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 (pH(water), 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 P-32-labelled inorganic P-source applied to the soil revealed that the specific activity of P-32 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 P-31 and thus increased the size of the plant-available pool of soil P and diluting the added P-32. 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

Rapid changes in carbon and phosphorus after rewetting of dry soil

Drying–rewetting (DRW) cycles are important for soil organic matter turnover; however, few studies have considered the short-term effects on nutrient availability. The pulses in soil respiration, extractable C, P and N pools were quantified after a single DRW cycle (ten sampling times over 49 h). Soil was pre-incubated with or without glucose (2.5 g kg−1) for 10 days to induce differences in the size and activity of the microflora and then either subjected to a single DRW cycle (7-day drying period) or kept constantly moist. A resin extractable P (Presin) method was used and compared to extraction of dissolved organic (DOP) and inorganic P (DIP) with a salt solution. The pulse in soil respiration, extractable organic C (EOC), Presin, DOP and DIP was immediate and greatest in the first 2 h. The Presin pulse was two to three times that measured by solution extraction (DIP). Also, Presin quantified temporal changes in P not apparent in DIP, indicating the advantage of anion-exchange membranes in quantifying short-term changes in P availability. The Presin pulse was smaller in the soil incubated with glucose showing that P pulses will be quantitatively smaller in a soil with an active microbial biomass. In contrast to P, pre-incubation with glucose did not alter EOC concentration or the pulse in EOC after rewetting. The Presin pulse had disappeared by 49 h after DRW despite continued elevated rates of respiration. The sustained increase in DIP following DRW may have implications for plant availability or environmental losses.Clayton R. Butterly, Ann M. McNeill, Jeff A. Baldock and Petra Marschne