The degree to which ecosystems sustain growth responses to rising atmospheric CO2 hinges, in part, on whether soil microbes release nutrients from soil organic matter (SOM). However, given that such priming effects also accelerate the turnover of SOM and release CO2 to the atmosphere, there is a need to better understand the biotic and edaphic factors that modulate the extent and duration of these effects, and to quantify their impacts on ecosystem C balance. We synthesized data collected from >60 CO2 enrichment experiments, including ecosystems where dominant plants associated with different types of mycorrhizal fungi and from sites varying in soil fertility. For a subset of sites, we looked at the degree to which the relationship between microbial and plant responses to CO2 depended on the degree to which SOM pools were protected from microbial decay. We hypothesized that ecosystems that experienced the largest gains in plant biomass in response to CO2 and had the most unprotected SOM (e.g., those dominated by ectomycorrhizal (ECM) fungi) would experience the largest losses of SOM. Then, we parameterized a coupled plant nutrient uptake-microbial decomposition model with data from these sites to investigate how the processes that control plant-microbial dynamics change over decadal timescales.
Results/Conclusions
Overall, we found that changes in SOM tracked changes in plant biomass, but the relationships differed in high vs. low fertility soils. In low fertility soils, the relationship between changes in SOM and plant biomass was negative, whereas in high fertility soils, the relationship was positive. As such, correlations between changes in SOM stocks and plant growth and were stronger in ECM-dominated ecosystems and weaker in arbuscular mycorrhizal (AM-) dominated ecosystems, which likely relates to the greater fraction of protected SOM in AM soils, and the inability of AM fungi to degrade complex forms of SOM. 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, and that predicting the future capacity of ecosystems to store C and slow climate change depend on interactions between plants and soil microbes, as well as the constraints imposed on these interactions by edaphic factors.