Sensitive Groups of Bacteria Dictate Microbial Functional Responses to Short-term Warming and N Input in a Semiarid Grassland

Environmental change factors can significantly affect the composition and physiology of soil microbes. How the resulting changes in the community composition are related to microbial functions, however, remains poorly understood. We investigated the effects of climate warming (+ 1.4 degrees C of air temperature and + 0.75 degrees C of soil temperature at 10 cm depth) and reactive nitrogen (N) input (12 g N m(-2) year(-1)) on the community composition and physiologies of soil bacteria in a semiarid Loess grassland. Soil bacterial communities were assessed by Miseq sequencing of 16S rRNA gene amplicons while their physiological properties were assessed by microbial metabolic quotients (qCO(2), microbial respiration per unit of microbial biomass) and microbial community-level physiological profiles (CLPPs). Our results showed that N input, but not warming, altered bacterial community structure, although both warming and N input significantly affected the abundances of certain phyla. While phyla Verrucomicrobia and Chloroflexi were sensitive to warming, Saccharibacteria, Bacteroidetes and Actinobacteria were primarily responsive to N input. Both warming and N input increased microbial metabolic quotients, but only warming significantly impacted soil microbial CLPPs with L-cysteine, oxalic acid, oxoglutaric acid and aminobutyric acid being the sensitive C sources. Structural equation modeling showed that warming and N input influenced soil bacterial phyla through soil moisture, soil NO3--N and plant biomass. The sensitive bacterial phyla, not the whole community property, were significantly correlated with qCO(2) and microbial C utilization. Our findings suggest that responses of bacterial groups sensitive to environmental change factors, rather than the whole community, may exert dominant effects on soil microbial functions under future climate change scenarios