Warming mitigates root exudate-induced priming effects via changes to microbial biomass, community structure, and gene abundance.

Root exudation, the export of soluble carbon compounds from living plant roots into soil, is an important pathway for soil carbon formation, but high rates of exudation can also induce rapid soil organic matter decomposition - a phenomenon known as the priming effect. Long-term soil warming associated with climate change could alter exudation rates and impact soil microbes by changing soil carbon chemistry. We hypothesized that warming-induced changes to exudation rate combined with direct effects of long-term warming on soil microbial communities would regulate the microbial priming effect. We tested this hypothesis with an artificial root exudate experiment using intact soil cores from a long-term soil warming experiment in a temperate forest. We found that chronic soil warming did not alter soil carbon formation from exudates, but did reduce the exudate-induced priming effect; exudation caused greater soil carbon loss in unwarmed than warmed soils. We used DNA stable isotope probing with 16S ribosomal RNA gene and shotgun metagenomic sequencing to determine whether long-term warming affected which microbes consume 13carbon-labeled artificial exudates. We found significant differences in bacterial community composition and relative gene abundances of 13carbon-enriched compared to natural abundance DNA. Both soil bacterial community composition and specific enzyme-coding gene families were strongly correlated with soil carbon priming in unwarmed treatments, but these effects were absent in warmed treatments. Our results suggest that the root exudate-induced priming effect is mediated by microbial biomass, community structure, and gene abundance, and that chronic warming reduces the priming effect by altering these microbial variables.