The wetter the better? Preferences in plant-microbial competition for phosphorus sources in rice cultivation under contrasting irrigation

Security in rice production requires solving challenges of water scarcity and phosphorus (P) limitations. Reductive dissolution of ferric (III) iron bound phosphate (Fe–P) and organic P (Porg) mineralization are two understudied P sources for rice plants and microorganisms. Using the new water-saving alternate wetting-drying irrigation should increase Porg mineralization but decrease the Fe–P dissolution and thereby shift the plant and microbial preferences for P sources. Rice biomass increased two-fold under alternate wetting-drying compared to continuous flooding, but the P use efficiency of plants was independent of water regimes. Plants were more competitive for P from Fe–P by Fe(III) reduction, whereas microorganisms preferred straw-derived P (enzymatic hydrolyzation). The high contribution (∼20 %) of P from straw to the P nutrition of rice plants and microorganisms raises the significance of Porg mineralization, e.g. from organic fertilizers. This makes the application of organic P fertilizers highly beneficial to increase rice productivity. Plants took up 62 % more soil-derived P under alternate wetting-drying than under continuous flooding. Accordingly, alternate wetting-drying is a more efficient management to increase the use of soil legacy P and reduce the use of mineral fertilizers compared to continuous flooding.