The role of water management technologies in regulating iron-phosphorus interaction in rice rhizosphere

Abstract

Phosphorus (P) availability to rice plants is influenced by its strong interaction with iron (Fe). In the rhizosphere microenvironment, the soil-plant interactions cause the formation of Fe-plaques that can retain porewater components, such as P. The Fe-P processes have been extensively described in paddy soils managed under continuous flooding, although, due to the increasing water scarcity, new water-saving techniques have been adopted. However, their effects on P retention/release mechanisms are largely unknown.   In order to assess the impacts of water-saving techniques on the rhizosphere Fe-P dynamics and P availability to rice, a macrocosm experiment was conducted to compare the effects of three different water management practices: continuous water flooding (WFL), alternated wet and dry (AWD), and delayed flooding (DFL). Three P fertilization levels were tested for each water management strategy. The concentrations of Fe and P in porewater were monitored until rice harvesting. The plant tissues were analyzed for P concentration, and the content of amorphous and crystalline Fe (hydr)oxides in root plaque was estimated via oxalate and dithionite extractions at mid-tillering, stem elongation, heading and harvesting. The molar P/Fe ratio in porewater and the formation of Fe plaques differed as a result of the combined effect of water management and P fertilization.  The WFL and DFL treatments led to a higher Fe plaque formation with respect to AWD, while in all water management treatments, Fe plaque formation was higher without P fertilization. The early rice development stages were characterized by a greater amount of amorphous Fe (hydr)oxides in root plaques. The proportion of crystalline Fe (hydr)oxides increased with plant development, despite the lower amount of total Fe plaques, suggesting a reduction of the poorly ordered fraction, especially when no P was supplied. Rice plants could be supposed to respond to P-limited conditions, exuding protons and/or organic acid anions that increase P availability through Fe plaque dissolution. This was confirmed by the negative correlation between porewater P concentration and the content of crystalline Fe in the plaques. These results indicate the complex spatio-temporal interconnection between P and Fe cycling at the root-soil interface. The amount of Fe plaques formed on the root surface and their crystallinity degree can explain the mechanisms that regulate their potential in P retention/release and the consequent effects on plant uptake. This study was funded by the PSR Lombardia 2014-2020 (“P-rice Fosforo in risaia: equilibrio tra produttività e ambiente nell'ottica delle nuove pratiche agronomiche”)