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.