Soil flooding and rice straw addition can increase isotopic exchangeable phosphorus in P-deficient soils

Soil flooding increases phosphorus (P) availability due to reductive dissolution of P-bearing Fe(III) minerals. It is, however, unclear whether such processes also act in P-deficient soils of the tropics that have large Fe/P ratios (dithionite- and oxalate-extractable P and Fe). The objective was to identify the extent of P release induced by flooding in such soils and the soil characteristics involved. Six topsoils (0.4-5% Fe) from rice fields in Madagascar were incubated aerobically and anaerobically for 66days amended with factorial combinations of H2PO4- (0, 50mgP/kg); half of the flooded soils were also amended with 1g rice straw/kg prior to flooding to stimulate soil oxygen depletion. The release of P after flooding was measured at day 40 with 33P isotopic exchange, which detects both changes of labile P (exchangeable P) and changes in P solubility. Flooding increased labile P concentration in soil compared with aerobic soils by 1.4-60mg P/kg, effects being significant in 6 of the 12 soil samples. Rice straw addition further increased the labile P in 5 of the 12 flooded soil samples by 2-27mg P/kg. The release of labile P by flooding increased with soil oxalate-extractable P concentration. Flooding combined with rice straw addition can increase the labile P in soil, even in soils with large amount of Fe; however, this release in unfertilized soils is likely insufficient for optimal nutrition of rice plants when evaluated against critical values for P solubility.