Substrate stoichiometric regulation of microbial respiration and community dynamics across four different ecosystems

Abstract

Microbes decompose soil organic matter (SOM), yet it is unclear how substrate inputs (i.e., stoichiometry) directly mediate microbial activities and community dynamics. We hypothesized that C+N input has the largest effect on microbial respiration and community structure, followed by C input and N input. Soils were collected from four ecosystems (grassland, piñon-juniper, ponderosa pine, mixed conifer) and amended with NH4NO3 (N only; 100 μg g−1 wk−1), 13C-glucose (C only; 1000 μg g−1 wk−1), or C+N in a five-week laboratory incubation. We found that C+N input induced the greatest total respiration while C input induced the greatest SOM-derived respiration (i.e., priming effect) across ecosystems. Shifts in community composition were the largest with C+N input, followed by C input, and showed little response to N input. C only and C+N inputs increased both of the relative and absolute abundances of Actinobacteria and Proteobacteria (α, β, γ), but reduced the relative abundances of Verrucomicrobia and δ-Proteobacteria. C+N input increased the relative abundances of Bacillales, Rhizobiales, Burkholderiales and of 9 families, and reduced the relative abundances of Myxococcales and of 12 families, but showed little effect on the absolute abundances of these bacterial taxa. N input reduced the absolute abundances of Actinobacteria, Proteobacteria, and Verrucomicrobia but did not affect their relative abundances in the mixed conifer soil; by contrast, N input reduced relative abundances of δ-Proteobacteria and increased the relative abundances of γ-Proteobacteria but did not affect their absolute abundances in the ponderosa pine soil. We also found that substrate inputs were the main driver of SOM decomposition, microbial respiration and diversity, while soil ecosystem was the main driver of community composition and abundances of most bacterial phyla. Our work suggests that substrate stoichiometry has predictable effects on soil C cycling, microbial diversity and community composition, but has variable effects on microbial abundances, and that incorporating bacterial gene copies in abundance calculations can help more accurately estimate microbial responses across taxonomic levels and ecosystems.