Phosphorus-arsenic interactions in variable-charge soils in relation to arsenic mobility and bioavailability.

Phosphorus (P) influences arsenic (As) mobility and bioavailability which depends on the charge components of soil. The objective of this study was to examine P-As interaction in variable-charge allophanic soils in relation to P-induced As mobilization and bioavailability. In this work, the effect of P on arsenate [As(V)] adsorption and desorption was examined using a number of allophanic and non-allophanic soils which vary in their anion adsorption capacity. The effect of P on As uptake by Indian mustard (Brassica juncea L.) plants was examined using a solution culture, and a soil plant growth experiment involving two As-spiked allophanic and non-allophanic soils which vary in their anion adsorption capacity, and a field As-contaminated sheep dip soil. Arsenate adsorption increased with an increase in the anion adsorption capacity of soils. The addition of P resulted in an increase in As desorption, and the effect was more pronounced in the case of allophanic soil. In the case of both As-spiked soils and field contaminated sheep-dip soil, application of P increased the desorption of As, thereby increasing its bioavailability. The effect of P on As uptake was more pronounced in the high anion adsorbing allophanic than low adsorbing non-allophanic soil. In the case of solution culture, As phytoavailability decreased with increasing concentration of P which is attributed to the competition of P for As uptake by roots. While increasing P concentration in solution decreased the uptake of As, it facilitated the translocation of As from root to shoot. The net effect of P on As phytoavailability in soils depends on the extent of P-induced As mobilization in soils and P-induced competition for As uptake by roots. The P-induced mobilization of As could be employed in the phytoremediation of As-contaminated sites. However, care must be taken to minimize the leaching of As mobilized through the P-induced desorption, thereby resulting in groundwater and off site contamination.