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.