Control of the Nitrogen Isotope Composition of the Fungal Biomass: Evidence of Microbial Nitrogen Use Efficiency

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  • Shinoda Kazuki
    United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology
  • Yano Midori
    Institute of Agriculture, Tokyo University of Agriculture and Technology Center for Ecological Research, Kyoto University
  • Yoh Muneoki
    Institute of Agriculture, Tokyo University of Agriculture and Technology
  • Yoshida Makoto
    Institute of Agriculture, Tokyo University of Agriculture and Technology
  • Makabe Akiko
    Institute of Agriculture, Tokyo University of Agriculture and Technology Project Team for Development of New-generation Research Protocol for Submarine Resources, Japan Agency for Marine-Earth Science and Technology
  • Yamagata Yohei
    Institute of Agriculture, Tokyo University of Agriculture and Technology
  • Houlton Benjamin Z.
    Department of Land Air and Water Resources, University of California
  • Koba Keisuke
    Institute of Agriculture, Tokyo University of Agriculture and Technology Center for Ecological Research, Kyoto University

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<p>Changes in 15N/14N in the soil microbial biomass during nitrogen (N) mineralization have been hypothesized to influence 15N/14N in soil organic matter among ecosystem sites. However, a direct experimental test of this mechanism has not yet been performed. To evaluate the potential control of microbial N mineralization on the natural N isotope composition, we cultured fungi (Aspergillus oryzae) in five types of media of varying C:N ratios of 5, 10, 30, 50, and 100 for 4 d, and tracked changes in δ15N in the microbial biomass, NH4+, and dissolved organic N (DON: glycine) over the course of the experiment. High rates of NH4+ excretion from A. oryzae were accompanied by an increase in δ15N in the microbial biomass in low C:N media (i.e., C/N<30). In contrast, NH4+ was strongly retained in higher C/N treatments with only minor (i.e., <1 ‰) changes being detected in δ15N in the microbial biomass. Differences in δ15N in the microbial biomass were attributed to the loss of low-δ15N NH4+ in low, but not high C/N substrates. We also detected a negative linear correlation between microbial nitrogen use efficiency (NUE) and Δ15N (δ15N-biomass–δ15N-glycine). These results suggest an isotope effect during NH4+ excretion in relatively N-repleted environments in which microbial NUE is low, which may explain the vertical patterns of organic matter δ15N in soil profiles.</p>

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