Modelled forest ecosystem carbon-nitrogen dynamics with integrated mycorrhizal processes under elevated CO₂

Abstract. Almost 95 % of all terrestrial plant species form symbioses with mycorrhizal fungi that mediate plant-soil interactions: Mycorrhizae facilitate plant nitrogen (N) acquisition and are therefore vital for plant growth, but also build a pathway for plant-assimilated carbon (C) into the rhizosphere. Therefore, mycorrhizae likely play an important role in shaping the response of ecosystems to environmental changes such as rising atmospheric carbon dioxide (CO₂) concentrations, which can increase plant N demand and the transfer of plant C assimilation to the soil. While the importance of mycorrhizal fungi is widely recognised, they are rarely represented in current terrestrial biosphere models (TBMs) explicitly. Here we present a novel, dynamic plant-mycorrhiza-soil model as part of the TBM QUINCY. This new model is based on mycorrhizal functional types that either actively mine soil organic matter (SOM) for N or enhance soil microbial activity though increased transfer of labile C into the rhizosphere and thereby (passively) prime SOM decomposition. Using the Duke Free-Air CO₂ Enrichment (FACE) experiment, we show that mycorrhizal fungi can have important effects on projected SOM turnover and plant nutrition under ambient as well as elevated CO₂ treatments. Specifically, we find that including enhanced active mining of SOM for N in the model allows to more closely match the observations with respect to observed decadal responses of plant growth, plant N acquisition, and soil C dynamics to elevated CO₂, whereas a simple enhancement of SOM turnover by increased below-ground C transfer of mycorrhizae is unable to replicate the observed responses. We provide an extensive parameter uncertainty study to investigate the robustness of our findings with respect to model parameters that cannot readily be constrained by observations. Our study points to the importance of implementing mycorrhizal functionalities in TBMs as well as to further observational needs to better constrain mycorrhizal models and to close the existing major knowledge gaps of actual mycorrhizal functioning.