Dissimilatory Nitrate Reduction to Ammonium (DNRA) Can Undermine Nitrogen Removal Effectiveness of Persistently Reducing Riparian Sediments

Denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) compete in reducing sediment conditions where DNF permanently removes nitrogen (N), while DNRA retains N with the conversion of nitrate (NO₃^–) to ammonium (NH₄⁺). Thus, an increase in the level of DNRA can undermine permanent N removal. We investigated the relative magnitude and controls of these two processes at two milldam-affected riparian sites. DNRA (5.2–37.6 μg L^–1 h^–1) accounted for 10–79% of total NO₃^– reduction and was highest in riparian sediments with higher iron (Fe) and sodium (Na⁺) in groundwater. DNF was the primary mechanism for NO₃^– reduction when Fe and Na⁺ concentrations were low but when NO₃^– was elevated. DNRA rates were higher for treatments with higher dissolved organic carbon (DOC):NO₃^– and Fe:NO₃^– ratios, indicating the stimulation of both heterotrophic and Fe²⁺ driven autotrophic DNRA. DNF and DNRA rates and their microbial functional genes decreased with increasing sediment depths. These findings imply that hydrologically stagnant and persistently reducing conditions associated with relict milldams and similar anthropogenic structures may enhance DNRA at the expense of DNF and undermine permanent N removal in riparian zones. Thus, the effects of such structures need to be accounted for in watershed N management strategies.