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

Atmospheric carbon dioxide enrichment (eCO 2 ) often increases soil nitrous oxide (N 2 O) emissions, which has been largely attributed to increased denitrification induced by CO 2 -enhancement of soil labile C and moisture. However, the origin of the N remains unexplained. Emerging evidence suggests that eCO 2 alters plant N preference in favor of ammonium (NH 4 + -N) over nitrate (NO 3 − -N). Yet, whether and how this attributes to the enhancement of N 2 O 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 N 2 O emissions in the presence of two N forms (NH 4 + -N or NO 3 − -N). Results obtained showed that N forms dominated eCO 2 effects on plant and microbial N utilization, and thus soil N 2 O emissions. Elevated CO 2 significantly increased the rate and the sum of N 2 O emissions by three to four folds when NO 3 − -N, but not NH 4 + -N, was supplied under both wheat and tall fescue. While enhanced N 2 O emission was more related to the reduced plant NO 3 − -N uptake under wheat, it concurred with increased labile C under tall fescue. In the presence of NO 3 − -N, significantly lower shoot biomass N and 15 N, but higher plant biomass C:N ratio, microbial biomass C and N, and/or soil extractable C indicated that eCO 2 constrained plant NO 3 − -N utilization and likely stimulated root exudation. We propose a new conceptual model in which eCO 2 -inhibition of plant NO 3 − -N uptake and/or CO 2 -enhancement of soil labile C enhances the N and/or C availability for denitrifiers and increases the intensity and/or the duration of N 2 O emissions. Together, these findings indicate that CO 2 -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 CO 2 scenarios. [ABSTRACT FROM AUTHOR]