Climate change factors such as elevated atmospheric CO2 and O3 can exert significant impacts on soil microbes and microbially-mediated ecosystem processes. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. A prevailing hypothesis, which states that CO2- or O3-induced changes in C availability dominates microbial responses, successfully predicts outcomes in some cases, but fails in others.
Results/Conclusions
Using a long-term field study conducted in a no-till wheat-soybean system, we show that N availability critically influenced soil microbial responses to elevated CO2 but not O3. Elevated CO2 significantly increased but O3 reduced above-ground residue mass and residue N inputs. However, only elevated CO2 significantly affected soil microbial parameters. While it only had marginal effects on microbial respiration in the first two years, elevated CO2 significantly stimulated microbial biomass and decomposition in the third and fourth years when N availability increased, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Our results indicate that atmospheric CO2 enrichment may create a positive feedback loop in which CO2-enhancement of N availability stimulates microbial activities and CO2 release from soil. These results also suggest that high N availability in many agricultural soils may accelerate organic C turnover and limit the potential of C sequestration in agroecosystems under future CO2 scenarios.