Impact of salinity gradients on nitric oxide emissions and functional microbes in estuarine wetland sediments

Estuarine wetland sediments are hotspots for nitrogen cycling and critical sources of atmospheric nitric oxide (NO). Yet studies on the impact of sediment salinity gradients on NO emissions and associated functional microbes at the land-ocean interface remain limited. Here, we measured sediment NO emission rates from incubated sediment samples that were collected from an estuarine wetland in Qingdao, China. Our findings indicate that sediment salinity is a pivotal factor shaping NO emission rates, by altering the community composition and gene abundance of functional microbes involved in NO emissions, with rates ranging from 0.04 to 0.25 μg N kg^–1 dry soil h^–1. Metagenomic analysis of the sediment samples reveals that greater NO emission rates (+486 %) under salinity changes are linked to a higher abundance of the nirS gene (+26 %) responsible for NO formation and a lower abundance of norBC genes (–23 %) responsible for NO consumption. Accordingly, the increase of NO emissions may be attributed to the accumulation of denitrifying NO, which could improve plant salt tolerance through co-evolutionary interactions between plants and sediment-dwelling microbes. Taken together, these findings contribute to a richer understanding of how biochemical NO emissions in estuarine wetland sediments respond to salinity gradients.