Seasonal nitrous oxide emissions from field soils under reduced tillage, compost application or organic farming

Soil management practices shown to increase carbon sequestration include reduced tillage, amendments of carbon and mixed rotations. As a means to mitigate greenhouse gases, however, the success of these practices will be strongly influenced by nitrous oxide (N2O) emissions that vary with soil wetness. Few seasonal data are available on N2O under different soil managements so we measured seasonal N2O emission in three field experiments between 2006 and 2009 in eastern Scotland. The experimental treatments at the three sites were (1) tillage: no-tillage, minimum tillage, ploughing to 20cm with or without compaction and deep ploughing to 40cm, (2) organic residue amendment: application of municipal green-waste compost or cattle slurry and (3) rotations: stocked and stockless (without manure) organic arable farming rotations. Most seasons were wetter than average with 2009 the wettest, receiving 20-40% more rainfall than average. Nitrous oxide emissions were measured using static closed chambers. There was no statistical evidence, albeit with low statistical power, that reduced tillage affected N2O emissions compared to normal depth ploughing. With organic residue amendments, only in the wet season in 2008 were emissions significantly increased by high rates of green-waste compost (4.5kgN2O-Nha-1) and cattle slurry (5.2kgN2O-Nha-1) compared to the control (1.9kgN2O-Nha-1). In the organic rotations, N2O emissions were greatest after incorporation of the grass/clover treatments, especially during conversion of a stocked rotation to stockless. Emissions from the organic arable crops (1.9kgN2O-Nha-1 in 2006, 3.0kgN2O-Nha-1 in 2007) generally exceeded those from the organic grass/clover (0.8kgN2O-Nha-1 in 2006, 1.1kgN2O-Nha-1 in 2007) except in 2008 when the wet weather delayed manure applications and increased emissions from the grass/clover (2.8kgN2O-Nha-1). Nevertheless, organic grassland was the land use providing the most effective overall mitigation. Although the magnitude of fluxes did not relate particularly well to rainfall differences between seasons, greater rainfall received during some growing seasons increased the differences between tillage, organic residue and crop rotation phase treatments, negating any possible mitigation by timing management operations in dry periods. This was partly attributed to applying tillage and manures late and/or in wet conditions. Of benefit would be different sampling strategies including closed chambers or eddy covariance with standardised methodology. Controlled soil management experiments with a wide geographic spread to specify land management for mitigation also important.