Hypernutrified estuaries as sources of N2O emission to the atmosphere:the estuary of the River Colne, Essex, UK

Abstract

Hypernutrified estuaries as sources of N 2 0 emission to the atmosphere: the estuary of the River Colne, Essex, UK 'Department of Biological Sciences. University of Essex, Colchester C 0 4 3SQ, United Kingdom 2Environmental Health. School of Chemistry. University of Birmingham, Birmingham B15 2TT. United Kingdom ABSTRACT: Measurements in the estuary of the River Colne, Essex, UK, showed strong gradients of nitrate and ammonium concentrations, increasing upriver due to inputs from the nver and a large sewage treatment works. In the low salinity region (<l0 to 12 psu) nitrate concentrations in the water column were sometimes > l mM, but rapidly decreased within the top 1 cm of sediment. Concentrations of N 2 0 in the water column correlated with nitrate concentrations, due to denitnfication and N 2 0 formation in the surface sediment. There was no formation of N,O in the water column. N 2 0 in the estuary water was always supersaturated with respect to N 2 0 in air, and in the low salinity/high nitrate part of the estuary could be up to 50 times air equ.ilibration. Water-air N 2 0 fluxes, calculated from a thin film model, also showed marked gradients up the estuary. The emission fluxes of N,O from the surface of tidally exposed sediments decreased with time after exposure. This was the result of the fast turnover (c40 min) of the sedimentary nitrate pool, and its depletion when it was no longer recharged by transport of nitrate from the water column. The sedunents were, therefore, extremely important processors of nitrate from the water column, but only when covered by the tide. Water-air emission fluxes in each sector of the estuary were calculated and showed that maximum N 2 0 emission at high tide was from the sector below that where peak unit area emission fluxes were detected, because of the greater area of water surface in the sector.Total N20 e~nission from the estuary did not vary with tidal state because the high n i t r a t e h g h N 2 0 water remained within the estuary even at low tide. Integration of the emission fluxes with time suggested that about 1.2 X 105 m01 of N20-N were emitted to the atmosphere in a year, of which 83 O/o was from the water surface. This emission accounted for 0.5'::~ of the Total Oxidised Nitrogen load to the estuary, or 0.3% of the Total Nitrogen load. Benthic NZO production corresponded to < 2 % of the nitrate denitrified in the bottom sediments, but could nonetheless give nse to significant export of N20 to the atmosphere. If the N 2 0 fluxes in the Colne estuary were extrapolated globally, the total estuarine N 2 0 production would be equivalent to between 0.13 and 0.45 Tg N20-N yr-' This is equivalent to the source strengths of adipic acid and nitr~c acid productions, but less than the hypothesised missing global source(s) of N,O.

Dissimilatory reduction of nitrate to ammonium, not denitrification or anammox, dominates benthic nitrate reduction in tropical estuaries

“…300% were recorded during the high nitrate intermediate and wet seasons, but saturations , 100% were recorded during the dry seasons, which suggested active removal of N 2 O. However, even these N 2 O saturations in the Cisadane during the intermediate and wet seasons were small when compared to values as high as several thousand percent of air saturation in some temperate estuaries (Bange et al 1996;Robinson et al 1998). The coefficient of variation of injected standards for the N 2 O analyses by gas chromatograph was 6 3.5%, which at typical tropical in situ values of 30uC and 0 salinity and atmosphere N 2 O concentration of 12.5 nmol L 21 represented 6 20% saturation, leading to the conclusion that only percent saturations .…”

“…On return to the United Kingdom the dissolved N 2 O concentrations in samples were measured in a gas chromatograph with electron capture detector (Robinson et al 1998). …”

“…In contrast to temperate regions, the significance of estuarine processing of nutrients in tropical estuaries is poorly understood. Moreover, temperate estuaries are also significant sources of the radiatively active trace gas N 2 O, generated as a minor product of DN (Robinson et al 1998;Seitzinger and Kroeze 1998). Although Seitzinger and Kroeze (1998) suggested that rivers and estuaries could account for as much as 20% of the total global N 2 O emissions to the atmosphere, Crutzen (2001) emphasized the comparative lack of data for tropical and subtropical ecosystems, particularly in relation to radiatively active trace gas production and its effect on global warming.…”

Seasonal variation in denitrification and dissolved nitrogen fluxes in intertidal sediments of the Tagus estuary, Portugal

“…Although a limited number of N 2 O flux measurements have been made so far in coastal sediments, the results generally indicate an N 2 O release to the atmosphere varying from < 0.1 to 9 µmol m -2 h -1 (Seitzinger et al 1983, 1984, Seitzinger 1990, Robinson et al 1998; this places the Tagus estuary among those systems with lower production rates. Apparently, the N 2 O release found in the present study was not comparable with known industrial N 2 O emissions (Robinson et al 1998).…”

Distributions and sea-to-air fluxes of nitrous oxide in the South China Sea and the West Philippines Sea

“…Denitrification produces and consumes N 2 O in oxygen minimum zones and the net result is that oxygen minimum zones have some of the highest N 2 O concentrations in the ocean (LaMontagne et al, 2003;Naqvi et al, 2010;Zamora et al, 2012;Arévalo-Martínez et al, 2015). In the water column with low oxygen concentrations and in sediments or inside suspended particles, nitrification and denitrification can occur simultaneously and are often closely coupled (Seitzinger, 1988;Ward et al, 1989;Capone, 1991;Middelburg et al, 1996;Barnes and Owens, 1998;Naqvi et al, 1998;Robinson et al, 1998;Usui et al, 2001;Nevison et al, 2003;Codispoti et al, 2005;Bange, 2008). The yield of N 2 O from both processes is highly dependent on the concentration of dissolved oxygen (Goreau et al, 1980;de Bie et al, 2002;Naqvi et al, 2010).…”