NITRIFICATION AND DENITRIFICATION AS SOURCES OF NITRIC-OXIDE AND NITROUS-OXIDE IN A SANDY LOAM SOIL

Emissions of nitric oxide (NO) and nitrous oxide (N2O) from a freely d rained sandy loam, fertilized with (NH4)2SO4 or KNO3 (100 kg N ha-1) w ith or without the addition of the nitrification inhibitor dicyandiami de (DCD), were measured. The addition of N fertilizers increased emiss ions of NO and N2O. For plots fertilized with (NH4)2SO4, NO emissions increased from 2.4 to 46.9 ng NO-N m-2 s-1 (2.1-40.5 g NO-N ha-1 day-1 ), in the first 7 days after fertilizer application. Nitrous oxide emi ssion rates were considerably lower, ranging from 0.95 to 7.4 ng N2O-N m-2 s-1 (0.82-6.4 g N2O-N ha-1 day-1). Nitrification rather than deni trification was the source of the NO emitted from the soil; additions of DCD inhibited the emissions by at least 92%. Nitrous oxide, on the other hand, was a product of both nitrification and denitrification. W hen soils were dry, N2O was produced predominantly by nitrification an d DCD reduced emissions by at least 40%. In contrast, in wet condition s denitrification was the main source of N2O and emissions were not in hibited by DCD. Nitric oxide emissions correlated significantly with s oil temperature (30 mm depth), the air temperature inside the chamber, soil available NH4+, and were significantly reduced by watering the s oil. Apparent activation energies, calculated from the temperature res ponse in the NO emission rates, ranged from 30 to 71 kJ mol-1. It was concluded from the close links between air temperature in the chamber and the NO emission rates that the NO was produced very close to the s oil surface. During nitrification the rate of depletion of NH4+-N emit ted as NO-N was 5.5 x 10(-5) s-1. It was estimated that for cultivated fields 0.15-0.75% of the applied NH4+ fertilizer is released as NO.