AMMONIA VOLATILIZATION FROM TOP-DRESSING WHEAT WITH UREA WITH AND WITHOUT UREASE INHIBITOR EDIS, R.B.1, TURNER, D.A. 1 and CHEN, D.1 1 Department of Resource Management and Geography, Faculty of Land and Food Resources, The University of Melbourne, Victoria, 3010, Australia Introduction Ammonia (NH3) volatilization represents a loss of fertility from agricultural lands, a degradation of air quality and, upon deposition of the nitrogen, an elevation of fertility offsite. Some of the re- deposited nitrogen will be converted to nitrous oxide; therefore NH3 is an indirect greenhouse gas. Substantial NH3 volatilization is likely when urea is applied to the soil surface without follow up wash-in or cultivation, as is the case with top-dressing wheat crops. Experiments were conducted in a wheat field near Horsham, Victoria, Australia to quantify NH3 volatilization from topdressing wheat growing in a Hypocalcic Calcarosol. A field experiment was conducted on a commercial farm at Gerang Gerung, western Victoria in August 2007 to investigate NH3 loss from winter fertiliser application to Rosella wheat. Two 50 m diameter test plots were established: one received 80 kg N/ha urea, the other 80 kg N/ha Green Urea (urea with urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT, also known as Agrotain) (Incitec Pivot)). Ammonia volatilisation from the fertilised plots was measured using space shuttles (Leuning et al. 1985) from July 31 (fertiliser application) to August 11, while emissions were significant, followed by a single week-long measurement to observe any further emissions. Soil (top 5mm) was sampled over the course of the experiment for: Ammonium (NH4+) – to detect the rate of breakdown/hydrolysis of the urea; nitrate (NO3-) – as an indication of the amount of nitrogen undergoing nitrification (the process by which ammonium is converted to nitrate – the main source of plant available nitrogen), and; urea content. Results Prior to fertiliser application, soil NH4+ and NO3- concentrations were 0.8 and 4.4 mg kg-1 in both the urea and Green Urea plots (Fig. 1a). Soil urea concentrations in both treatments fell sharply after fertiliser application (July 31), however the decline was more rapid and complete in the urea treatment (Fig 1b). Due to urea hydrolysis soil NH4+ concentrations had increased to 215 mg kg-1 in the urea plot one week after fertiliser application. After the first week there was a sharp decline as hydrolysis ceased and NH4+ was either volatilised as NH3 or nitrified to NO3-. In the Green Urea treatment the soil NH4+ concentration was lower than in the urea plots due to the urease inhibitor (Fig 1b). Soil NH4+ concentrations increased to only 77 mg kg-1 in the week after Green Urea application and continued to rise until the end of the measurement period (August 16). Soil NO3- concentrations were similar for both urea and Green Urea and gradually increased over the measurement period for both treatments, indicating nitrification of NH4+ to NO3- as the applied urea was hydrolysed. NO3 (U) NH4 (U) NO3 (GU) NH4 (GU) Urea (U) Urea (GU) 250 1600 200 (a) (b) Mineral N (mgN/kg) 1200 Urea (mgN/kg) 150 800 100 400 50 0 0 30/07 1/08 3/08 5/08 7/08 9/08 11/08 13/08 15/08 30/07 1/08 3/08 5/08 7/08 9/08 11/08 13/08 15/08 Date Date Figure 1. Top 5 mm soil (a) nitrate (NO3) and ammonium (NH4) and (b) urea concentrations for urea (U) and green urea (GU) treatments (mg N kg-1). Due to the higher concentration of NH4+ in the surface soil, NH3 volatilisation was greater in the urea treatment than in the Green Urea treatment (Fig. 2a & b). Over the measurement period 6.9 kg N ha-1 (8.6% of fertiliser applied) was volatilised as NH3 from the urea treatment compared to 2 kg N ha-1 (2.5%) for the Green Urea treatment. These losses were probably driven by the very light rainfall events over the course of the experiment. These were sufficient to moisten the surface soil, where NH3 could be easily volatilised, but not sufficient to wash the urea and NH4+ deeper into the soil profile where NH3 is not easily volatilised. Urea Green urea (a) U GU (b) 0.6 8.0 NH3 loss (kg N ha-1) NH3 loss (kg N ha-1) 6.0 0.4 4.0 0.2 2.0 0.0 0.0 31/07 2/08 4/08 6/08 8/08 10/08 12/08 14/08 16/08 31/07 2/08 4/08 6/08 8/08 10/08 12/08 14/08 16/08 Date Date Figure 2. Cumulative NH3 loss (kg N ha-1) (a) individual measurement periods and (b) cumulative losses for urea (U) and green urea (GU) treatments. Reference Leuning R, Freney JR, Denmead OT, Simpson JR (1985) A sampler for measuring atmospheric ammonia flux. Atmospheric Environment 19, 1117-1124.