Phosphorus (P) cycling in paddy soil is closely related to iron (Fe) redox wheel; its availability to rice has thus generally been ascribed to Fe minerals reductive dissolution. However, the literature aimed to identify the best method for predicting rice available P does not uniformly point to Fe reductants. Rice plants can indeed solubilize and absorb P through many strategies as a function of P supply, modifying the chemical environment. Therefore, this study aims to estimate P availability in paddy soils coupling the redox mechanisms driving P cycling with concurrent plant responses. Soil available P was estimated in three groups of paddy soils with low, medium, or high P content assessing easily desorbable pools (0.01 M calcium chloride, Olsen, Mehlich-III, anion exchanging resins) and Fe-bound P pools (EDTA, citrate-ascorbate, and oxalate). Rice P uptake and responses to P availability were assessed by a mesocosm cultivation trial. Although P released in porewater positively correlated with dissolved Fe(II), it did not with plant P uptake, and readily desorbable P pools were better availability predictors than Fe-bound pools, mainly because of the asynchrony observed between Fe reduction and plant P demand. Moreover, in low P soils, plants showed higher Fe(II) oxidation, enhanced root growth, and up-regulation of P root transporter encoding genes, plant responses being related with changes in P pools. These results indicate the generally assumed direct link between Fe reduction and rice P nutrition in paddy soils as an oversimplification, with rice P nutrition appearing as the result of a complex trade-off between soil redox dynamics, P content, and plant responses.
<p>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. &#160;</p> <p>&#160;</p> <p>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.</p> <p>&#160;</p> <p>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. &#160;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.</p> <p>&#160;</p> <p>This study was funded by the PSR Lombardia 2014-2020 (&#8220;P-rice Fosforo in risaia: equilibrio tra produttivit&#224; e ambiente nell'ottica delle nuove pratiche agronomiche&#8221;)</p> <p>&#160;</p>
<p>The optimization of P fertilization in paddy rice fields requires an accurate estimation of soil P availability to balance rice productivity and ecosystem preservation.&#160; While there are several generally accepted methods to evaluate P availability to crops grown in aerobic soils, the available P pool in paddy soils cannot be so easily assessed. Phosphorus cycle in paddy soils is closely linked to Fe redox wheel and conditioned by the complex interactions between soil characteristics and plant strategies to promote P uptake. The aim of this study was the identification of the method that best estimates P availability for rice plants while taking into account the complex interactions between soil (bio)geochemistry and plant responses.</p><p>Twelve representative paddy soils have been selected and analyzed for available P with different methods (calcium chloride, Olsen, Mehlich-3, anion exchanging resins, EDTA, citrate/ascorbate and oxalate). In the same soils, rice plants were grown for 60 days; during this period temporal variation of soluble P and Fe(II) in the soil solution was monitored. The plants were then harvested and the roots and shoots biomass, the P content in plant tissues, the expression of the root phosphate transporter encoding genes and the root activity of phytase and phosphatase were determined.</p><p>During the growing period, the soluble P concentration in the soil solution increased during the first 3-4 weeks, following the same trend of Fe(II), then it decreased, probably due to plant uptake and P-Fe co-precipitation. Both biomass and P concentration in the tissues were affected by soil P content. The extraction with resins was the best predictor for plant productivity and P uptake, followed by CaCl<sub>2</sub> and Olsen extraction. The extractants involving the partial dissolution of the sorbing minerals (i.e., oxalate and citrate/ascorbate) showed a poorer, although still significant correlation with P concentration in rice plants, but a higher performance in terms of organic P. Phosphatase activity was greater than phytase in all cases; the former did not significantly differ among soils, while the latter was higher in those soils releasing more P in solution during the growing period and was correlated with P concentration in plants. In low P soils a higher expression of root transporter encoding genes was observed, particularly those at high-affinity.</p><p>Although resins, CaCl<sub>2</sub> and Olsen extractions are confirmed as useful tools for the prediction of P availability even for paddy rice cultivation, in P-deficient soils the enhancement of enzymatic activity and the overexpression of root P-transporters increased the capacity of plant P uptake above the prediction of the chemical extractants.</p>
Background and aims Iron (Fe) plaque which normally coats rice roots has a strong affinity for phosphorus (P), with a debated effect on plant P uptake. Furthermore, plant responses to P availability shape the rhizospheric environment, possibly affecting the rates of Fe plaque formation and dissolution. The role of Fe plaque to serve as a sink or source of available P may depend on root traits, themselves influenced by P availability. However, the underlying mechanism regulating these interactions remains unclear. In this study, we investigated the effects of P availability on root traits, Fe plaque dynamics and their implications for P uptake and rice plant growth.Methods Plants were hydroponically grown for 60 days under P-sufficiency or P-deficiency, with or without Fe plaque. Root traits, rhizosphere acidification, and the rates of Fe plaque formation and dissolution were investigated and linked to differences in rice P content and growth.Results P-deficient conditions stimulated root development and promoted Fe plaque formation on the root surface compared to P-sufficient conditions. However, P limited plants exhibited a faster Fe plaque dissolution, along with increased net proton exudation. After 60 d, P-deficient plants showed higher P uptake in the presence of Fe plaque, whereas the opposite was observed in P-sufficient plants, where Fe plaque limited plant P uptake.Conclusions The role of Fe plaque in regulating P uptake highly depends on the dynamic nature of this Fe pool that is strictly linked to P availability and regulated by plant responses to P deficiency.
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