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 δ15 N 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 δ15 N 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 δ15 N in the microbial biomass. Differences in δ15 N in the microbial biomass were attributed to the loss of low-δ15 N NH4 + in low, but not high C/N substrates. We also detected a negative linear correlation between microbial nitrogen use efficiency (NUE) and Δ15 N (δ15 N-biomass-δ15 N-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 δ15 N in soil profiles.