Nitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA) support nitrogen dynamics in Texas estuaries

We conducted continuous-flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH ) flux ranged from slight uptake (negative values) 4 to NH production rates of about 300 mmol m22 h21 (units used for all N rates) and increased with salinity (p 5 4 0.10). Net nitrate (NO ) flux (220 to 32) and net N2 flux (270 to 100) did not relate to salinity. Mean net N2 flux 3 was positive but near zero, indicating that N2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO reduction to NH (DNRA) rates were estimated after inflow water was enriched 2 1 3 4 with 15NO (100 mmol L21). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 3 to 97. Potential DNRA, measured conservatively as 15NH accumulation, ranged from 0 to 80 and related signifi4 cantly to salinity (p , 0.01). Increases in total NH release after 15NO additions were higher but closely related 1 2 4 3 (r 5 0.9998) to 15NH accumulation, implying exchange reactions of DNRA-regenerated 15NH with sediment1 1 4 4 bound 14NH . The fate of NO was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was 1 2 4 3 high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries. Nitrogen (N), a key nutrient limiting primary production in coastal ecosystems, can be introduced to coastal waters via tributaries, atmospheric deposition, or groundwater inputs as nitrates (NO or NO ), ammonium (NH ), and or2 2 1 3 2 4 ganic N, or through N-fixation (Brock 2001; Paerl et al. 2002; Seitzinger et al. 2002). Available N is removed from coastal waters via physical transport, sediment burial, or conversion to gaseous forms, such as N2 or N2O, via denitrification (Seitzinger et al. 1984; Brock 2001). An improved understanding of N dynamics in coastal systems is needed 1 Corresponding author ([email protected]). 2 Current address: Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea.