Nitrogen oxides emissions from coastal wetland sediments: Experimental assessment of the influence of vegetation and nitrogen input

Nitrogen oxides (NO_x = NO + NO₂) have essential impacts on global climate and the environment, making it essential to study the contribution of wetland-generated NO_x to environmental problems. With exogenous nitrogen input from anthropogenic activities, wetland sediments become active emission hotspots for NO_x. In this study, we conducted field experiments in a typical salt marsh wetland to measure nitric oxide (NO, the primary component of NO_x from sediments) exchange fluxes in both mudflat and vegetated sediments. We found that NO fluxes in vegetated sediments (0.40 ± 0.15 × 10^-12 kg N m^-2 s^-1) were relatively higher than in mudflat sediments (-1.31 ± 1.39 × 10^-12 kg N m^-2 s^-1), with this difference occurring only during the vegetation-dying season (autumn). Correlations between sediment NO fluxes and environmental parameters revealed that NO flux variation during the observation period was primarily influenced by sediment respiration, temperature, water content, and substrate availability. However, the influence of these factors on NO fluxes differed between mudflat and vegetated sediments. In-situ data analysis also suggested that tidal horizontal migration, which affects sediment substrate and salinity, may regulate sediment NO emissions. Furthermore, in-situ incubations with nitrogen addition (ammonia, nitrite, and nitrate) were conducted to study the response of sediment NO emissions to exogenous nitrogen. We observed that nitrogen addition caused a 259.7 % increase in NO emissions from vegetated sediments compared to the control during the effective period of nitrogen addition (days 1-3). However, although nitrogen addition markedly stimulated sediment NO emissions, the overall NO production capacity constrained the extent of this increase.