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

Nitrous oxide (N2O), the third most important greenhouse gas in terms ofanthropogenic climate forcing, is also one of the most important trace gases indriving atmospheric chemistry. It is a potent greenhouse gas, having a globalwarming potential per mole about 300 times that of carbon dioxide, and it is anintermediate in the destruction of stratospheric ozone. Unfortunately, the N2O globalbudget, and in particular the cause of its steady rise over the past century, is not welldefined. The natural and anthropogenic sources for its increase are probably not allidentified and certainly not well quantified. Recent studies show that when estuarineand coastal regions are included in the global N2O budget, a considerable portion ofthe global marine N2O flux is from estuarine and coastal regions, mainly due to highemissions from estuaries. To examine the contribution of estuaries to N2O emissions,nitrous oxide concentrations in the water were measured by gas chromatography ona monthly basis in the River Itchen and Itchen Estuary - UK, from November 2001 toDecember 2002. Water column concentrations of N2O in both, river and estuary weresupersaturated with respect to air (mean saturation 325% and 162%, respectively),indicating that they were sources of N2O to the atmosphere. High N2Oconcentrations 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 highsalinity (average saturation = 87%). Nitrite had the strongest correlation with N2O forall surveys (r=0.78; p<0.05), suggesting that nitrite is linked to nitrous oxideproduction in estuaries. Fluxes from the River Itchen and Itchen Estuary extrapolatedto the UK systems and compared with other anthropogenic sources of nitrous oxideto the atmosphere, showed that these systems are significant sources and should beincluded in the N2O budget. Incubation experiments were done with sedimentcollected from the Itchen Estuary to investigate N2O production. Initial experimentson a whole core showed the importance of temperature on N2O production, and alsothe potentially complicating impact of biological activity. In subsequent experiments,homogenized and sieved sediment material were used, in which macro benthoswere excluded. N2O fluxes from the sediment were estimated and denitrification wassuggested as the main process producing N2O in the sediments of the incubatedcores