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Soil Biology and Biochemistry
Vol. 82, 2015, Pages: 135–142

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

Feike A. Dijkstra, Mingzhu He, Mathew P. Johansen, Jennifer J. Harrison, Claudia Keitel

Department of Environmental Sciences, University of Sydney, Centre for Carbon, Water and Food, 380 Werombi Road, Camden, NSW 2570, Australia.

Abstract

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 3NO−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 3NO−NO3 and P was taken up in plants and microbes. However, plant uptake of 3NO−NO3 was more sensitive to drought than microbial 3NO−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.

Keywords: Microbial biomass; N/P stoichiometry; Nitrogen; Phosphorus; Plant–microbe interactions; Soil moisture.


 
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