Denitrification, dissimilatory nitrate reduction to ammonium, and nitrogen fixation along a nitrate concentration gradient in a created freshwater wetland
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- Scott, J.T., McCarthy, M.J., Gardner, W.S. et al. Biogeochemistry (2008) 87: 99. doi:10.1007/s10533-007-9171-6
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 N2 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 μmol N m−2 h−1. “Potential DNRA” (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 μmol N m−2 h−1. Net N2 flux ranged from 184 (net denitrification) to −270 (net N2 fixation) μmol N m−2 h−1. Nitrogen fixation was variable, ranging from 0 to 426 μmol 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 (r2 = 0.84), and was related to NO3− concentration in overlying water in one summer (r2 = 0.81). Sediments were a NO3− sink and accounted for 50% of wetland NO3− removal (r2 = 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 N2 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.