Reduced phosphorus bioavailability in rice paddies intensified by elevated CO2-driven warming

Abstract

Rising atmospheric CO2 reduces soil phosphorus (P) availability in paddy soils by promoting soil organic P accumulation and crop harvest removal. Atmospheric CO2 and temperatures are increasing simultaneously, yet their interaction with the soil P cycle remains unresolved. Here we report a decade-long free-air CO2 enrichment experiment integrated with in situ warming (+2 degrees C) in a typical paddy-upland rotation system. We find that both elevated CO2 and warming exacerbate P constraints, and that warming alone and in combination with elevated CO2 has a greater impact than elevated CO2 alone. All climate change treatments significantly depleted soil available P (32-54%) and increased the soil C:P ratios (4-30%). Moreover, warming initially accelerated P mineralization but reduced P availability by enhancing Fe-organic carbon complexes and microbial immobilization. These processes, together with increased crop P demand driven by accelerated growth under elevated CO2, exacerbate P depletion. We identify Fe-organic carbon interactions as a previously overlooked mechanism that significantly reduces P bioavailability. Our findings offer a mechanistic framework linking aboveground-belowground C-P coupling with microbially driven Fe-organic matter dynamics, highlighting the urgent need for adaptive nutrient management strategies to sustain rice production under future climate change.