COS 186-3 - Long-term soil warming mediates relationships between root exudation and soil carbon dynamics

Abstract: Plant root exudation – the release of low-molecular weight organic carbon (C) compounds from living plant roots into soil – is an important flux between plant biomass C and soil C pools. However, the responses of this flux to rapid global climate change are not well quantified. In addition to being a C flux into soil, root exudation can also instigate microbial priming of native soil C, resulting in a C flux out of the soil that can be equal to or greater than exudate C inputs. Understanding how soil warming mediates C fluxes both in and out of the soil due to root exudation is vital to predicting the future of the soil C sink. We measured changes in root exudation and respiration rates in situ in response to over 20 years of soil warming at the Barre Woods Soil Warming Experiment at the Harvard Forest Long-term Ecological Research (LTER) site, Petersham, MA, USA. Additionally, we simulated how root exudate-soil C pathways interact with warming in a fully factorial artificial root exudate (ARE) experiment with intact soil cores from the warmed and control plots at Barre Woods. In our field measurements, we observed decreased root exudation in response to soil warming driven by ectomycorrhizal (EM) tree species (mostly Quercus rubra). We also found a positive relationship between root exudation and root respiration that was suppressed by warming. We hypothesize downregulation of root exudation is a response to enhanced N mineralization under warming because exudation may be an organic N acquisition strategy used by EM tree species. However, when we scaled our exudation measurements by root biomass distributions, we found that the negative effect of warming on exudation rates was partially offset by increased root biomass of EM trees. In our ARE experiment, we found evidence that higher exudation rates increase soil C priming, but this effect was mitigated by warming. In soil cores from the warmed plot, we consistently found reduced effects on all indicators of soil C loss (including pore water C, mineral-associated C loss, and CO₂ respiration), which are likely driven by reduced microbial biomass under warming. Our results suggest that soil warming may induce decreased C flux into soil from root exudation, and that decreased root exudation along with warming-mediated reductions in microbial biomass will reduce the C flux out of soil due to priming, potentially constituting a negative climate change feedback.