Denitrification and nitrogen fixation rates at tidal flats and coastal bottom areas

Abstract

We investigated both rates of denitrification and N2 fixation using intact sediment cores at tidal flats and coastal bottom areas, and the relationships between their activities and environmental factors, to clarify their roles on nitrogen budget in northern Hiroshima Bay. Foreshore tidal flats in islands acted as net sources of nitrogen for the adjacent aquatic environments for these two processes of denitrification and N2 fixation, because the rates of the latter were almost higher than those of the former. It is considered that this phenomenon represents a characteristic of tidal flats located in islands, where nitrate concentrations in the surrounding environmental waters are low. However, other tidal flats at estuaries in mainland areas demonstrated reverse relations: denitrification rates were always higher than N2 fixation rates, and coastal bottom areas also acted as a net sink on nitrogen budget as well as the estuarine tidal flats, showing that these places played important roles in removing nitrogen from surrounding aquatic environments through these two processes. In situ rates of denitrification and N2 fixation were in the ranges of 0.01-20, 0.01-3.8 mg-N m-2 d-1 at tidal flats, and 0.10-3.4, 0.08-0.59 mg-N m-2 d-1 at coastal bottom areas, respectively. Between in situ rates of denitrification (DN) and N2 fixation (FN), there was a negative allometric power relation expressed as the following equation: FN = 0.32 DN-0.318. N2 fixation rates were significantly correlated with water temperatures, and denitrification rates also showed highly significant correlations to ambient nitrate concentrations in environmental waters. Denitrification abilities measured under the same incubation conditions in 0.5 mg-N l-1 of nitrate at 20°C did not show any significant correlations with numbers of denitrifying bacteria, differences in types of tidal fiats and sediment qualities, but did with the density and biomass of macrobenthos. This suggests that benthos may have some important influences on the activities of denitrifying bacteria.