Disentangling the contribution of mycorrhizal fungi to soil organic carbon storage

Ectomycorrhizal fungi (ECM) play a fundamental role in plant-soil carbon and nitrogen cycling in forest ecosystems, yet their influence on soil organic carbon (SOC) sequestration remains underexplored, particularly in process-based models. Here, we develop a Mycorrhizal fungi-mediated Microbial-ENzyme Decomposition (Myc-MEND) model to explore ECM effects on plant carbon fixation and nitrogen uptake. The model was calibrated with biomass of foliage, wood, and roots, and annual net primary productivity from forests in eastern NC, USA. Our findings show that ECM enhance plant nitrogen availability, increasing plant productivity but not directly promoting SOC sequestration. We find that the increased nitrogen provided by ECM to plants decreased plant C:N ratio and led to a 17 % increase in plant photosynthate. This increase in plant quality and quantity increased SOC storage up to 20 %. However, these increases also stimulated saprotrophic microbial activity and extracellular enzyme production, resulting in a 14 % decline in SOC, particularly a 19 % reduction in the particulate organic carbon pool. The most influential pathway for SOC stability was the stabilization of recalcitrant mycorrhizal mycelium necromass, which accounts for 36 % of mineral-associated organic carbon (MOC) storage and 31 % of overall SOC accumulation. Although mycorrhizal colonization led to a net 14 % decrease in total SOC storage, it contributed to a 10 % increase in MOC, highlighting its role in enhancing MOC formation. Our simulations demonstrated that ECM influence the microbial carbon pump by lowering plant C:N ratios, reducing microbial carbon use efficiency, and altering plant-soil carbon fluxes. Overall, our results underscore the critical role of ECM in regulating microbial carbon pump mechanisms and their indirect contributions to SOC persistence via MOC formation. By bridging empirical observations and theoretical modeling, this study lays the groundwork for integrating mycorrhizae processes into future research aimed at predicting ecosystem carbon fluxes and assessing their climate change mitigation potential.