Factors affecting NO fluxes from heavy metal-contaminated mangrove soils in a subtropical estuary.

A 1-year field monitoring program was carried out to observe seasonal variation in NO 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 NO fluxes between the two investigated sites with both being significantly higher in summer than in other seasons, the average of NO fluxes in the island-type mangrove wetland was 7.19 μg·m·h， which tended to be lower compared to the small bay-type mangrove wetland (15.63 μg·m·h). Overall, NO 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 NO 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 NO 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 NO emission from the mangrove soils.