Crop residues as driver for N2O emissions from a sandy loam soil

Nitrogen (N) cycling within agriculture constitutes a source of direct and indirect emissions of the potent greenhouse gas nitrous oxide (N2O). We analysed relationships between N2O emissions and C and N balances of four arable cropping systems under conventional or organic management within a long-term experiment on a loamy sand soil at Foulum in Denmark. All cropping systems included winter wheat, a leguminous crop (faba bean or grass-clover), potato and spring barley grown in different 4-crop rotations varying in strategies for N supply (fertilizer/manure type and rate, use of catch crops and green manure). Crops in both organic and conventional systems received N at rates below the optimum for crop production. Soil N2O emissions were monitored in 2008–2009 in six selected crops which could be combined with data from other monitoring programs to calculate N2O emission factors for each of the 16 crops and four entire crop rotations. Accumulated annual N2O emissions were on average 0.8 to 0.9 kg N ha−1 y−1 and did not differ significantly between rotations. Soil N inputs in above- and belowground crop residues from main crops and catch crops were quantified by field measurements. Average nitrate-N leaching losses ranged from 39 to 56 kg N ha−1 y−1 and were lowest in rotations with catch crops; leaching was not correlated with N surplus or N input in fertilizer or manure. Crop yields of the organic rotations were 25 to 37% lower than in identical conventional rotations. As a consequence, yield-scaled N2O emissions were lower under conventional management compared with the organic alternatives. There was no significant correlation between N2O emissions and N input in fertilizer/manure, neither for annual emissions nor spring emissions. In contrast, N2O emissions were correlated with N input in residues from the previous main crop and catch crop (r = 0.56, p < 0.01). This indicates that, besides nitrogen, degradable carbon provides an important and estimable driver for N2O emissions in arable cropping systems.