Phosphorus retention and release of anions and organic carbon by two Andisols

Andisols can absorb large amounts of phosphorus rapidly, and then release it slowly, yet the mechanisms by which they retain P and release it for plant growth are poorly understood. Ligand exchange of organic compounds from Al-humic complexes by P and/or Si release - due to breakdown of allophanic microstructure to provide sorption sites - might account for the retention of P, but its extent is not known. We applied a soil column flow-through technique to quantify the release of anions and organic carbon (C) associated with P sorption by two andic soils, and we related the anion release to possible mechanisms for the retention of P. Phosphate (H2PO4- HPO42-) sorption and concurrent anion desorption were obtained by passing a 1-g P l(-1) (32 mmol KH2PO4 in 1 mM CaCl2) solution through the soil columns (25 cm(3)). Total dissolved P, Fe, Al, S, Ca, Mg, K, Mn, organic C and pH were determined in the eluent. Changes in eluent pH and the patterns of the retention of P and corresponding concentrations of Al, Si and organic C in the eluent were similar for the two Andisols. The general pattern and changes in pH of the eluent coincided with changes in the patterns of release of organic C and Si and the rate of P retention. Release of silica accounted for <6% of the P sorbed and had only a minor role in P retention in these two Andisols. Release of organic C, however, accounted on a molar basis for 40% and 83%, respectively, of the P sorbed. Direct measurements of the pH of the eluent and release of anions and organic C concurrent to P retention contribute to rapid assessment of the controlling mechanisms of P retention. The results indirectly confirm the hypothesis of ligand exchange of solution P with organic complexes held on allophanic surfaces. The organic C release, however, is not specifically related to either the fast or the slow P retention phase. The shift in the controlling P retention reaction associated with a change from the fast to the slow P retention phase is clearly indicated by an abrupt change of the pH of the eluent. This shift, in previous studies' identified graphically by a change in slope of the P sorption isotherm, can be identified directly by measuring the pH of the matrix.