Denitrification loss from a grassland soil in the field receiving different rates of nitrogen as ammonium nitrate

Summary Denitrification loss from a loam under a cut ryegrass sward receiving 0, 250 and 500 kg N ha −1 a −1 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 −1 day −1 only when the soil water content was >20% (w/w) and nitrate was >5μ N g −1 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 −1 day −1 . Highest rates (up to 2.0 kg N ha −1 day −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 O loss (up to 0.30 kg N ha −1 day‐1) were invariably associated with high rates of denitrification (> 0.2 kg N ha −1 day −1 ). However, within 2–3 weeks following application of fertilizer to the plot receiving 250 kg N ha −1 a −1 the soil acted as a sink for atmospheric N 2 O when its water content was >20% and its temperature >5–8°C. Annual N losses arising from denitrification were 1.6, 11.1 and 29.1 kg N ha −1 for the plots receiving 0, 250 and 500 kg N ha −1 a −1 , 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%.