Abstract

Synthetic urine was applied at 5 rates, from 0 to 1000 kg synthetic urine-N/ha, to a pasture soil under controlled laboratory conditions. Gaseous emissions of NO_x and NH₃ were monitored for up to 21 days following application using selected ion flow tube mass spectrometry with N₂O measured using electron-capture gas chromatography. During this period soil replicates were destructively sampled to measure changes in soil pH and inorganic-N concentrations. Comparisons were made between measured soil variables, calculated soil concentrations of NH_3(g), HNO₂, and the measured gas fluxes. At N rates up to 500 kg N/ha, inorganic-N concentrations increased as nitrification progressed over time. With the exception of the 1000 kg N/ha treatment, NO production followed the pattern of increasing nitrification, reaching a maximum of 905 ng NO-N/cm².h in the 500 kg N/ha treatment 14 days after synthetic urine application. At this time the NO flux was associated best with soil pH, NH₄⁺, and NO₂^– levels. Over 21 days the maximum cumulative loss as NO-N and N₂O-N occurred under the 100 kg N/ha urine treatment, with 6.6 and 6.4% of N applied lost as gas, respectively. NO₂ gas fluxes paralleled the NO emissions but were an order of magnitude smaller. Nitrification was inhibited in the 1000 kg N/ha treatment due to the sustained high ammoniacal-N and pH conditions present. These conditions prolonged the NH₃ volatilisation from this treatment. NH₃ volatilisation, as determined by selected ion flow tube-mass spectrometry, was linearly related to calculated soil NH₃ gas concentrations up to 500 kg N/ha on Day 1.