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.