Nitrous oxide (N2O) in the Seine river and basin: Observations and budgets

In the Seine basin (71 730 km2) intensive use of N fertilizers in agriculture and the release of reduced N in urban wastewater, not fully eliminated in treatment plants, have deeply perturbed the regional nitrogen cycle, namely by promoting intense nitrification and denitrification, two key microbial processes involved in N2O emission.Sampling surveys in the whole Seine drainage network have shown that N2O water concentrations are always far above saturation with respect to atmospheric concentrations. Highest values are found in 1st order streams (1–4 μg N l−1), decrease by out-gazing down to 4th order rivers (0.5 μg N l−1) and increase again in polluted higher order rivers (4 μg N l−1). Calculated values of gas transfer velocities and the observed N2O supersaturation by stream order was used to estimate N2O emissions for the whole water surface area of the Seine drainage network to 100–200 × 103 kg N-N2O yr−1. The results show that a major fraction of the emission occurs in head-water streams. On the basis of previous work, the emission by wastewater treatment plants – WWTPs – has been estimated at the scale of the Seine basin, to 150–260 × 103 kg N-N2O yr−1, i.e. to be in the same range as N2O emissions from the entire water surface.Using an emission factor approach, based on an intensive literature survey considering grassland, cropland and forest soils distribution, the total direct emissions of N2O of the Seine basin was estimated to 10 400 × 103 kg N-N2O yr−1 among which 2500 × 103 kg N-N2O yr−1 are issued from the forested soils and 7900 × 103 kg N-N2O yr−1 from agricultural land. The latter figure agrees well with the [Intergovernmental Panel on Climate Change (IPCC), 2006. AFOLU, N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application, vol. 4, 54 pp (Chapter 11)] estimate of 8250 × 103 kg N-N2O yr−1.The quantified indirect emissions (from the river-atmosphere interface and from the WWTPs) thus represent 3–6% of the direct agricultural emissions of the Seine basin. However, N2O indirect emissions from ‘riparian zones’ have only been roughly calculated and, thus, are remaining uncertain. A conservative estimate might increase the indirect emissions to about 13–17% of the direct ones.