Recent field observations have raised concerns of a saturating land carbon sink, caused by an apparent lack of plant growth stimulation despite a nearly 30% increase in CO2 over the past half-century. However, evidence from a global mass balance approach points to a substantial increase in the land carbon sink over the same time period.
Using an adaptive modeling approach, we here evaluate whether this discrepancy could arise from an overlooked mechanism: an increase in belowground soil carbon despite limited aboveground growth response. We further investigate whether the ability of forests to store carbon above- vs. belowground depends on functional biodiversity, with particular attention to differences between trees associated with different fungal symbioses.
Our approach considers the first-principle mechanisms and tradeoffs by which plants and plant roots invest carbon to gain belowground resources. A unique feature of our model is that we specifically consider the two most dominant plant symbiotic nutrient strategies – arbuscular and ectomycorrhizal fungal association. Moreover, we allow plants to compete for nutrients by allocating biomass across the soil depth profile. Finally, we evaluate belowground carbon storage across the continental United States, by comparing results from our model against empirical results derived from Forest Inventory Analysis (FIA).
Results/Conclusions
Our model recreates the observed responses across a range of free-air CO2 enrichment experiments, including a distinct response between plants associated with arbuscular mycorrhizal vs. ectomycorrhizal fungi. We also identify a strong interaction effect between plant mycorrhizal identity and the way plants forage for nutrients vertically. 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.