Many regions in the Amazon Basin are characterized by highly weathered soils (Ultisols and Oxisols) with high nitrogen (N), but low rock-derived phosphorus (P) (and base cations) availability, which may be (co-)limiting plant productivity. However, much less is known how low P availability influences the activity, physiology and resource acquisition strategies of heterotrophic soil microbial communities, and thereby affects organic matter decomposition, nutrient mineralization and long-term carbon (C) sequestration in tropical soils.

To tackle this gap, we studied various properties of soil microbial communities in tropical lowland forest soils located in the north-eastern Amazon in French Guiana, near Paracou and Nouragues field station, as well as their response to several years of N and P additions as part of the Imbalance-P project. We determined microbial biomass, stoichiometry, respiration rates, and estimated soil microbial growth using a substrate-independent method based on the incorporation of 18O labelled water into microbial DNA, as well as their extracellular enzyme activity potential.

Our results showed that at the studied sites the soil microbial communities slightly increased their biomass C and N contents in response to N, but not P additions. In contrast, P additions increased microbial P (and decreased C:P ratios), which suggests that microbes are an active sink for P. However, P additions also increased total and available soil P pools, indicating that the P demand of both plant and microbial communities, adapted to naturally low P conditions, potentially became saturated after multiple years nutrient additions. Albeit weak, we found a slightly stronger effect of P than of N additions on both microbial biomass-normalized respiration- and growth-rates, while overall the C use efficiency of the communities remained unaffected. This suggests a higher turnover of C by soil microbial communities under alleviated nutrient limitation. In addition, we found that in particular P additions decreased the investment in extracellular enzymes targeting P mining.

Our results highlight that in tropical forests soil microbial communities have a crucial role in soil C, N and P cycling and might act as potential strong sinks for available P in such highly weathered soils. While microbial C and N dynamics are more tightly coupled, our data demonstrate the potential for non-homoeostatic stoichiometric behavior of soil microbial communities in terms of P cycling, which is important to consider in soil and ecosystem models relying on strict stoichiometric relationships.