Emission of nitrogen oxides and ammonia from varying rates of applied synthetic urine and correlations with soil chemistry

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 NOx 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 NH3(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.