Low phosphorus (P) bioavailability is prevalent in humid tropical forest on deeply weathered clay-rich soils. However, these ecosystems also have among the highest primary productivity, respiration, and decomposition rates globally. The current paradigm posits that low P availability limits microbial growth and other soil biogeochemical processes regulated by microbes. We present evidence from field and laboratory experiments showing distinct microbial and biogeochemical responses to P availability under contrasting redox conditions. Along a wet rainfall gradient in Puerto Rico, we observed more frequent and intensive low redox events at wetter sites. Soil organic carbon (C) concentration increased by more than two-fold with increasing rainfall, and microbial biomass C to P ratio also increased significantly, suggestive of microbial P limitation. In a laboratory incubation experiment using three soils collected along this gradient, P amendment consistently increased CO₂ production under oxic conditions. However, anaerobic microbial processes including anoxic CO₂ production, Fe reduction, and methanogenesis, only increased with P amendment at the driest site. Anoxic CO₂ production was surprisingly high compared to oxic CO₂ production, while microbial biomass C and P concentrations were greatly decreased in the anoxic treatment. Results from NanoSIMS single cell elemental analysis showed that anoxic conditions reduced cellular microbial P assimilation, resulting in high biomass C to P ratios. The decreased cellular P was correlated with lower carbon use efficiency (CUE), as more C was allocated to respiration rather than growth. Redox-induced changes in microbial CUE offer a new mechanism to explain the surprisingly high CO₂ productions that have been observed under low and oscillating redox conditions. Our results reveal the importance of redox conditions in regulating the extent of P limitation on soil microbial processes in tropical forest soils.

Jennifer Pett-Ridge

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