Organic anions and phosphorus desorption and bioavailability in a humid Brazilian ultisol

Phosphorus (P) bioavailability is controlled by combined reactions of adsorption/precipitation and desorption/dissolution. Soil P has been fractionated; however, current P metrics do not necessarily measure discrete pools of bioavailable P based on mechanisms of soil inorganic P extraction by plants. Plants can desorb P by creating disequilibria between solid and solution phases (disequilibria desorption) and through the action of exuded organic anions on soil surfaces (ligand desorption). Both desorption mechanisms involve surface reactions of exchange and dissolution. These pools of P are potentially bioavailable pools because they react to plant mechanisms. However, these pools are not well documented. Our study had three objectives: (i) to measure liganddesorbable P, (ii) to determine if we could separate ligand-exchangeable and ligand-dissolvable pools, and (iii) to investigate the fate of applied fertilizer P as it was incorporated into disequilibria-desorbable and ligand-desorbable pools. All studies used a tropical Brazilian Ultisol. Seven hours of interaction time and 500 mu mol of anions g(-1) soil were found optimum for the measurement of ligand-desorbable P. Our data suggested that ligand exchange and ligand dissolution could be operationally separable when anion concentrations were below 1 mu mol g(-1). In the short term, P fertilization preferentially put P in the disequilibria-desorbable pool over ligand-desorbable pool. Fractionation of soil bioavailable P must,account for. the mechanisms, that plants and micro organisms have for extracting P from soils, which means they must go beyond the common P fractionation schemes commonly used today, which have their roots in soil pedology.