The diversity of the N(2)O reducers matters for the N(2)O:N(2) denitrification end-product ratio across an annual and a perennial cropping system

Agriculture is the main source of terrestrial emissions of N(2)O, a potent greenhouse gas and the main cause of ozone layer depletion. The reduction of N(2)O into N(2) by microorganisms carrying the nitrous oxide reductase gene (nosZ) is the only biological process known to eliminate this greenhouse gas. Recent studies showed that a previously unknown clade of N(2)O-reducers was related to the capacity of the soil to act as an N(2)O sink, opening the way for new strategies to mitigate emissions. Here, we investigated whether the agricultural practices could differently influence the two N(2)O reducer clades with consequences for denitrification end-products. The abundance of N(2)O-reducers and producers was quantified by real-time PCR, and the diversity of both nosZ clades was determined by 454 pyrosequencing. Potential N(2)O production and potential denitrification activity were used to calculate the denitrification gaseous end-product ratio. Overall, the results showed limited differences between management practices but there were significant differences between cropping systems in both the abundance and structure of the nosZII community, as well as in the [rN(2)O/r(N(2)O+N(2))] ratio. More limited differences were observed in the nosZI community, suggesting that the newly identified nosZII clade is more sensitive than nosZI to environmental changes. Potential denitrification activity and potential N(2)O production were explained mainly by the soil properties while the diversity of the nosZII clade on its own explained 26% of the denitrification end-product ratio, which highlights the importance of understanding the ecology of this newly identified clade of N(2)O reducers for mitigation strategies.