Nitrous oxide emissions from a fertile grassland in Western Norway following the application of inorganic and organic fertilizers

In Norway, 65 % of the agricultural land is under grassland for feeding ruminants. The objective of the present study was to quantify N2O emissions from grassland on a fertile sandy loam in Western Norway, and to estimate the response of seasonal N2O emissions to added inorganic N, cattle slurry (CS) N and clover N. Ammonium nitrate (AN) and CS were applied manually at annual rates of 0, 100, 150, 200 and 250 kg AN-N ha(-1), 80 kg CS-N ha(-1) or as a combination of 200 kg AN-N ha(-1) and 80 kg CS-N ha(-1). Background N2O emissions were five times higher in summer season 2009 than in 2010, but the relative amount of N2O derived from AN was constant in both periods, amounting to 0.11 % of applied N. CS had no measurable impact on N2O emissions in 2009, but 0.15 % of CS-N was emitted as N2O during summer 2010. In the warm year of 2009, which included a drought period, 1-24 % of the N2O emissions were attributed to the effect of clover depending on fertilization. Clover had no effect on N2O fluxes in the cool and moist year 2010. Our results suggest that N2O emissions in fertile Norwegian grasslands are to a great extent controlled by inter-annual variations in background emissions and variable contribution of biologically fixed N and CS-N.