PLANT-MICROBE-MINERAL MEDIATION OF ECOSYSTEM RESPONSES TO ELEVATED CO2: NEW UNDERSTANDING THROUGH SYNTHESIS AND MODELING (Invited Presentation)

The degree to which ecosystems sustain growth responses to rising atmospheric CO₂ hinges on how soil microbes release nutrients from soil organic matter (SOM). Given that much of the SOM in soils may be physically or chemically sorbed to minerals, there is a need to better understand the biotic and edaphic factors that modulate the extent and duration of these effects. We synthesized data collected from dozens of CO₂ enrichment experiments, including ecosystems where dominant plants associated with different types of mycorrhizal fungi and from sites varying in fertility. We hypothesized that differences in the type of mycorrhizal association of the dominant plants – arbuscular mycorrhizal (AM) fungi vs. ectomycorrhizal (ECM) fungi – would impact ecosystem sensitivity to elevated CO₂ owing to differences in the ability of these fungi to mine nutrients from mineral-bound SOM. Then, we ran a coupled plant nutrient uptake-microbial decomposition model to investigate how the processes that control plant-microbial dynamics may change over decadal timescales. We found that on average ECM-associating plants (but not AM-associating) were able to sustain biomass gains by 29% in response to elevated CO₂ owing to lesser costs of mining nitrogen (N) from SOM pools. In our synthesis, we found that despite no differences in belowground C allocation between the mycorrhizal types, ECM plants acquired far more N under elevated CO2 than AM plants, indicating a greater return on investment. Using the modeling framework, we then determined that these effects were driven by the ability of ECM fungi to mine nutrients directly from the SOM and the lack of SOM in ECM soils that was physically or chemically protected from microbial decay. In model simulations, we found that the negative relationship between changes in SOM and plant biomass diminished after ~50 years, owing to increases in the amount of SOM transferred into protected SOM pools in ECM systems coupled with depletion of unprotected SOM due to accelerated decomposition in response to rising CO₂. Collectively, our results indicate that microbial and plant responses to elevated CO₂ are intimately linked to SOM and minerals; as such, the future capacity of ecosystems to store C and slow climate change may depend on the constraints imposed on plant-microbe interactions by edaphic factors.