N2O emission from grass-clover swards is largely unaffected by recently fixed N2

Biological N2 fixation in grass-legume swards provides a major N input to many organic farming systems, but knowledge is sparse regarding the amount of fixed N2 lost from the grasslands as N2O. Nitrifying and denitrifying bacteria are the main contributors to the N2O production in soils. According to the current guidelines issued by The Intergovernmental Panel on Climate Change (IPCC), biological N2 fixation in grass-legume swards should not be considered as a source of N2O in the national greenhouse gas inventories (IPCC, 1997), partly because of uncertainties in quantifying the N2 fixation in the grasslands (Mosier et al., 1998). Hence, the agricultural greenhouse gas release may presently be underestimated. As organic farming to a very large extent utilises grass-legume mixtures as N source, the contribution from organic farming systems in particular may be underestimated. For all other N inputs (viz. inorganic fertiliser, manure and biological N2 fixation in other crops), it is assumed that 1.25 % of the total N supply is emitted as N2O (IPCC, 1997). This standard emission factor relies on experiments with fertiliser and manure only (Bouwman, 1996), and could thus be considerably unrepresentative for biologically fixed N2. Therefore, as part of the DINOG project, a 15N2-tracer-experiment was initiated, on grass-clover to assess the contribution o