The relationship between anammox and denitrification in the sediment of an inland river

This study measured the microbial processes of anaerobic ammonium oxidation (anammox) and denitrification in sediment sampled from two sites in the estuary of an inland river (Koisegawa River, Ibaragi prefecture, Japan) using a nitrogen isotope pairing technique (IPT). The responses of anammox and denitrification activities to temperature and nitrate concentration were also evaluated. Further, to elucidate the correlation between anammox and denitrification processes, an inhibition experiment was conducted, using chlorate to inhibit the first step of denitrification. Denitrification activity was much higher than anammox activity, and it reached a maximum at the surface layer in February 2012. Denitrification activity decreased as sediment depth increased, and a similar phenomenon was observed for anammox activity in the sediment of site A, where aquatic plants were absent from the surroundings. The activities of both denitrification and anammox were temperature-dependent, but they responded differently to changes in incubation temperature. Compared to a linear increase in denitrification as temperature rose to 35 °C, the optimal temperature for anammox was 25 °C, after which the activity decreased sharply. At the same time, both anammox and denitrification activities increased with NO3(-) concentration. The Michaelis-Menten kinetic constants (Vmax and Km) of denitrification were significantly higher than those of the anammox process. Furthermore, anammox activity decreased accordingly when the first step of denitrification was inhibited, which probably reduced the amount of the intermediate NO2(-). Our study provides the first direct exploration of the denitrification-dependent correlation of anammox activity in the sediment of inland river.