Nitrous oxide (N2O) emissions were measured and compared on 2 typical crop rotations of a grain farm and a dairy farm with feed cropping, over 5 years (from 2012 to 2016) in southwestern France. The annual N2O emissions of the 5 typical rotational crops of the region (summer crops: irrigated maize and sunflower; winter crops: winter wheat, rapeseed and barley) varied from 0.95 ± 0.88 to 7.96 ± 1.73 kgN ha−1, with the highest values observed on the dairy farm plot and for summer crops. N2O emissions were analysed on a daily, monthly, seasonal and annual basis, and correlated with their main direct or indirect drivers, i. e. water and nitrogen (mineral or organic) supply amount, rotational crops, vegetation covering and tillage. We observed a marked seasonal pattern of N2O emission peaks. On average, more than 50% of N2O emissions occurred during spring for summer crops, and more than 40% occurred in winter for winter crops. We have identified agricultural practices that increase N2O emissions. In particular, our results show that when the soil is left bare or with limited crop development, spring mineralization of organic N residues (from previous crop or winter cover crop) results in N losses, partly as emissions of N2O, which are detrimental to agronomic performance (low NUE).
We also conducted an agronomic assessment of annual N2O emissions versus nitrogen surplus and nitrogen use efficiency (NUE), which lead us to discuss agricultural practices that may mitigate N2O emissions while optimizing agronomic and economic performance of crops. Indeed, we point out that N surplus and N fate may be controlled through the right timing of sowing, cover crop, irrigation and fertilization.
