The European Union is making real efforts to reduce its greenhouse gas (GHG) emissions. Over the past two decades, emissions have gone down by 16%, whereas the economy has grown by 40% over the same period.
The Europe 2020 Strategy for smart, sustainable and inclusive growth includes five headline targets that set out where the EU should be in 2020. One of them relates to climate and energy: Member States have committed themselves to reduce GHG emissions by 20%, increasing the share of renewables in the EU's energy mix to 20%, and achieving the 20% energy efficiency target by 2020. In addition, in March 2011 the European Commission released the communication “A Roadmap for moving to a competitive low carbon economy in 2050”, that foresee a significant reduction of agricultural non-CO2 gases (N2O and CH4) from -42 to -49 % until 2050 respect to 1990 emissions. While the major GHG issue for the total economy in industrialized countries is CO2, mainly related to energy production and use, for agriculture the most important GHG is nitrous oxide (N2O), mainly from soil and N input to crop and soil systems. Even if nitrous oxide is a small part of the overall GHG emissions (8% at world scale), agriculture is considered to be its major source through soil management and fertilizer use. The reduction of greenhouse gas emissions will be considered not only in the European environmental policy but also in the definition of the next Commmon Agricultural Policy (CAP) after 2013. Indeed, an important task is the integration of climate change mitigation actions with agricultural policy. The measures highlighted in the Roadmap include further sustainable efficiency gains, efficient fertilizer use, biogasification of organic manure, improved manure management, better fodder, improved livestock productivity and local diversification and commercialisation of production, as well as maximising the benefits of extensive farming.
All of these measures are understood to have potential to reduce the non-CO2 emissions (chiefly nitrous oxide and methane) which are the quantitative milestones for the agricultural sector.Nitrous oxide is a chemically stable molecule with a mean atmospheric life time of ~ 120 years. This fact contributes to the efficiency of N2O as an absorber of infrared radiation emitted by the earth, i.e., as a greenhouse gas. The global warming potential of N2O has been estimated at 298 times that of carbon dioxide by the Intergovernmental Panel on Climate Change (IPCC, 2007). However, due its atmospheric concentration of ~ 315 ppb, which is 1000 times less than that of CO2, presently N2O is estimated to contribute only about 6% to the total radiative forcing of the atmosphere. Ice core and in situ data indicate a steady rise in N2O atmospheric concentration since 1850, and more rapid increases in the second-half of the 20th century, such that N2O concentration is presently about 12% above the pre-industrial levels while continuing to rise at ~ 0.7ppb/year. The only known sink for N2O is the photolytic oxidation of N2O in the stratosphere (Crutzen, 1981).
According to Mosier et al. (1998) and Kroeze et al. (1999), total emissions are estimated to 17.7 Tg/year with a contribution of 9.6 Tg/year from natural sources and 8.1 Tg/year from anthropogenic sources. The anthropogenic or natural emissions from soils are the dominant N2O source with a total contribution of 10.2 Tg/year, i.e., 60% of the global sources. Use of nitrogen fertilizers on arable crops largely favors N2O production.
Bacterially-mediated processes of denitrification and nitrification are the dominant sources of N2O in most soils.
Biological denitrificiation is the reduction of nitrate (NO3-) or nitrite (NO2-) to gaseous NO, N2O and N2 mainly governed by heterotrophic bacteria. In absence or shortage of molecular O2, these bacteria use NO3- and NO2- as alternative electron acceptors. Nitrification is the biological oxidation of ammonium (NH4+) to NO2- or NO3- primarily mediated by chemo-autotrophic bacteria under aerobic conditions.
Magnitude of nitrification and denitrification are largely controlled by soil and climate characteristics, although agricultural management may also interfere with soil processes and thus influence N availability and consequently the relative rates of N2O emissions. Soil water content also affects N2O release. Nitrification is a relatively constant process across ecosystems, whereas denitrification rates are temporally and spatially very variable.
Various studies indicate that N2O is mainly produced by denitrification in temperate zones. N2O is produced in “hot spots” created by decomposing organic matter generating anaerobic micro-sites. The N2O emissions are also very sporadic. Laville et al. (2011) report that 63% of the time N2O fluxes could be smaller than the detection limit of many methodologies used to measure N2O soil emissions. The bulk of the annual N2O budget is then related to emission peaks that typically occur both after application of nitrogen fertilizer and during large mineralization of crop residues. The sporadic nature of the N2O emissions is largely related to their response to soil water content, according to an exponential relation above a given threshold. This large spatial and temporal variability of N2O emissions makes very difficult to evaluate annual budgets from integrated areas (agricultural fields, meadows, and forests). Based on previous considerations, there is an urgent need for measuring systems allowing both continuously monitoring in a given site of N2O flux and assessment of its spatial variability, with a sensitivity and reliability higher than those of available methodologies.