Low Redox Decreases Potential Phosphorus Limitation on Soil Biogeochemical Cycling Along a Tropical Rainfall Gradient

Humid tropical forests on highly weathered soils are often characterized by low bioavailable phosphorus (P) concentrations. These ecosystems also often experience low and fluctuating redox conditions. Little is known about how soil redox conditions affect P availability and how this might feed back on biogeochemical cycling. Here we used soils from a wet tropical rainfall gradient in Puerto Rico to explore the effects of redox on P bioavailability and associated biogeochemical processes. Concentrations of soil carbon (C) and poorly crystalline iron (Fe) and aluminum (Al) minerals increased at least two-fold with increasing rainfall, reflecting stronger anaerobic conditions at wetter sites and associated declines in decomposition. The fraction of the total P pool in the NaOH-extractable organic form also generally increased with increasing rainfall. In a laboratory incubation experiment using three sites along the gradient, P amendment increased aerobic CO2 production. However, anaerobic processes, including anaerobic respiration, Fe reduction, and methanogenesis, increased with P amendment at the driest site only. Microbial C:P ratios decreased with P amendment under anoxic conditions at the driest site, an indicator of possible microbial P limitation at this site. Both microbial biomass C and P concentrations were lower under anoxic conditions than under oxic conditions across all soils, suggesting that anoxic conditions could be a more limiting factor to microbes than P concentrations. Overall, our results demonstrate that redox conditions regulate the extent of P limitation to biogeochemical processes in tropical forest soils. Phosphorus limitation was pronounced in aerated environments with low mean annual rainfall, whereas low redox conditions or associated factors under high rainfall conditions may have a stronger impact on biogeochemical cycling than P availability.