Nitrous Oxide Formation in the Colne Estuary, England: the Central Role of Nitrite

Abstract

Nitrate and nitrite concentrations in the water and nitrous oxide and nitrite fluxes across the sediment-water interface were measured monthly in the River Colne estuary, England, from December 1996 to March 1998. Water column concentrations of N 2 O in the Colne were supersaturated with respect to air, indicating that the estuary was a source of N 2 O for the atmosphere. At the freshwater end of the estuary, nitrous oxide effluxes from the sediment were closely correlated with the nitrite concentrations in the overlying water and with the nitrite influx into the sediment. Increases in N 2 O production from sediments were about 10 times greater with the addition of nitrite than with the addition of nitrate. Rates of denitrification were stimulated to a larger extent by enhanced nitrite than by nitrate concentrations. At 550 M nitrite or nitrate (the highest concentration used), the rates of denitrification were 600 mol N · m ؊2 · h ؊1 with nitrite but only 180 mol N · m ؊2· h ؊1 with nitrate. The ratios of rates of nitrous oxide production and denitrification (N 2 O/N 2 ؋ 100) were significantly higher with the addition of nitrite (7 to 13% of denitrification) than with nitrate (2 to 4% of denitrification). The results suggested that in addition to anaerobic bacteria, which possess the complete denitrification pathway for N 2 formation in the estuarine sediments, there may be two other groups of bacteria: nitrite denitrifiers, which reduce nitrite to N 2 via N 2 O, and obligate nitrite-denitrifying bacteria, which reduce nitrite to N 2 O as the end product. Consideration of free-energy changes during N 2 O formation led to the conclusion that N 2 O formation using nitrite as the electron acceptor is favored in the Colne estuary and may be a critical factor regulating the formation of N 2 O in high-nutrient-load estuaries.Nitrous oxide (N 2 O) is, after molecular nitrogen, the most abundant nitrogen compound in the atmosphere. Global longterm measurement series of tropospheric N 2 O show an annual growth rate of about 0.25 to 0.31% year Ϫ1 (18, 48), indicating that current global sources exceed sinks (4). Nitrous oxide has an atmospheric lifetime of about 150 years and a large greenhouse warming potential (36). When century-long effects are considered, the greenhouse warming potential of N 2 O is 310 times greater than that of CO 2 (1).According to Mathews (26), known global sinks exceed known sources by 40%, which implies either the presence of unknown sources of N 2 O or the underestimation of alreadyknown sources. Generally, global budgets do not include estimates of sources of N 2 O in estuaries and coastal seas. A recent study (2), however, showed that when estuarine and coastal regions were included, a considerable portion (approximately 60%) of the global marine N 2 O flux was from estuarine and coastal regions, mainly due to high emission from estuaries. High N 2 O concentrations and high fluxes to the atmosphere have been described in some estuaries and coastal seas (2,3,6,14,19,24,27,28,35,39,40,41,42,4...

Nitrogen enrichment and the emission of nitrous oxide from streams

“…Although nitrite is only ever present in low concentrations, it explains a significant amount of the variation in daily N 2 O fluxes in Stouffville Creek (Table 5). Nitrite addition experiments have been shown to yield higher N 2 O production ratios than the addition of NO 3 − , and strong correlations between N 2 O fluxes and nitrite concentrations have been shown within estuaries [Dong et al, 2002]. Ammonium was also a significant correlate in one of our study streams, but was the best single predictor of N 2 O fluxes in a group of small eutrophic Mexican streams [Harrison and Matson, 2003].…”

Changes in Benthic Denitrification, Nitrate Ammonification, and Anammox Process Rates and Nitrate and Nitrite Reductase Gene Abundances along an Estuarine Nutrient Gradient (the Colne Estuary, United Kingdom)

“…Field sampling was conducted in February and March 2005 at three sites along the Colne estuary, United Kingdom, at Hythe (the head), Alresford Creek (the middle), and Brightlingsea (the mouth), as described previously (8,9). At each site, replicate small (10-cm-long) cores of sediment were collected into core tubes (internal diameter, 3.4 cm, and length, 22 cm) for process rate measurements (n, 5 per treatment).…”

Nitrous oxide emission by aquatic macrofauna

“…These bacteria might be indirectly involved in nitrous oxide production via the accumulation of nitrite in the animal gut. Nitrite has been shown to stimulate nitrous oxide production in estuarine sediment (22) and the earthworm gut (23) more effectively compared with nitrate.…”