Studies of nitrous oxide and the nitrogen cycle in a temperate river-estuarine system

Abstract

Nitrous oxide (N2O), the third most important greenhouse gas in terms of
anthropogenic climate forcing, is also one of the most important trace gases in
driving atmospheric chemistry. It is a potent greenhouse gas, having a global
warming potential per mole about 300 times that of carbon dioxide, and it is an
intermediate in the destruction of stratospheric ozone. Unfortunately, the N2O global
budget, and in particular the cause of its steady rise over the past century, is not well
defined. The natural and anthropogenic sources for its increase are probably not all
identified and certainly not well quantified. Recent studies show that when estuarine
and coastal regions are included in the global N2O budget, a considerable portion of
the global marine N2O flux is from estuarine and coastal regions, mainly due to high
emissions from estuaries. To examine the contribution of estuaries to N2O emissions,
nitrous oxide concentrations in the water were measured by gas chromatography on
a monthly basis in the River Itchen and Itchen Estuary - UK, from November 2001 to
December 2002. Water column concentrations of N2O in both, river and estuary were
supersaturated with respect to air (mean saturation 325% and 162%, respectively),
indicating that they were sources of N2O to the atmosphere. High N2O
concentrations in the river appear related to high concentrations in the groundwater.
Highest N2O concentrations in the estuary were generally observed at lower salinities
(up to 79nM and saturation = 679%) when compared with concentrations at the high
salinity (average saturation = 87%). Nitrite had the strongest correlation with N2O for
all surveys (r=0.78; p<0.05), suggesting that nitrite is linked to nitrous oxide
production in estuaries. Fluxes from the River Itchen and Itchen Estuary extrapolated
to the UK systems and compared with other anthropogenic sources of nitrous oxide
to the atmosphere, showed that these systems are significant sources and should be
included in the N2O budget. Incubation experiments were done with sediment
collected from the Itchen Estuary to investigate N2O production. Initial experiments
on a whole core showed the importance of temperature on N2O production, and also
the potentially complicating impact of biological activity. In subsequent experiments,
homogenized and sieved sediment material were used, in which macro benthos
were excluded. N2O fluxes from the sediment were estimated and denitrification was
suggested as the main process producing N2O in the sediments of the incubated
cores.

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