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 drained sandy loam, fertilized with (NH4)2SO4 or KNO3 (100 kg N ha-1) with or without the addition of the nitrification inhibitor dicyandiamide (DCD), were measured. The addition of N fertilizers increased emissions 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 emission 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 denitrification 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. When soils were dry, N2O was produced predominantly by nitrification and DCD reduced emissions by at least 40%. In contrast, in wet conditions denitrification was the main source of N2O and emissions were not inhibited by DCD. Nitric oxide emissions correlated significantly with soil temperature (30 mm depth), the air temperature inside the chamber, soil available NH4+, and were significantly reduced by watering the soil. Apparent activation energies, calculated from the temperature response 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 soil surface. During nitrification the rate of depletion of NH4+-N emitted 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.