Environmental controls on nitrogen and sulfur cycles in surficial aquatic sediments

Abstract

Enhanced anthropogenic inputs of nitrogen (N) and sulfur (S) have disturbed their biogeo-chemical 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 NO3- reduction coupled to S oxidation, consuming NO3-, and producing sulfate SO2-4. 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 NO3- and SO2-4 are present separately or simultaneously, (2) how the N and S cycles could interact through S oxidation coupled to NO3- 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 NO3- and SO2-4 were added either separately or simultaneously and NO3- and SO2-4 reduction rates as well as NO3- 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 NO3- and SO2-4 were both present. When NO3- and SO2-4 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 SO2-4 addition had little effect on NO3- reduction; neither did the NO3- addition inhibit SO2-4 reduction. The presence of NO3- led to SO2-4 production most likely due to the oxidation of sulfur. Our observations suggest that metal-bound S, instead of free sulfide produced by SO2-4 reduction, was responsible for the S oxidation. © 2012 Gu, Laverman and Pallud.