15N enrichment as an integrator of the effects of C and N on microbial metabolism and ecosystem function

Abstract Organic carbon (C) and nitrogen (N) are essential for heterotrophic soil microorganisms, and their bioavailability strongly influences ecosystem C and N cycling. We show here that the natural 15 N abundance of the soil microbial biomass is affected by both the availability of C and N and ecosystem N processing. Microbial 15 N enrichment correlated negatively with the C : N ratio of the soil soluble fraction and positively with net N mineralization for ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of ecosystem development. In addition, during soil incubation, large increases in microbial 15 N enrichment corresponded to high net N mineralization rates. These results support the idea that the N isotope composition of an organism is determined by the balance between N assimilation and dissimilation. Thus, 15 N enrichment of the soil microbial biomass integrates the effects of C and N availability on microbial metabolism and ecosystem processes.