Unravelling the importance of organic phosphorus forms in rice root iron plaque

Rice (Oryza sativa L.) plants accumulate phosphorus (P) on iron (Fe) plaque deposited on root surfaces. We aimed to investigate P speciation in rice root Fe plaque using x-ray absorption near edge structure (XANES) spectroscopy coupled to conventional techniques. We examined Fe plaque samples from rice experiments in different soils. Samples deposited on filters yielded sufficiently thin, concentrated samples with high-quality spectra despite low P concentrations. The low P concentrations in Fe plaque prevented us from using L-edge P XANES for organic P differentiation, but allowed K-edge P XANES. Fits of K-edge P XANES data showed that organic P comprised from 40% to 70% of root plaque P, while inorganic P represented only a minor fraction. These findings demonstrate the importance of organic P in the rice rhizosphere and may lead to targeted P fertilization with minimal environmental impact.