Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters

Anthropogenic nitrogen (N) loading to agricultural and populated landscapes has resulted in elevated nitrate (NO3–) concentrations in ground water, streams and rivers, ultimately causing problems in coastal marine environments such as eutrophication, hypoxia and harmful algal blooms. Nitrate removal along hydrologic flow paths through landscapes intercepts much of the N before it reaches coastal zones. We used traditional porewater equilibrators in a novel way to add nitrate to the sediment porewater of 8 wetlands in southwestern Michigan. Nitrate losses and changes in porewater chemistry were examined to elucidate N removal processes, with particular focus on the potential coupling of bacterial sulfur (S) oxidation to (1) dissimilatory nitrate reduction to ammonium (DNRA) and (2) denitrification. We hypothesized that, if S oxidizers utilized the added nitrate, porewater sulfide concentrations should decrease and sulfate concentrations should increase. Additionally, if the nitrate is used in DNRA, ammonium concentrations should increase as well. Nitrate additions caused decreases in dissolved hydrogen sulfide and increases in sulfate relative to controls at all sites. Ammonium also tended to increase, though the response was less consistent due to a high background ammonium pool. These results provide evidence that microbial S transformations may play an important role in nitrate removal in these freshwater wetland sediments