Saltwater incursion regulates N 2 O emission pathways and potential nitrification and denitrification in intertidal wetland

Variations of N transformation processes, N 2 O release rates, and N 2 O emission pathways were investigated at different levels of salinity (stage 1, low salinity inhibition stage; stage 2, medium salinity promotion stage; and stage 3, high salinity promotion stage) using 15 N- 18 O dual-isotope labeling technique. Potential nitrification rates were reduced by saltwater incursion in stage 1, increased markedly in stage 3, and significantly inhibited potential denitrification rate under higher salinity. N 2 O emission significantly increased along salinity gradient in stage 3 due to the changes of potential nitrification rates. Saltwater incursion significantly increased the contribution of heterotrophic denitrification to N 2 O emission in stage 1. In stages 2 and 3, contributions of nitrifier denitrification and nitrification-coupled denitrification to N 2 O emission increased gradually, and nitrifier denitrification became the dominant pathway of N 2 O emission under high salinity. Changes of N transformations, N 2 O emission rates, and their pathways were regulated both by composition of microbial community and physicochemical properties of the sediment. Due to increased ammonification rate, organic N decomposition accelerated by saltwater can reduce the wetland N sink and may turn coastal wetland to significant reactive N source.