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

Abstract

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.