Carbon and sulfide as controls on nitrogen cycling in freshwater sediments

Background/Question/Methods

Dissimilatory nitrate reduction to ammonium (DNRA) is the microbial metabolic conversion of nitrate to ammonium, in contrast to the conversion of nitrate to gaseous N₂, as in respiratory denitrification. The resultant ammonium is not permanently removed, but retained within the ecosystem. Two forms of DNRA are known to occur: fermentative and sulfur-oxidizing DNRA. Some have hypothesized that carbon controls fermentative DNRA; however, others have hypothesized sulfide controls DNRA by inhibiting key denitrifying enzymes. We tested the relative importance of carbon vs. sulfide in regulating nitrate removal, DNRA and denitrification using a microbial assay approach. Sediment was collected from seven freshwater wetlands and lakes representing a range in ambient sulfide concentrations (0-100 µM) and divided into high sulfide (>50 µM) and low sulfide (<10 µM) sites for analysis. We examined the effect of labile carbon and sulfide amendments on nitrate removal, denitrification and DNRA rates.

Results/Conclusions
Potential nitrate removal rates ranged from 14.9 – 87 µmoles N L^-1 hr^-1, potential denitrification from 2.8 – 28.5 µmoles N L^-1 hr^-1 and potential DNRA from 0 – 14.1 µmoles N L^-1 hr^-1. Examination of the nitrate only additions (no added carbon or sulfide) showed that high ambient sulfide sites had significantly lower nitrate removal rates (ANOVA p = 0.004), but there was no significant difference in DNRA and denitrification rates between high and low ambient sulfide sites (p = 0.11 and 0.21, respectively). Carbon addition significantly increased denitrification rates (p= 0.039) and increased nitrate removal rates (p=0.099), but did not effect DNRA rates compared to sediments which only had nitrate additions (p=0.971). Sulfide addition did not significantly affect nitrate removal (p = 0.268), but did significantly increase denitrification and DNRA rates (p = 0.002, 0.013) compared to sediments which only had nitrate amendments. Generally, the highest nitrate removal rates occurred when both carbon and sulfide were added to sediment. These results indicate that both carbon and sulfide may interact to control nitrate removal, denitrification and DNRA in freshwater ecosystems. These results also indicate that DNRA potential can be equal to or sometimes more important than denitrification potential in freshwater sediments. These findings are significant because they highlight that the interactions between the sulfur and nitrogen cycles are not well understood, particularly in the relatively low sulfide freshwater ecosystems. Additionally, the potential importance of DNRA has profound implications for our understanding of N cycling in aquatic ecosystems.