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

Soil phosphorus (P) availability is commonly thought to limit net primary productivity and belowground biogeochemical dynamics in humid tropical forests. Soils in these ecosystems often experience low and fluctuating redox conditions. However, little is known about how P availability and redox conditions may interact to influence soil biogeochemical cycling. We used a tropical forest rainfall gradient and soil P amendments in a laboratory incubation experiment to determine if P limits soil biogeochemical processes under oxic and anoxic conditions. We examined the biogeochemical responses of three soils that were characterized by different rainfall and redox regimes in the Luquillo Long Term Ecological Research site and Critical Zone Observatory in Puerto Rico. We found that aerobic soil respiration was consistently enhanced by P amendment in all three soils. In contrast, anaerobic biogeochemical processes, including anaerobic CO2 production, iron reduction, and methanogenesis, was only limited by P availability in the driest soil. Microbial C:P ratios decreased with P amendment under anoxic conditions at the driest site, another indicator of 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 suggest that while P may limit some aerobic biogeochemical process, it may not be the primary control on anaerobic biogeochemical cycling. 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.