CO2-induced alterations in plant nitrate utilization and root exudation stimulate N2O emissions

Atmospheric carbon dioxide enrichment (eCO(2)) often increases soil nitrbus oxide (N2O) emissions, which has been largely attributed to increased denitrification induced by CO2-enhancement of soil labile C and moisture. However, the origin of the N remains unexplained. Emerging evidence suggests that eCO2 alters plant N preference in favor of ammonium (NH4+-N) over nitrate (NO3--N). Yet, whether and how this attributes to the enhancement of N2O emissions has not been investigated. We conducted a microcosm experiment with wheat (Triticum aestivum L.) and tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.) to examine the effects of eCO(2) on soil N2O emissions in the presence of two N forms (NH4+-N or NO3--N). Results obtained showed that N forms dominated eCO2 effects on plant and microbial N utilization, and thus soil N2O emissions. Elevated CO2 significantly increased the rate and the sum of N2O emissions by three to four folds when NO3--N, but not NH4+-N, was supplied under both wheat and tall fescue. While enhanced N2O emission was more related to the reduced plant NO3--N Uptake under wheat, it concurred with increased labile C under tall fescue. In the presence of NO3--N, significantly lower shoot biomass N and N-15, but higher plant biomass C:N ratio, mitrobial biomass C and N, and/or soil extractable C indicated that eCO(2) constrained plant NO3--N utilization and likely stimulated root exudation. We propose a new conceptual model in which eCO(2)-inhibition of plant NO3--N uptake and/or CO2-enhancement of soil labile C enhances the N and/or C availability for denitrifiers and increases the intensity and/or the duration of N2O emissions. Together, these findings indicate that CO2-enhancement of soil N and labile C favors denitrification, suggesting that management of N fertilizers in intensive systems will likely become more challenging under future CO2 scenarios. (C) 2016 Elsevier Ltd. All rights reserved.