Effect of labile carbon on iron reduction and phosphorus availability in two paddy soils

Phosphorus (P) limitation in acidic tropical paddy soils is thought to be contributed by the affinity of iron oxides and hydroxides for phosphorus. However, the addition of labile carbon (C) compounds appears to be stimulated iron (Fe) reduction which can release occluded P through Fe cycling. The aim of this study was to investigate the phosphorous mobilization triggered by labile C coupled to iron (Fe) cycling during flooding anaerobic conditions in two P limited paddy soils. Two concentrations (low and high) of equal mixture of glucose and oxalate were added to soil microcosms and they were incubated for one month under flooded condition. During the 30-day experiment, replicate microcosms were destructively harvested for five times and measured total Fe, Fe II, available P and pH. It was found that labile C addition significantly increased total Fe and Fe II in soil solution at early incubation period although the changes were not significant in soil. Available P in soil was not increased and steady throughout the incubation period because the added labile C may not be enough to enhance the P solubilization or available P was dissolved in soil water which we did not measure in this experiment. High amount of labile C addition significantly decreased pH at the start of incubation. pH and available P are negatively correlated at day 7 which is compatible with decreased pH can increase available P. Our results suggest that reducing conditions coupled with labile carbon inputs can stimulate iron reduction and corresponding decreased pH which can triggered soil phosphorus mobilization in P limited paddy soils.