Non-Peer ReviewedThe rate of denitrification, using the acetylene blockage method, was determined before and after irrigation at two sites; at Birsay on a clay-loam soil and at Outlook on a sandy soil. At Birsay, 110 kg N/ha as urea was applied 10 days before seeding and an additional 4 kg N/ha as ammonium phosphate at time of seeding. At Outlook, 50 kg N/ha as ammonium nitrate was applied just before seeding. Before irrigation, the rate of denitrification at both sites was almost undetectable. A sharp increase in the rate, however, occurred at both sites within a few hours after irrigation (approximately 25 mm) and lasted for approximately 24 hrs and 12 hrs at Birsay and Outlook, respectively. At the time of maximum activity, the losses of N (N2O and N2) were estimated to be 50 g ha-1 hr-1 and 3 g ha-1 hr-1 at Birsay and Outlook, respectively. The total amount of N lost per irrigation cycle, due to denitrification, at Birsay and Outlook
were calculated to be 730 g ha-1 and 21g ha-1, respectively. The difference in the amount of N2O evolution at the two sites is partially attributed to the difference in soil type. The water holding capacity at Outlook is lower as compared with the soil at Birsay. Subsequently, the degree of anaerobic conditions, a pre-requisite for denitrification, will be less at Outlook. A lag period of 20 hrs occurred between the application of water and the maximum evolution of N2O of incubated soil cores. This was determined by analyzing incubated soil cores repeatedly over a period of 48 hrs. As the increase in denitrification at Birsay lasted for 24 hrs after the application of water, the maximum rate of denitrification did occur under those field conditions at Birsay. At Outlook, however, where the increase in denitrification only lasted for 12 hrs, the maximum rate of denitrification was not obtained under the existing field conditions. There were 10 irrigation and 7 precipitation events which caused denitrification. According to the percentage of the landform elements and its proper denitrification activity, the total N lost per year at Birsay was estimated to be 120 kg ha-1
