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

Nitrous oxide (N2O) 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 N2O emissions is still very uncertain, while regional transfer of nitrogen depends on soil and hydrology. Riverine and spring N2O dissolved in water was therefore measured over two years in the 3453 km2 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 N2O concentrations (Beauce region, mean: 2.93 µg-N L-1) than the western rivers (Perche region, mean: 0.87 µg-N L-1), 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 N2O. The ratios of N2O 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 EF5r) and the Loir spring (0.085% versus 0.6% for the IPCC EF5g for groundwater and springs). EF5r were significantly different between the two parts of the watershed only in winter, because of the seasonal variability of NO3-. Moreover dissolved N2O is controlled not only by NO3-, as it is considered in the calculation of the EF5, but also by water pH and dissolved organic carbon. A good prediction of dissolved N2O was obtained using these physicochemical variables and hydrological regions. Thus, these results suggest that the spatial variability of riverine N2O depends on local hydrology, while further research is needed to understand the seasonal variability.

Management of soil pH promotes nitrous oxide reduction and thus mitigates soil emissions of this greenhouse gas.

While concerns about human-induced effects on the Earth's climate have mainly concentrated on carbon dioxide (CO2) and methane (CH4), reducing anthropogenic nitrous oxide (N2O) flux, mainly of agricultural origin, also represents an opportunity for substantial mitigation. To develop a solution that induces neither the transfer of nitrogen pollution nor decreases agricultural production, we specifically investigated the last step of the denitrification pathway, the N2O reduction path, in soils. We first observed that this path is mainly driven by soil pH and is progressively inhibited when pH is lower than 6.8. During field experiments, we observed that liming acidic soils to neutrality made N2O reduction more efficient and decreased soil N2O emissions. As we estimated acidic fertilized soils to represent 37% [27-50%] of French soils, we calculated that liming could potentially decrease France's total N2O emissions by 15.7% [8.3-21.2%]. Nevertheless, due to the different possible other impacts of liming, we currently recommend that the deployment of this solution to mitigate N2O emission should be based on local studies that take into account agronomic, environmental and economic aspects.

A snapshot of soil water composition as an indicator of contrasted redox environments in a hedged farmland plot.

While soil water composition has long been recognised as being related to soil type (characteristics of the horizons), the influence of structures resulting from agricultural activities (hedges, ditches, wheel ruts, etc) is still under discussion. This work was undertaken to show that a snapshot of spatial variability of the geochemical characteristics of soil water at the scale of a plot can improve our understanding of soil geochemistry in a farmland setting. We selected a 3 hectare hedged plot located on a hillside, limited by a stream and used as pasture where soils have developed in weathered gneiss. The water depth, electrical conductivity, major ions, temperature, pH, dissolved organic carbon (DOC) content, dissolved oxygen content, fluorescence, alkalinity, Fe(2+), Mn(2+), NO(2)(-), Fe(III) and F(-) contents were measured in 62 auger holes randomly drilled on the site. Four sectors were identified in order to describe the distribution of the main geochemical parameters. Electrical conductivity and some major ions, especially sulphate, had larger concentrations near hedges where oxic conditions prevailed. These features were attributed to the impact of the linear anthropogenic network on the circulation of subsurface soil waters and evapo-transpiration and represent sector I. Dissolved Mn was an indicator of well channelled runoff subsurfaces facilitating the circulation of more highly reducing water (sector III), while DOC probably marked areas drained less well, with a prolonged contact time between soil solutions and organic topsoil horizons (sector II). The presence of dissolved Mn and Fe(II) indicates bottomland anoxic conditions (sector IV). It is concluded that a survey of the chemical composition of soil water may be a direct approach to show the influence of permanent structures on current soil properties and dynamics.