Redox Oscillations and Pyrogenic Carbon Synergistically Enhance Phosphorus Release from Ferric Iron-Phosphates

Phosphorus (P) bioavailability in soils is constrained by binding to Fe(III) minerals. While pyrogenic carbon (PyC) influences microbial Fe(III) reduction, its role in Fe(III)-bound P dynamics during redox fluctuations remains unresolved. Here, we demonstrate that PyC enhances Fe-bound P release during redox cycling via coupled biotic and abiotic pathways. Experiments with crystalline (FePO4·2H2O) and amorphous (FePO4·4H2O) ferric iron-phosphate minerals demonstrated that microbial Fe(III) reduction by Shewanella oneidensis MR-1 under anoxic conditions released dissolved P, exceeding reimmobilization during oxidation and driving net P release. PyC amplified net P release by 4.4-fold (crystalline) and 2.7-fold (amorphous), primarily by stimulating microbial Fe(III) reduction. Dissolved PyC (DPyC) contributed substantially to microbial reduction, surpassing bulk PyC effects for amorphous FePO4·4H2O. Beyond biotic pathways, redox-cycled PyC mediated abiotic Fe(III) reduction of FePO4·4H2O via increased phenolic moieties. While redox cycling alone suppressed amorphous FePO4·4H2O dissolution via recrystallization, it enhanced crystalline FePO4·2H2O reduction without altering crystallinity. PyC addition reduced crystallinity in both phases, promoting late-stage reductive dissolution. These results reveal that PyC governs P mobility in redox-dynamic environments by enhancing microbial Fe(III) reduction, facilitating abiotic Fe(III) reduction, and inhibiting mineral recrystallization. This advances understanding of P cycling in Fe-rich soils and informs strategies for improving P bioavailability.