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Keywords:

  • 13C-15N-amino acids;
  • 15N;
  • deciduous forest;
  • Festuca gigantea ;
  • root;
  • soil drying–rewetting

Abstract

We tested whether the presence of plant roots would impair the uptake of ammonium (inline image), glycine, and glutamate by microorganisms in a deciduous forest soil exposed to constant or variable moisture in a short-term (24-h) experiment. The uptake of 15NH4 and dual labeled amino acids by the grass Festuca gigantea L. and soil microorganisms was determined in planted and unplanted soils maintained at 60% WHC (water holding capacity) or subject to drying and rewetting. The experiment used a design by which competition was tested in soils that were primed by plant roots to the same extent in the planted and unplanted treatments. Festuca gigantea had no effect on microbial N uptake in the constant moist soil, but its presence doubled the microbial inline image uptake in the dried and rewetted soil compared with the constant moist. The drying and rewetting reduced by half or more the inline image uptake by F. gigantea, despite more than 60% increase in the soil concentration of inline image. At the same time, the amino acid and inline image- N became equally valued in the plant uptake, suggesting that plants used amino acids to compensate for the lower inline image acquisition. Our results demonstrate the flexibility in plant-microbial use of different N sources in response to soil moisture fluctuations and emphasize the importance of including transient soil conditions in experiments on resource competition between plants and soil microorganisms. Competition between plants and microorganisms for N is demonstrated by a combination of removal of one of the potential competitors, the plant, and subsequent observations of the uptake of N in the organisms in soils that differ only in the physical presence and absence of the plant during a short assay. Those conditions are necessary to unequivocally test for competition.