accumulation is measured in order to assess the degree of As contamination in soil. A sequential fractionation study of As revealed the concentration of different arsenic fractions in the order: As held at the internal surfaces of soil aggregates (20.7%) > freely exchangeable As (20.3%) > calcium associated As (18.7%) > chemisorbed As (17%) > residual As (15.7%) > labile As (3.29%). The variation in fractions may be attributed to the mineralogical make-up of soils along with some physicochemical factors. Statistical correlations and path analyses revealed that total and Olsen extractable arsenic (plant available arsenic) are dependent upon the As held at the internal surfaces of soil aggregates and chemisorbed arsenic fraction, which are directly influenced by the mineralogy of these experimental soils. The crop uptake by Kharif rice and mustard grown in these areas also corroborates the above fact. The poor reflection of exchangeable forms of soil arsenic in crop availability revealed that arsenic has undergone transformation via minerals through the continuous use of arsenic-laden water for irrigation.