Mineralization of soil organic P induced by drying and rewetting as a source of plant-available P in limed and unlimed samples of an acid soil

The effects of lime and P applications and drying and rewetting cycles, and their interactions, on the extractability and plant-availability of native and applied P were investigated in a glasshouse experiment using an acid (pHwater, 5.3), P-deficient soil. As expected, liming increased soil pH, decreased exchangeable Al to negligible concentrations and also caused a decrease in Bray No. 1- and NaHCO3 - extractable P. Dry matter yields and P uptake by wheat were enhanced by both lime and P applications. Subjecting the soil to two drying and rewetting cycles increased the extractability of soil P. This effect was more marked when P had been applied but was equally evident in both limed and unlimed soils. In addition, subsequent dry matter yields and plant P uptake were markedly increased by the drying and rewetting of the soils and this effect was more evident for the limed treatments. A subsidiary experiment using a 32P-labelled inorganic P-source applied to the soil revealed that the specific activity of 32P in both Bray No. 1 and resin soil extracts was decreased by subjecting the soil to drying and rewetting cycles. This occurred despite there being an increase in extractable soil P, dry matter yields and plant P uptake. These results strongly suggest that drying and rewetting stimulated mineralization of soil organic 31P and thus increased the size of the plant-available pool of soil P and diluting the added 32P. In relation to the fertility of highly weathered, P-deficient soils, these findings are important since organic P often accounts for 50%, or more, of their total P content and therefore represents a substantial pool of potentially plant-available P.