High NO reduction potential by denitrification in the nearshore site of a riparian zone.

As a potent atmospheric greenhouse gas and a major source of ozone depletion, nitrous oxide (NO) emission has been given increasing attention in aquatic systems, particularly at the aquatic-terrestrial interfaces, such as riparian zones. However, the microbial mechanisms regulating NO emission in riparian zones remain unknown. Here, we measured the contributions of denitrification and ammonium oxidation to NO emission along with the abundance and community structure of nirK-, nirS-, nosZ I- and nosZ II-harbouring bacteria in both surface sediments (0-10 cm) and overlying water along a lake riparian zone (including nearshore sites and offshore sites). Overall, the nearshore sites of the riparian zones emitted less NO than the offshore sites. Nearshore NO emission was dominated by denitrification with a high NO reduction rate, whereas offshore NO emission was driven by ammonium oxidation. Furthermore, NO derived from ammonium oxidation was influenced by the NH-N content, and denitrification NO was modulated by denitrifier communities. The NO-producing community was dominated by nirS-harbouring bacteria, while the NO-reducing community was dominated by nosZ I-harbouring bacteria. The relative abundance of Hydrogenophilales from nirS-denitrifiers and Chloroflexi unclassified from nosZ II-type communities influenced the NO produced by denitrification, according to high-throughput sequencing analysis. Additionally, we also found lower levels of NO production per unit volume in overlying water, which were 3-4 orders of magnitude less than in the surface sediment. Overall, we propose that using riparian zones can be an effective management tool for NO mitigation by enhancing the NO reduction process of denitrification and decreasing ammonium oxidation.