Denitrification, dissimilatory nitrate reduction to ammonium, and nitrogen fixation along a nitrate concentration gradient in a created freshwater wetland

Abstract

Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO 3 -) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in NO 3 -concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO 3 -reduction to ammonium (DNRA), and N 2 fixation were examined along the NO 3 -concentration gradient in the LWW. ''Potential DNF'' (hereafter potDNF) was observed in all months and ranged from 54 to 278 lmol N m -2 h -1 . ''Potential DNRA'' (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 lmol N m -2 h -1 . Net N 2 flux ranged from 184 (net denitrification) to -270 (net N 2 fixation) lmol N m -2 h -1 . Nitrogen fixation was variable, ranging from 0 to 426 lmol N m -2 h -1 , but high rates ranked among the highest reported for aquatic sediments. On average, summer potDNRA comprised only 5% (±2% SE) of total NO 3 -loss through dissimilatory pathways, but was as high as 36% at one site where potDNF was consistently low. Potential DNRA was higher in sediments with higher sediment oxygen demand (r 2 = 0.84), and was related to NO 3 -concentration in overlying water in one summer (r 2 = 0.81). Sediments were a NO 3 -sink and accounted for 50% of wetland NO 3 -removal (r 2 = 0.90). Sediments were an NH 4 + source, but the wetland was often a net NH 4 + sink. Although DNRA rates in freshwater wetlands may rival those observed in estuarine systems, the importance of DNRA in freshwater sediments appears to be minor relative to DNF. Furthermore, sediment N 2 fixation can be extremely high when NO 3 -in overlying water is consistently low. The data suggest that newly fixed N can support sustained N transformation processes such as DNF and DNRA when surface water inorganic N supply rates are low.

The short-term effects of management changes on watertable position and nutrients in shallow groundwater in a harvested peatland forest

“…Indeed, the C:NO 3 -ratio, rather than the redox potential of the soil per se, is considered to be the principle factor regulating nitrate partitioning between denitrification and DNRA, with C:NO 3 -ratios greater than 12 considered to be required for substantial DNRA (Fazzolari et al, 1998). Dissimilatory nitrate reduction to ammonium, carried out by strictly anaerobic bacteria (Necpalova et al, 2012), results in the production of NH 4 + -N (Scott et al, 2008).…”

Distribution of typical denitrifying functional genes and diversity of the <i>nirS</i>-encoding bacterial community related to environmental characteristics of river sediments

“…Bacteria of Cluster-I was facultative denitrifying bacteria, which preferred low dissolved oxygen conditions and usually accumulated in the upper sediments (Fukushima et al, 2007). Nitrification and denitrification processes can simultaneously occur in the surface sediment, thereby increasing the reduction rates of NO − 3 and NO − 2 (Rysgaard et al, 1993;Scott et al, 2008). R. eutropha H16 of Cluster-B, which preferred non-halophilic habitats and did not require oxygen, appeared in the deeper sediment (Schwartz et al, 2003).…”

Partitioning whole‐lake denitrification using in situ dinitrogen gas accumulation and intact sediment core experiments

“…Continuous-flow sediment core incubations were conducted similarly to those described by Scott et al (2008). Cores were sealed airtight with rubber stoppers and fitted with Teflon tubing through the stopper to provide inflow and outflow paths for the incubation water.…”