Phosphorus additions increase microbial phosphorus accumulation and carbon turnover in tropical soils in French Guiana

Tropical forests often grow on highly weathered soils with rather high nitrogen (N), but low rock-derived phosphorus (P) (and base cation) availability. While the role of P limitation in constraining plant productivity is well established, its impact on heterotrophic microbial communities remains less clear. Specifically, it is crucial to understand how P availability shapes microbial activity, physiology and resource acquisition strategies, but also potential repercussions on organic matter decomposition, nutrient mineralization, and long-term carbon (C) sequestration.

To address this knowledge gap, we studied soil microbial communities in tropical lowland forest soils located in the north-eastern Amazon in French Guiana following three years of N and P additions. We assessed soil microbial biomass, stoichiometry, extracellular enzyme activity potential, and respiration rates. Additionally, we quantified soil microbial growth using a substrate-independent method based on the incorporation of ¹⁸O from labelled water into microbial DNA.

Our results showed that soil microbial biomass slightly increased in response to N, but remained unaffected by P additions. In contrast, P additions increased microbial P content (and reduced associated C:P ratios), suggesting that microbes are highly competitive for P and can act as a significant P sink in these soils. Additionally, P additions also increased total and available soil P pools, indicating that both plant and microbial communities are well adapted to naturally occurring low P availability, and may have reached P saturation after multiple years of nutrient enrichment. Despite these changes, microbial biomass-normalized specific respiration- and growth-rates increased with both N and P fertilization, with a stronger response to P, while overall, the C use efficiency of the microbial communities remained unaffected by both.

Our results highlight (i) the pivotal role of soil microbes in C, N and P cycling in tropical forest soil and (ii) the remarkable P storage capacity of microbial communities in highly weathered soils. While microbial C and N dynamics appear tightly coupled, likely due to the similar composition of microbial cell walls, our data demonstrate non-homoeostatic stoichiometric behavior of microbial communities. This underscores the importance of reconsidering assumptions about strict stoichiometric relationships in soil and ecosystem models.