The mineralisation and fate of nitrogen following ploughing of grass and grass-clover swards

This project aimed to investigate the release of mineral N following the ploughing of clover-rich and grass-dominated swards, previously subject either to cutting or grazing regimes. The hypotheses tested were firstly that N mineralisation and losses following incorporation of grass-clover swards are greater than from grass swards, and secondly that N mineralisation and losses following incorporation of previously grazed swards are greater than from previously cut swards. Following ploughing of previously grazed swards in 1992 and swards that had been subjected to an unfertilised, ungrazed regime in 1993, N uptake, N leaching losses (measured by soil solution samplers with drainage estimation from a nearby experiment) and N₂O losses (measured by the closed chamber method) were determined on both resown and fallow plots. Results showed: (1) higher N release after ploughing from the grass-fallow treatment (449 kg N ha^-1) than from the grass-clover fallow treatment (244 kg N ha^-1) over 18 months; (2) the net release of N after ploughing and reseeding, compared with a continued unfertilised sward, was about 85 kg ha^-1 for the grass-clover plots and 140 kg ha^-1 for the grass-only plots, over the following 18 months. Of this, the net releases in the second cropping season after incorporation were 19 and 25 kg N ha^-1 on the resown grass-clover and grass-only plots, respectively; (3) the net release of mineral N after ploughing in 1993/1994, when swards had not been grazed for over a year, was only about 40 kg ha^-1 and no effect of the previous sward was evident; (4) in the 7 weeks after the 1992 ploughing, there was a considerable short-term input of N₂O to the atmosphere (1.5-3.7 kg N ha^-1), due to the supply of readily available C. Leaving swards ungrazed and unfertilised over winter before ploughing in spring has the potential to reduce such emissions considerably. We conclude that N release following cultivation of grazed swards is more a function of grazing intensity and history prior to ploughing rather than of sward composition.