AMMONIA VOLATILIZATION FROM TOP-DRESSING WHEAT WITH UREA WITH AND

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					 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.

				
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