Solar-photoaged maize leaf residues reshape soil physicochemical-biological processes and Fe-Cd biogeochemistry

Solar photoaging endows maize leaf residues with persistent redox activity through the spontaneous generation of oxygen-centered environmentally persistent free radicals (EPFRs). Upon soil incorporation, these long-lived radicals and their derived dissolved organic matter (DOM) act as electron donors that promote Fe(III) reduction to Fe(II), followed by Fe(II) reoxidation and reprecipitation, thereby promoting reduction-initiated Fe(III)/Fe(II) redox cycling. This process profoundly shapes soil properties by expanding the organic carbon pool, yet it accelerates soil acidification and triggers root-localized oxidative stress, which induces lipid peroxidation and partially offsets the growth-promoting effect of biomass amendment. Concurrently, the persistent oxidative pressure and oxygen consumption reshape the indigenous soil microbiome, shifting it toward iron-reducing consortia (e.g., Gammaproteobacteria and Bacillus circulans). Crucially, this radical-Fe process is associated with the conversion of labile ion-exchangeable Cd into more stable Fe/Mn oxide-bound fractions. Consequently, heavy metal translocation in the soil-plant system is effectively blocked, mitigating root and shoot Cd accumulation in Ipomoea aquatica by up to 50%. This study demonstrates a novel nature-based strategy coupling organic waste recycling with sustainable in-situ heavy metal remediation.