A 1-year field monitoring program was carried out to observe seasonal variation in N2O fluxes at two typical mangrove wetlands in a subtropical estuary. The soils in the island-type mangrove wetland had a higher level of heavy metal(loid) contamination and a lower level of salinity compared to the small bay-type mangrove wetland. While there was a high level of similarity in the seasonal variation pattern of N2O fluxes between the two investigated sites with both being significantly higher in summer than in other seasons, the average of N2O fluxes in the island-type mangrove wetland was 7.19 μg·m-2·h-1， which tended to be lower compared to the small bay-type mangrove wetland (15.63 μg·m-2·h-1). Overall, N2O flux was closely related to soil-borne heavy metal(loid)s, showing a trend to decrease with increasing concentration of these heavy metal(loid)s. The N2O fluxes increased with decreasing abundance of either denitrifiers or nitrifiers. But the opposite was observed for the anammox bacteria present in the soils. The anammox bacteria were more sensitive to heavy metal(loid) stress but more tolerated high salinity encountered in the investigated soils compared to the denitrifiers or nitrifiers. It appears that anammox reactions mediated by anammox bacteria played a key role in affecting the spatial variation in N2O fluxes from the mangrove soils in the study area. And an increased level of ammonium in soils tended to promote the activity of anammox bacteria and consequently enhanced N2O emission from the mangrove soils.
Keywords: Anaerobic ammonia oxidation; Denitrification; Heavy metal(loid)s; Mangrove soils; Nitrification; Nitrous oxide.
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