Stimulating denitrification in a stream mesocosm with elemental sulfur as an electron donor

The heavy use of fertilizers in agricultural lands can result in significant nitrate (NO3−) loadings to the aquatic environment. We hypothesized that biological denitrification in agricultural ditches and streams could be enhanced by adding elemental sulfur (So) to the sediment layer, where it could act as a biofilm support and electron donor. Using a bench-scale stream mesocosm with a bed of So granules, we explored NO3− removal fluxes as a function of the effluent NO3− concentrations. With effluent NO3− ranging from 0.5 mg N L−1 to 4.1 mg N L−1, NO3− removal fluxes ranged from 228 mg N m−2 d−1 to 708 mg N m−2 d−1. This is as much as 100 times higher than for agricultural drainage streams. Sulfate (SO42−) production was high due to aerobic sulfur oxidation. Molecular studies demonstrated that the So amendment selected for Thiobacillus species, and that no special inoculum was required for establishing a So-based autotrophic denitrifying community. Modeling studies suggested that denitrification was diffusion limited, and advective flow through the bed would greatly enhance NO3− removal fluxes. Our results indicate that amendment with So is an effective means to stimulate denitrification in a stream environment. To minimize SO42− production, it may be better to place So deeper in the sediment layer.