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

<p dir="ltr">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.</p>