Humid tropical forests are characterized by fluctuating redox conditions occurring on the scale of hours to days. Redox state, as well as the frequency and magnitude of changes in oxidation and reduction could have profound effects on the biogeochemical cycling of phosphorus (P). Reactive iron (hydr)oxide minerals have the potential to strongly bind with P under well-aerated conditions. Under poorly aerated conditions, the bond between P and iron (Fe) species is weakened due to Fe reduction. Theory suggests that if P is mobilized during low redox events, it can be rapidly assimilated into roots and microbial biomass, especially if P is limiting to biota. Humid tropical forests on highly weathered, Fe- and aluminum-rich soils are typically characterized by low P bioavailability. To better understand P cycling and linkages to the carbon (C) cycle in tropical forests, we need develop a mechanistic understanding of how redox dynamics, mineral sinks, and biological competition for P interact.
Several lines of evidence suggest that Fe reduction releases P to the soil environment in humid tropical forests, while Fe oxidation can remove it. In a Puerto Rican soil, oxalate-extractable P increased with the production of poorly crystalline Fe(II) under anaerobic conditions (Chacon et al. 2006). Under field conditions, Liptzin and Silver (2015) found that reduced Fe concentrations explained 57% of the variability in labile P concentrations along a wet tropical elevation gradient. Drought in a tropical rainforest led to rapid Fe oxidation and a dramatic decline in labile P pools. The potential (dry soil) mineralogical sink for P varied across the landscape and likely varies over time. In a humid tropical forest in Puerto Rico, faster P sorption was measured in the typically reduced, lower topographic positions than on slopes with greater aeration. Once in organic form, controls on P pools change. Microbial biomass P was less sensitive to redox fluctuation and more sensitive to labile C availability. Microbial P increased with microbial activity, even under well-aerated conditions where microbes competed effectively with mineral sinks. Factors decreasing microbial activity, however, such as low redox conditions and severe drought can lead to the accumulation of organic P over time. These results show that patterns in Fe-redox dynamics and labile C availability exert important controls on P cycling in humid tropical forests. Adding these processes to biogeochemical models will improve our ability to predict the effects of climate and other changes on tropical forest C fluxes.