Indirect emissions of nitrous oxide in a cropland watershed with contrasting hydrology in central France.

Nitrous oxide (NO) is an important greenhouse gas. Its atmospheric concentration have increased with the industrialisation and the use of N fertilizer. The contribution of freshwater systems to NO emissions is still very uncertain, while regional transfer of nitrogen depends on soil and hydrology. Riverine and spring NO dissolved in water was therefore measured over two years in the 3453 km Haut-Loir watershed (France). This temperate cropland watershed is characterized by two different hydrological systems east and west of the Loir River. The eastern rivers, fed by the emergence of the deep Beauce aquifer, exhibited significantly higher dissolved NO concentrations (Beauce region, mean: 2.93 μg-N L) than the western rivers (Perche region, mean: 0.87 μg-N L), which were largely influenced by runoff during winter flooding. The eastern rivers had large nitrate concentrations all over the year; in the Perche, nitrate underwent a seasonal cycle with large loads during winter floods, but there were no consistent seasonal patterns in NO. The ratios of NO in excess of equilibrium on nitrate, often used as a proxy of emission factor (EF), were much smaller than the default IPCC values, both for rivers (0.014% versus 0.25% for IPCC EF) and the Loir spring (0.085% versus 0.6% for the IPCC EF for groundwater and springs). EF were significantly different between the two parts of the watershed only in winter, because of the seasonal variability of NO. Moreover dissolved NO is controlled not only by NO, as it is considered in the calculation of the EF, but also by water pH and dissolved organic carbon. A good prediction of dissolved NO was obtained using these physicochemical variables and hydrological regions. Thus, these results suggest that the spatial variability of riverine NO depends on local hydrology, while further research is needed to understand the seasonal variability.