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Plant–microbial competition for nitrogen uncoupled from soil C:N ratios

Authors

  • Katarina Månsson,

  • Per Bengtson,

  • Ursula Falkengren-Grerup,

  • Göran Bengtsson


K. Månsson, P. Bengtson, U. Falkengren-Grerup and G. Bengtsson (goran.bengtsson@ekol.lu.se), Dept of Ecology, Lund Univ., Ecology Building, Sölvegatan 37, SE–223 62 Lund, Sweden.

Abstract

A green house experiment was designed to test the idea that competition for inorganic nitrogen (N) between plants and heterotrophic microorganisms occurs in soils with high C:N ratios, qualifying for N limited microbial activity, but not at low C:N ratios. The short-term (24 h) 15N uptake by the grass Festuca gigantea and microorganisms in planted and unplanted soils was determined, and the bacterial activity was measured by the 3H-thymidine incorporation technique. Two deciduous forest soils, with C:N-ratios of 20 and 31, and the 20 soil amended with litter to a C:N ratio of 34, were used. A novel and important part of the experimental design was the preparation of the unplanted reference soil with plants present until the competition assay started by the addition of 15N labelled ammonium ( inline image) or nitrate inline image). The results suggested that plants and soil microorganisms competed for mineral N but under influence of other factors than the soil C:N ratio. The plants reduced the microbial uptake of inline image and inline image in the soil with low C:N ratio, which also had the lowest bacterial activity. The plants had a larger N uptake than microorganisms in the two natural soils but smaller in the litter-amended, and their presence enhanced the bacterial activity, especially in the latter soil. The litter-amended soil with its high C:N ratio and easily decomposable C was the soil that best fulfilled the criteria for competition, including a net consumption of mineral N during the assay, the lowest plant uptake of mineral N due to the high N immobilization by microorganisms, and a reduced microbial 15N uptake-to-bacterial activity in the presence of plants. Thus, other factors, such as the decomposability of the soil C and the bacterial activity, were more important than the soil C:N ratio to the outcome of plant–microbial competition for N.

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