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

Changes in (15)N/(14)N in the soil microbial biomass during nitrogen (N) mineralization have been hypothesized to influence (15)N/(14)N 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 δ(15)N in the microbial biomass, NH(4)(+), and dissolved organic N (DON: glycine) over the course of the experiment. High rates of NH(4)(+) excretion from A. oryzae were accompanied by an increase in δ(15)N in the microbial biomass in low C:N media (i.e., C/N<30). In contrast, NH(4)(+) was strongly retained in higher C/N treatments with only minor (i.e., <1 ‰) changes being detected in δ(15)N in the microbial biomass. Differences in δ(15)N in the microbial biomass were attributed to the loss of low-δ(15)N NH(4)(+) in low, but not high C/N substrates. We also detected a negative linear correlation between microbial nitrogen use efficiency (NUE) and Δ(15)N (δ(15)N-biomass–δ(15)N-glycine). These results suggest an isotope effect during NH(4)(+) excretion in relatively N-repleted environments in which microbial NUE is low, which may explain the vertical patterns of organic matter δ(15)N in soil profiles.