Soil Phosphorus Supply Under Different Soil Redox Conditions: Implications of Mineral Dissolution and Soil Organic Matter Turnover

In humid tropical environments, phosphorus (P) limitation to net primary productivity (NPP) has been associated with the formation of secondary phosphate minerals with aluminum (Al) and iron (Fe) oxides. Under low oxygen (O₂) availability, iron-reducing bacteria use Fe oxides and oxyhydroxides as the terminal electron acceptors for anaerobic respiration and phosphate ions that are associated with the mineral surfaces are released when the surface is reduced. Surface soils of humid tropical forests are often characterized by low and fluctuating O₂ availability. In this study we examined the effect of soil O₂ depletion on Fe-P relationships in humid tropical forest soils from Puerto Rico. Surface mineral soils (0-10 cm depth) were incubated under oxic (ambient air), suboxic (94% N₂- 6% O₂) and anoxic (100% N₂) conditions for 39 days and analyzed periodically for P fractions and CO₂ efflux. The moderately labile inorganic P fraction (NaOH-Pi ) was the most sensitive to changes in soil redox conditions showing a rapid increase over the first 2 days of incubation. Under suboxic and oxic conditions, NAOH-Pi increased after 25 days of incubation, and was correlated with soil CO₂ efflux suggesting that mineralization of organic matter led to increased labile P in these samples. Under anoxic conditions HCl-Pi was correlated with the most labile P pool (water available Pi) suggesting that soil reduction causes a rapid pulse of P from the geochemical pool. This short term release of P is likely to be important in maintaining the high NPP characteristic of humid tropical forests.