03 Aug 2023
 | 03 Aug 2023
Status: this preprint is currently under review for the journal BG.

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

Melanie Alexandra Thurner, Silvia Caldararu, Jan Engel, Anja Rammig, and Sönke Zaehle

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.

Melanie Alexandra Thurner et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-109', Joshua Fisher, 21 Aug 2023
  • RC2: 'Comment on bg-2023-109', Anonymous Referee #2, 09 Sep 2023

Melanie Alexandra Thurner et al.

Melanie Alexandra Thurner et al.


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Short summary
We implemented mycorrhizal fungi into the terrestrial biosphere model QUINCY, because of their crucial role in terrestrial ecosystems. They interact with mineral and organic soil to support plant nitrogen uptake, and thus plant growth. Our results suggest that the effect of mycorrhizal interactions for simulated ecosystem dynamics is minor under constant environmental conditions, but necessary to reproduce and understand observed pattern under changing conditions, such as rising atmospheric CO2.