Nitrous oxide emissions from organic farming: the importance of well-timed ley cultivation

Summary

The period after ploughing of grass-clover within a ley-arable rotation is when nitrogen accumulated during ley cropping is most vulnerable to loss. I investigated how date of ploughing and date of cessation of grazing before ploughing influenced losses of nitrogen as nitrous oxide during establishment of the first cereal crop. Crop nitrogen uptake was also assessed. Nitrous oxide (N2O) emissions in the 1-2 month period after autumn or spring ploughing or after sowing were typically between 20 and 150 g N ha-1 day-1 and increased with temperature and rainfall. Tillage and sowing operations on previously ploughed soil stimulated N2O emissions even several months after ploughing. Cumulative N2O emissions were highest (~8 kg ha-1 over 17 months) after late cessation of grazing and late ploughing, and lowest (~5.5 kg ha-1) after early cessation of grazing and early ploughing. The optimum time of ploughing appears to be midwinter when the cold restricts nitrogen mineralisation, but sufficient nitrogen subsequently becomes available for early spring crop growth and satisfactory N offtake. Restricting tillage operations to cool conditions should help to reduce N2O emissions. However, the soil should also be dry enough to reduce the risk of damage to soil structure.