Belowground root dynamics are central to understanding how land ecosystems respond to elevated CO2, yet models are thus far not able to predict root responses across plant functional groups, soil depth, or across major land biomes. Of particular concern is the apparent lack of stimulation of the aboveground carbon sink despite ~30% increase of atmospheric CO2over the past half-century, and despite the clear acceleration of the land carbon sink over the same period. This apparent discrepancy in land ecosystem response has led to the proposition that changes in belowground root dynamics might be responsible for the overlooked land sink.
We here present a new modeling approach for predicting the response of root biomass, root dynamics, and soil carbon storage to increased CO2. Our approach considers the first-principle mechanisms and tradeoffs by which plants and plant roots invest carbon to gain belowground resources. We also consider a range of plant strategies for resource acquisition, including arbuscular mycorrhizal and ectomycorrhizal symbioses, nitrogen fixation, and naked root strategies. Finally, we allow plants to locally compete for nutrients in an adaptive dynamics framework, with the ability to allocate biomass at different depths in the soil profile.
Results/Conclusions
Our results support the idea that plants “dig deeper” when exposed to increased CO2, and we offer an economic-based mechanism for predicting the plant root response across soil conditions, plant functional groups and major biomes. Our model also recreates the observed responses across a range of free-air CO2 enrichment experiments, including a distinct response between plants associated with EM and AM mycorrhizal fungi. Most broadly, our findings indicate that roots may be increasingly important in the land carbon sink, and call for a greater effort to quantify belowground responses to elevated atmospheric CO2.