Effects of cover crops on nitrous oxide (N2O) emissions in cereal cropping

More than 60% of anthropogenic nitrous oxide (N2O) emissions are attributed to agricultural activities. N2O production in soils highly depends on the N availability along with other factors such as soil moisture content, which provide suitable conditions for nitrification and denitrification. Cover crops (CCs) are used in agriculture to take up the excess N from the field to reduce nitrate leaching and assimilate carbon, which has been reported to result in increased soil organic carbon stocks. Despite these promising benefits, high N2O emissions are recorded from agricultural soils containing CCs, particularly during winter and early spring. The freeze and thaw cycles in these off-seasons may not only cause the release of trapped N2O, but also stimulate its de novo formation. Hence, due to high off-season emissions, the benefit of carbon capture by the CCs can be offset. The aims of this thesis were to investigate the effect of different cover crop types (legumes, non-legumes, brassicas, and herb mixture) on N2O emissions, and to assess potential trade-offs between carbon accretion by CCs and N2O emissions. N2O fluxes of approximately one year were analyzed to assess seasonal and treatment effects of CCs. The results showed that the effect of having CCs in the cereal cropping system was species dependent. Ryegrass (Italian and perennial) and a herb mixture suppressed N2O emissions compared with the control, particularly during winter, while oilseed radish increased N2O emissions. Legumes (winter and summer vetch, Phacelia) grew poorly and had no effect. Nitrous oxide emissions showed strong seasonal patterns with off-season emissions accounting for more than 80% of the annual emission. Among the different cover crop species, the highest cumulative N2O emissions were recorded from oilseed radish (10.5–14.2 kg N2O-N ha-1 y-1), which is frost-killed and has a low C/N ratio. Perennial ryegrass (2.6 kg N2O-N ha-1 y-1) and the herb mixture (3.54 kg N2O-N ha-1 y-1) had the lowest emissions. Assuming an extra C sequestration by CCs of 320 kg C ha-1 yr-1, increase in N2O emissions by CCs must be kept below 2.5 kg N2O-N ha-1 y-1 to avoid offsetting the expected C gain. Oilseed radish increased N2O emissions by more than 2.5 kg N2O-N ha-1 y-1, mainly because of large winter emissions, and can therefore not be recommended for enhancing soil C sequestration under Nordic conditions. All other CCs had no effect or reduced N2O emissions, supporting the idea that cover crops can be used to combat GHG emissions by “carbon farming”