Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers

•15N and 32P tracers were added to plant–soil systems with and without drought.•Drought caused greater reductions in plant than in microbial NO3−NO3− uptake.•Microbial P uptake was more strongly reduced by drought than plant P uptake.•Drought may result in decoupling of N and P cycles.

Competition for nutrients between plants and microbes is an important determinant for plant growth, biodiversity and carbon cycling. Perturbations such as drought affect the availability of nitrogen (N) and phosphorus (P), and may cause shifts in uptake of N and P between plants and microbes. Competitiveness for these nutrients may depend on how flexible plants and microbes are in taking up N and P. We used a novel dual isotope labelling technique (15N and 32P) to assess short-term uptake of N and P by plants and microbes affected by drought in two different plant–soil systems. Mesocosms were extracted from two grassland sites differing in soil nutrient availability and plant species. Half of the mesocosms were subjected to drought one week prior to injection of 15N (as KNO3) and 32P (as H3PO4) tracers. Uptake rates of NO3−NO3− and P in plants and microbes were estimated based on average source pool enrichment during the labelling period and on plant and microbial recovery of 15N and 32P measured after 4 days of labelling. Overall competition for N and P was reduced with drought as less NO3−NO3− and P was taken up in plants and microbes. However, plant uptake of NO3−NO3− was more sensitive to drought than microbial NO3−NO3− uptake, while microbial P uptake was more sensitive than plant P uptake. These different sensitivities to drought by plants and microbes may decouple the N and P cycle with increased drought conditions.