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.