Dissolution of Phosphate in a Phosphorus-Enriched Ultisol as Affected by Microbial Reduction

Abstract

Knowledge of the effect of reduced soil redox conditions on P dissolution is needed to better assess P mobility to surface or ground water. The objectives of our study were to determine the effect of microbial reduction on P dissolution and determine mechanisms of P release in a reduced soil. Duplicate suspensions of silt+clay from a Cape Fear sandy clay loam were reduced in a continuously-stirred redox reactor for 40 d. We studied the effects of three treatments on P dissolution: (i) 2 g dextrose kg-1 solids added as a microbial carbon source at time 0 d; (ii) 2 g dextrose kg-1 solids split into three additions at 0, 12, and 26 d; and (iii) no added dextrose. Regardless of treatment or variation in the intensity of reduction rates, dissolved reactive P (DRP) increased up to 7-fold from 1.5 to 10 mg L-1 and was linearly related (R2 = 0.79) with dissolved organic C. Dissolved Fe and Al and pH also increased, suggesting the formation of aqueous Fe- and Al-dissolved organic matter (DOM) complexes. Separate batch experiments were performed to study the effects of increasing pH and citrate additions on PO4 dissolution under aerobic conditions. Increasing additions of citrate increased concentrations of DRP, Fe, and Al, while increasing pH had no effect. Results indicated that increased DOM during soil reduction contributed to the increase in DRP, perhaps by competitive adsorption or by formation of aqueous ternary PO4-Fe-DOM or PO4-Al-DOM complexes.