Emission of nitrous oxide and other trace gases during composting of grass and green waste

The emission of trace gases during composting of green waste from land maintenance (fresh cuttings of mixed herbage from fallow land) were studied. Concentrations of nitrous oxide and other trace gases were measured in experimental compost heaps by means of an infrared gas analyser and a high-resolution FT-IR spectrometer. It was verified that the maintenance of aerobic conditions is essential to keep emissions of methane and nitrous oxide at a low rate. Estimates made using a simple air transport model, indicated that the emission of nitrous oxide during composting of green waste from land maintenance was about 0.5% of the total nitrogen content of the initial material. Carbon monoxide was also detected in the compost air and its emission during biodegradation corresponded to about 0.04% of the initial carbon content of green waste. For a detailed evaluation of the findings during field experiments, laboratory apparatus for measuring the composition of compost air was developed. The measurements were performed at a temperature of 35 degrees C and at different ventilation rates. Moistened plant material (dry cuttings of mixed herbage from fallow land) amended with lime saltpetre was used as compost substrate. At a ventilation rate of 100 cm(3)/min of air per kg of substrate, the maximum emission rate of nitrous oxide was 2.2 mg/h per kg of substrate. The maximum emission rate increased to 13.3 mg/h kg, when the ventilation rate was lowered to 20 cm(3)/min kg. The emission rate of carbon monoxide was about 40 mu g/h per kg of substrate at the low ventilation rate and rose above 200 mu g/hkg at the higher ventilation rate. (C) 1998 Silsoe Research Institute.