Diverse Mycorrhizal Associations Enhance Terrestrial C Storage in a Global Model

Accurate projections of the terrestrial carbon (C) sink are critical to understanding the future global C cycle and setting CO2 emission reduction goals. Current earth system models (ESMs) and dynamic global vegetation models (DGVMs) with coupled carbon-nitrogen cycles project that future terrestrial C sequestration will be limited by nitrogen (N) availability, but the magnitude of N limitation remains a critical uncertainty. Plants use multiple symbiotic nutrient acquisition strategies to mitigate N limitation, but current DGVMs omit these mechanisms. Fully coupling N-acquiring plant-microbe symbioses to soil organic matter (SOM) cycling within a DGVM for the first time, we show that increases in N acquisition via SOM decomposition and atmospheric N-2 fixation could support long-term enhancement of terrestrial C sequestration at global scales under elevated CO2. The model reproduced elevated CO2 responses from two experiments (Duke and Oak Ridge) representing contrasting N acquisition strategies. N release from enhanced SOM decomposition supported vegetation growth at Duke, while inorganic N depletion limited growth at Oak Ridge. Global simulations reproduced spatial patterns of N-acquiring symbioses from a novel niche-based map of mycorrhizal fungi. Under a 100-ppm increase in CO2 concentrations, shifts in N acquisition pathways facilitated 200 Pg C of terrestrial C sequestration over 100years compared to 50 Pg C for a scenario with static N acquisition pathways. Our results suggest that N acquisition strategies are important determinants of terrestrial C sequestration potential under elevated CO2 and that nitrogen-enabled DGVMs that omit symbiotic N acquisition may underestimate future terrestrial C uptake. Plain Language Summary Plants grow faster when there is more carbon dioxide (CO2) in the air. Because plant growth removes CO2 from the air, this faster plant growth could lessen the warming that is caused by CO2 emissions from human activities. However, plants also need nitrogen to grow, and the availability of this nutrient could constrain, or limit, how much CO2 plants can remove from the air. Correctly predicting the degree of nitrogen limitation of plant growth is a major challenge for global land models. Some plants can overcome nitrogen limitation by partnering with fungi that extract nitrogen from the soil or with bacteria that extract nitrogen from the air. We incorporated these plant-microbe partnerships into a global land model, and we show that these partnerships could sustain plant growth under rising CO2 concentrations. This suggests that existing models and experiments that do not include these partnerships may underestimate the potential for future CO2 uptake by plants.