EMISSION OF NITRIC-OXIDE AND NITROUS-OXIDE FROM SOIL UNDER FIELD AND LABORATORY CONDITIONS

A detailed short-term (12 d) laboratory study was carried out io inves tigate the effects of applying animal urine, fertilizer (ammonium nitr ate) and fertilizer + urine on emission of NO and N2O from soil. A com plementary 24 d field study measured the effect of fertilizer: or fert ilizer + sheep grazing on NO and N2O emissions from pasture. The data generated were used to interpret the transformations responsible for t he release of these gases. Application of urine to the soil (at a rate equivalent to 930 kg N ha(-1)) increased the amount of mineral and mi crobial N in the soil. This was followed by increases in emissions of NO (from 0.02 to 1.76 mg NO-N m(-2) d(-1)) and N2O (from 15 to 330 mg N2O-N m(-2) d(-1)). Molar ratios of NO-N-to-N2O-N were very low (<0.00 1 to 0.011) indicating that denitrification was the main process durin g the first 12 d after application. In the laboratory, nitrification w as inhibited during the first 7 d due to an inhibitory effect of the u rine, but even though nitrification was clearly underway 7-12 d after application, denitrification was still the dominant process. The ferti lizer was applied at a lower rate (120 kg N ha(-1)) than the urine. Co nsequently, the effect on soil mineral N was smaller. Nevertheless the fertilizer still increased NO and N2O emission with denitrification t he dominant process. The effects of fertilizer and grazing on NO and N 2O emissions was less obvious in the field compared with the laborator y and fluxes returned to background rates within 4 d. This was attribu ted to the rapid decline in soil mineral N in the field trial due to p lant uptake and leaching, processes that did not occur in the laborato ry. (C) 1998 Elsevier Science Ltd. All rights reserved.