Taxonomic patterns in the nitrogen assimilation of soil prokaryotes

Nitrogen (N) is frequently a limiting nutrient in soil; its availability can govern ecosystem functions such as primary production and decomposition. Assimilation of N by microorganisms impacts the availability of N in soil. Despite its established ecological significance, the contributions of microbial taxa to N assimilation are unknown. Here we measure N uptake and use by microbial phylotypes and taxonomic groups within a diverse assemblage of soil microbes through quantitative stable isotope probing (qSIP) with ¹⁵N. Following incubation with ¹⁵NH⁺₄, distinct patterns of ¹⁵N assimilation among taxonomic groups were observed. For instance, glucose addition stimulated ¹⁵N assimilation in most members of Actinobacteria and Proteobacteria but generally decreased ¹⁵N use by Firmicutes and Bacteriodetes. While NH⁺₄ is considered a preferred and universal source of N to prokaryotes, the majority (> 80%) of N assimilation in our soils could be attributed to a handful of active orders. Characterizing N assimilation of taxonomic groups with ¹⁵N qSIP may provide a basis for understanding how microbial community composition influences N availability in the environment.