Shifts in the Composition and Activities of Denitrifiers Dominate CO₂ Stimulation of N₂O Emissions

Elevated atmospheric CO₂ (eCO₂) often increases soil N₂O emissions, but the underlying mechanisms remain largely unknown. One hypothesis suggests that high N₂O emissions may stem from increased denitrification induced by CO₂ enhancement of plant carbon (C) allocation belowground. However, direct evidence illustrating linkages among N₂O emissions, plant C allocation, and denitrifying microbes under eCO₂ is still lacking. We examined the impact of eCO₂ on plant C allocation to roots and their associated arbuscular mycorrhizal fungi and its subsequent effects on N₂O emissions and denitrifying microbes in the presence of two distinct N sources, ammonium nitrogen (NH₄⁺–N) and nitrate nitrogen (NO₃^––N). Our results showed that the form of the N inputs dominated the effects of eCO₂ on N₂O emissions: eCO₂ significantly increased N₂O emissions with NO₃^––N inputs but had no effect with NH₄⁺–N inputs. eCO₂ increased plant biomass N more with NH₄⁺–N than with NO₃^––N inputs, likely reducing microbial access to available N under NH₄⁺–N inputs and/or contributing to higher N₂O emissions under NO₃^––N inputs. eCO₂ enhanced root and mycorrhizal N uptake and also increased N₂O emissions under NO₃^––N inputs. Further, eCO₂ enhancement of N₂O emissions under NO₃^––N inputs concurred with a shift in the soil denitrifier community composition in favor of N₂O-producing (nirK- and nirS-type) over N₂O-consuming (nosZ-type) denitrifiers. Together, these results indicate that eCO₂ stimulated N₂O emissions mainly through altering plant N preference in favor of NH₄⁺ over NO₃^– and thus stimulating soil denitrifiers and their activities. These findings suggest that effective management of N sources may mitigate N₂O emissions by negating the eCO₂ stimulation of soil denitrifying microbes and their activities.