Denitrification loss from a loam under a cut ryegrass sward receiving 0, 250 and 500 kg N h a p 1 a -' in four equal amounts was measured during 14 months using the acetylene-inhibition technique. The rate of denitrification responded rapidly to changes in soil water content as affected by rain. Mean rates of denitrification exceeded 0.2 kg N ha-' day-I only when the soil water content was >20% (w/w) and nitrate was > 5 pg N g-in the upper 20 cm of the profile and when soil temperature at 2 cm was >5-8"C. When the soil dried to a water content <20%, denitrification decreased to <0.05 kg N ha-dayp1. Highest rates (up to 2.0 kg N ha-' d a y p 1 ) were observed following application of fertilizer to soil at a water content of about 30% (w/w) in early spring. Denitrification in the control plot during this period was generally about a hundredth of that in plots treated with ammonium nitrate. High rates of N 2 0 loss (up to 0.30 kg N ha-' day-I ) were invariably associated with high rates of denitrification (> 0.2 kg N ha-I day-'). However, within 2-3 weeks following application of fertilizer to the plot receiving 250 kg N ha-a-I the soil acted as a sink for atmospheric N 2 0 when its water content was > 20% and its temperature > 5-8°C. Annual N losses arising from denitrification were 1.6, 1 1.1 and 29.1 kg N ha-I for the plots receiving 0, 250 and 500 kg N ha-' a -I , respectively. More than 60% of the annual loss occurred during a period of 8 weeks when fertilizer was applied to soil with a water content >20%.

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Two field experiments commencing in winter (December) and spring (April) were conducted to determine the fate of nitrogen (N) in cattle slurry following application to grassland. In each experiment three methods of application were used: surface application, and injection f the nitrification inhibitor, nitrapyrin. Slurry was applied at 80 t ha-', (~2 4 8 kg total N ha-' in the winter experiment, and 262kgNha-' in the spring experiment). From slurry applied to the surface, total losses of N through NH, volatilization, measured using a system of wind tunnels, were 77 and 53 kg N ha-' respectively for the winter and spring experiments. Injection reduced the total NH, volatilization loss to -2 kg N ha-'. Following surface application, loss by denitrification, measured using an adaptation of the acetylene-inhibition technique, was 30 and 5 kg N ha-' for the two experiments. Larger denitrification losses were observed for the injected treatments; in the winter experiment the loss from the injected slurry without nitrapyrin was 53 kgN ha-', and with nitrapyrin 23 kg N ha-'. Total denitrification losses for the corresponding injected treatments in the spring experiment were 18 and 14 kg N ha-'. Apparent recoveries of N in grass herbage in both experiments broadly reflected the differences between treatments in total gaseous loss.

The micrometeorological mass balance method has been assessed with a view to its use in the determination of ammonia (NH,) loss from grazed swards. The method involves the measurement of wind speed and the concentration of NH3 in air at different heights above the sward at its windward boundary and a position leeward of the grazed area. The flux of NH3 is calculated from these measurements and a continuous record of wind direction. Quantitative recovery of NH3 from air flows up to 10 litre min-' was achieved using a simple trap containing dilute orthophosphoric acid and a gas dispersion tube. Wind speed measured at a height of 0.25 m at six different positions above a previously grazed sward varied by <2%, The NH3 concentration at the same positions was affected by the distance between the windward edge of the sward and the sampling position (i.e. the fetch) and by heterogeneity in the distribution of sources of NH3 (urine-or dung-affected areas). However, the normalised horizontal flux through 0.25 m (wind speedxNH3 concentration/fetch) varied by less than +8.1% probably due to mixing through fluctuations in wind direction (approximately k30" on the mean) as air passed over the sources of NH3 during each measurement period. Plots of wind speed or NH3 concentration versus logarithm of height indicated that each approximated a linear relationship. This facilitated the calculation of the NH3 flux per unit land area and reduced the total error to about 10%. The total loss of NH3 derived by summing losses during individual sampling intervals of 2 to 18 h within a 24 h period was essentially the same as that derived by averaging wind speeds and NH3 concentrations measured continuously over the same 24 h period. The flux of NH3 from a ryegrass sward grazed by yearling steers ranged from 0.01 to 0.14 kg N ha-' h-' during a 2 day grazing period within a 28 day rotation and during the 5 days following removal of animals. A pronounced diurnal variation was observed in the flux of NH3, the maximum occurring between 13.00 and 20.00 hours on each day. Rainfall and low rates of evapotranspiration reduced the flux to <0.02 kg N ha-' h-'. The total loss of NH3 during 28 days was 20.7 kg N ha-'.