Arsenic supply characteristics of four cotton-producing soils

Cotton-producing soils in Louisiana average 23 mg kg(-1) arsenic (As) due mainly to use of As-containing agrichemicals. Bioavailability of soil As could become an important management factor if As-tolerant cotton is rotated with other more sensitive crops. The first step in modelling As bioavailability and its uptake by plants is to characterize soil supply of As. We sought to: i) determine the relationships among solution As, resin-exchangeable solid-phase As, and As addition, and ii) examine soil properties that affect these relations. Four diverse surface soils were collected from the cotton-producing areas of the state. Five rates of As from 0 to 200 mg kg(-1) were applied and the soils allowed to equilibrate at 80% of ''field capacity'' water content for 30 days. Total initial As, As in displaced soil solution, and resin-exchangeable solid-phase As were determined for each treatment. For all soils, the concentration of solution As increased curvilinearly with As addition, conforming to the equation As-sol = ax(c)+d. Values of ''c'' describing the curvilinearity ranged from 1.09 to 3.19. Curvilinearity was negatively related to both initial solution As and diethylenetriaminepentaacetic acid (DTPA)-extractable manganese (Mn) (r(2) = 0.99). DTPA-extractable Mn complexed As from the solution phase. The concentration of resin-exchangeable solid-phase As increased at a decreasing rate with As addition. The relation was described by the equation As-resp = mx(l)+n, where values for the curvilinearity, l, ranged from 0.048 to 0.986. Concentrations approached a point where the adsorption sites for this phase became saturated and any additional As remained in solution. These relationships provide valuable information for modelling As uptake by plant roots.