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

Root exudation, the export of soluble carbon (C) compounds from living plant roots into soil, is an important pathway for soil C 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 that consume exudates. We hypothesized that warming-induced changes to exudation rate combined with the 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 C formation from exudates, but it did reduce the exudate-induced priming effect; exudation caused greater soil C loss in unwarmed compared to warmed soils. We used DNA stable isotope probing with 16S amplicon and shotgun metagenomic sequencing to determine whether long-term warming affected which microbes consume ¹³C-labeled artificial exudates. We found significant differences in bacterial community composition and relative gene abundances of ¹³C-enriched compared to natural abundance DNA. Both soil bacterial community composition and specific enzyme-coding gene families were strongly correlated with soil C 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.