ABSTRACT Land emissions of N 2 O, CO 2 and NH 3 have been subject to little study under organic systems, yet form important aspects of sustainability of such systems. We describe innovative methods developed at SAC to assess trace gas emission using both automatic closed chamber systems (intensive, short term monitoring) and manually-operated closed chamber systems (occasional, long term monitoring). Long-term data were collected from organic ley-arable rotation trials in North-east of Scotland. Short term data were collected to show the effect of timing and depth of ploughing-out of the ley phase on gas emissions. Ploughing gave a shortterm stimulation of CO 2 and, more markedly, of N 2 O emission. Emissions of N 2 O from organic grass-clover leys were considerably lower than from conventional grass. However, some N 2 O emissions from organic arable are higher than from conventional systems, particularly in the first year after ploughing out ley. Ammonia emissions after spreading manure on grass were significant in the summer, though only short-lived
