COS 120-6 - Long-term soil warming interacts with mycorrhizal tree type to constrain ecosystem carbon loss in a mixed arbuscular mycorrhizal and ectomycorrhizal forest

Abstract: Nearly all trees in temperate deciduous forests are associated with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi, and there is an emerging consensus that these plant-fungal associations play a central role in forest nitrogen (N) availability. ECM fungi can directly mineralize N from organic sources whereas AM fungi mostly scavenge inorganic N mineralized by other microbes. Thus, the concentration of N in organic vs. inorganic pools may control tree-mycorrhizal performance. Climate change, especially soil warming, can accelerate soil organic matter (SOM) mineralization, potentially altering mycorrhizal controlled N availability and forest carbon (C) cycling. We leveraged a 20-year soil warming experiment (+5 ºC) in a mixed ECM/AM forest stand at the Harvard Forest Long-Term Ecological Research (LTER) site in Petersham, MA. We measured the effects of warming on N mineralization rates, soil respiration rates and soil C stocks, microbial community characteristics (total microbial biomass and community composition via PLFA), and above- and belowground plant resource use (root biomass, root exudation, root respiration, leaf traits, and tree growth rates) to determine how ECM and AM trees acclimate to warming. In soils from both tree types, warming enhanced SOM mineralization and shifted the microbial community from fungal dominated to bacterial dominated, which increased inorganic N concentrations. Under warming, ECM trees increased absorptive root biomass while decreasing mass-based root exudation, potentially due to increased N availability. Aboveground, ECM trees used more conservative resource strategies by increasing nutrient resorption, decreasing specific leaf area, and decreasing aboveground growth. AM tree nutrient acquisition strategies remained relatively unaltered belowground despite increased AM tree growth under warming. Consequently, AM tree leaf litter increased in warmed plots, potentially sustaining high N mineralization rates, and reinforcing AM tree performance. When we scaled C fluxes to the ecosystem level, we found that the effect of increased root respiration of ECM trees and the simultaneous increased C gains from AM tree growth limited ecosystem C loss under warming. Our results suggest that the additive effects of ECM and AM trees on the forest C sink may constrain ecosystem C loss under warming.