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

Abstract

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 (NH4+) flux ranged from slight uptake (negative values) to NH4+ production rates of about 300 µmol m−2 h−1 (units used for all N rates) and increased with salinity (p = 0.10). Net nitrate (NO3−) flux (–20 to 32) and net N2 flux (–70 to 100) did not relate to salinity. Mean net N2 flux was positive but near zero, indicating that N2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO3− reduction to NH4+ (DNRA) rates were estimated after inflow water was enriched with 15NO3− (100 µmol L−1). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 to 97. Potential DNRA, measured conservatively as 15H accumulation, ranged from 0 to 80 and related significantly to salinity (p < 0.01). Increases in total NH4+ release after 15NO3− additions were higher but closely related (r = 0.9998) to 15NH4+ accumulation, implying exchange reactions of DNRA‐regenerated 15N4+ with sediment‐bound 14NH4+. The fate of NO2 was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was 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.