Labile organic matter intensifies phosphorous mobilization in paddy soils by microbial iron (III) reduction

Iron (Fe) is one of the most abundant elements in the earth's crust and intensively interferes with the biogeochemical cycles of carbon (C) and phosphorous (P), especially in highly weathered tropical and subtropical soils. The strong affinity of phosphate to Fe oxides and hydroxides limits the P availability in paddy soils. Reductive dissolution of Fe(III) can release occluded and adsorbed P into solution and make it available for plant uptake. Such effects remain elusive, especially in highly weathered subtropical paddy soils with oscillating redox (Eh) conditions. Under submerged conditions, incomplete litter decomposition occurs and rice roots release large amounts of labile organic compounds as exudates. We investigated the role of acetate, formate, oxalate, and propionate on Fe(III) reduction, P mobilization, and CO2 efflux in two paddy soils (i.e. oxisol and ultisol) of varying organic C (OC) and Fe(II) contents. Microbial mineralization of added labile C decreased soil Eh and increased the rates of Fe(III) reduction followed by P mobilization. This indicated that microbially-mediated Fe(III) reduction was intensified by labile OC compounds, which acted as energy sources and electron donors. The release of available P via Fe reduction was accompanied by peaks in Fe(II), dissolved OC, and pH and was followed by a decrease in iron-bound P (FeP). This implied that FeP was the main source of available P in the paddy soils. The faster release of available P in the oxisol than the ultisol indicated that the higher OC and Fe(II) contents in the oxisol allowed a fast Eh decrease, leading to rapid microbial oxygen (O2) consumption and consequently faster and more intensive Fe(III) to Fe(II) reduction. This conclusion is supported by a faster Eh decrease in the oxisol than the ultisol corresponding to early P mobilization after input of labile C, and it suggests that Eh-driven Fe transformations and P mobilization are strongly modulated by labile OC mineralization.