Phosphate addition increases tropical forest soil respiration primarily by deconstraining microbial population growth

Tropical ecosystems are an important sink for atmospheric CO₂; however, plant growth is restricted by phosphorus (P) availability. Although soil microbiota facilitate organic P turnover and inorganic P mobilization, their role in carbon-phosphorus coupled processes remains poorly understood. To advance this topic, soils collected from four sites representing highly weathered tropical soils in the El Yunque National Forest, Puerto Rico were incubated with exogenous PO₄ ³⁻ under controlled laboratory conditions. P amendment increased CO₂ respiration by 14–23% relative to control incubations for soils sampled from all but the site with the greatest total and bioavailable soil P. Metatranscriptomics revealed an increase in the relative transcription of genes involved in cell growth and uptake of other nutrients in response to P amendment. A new methodology to normalize gene expression by population-level relative (DNA) abundance revealed that the pattern of increased transcription of cell growth and division genes with P amendment was community-wide. Soil communities responsive to P amendment possessed a greater relative abundance of α-glucosyl polysaccharide biosynthesis genes, suggestive of enhanced C storage under P-limiting conditions. Phosphorylase genes governing the degradation of α-glucosyl polysaccharides were also more abundant and increased in relative transcription with P amendment, indicating a shift from energy storage towards growth. Conversely, microbial communities in soils nonresponsive to P amendment were found to have metabolisms tuned for the phosphorolysis of labile plant-derived substrates, such as β-glucosyl polysaccharides. Collectively, our results provided quantitative estimates of increased soil respiration upon alleviation of P constraints and elucidated several underlying ecological and molecular mechanisms involved in this response.