Potential phosphorous mobilization in subtropical forest soils triggered by labile carbon coupled to iron cycling.

Phosphorus (P) exhibits a strong affinity for iron (Fe) oxides, which limits its bioavailability in soils. Reductive dissolution of ferric iron (Fe3+) can release occluded and adsorbed P into soil matrix. However, the mechanisms governing how Fe3+ reduction influences P mobilization remain poorly understood, particularly in highly weathered subtropical soils characterized by fluctuating redox potentials (Eh) and variable carbon (C) availability. This investigation explored the impacts of readily decomposable C substrates (formate, acetate, propionate, and oxalate) on Fe reduction, P release, and greenhouse gas emissions (carbon dioxide, CO2, and methane, CH4) in two distinct forest soils (Oxisol and Ultisol). The results showed that these substrates addition lowered soil Eh, and accelerated Fe3+ reduction and triggered P release for both soils. Besides, the observed peak in ferrous ion (Fe2+) concentrations and dissolved organic carbon (DOC) content was coincided with the release of available P. These results demonstrated that Fe-P represents the primary source of bioavailable P in these forest soils. However, the Oxisol exhibited faster P mobilization compared to the Ultisol, likely due to its higher SOC (21.11 vs 11.41 g C kg-1) and Fe (25.17 vs 15.65 g kg-1) content facilitating more rapid Eh decreases, consequently accelerating faster and more pronounced reduction of Fe3+ to Fe2+. The enhanced response in the Oxisol was further supported by evidence of earlier P mobilization and greater CH4 efflux following labile C addition compared to the Ultisol (Average: 42.02 vs 21.13 mg C kg-1). In summary, the present study suggested that P release associated with Fe reduction appears to provide a transient supply of P in subtropical forest soil, of which this process was intensified by labile C mineralization, accompanying with a decrease in Eh.