Effect of fertilization rate and ploughing time on nitrous oxide emissions in a long-term cereal trail in south east Norway

Fertilization and soil tillage are important factors for crop production and N2O emissions. Nothing is known about fertilizer-induced N2O emissions (FIEs) in cereal production under cool-temperate conditions in Norway. Here, we report N2O emissions measured throughout two vegetation periods in a long-term cereal trail in SE Norway, comparing three nitrogen (N) fertilization rates (0, 60 and 120 kg N ha−1) under long-term autumn and spring-ploughing regimes. Emission rates were higher in undisturbed plots directly after spring thaw before ploughing (35–52 μg N2O-N m−2 h−1) than those in plots ploughed in previous autumn (18–26 μg N2O-N m−2 h−1), suggesting that ploughing date affected post-thaw availability of N from crop residues in the following year. In contrary, autumn-ploughed plots had higher N2O emissions during drying–rewetting events in summer, cancelling out any effect of ploughing date on growing season N2O emissions. Averaged FIE differed greatly between the years, with 0.37–0.40 and 0.72–0.96 % of fertilizer N lost as N2O in 2009 and 2010, respectively, reflecting different weather conditions in the 2 years. Growing season N2O emission response was not stable over the tested fertilization range but increased disproportionally above an optimum fertilization rate of 90–100 kg N ha−1, probably because of incomplete N uptake by the crop. This would suggest that decreasing the N fertilization rate by 25 % (from 120 to an optimum level of 90 kg N ha−1) could reduce N2O emission by 30 % with only minor reduction in grain yield (6–8 %).