Nitrous oxide emissions, cereal growth, N recovery and soil nitrogen status after ploughing organically managed grass/clover swards

The period after ploughing of grass-clover leys within a ley-arable rotation is when nitrogen accumulated during the ley phase is most vulnerable to loss. We investigated how ploughing date and timing of cessation of grazing before ploughing affected nitrous oxide (N2O) losses of the first cereal crop. Ploughing dates were July and October for a winter wheat pilot study and January and March for spring barley in the main experiment. Timings of cessation of grazing (main experiment only) were October, January and March. Spring barley yield, nitrogen uptake and soil mineral nitrogen were also assessed. A separate large-scale laboratory incubation was made to assess the effect of temperature and rainfall on nitrous oxide emissions and nitrate leaching under controlled conditions. Nitrous oxide emissions in the 1- to 2-month period after autumn or spring ploughing, or sowing were typically between 20 and 150 g N ha(-1) day(-1) and increased with temperature and rainfall. Tillage for crop establishment stimulated N2O emissions with up to 2.1 kg N ha(-1) released in the month after spring tillage. Cumulative nitrous oxide emissions were greatest (similar to 8 kg ha(-1) over 17 months) after cessation of grazing in March before March ploughing, and lowest (similar to 5.5 kg ha(-1)) after cessation of grazing in January before January ploughing. These losses were 1.2-3.9% of the N inputs. In the laboratory study, winter ploughing stimulated nitrate leaching more than nitrous oxide emissions. The optimum time of ploughing appears to be early spring when the cold restricts nitrogen mineralization initially, but sufficient nitrogen becomes available for early crop growth and satisfactory N offtake as temperature increases. Early cessation of grazing is advantageous in leaving an adequate supply of residues of good quality (narrow C:N ratio) for ploughing-in. Restricting tillage operations to cool, dry conditions, being aware of possible compaction and increasing the use of undersown grass-clover should improve the sustainability of organic farming.