Rice root Fe plaque enhances oxidation of microbially available organic carbon via Fe(III) reduction-coupled microbial respiration

Abstract

Poorly crystalline iron (Fe) oxides are commonly deposited on the surface of rice roots, forming an Fe plaque. This Fe plaque is an important area for Fe redox reactions because the poorly crystalline Fe is available for microbial transformations. However, it remains unclear what the mechanisms are that cause the root Fe plaque to affect CO₂ emissions from paddy soils. Thus, this study investigated the effects of Fe plaque on paddy soil CO₂ emissions and the associated mechanisms. Paddy soil with rice plant roots containing an Fe plaque-coating had 1.5-fold larger CO₂ emissions and higher expression levels of genes involved in soil carbon (C) degradation than Fe plaque-free roots, indicating that Fe plaque stimulated paddy soil derived CO₂ emissions. Fe plaque-coated rice roots, immersed in a ¹³C-labeled glucose solution, emitted more CO₂, with a higher abundance of ¹³CO₂, than the Fe plaque-free roots. These differences were not observed when the surfaces of the rice roots were sterilized. These results indicate that the Fe plaque-stimulated CO₂ emissions were due to soil microbial respiration, not autotrophic root respiration. Chelating the dissolved Fe on the root surface eliminated Fe plaque stimulation of CO₂ emissions, while Fe(III) supplementation correlated with enhanced CO₂ emissions. The stoichiometric mole ratio of the enhanced CO₂ emissions to the Fe(II) generated was comparable to the theoretical ratio of Fe(III) when used as an electron acceptor for organic C decomposition. These results showed that the Fe plaque enhancement of CO₂ emissions was coupled to Fe(III) reduction. Limiting the stimulating effects of Fe plaque on CO₂ emissions may be a potentially useful approach for mitigating organic C loss from paddy soils.