Environmental controls on nitrogen and sulfur cycles in surficial aquatic sediments.

Enhanced anthropogenic inputs of nitrogen (N) and sulfur (S) have disturbed their biogeochemical cycling in aquatic and terrestrial ecosystems. The N and S cycles interact with one another through competition for labile forms of organic carbon between nitrate-reducing and sulfate-reducing bacteria. Furthermore, the N and S cycles could interact through nitrate [Formula: see text] reduction coupled to S oxidation, consuming [Formula: see text] and producing sulfate [Formula: see text] The research questions of this study were: (1) what are the environmental factors explaining variability in N and S biogeochemical reaction rates in a wide range of surficial aquatic sediments when [Formula: see text] and [Formula: see text] are present separately or simultaneously, (2) how the N and S cycles could interact through S oxidation coupled to [Formula: see text] reduction, and (3) what is the extent of sulfate reduction inhibition by nitrate, and vice versa. The N and S biogeochemical reaction rates were measured on intact surface sediment slices using flow-through reactors. The two terminal electron acceptors [Formula: see text] and [Formula: see text] were added either separately or simultaneously and [Formula: see text] and [Formula: see text] reduction rates as well as [Formula: see text] reduction linked to S oxidation were determined. We used redundancy analysis, to assess how environmental variables were related to these rates. Our analysis showed that overlying water pH and salinity were two dominant environmental factors that explain 58% of the variance in the N and S biogeochemical reaction rates when [Formula: see text] and [Formula: see text] were both present. When [Formula: see text] and [Formula: see text] were added separately, however, sediment N content in addition to pH and salinity accounted for 62% of total variance of the biogeochemical reaction rates. The [Formula: see text] addition had little effect on [Formula: see text] reduction; neither did the [Formula: see text] addition inhibit [Formula: see text] reduction. The presence of [Formula: see text] led to [Formula: see text] production most likely due to the oxidation of sulfur. Our observations suggest that metal-bound S, instead of free sulfide produced by [Formula: see text] reduction, was responsible for the S oxidation.