Emissions of nitrous oxide (N2O) and nitric oxide (NO) have been identified as one of the most important sources of atmospheric pollution from grasslands. Soils are major sources for the production of N2O and NO, which are by-products or intermediate products of microbial nitrification and denitrification processes. Some studies have tried to evaluate the importance of denitrification or nitrification in the formation of N2O or NO but there are few that have considered emissions of both gases as affected by a wide range of different factors. In this study, the importance of a number of factors (soil moisture, fertiliser type and temperature) was determined for N2O and NO emissions. Nitrous oxide and NO evolution in time and the possibility of using the ratio NO:N2O as an indicator for the processes involved were also explored. Dinitrogen (N2) and ammonia (NH3) emissions were estimated and a mass balance for N fluxes was performed. Nitrous oxide and NO were produced by nitrification and denitrification in soils fertilised with NH+4 and by denitrification in solis fertilised with NH−3. Water content in the soil was the most important factor affecting N2O and NO emissions. Our N2O and NO data were fitted to quadratic (r=0.8) and negative exponential (r=0.7) equations, respectively. A long lag phase was observed for the N2O emitted from soils fertilised with N2O emitted from soils fertilised with NO−3 (denitrification), which was not observed for the soils fertilised with NH+4 (nitrification) and was possibly due to a greater inhibiting effect of low temperatures on microbial activity controlling denitrification rather than on nitrification. The use of the NO:N2O ratio as a possible indicator of denitrification or nitrification in the formation of N2O and NO was discounted for soils fertilised with NO−3. The N mass balance indicated that about 50 kg N ha−1 was immobilised by microorganisms and/or taken up by plant roots, and that most of the losses ocurred in wet soils (WFPS >60%) as N2 and NH3 losses (>55%).