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 to investigate the effects of applying animal urine, fertilizer (ammonium nitrate) and fertilizer+urine on emission of NO and N2O from soil. A complementary 24 d field study measured the effect of fertilizer or fertilizer+sheep grazing on NO and N2O emissions from pasture. The data generated were used to interpret the transformations responsible for the 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 microbial 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 (<0.001 to 0.011) indicating that denitrification was the main process during the first 12 d after application. In the laboratory, nitrification was inhibited during the first 7 d due to an inhibitory effect of the urine, but even though nitrification was clearly underway 7–12 d after application, denitrification was still the dominant process. The fertilizer was applied at a lower rate (120 kg N ha−1) than the urine. Consequently, the effect on soil mineral N was smaller. Nevertheless the fertilizer still increased NO and N2O emission with denitrification the dominant process. The effects of fertilizer and grazing on NO and N2O emissions was less obvious in the field compared with the laboratory and fluxes returned to background rates within 4 d. This was attributed to the rapid decline in soil mineral N in the field trial due to plant uptake and leaching, processes that did not occur in the laboratory.