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.

Redox regulation of phosphorus limitation on soil microbial processes in humid tropical forests

ACS Fall 2021

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