Elevated CO₂ mediates ectomycorrhizal fungi species-specific decreases of native soil carbon and causes negative priming under low nutrients

Ectomycorrhizal fungi (ECM) can influence soil carbon (C) accrual and loss. The mechanisms and environmental controls of this balance are unclear, and direct evidence is needed. We assessed the influence of atmospheric CO₂ and inorganic nitrogen (N) on the impact of two ECM fungi on the cycling of native and new soil C. We inoculated Eucalyptus under continuous C isotopic labeling with two species of Pisolithus and manipulated inorganic N (high/low) and CO₂ (ambient/elevated, aCO₂/eCO₂). We differentiated plant-derived C and native soil-C in soil, dissolved-C, microbial-C, and CO₂. Under eCO₂, Pisolithus albus increased plant-derived dissolved organic C compared to the uninoculated control and reduced soil-derived C compared to both the uninoculated control and Pisolithus microcarpus, consistent with the facilitation of soil C decay. While ECM effects were not dependent on N, eCO₂ reduced soil-derived C loss under low N, demonstrating suppression of decomposition (negative C priming) under low nutrients. These findings highlight that closely related ECM fungal species have contrasting capacities to impact different pools of soil C. For some ECM species, eCO₂ may favor soil C decay. Our findings call for caution in generalizing the role of ECM fungi on soil C and of N availability on eCO₂ impacts.