Can ferric iron reduction in paddy soils compensate phosphorus limitation of rice plants and microorganisms?

Biogeochemical cycles of phosphorus (P) and iron (Fe) are tightly intertwined, especially in highly weathered and acidic subtropical and tropical soils rich in ferric Fe (Fe(III))oxides. In low-redox and P-deficient paddy soils, the quantitative contribution of the reductive dissolution of Fe(III)-bound P (Fe-P) to the demands of rice plants (Oryza sativa L.) and microorganisms remains unclear. We hypothesized that Fe(III) reductive dissolution can cover the P demand of microorganisms but not of rice plants during the initial growth stages, when P demand is high but the root system is still limited. We grew pre-germinated rice plants for 33 days in flooded rhizoboxes filled with a paddy soil of poor P availability. ³²P-labeled orthophosphate sorbed to ferrihydrite (80 kg ha^-1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots from directly mobilizing Fe-P (Pellets-mesh bag treatment), or (2) in the form of pellets directly to the soil without mesh bags to enable roots’ accessing the Fe-P (Pellets-no-mesh bag treatment). With the application of Fe-P directly to the soil, P was more available resulting in the increases in microbial biomass carbon (MBC) by 18–55% and nitrogen (MBN) by 4–108% in rooted soil as compared to the pellet not available to roots directly. The maximum enzyme activities (V_max) of phosphomonoesterase and β-glucosidase followed this pattern. During rice root growth, MBC and microbial biomass phosphorus (MBP) in both rooted and bottom bulk soil gradually decreased by 28–56% and 47–49%, respectively. In contrast to our hypothesis, the contribution of Fe-P to MBP strongly decreased from 4.5% to almost zero during 10–33 days after rice transplantation, while Fe-P compensated up to 16% of the plant P uptake 33 days after rice transplantation, thus outcompeting microorganisms.