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

Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO3-) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in NO3- concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO3- reduction to ammonium (DNRA), and N-2 fixation were examined along the NO3- concentration gradient in the LWW. "Potential DNF" (hereafter potDNF) was observed in all months and ranged from 54 to 278 mu mol N m(-2) h(-1). "Potential DNRA" (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 mu mol N m(-2) h(-1). Net N-2 flux ranged from 184 (net denitrification) to -270 (net N-2 fixation) mu mol N m(-2) h(-1). Nitrogen fixation was variable, ranging from 0 to 426 mu mol 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 NO3- 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 NO3- removal (r(2) = 0.90). Sediments were an NH4+ source, but the wetland was often a net NH4+ 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 NO3- 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.