Linking Models and Measurements of Soil Nitrous Oxide Emissions on a Field Scale.

N cycle processes have been modelled extensively through such well known models as DNDC and DAYCENT.  However, where models have been tested in the field, it has usually been in hindsight on data collected for other purposes and by groups not associated with the modellers.  We describe a collaborative undertaking between modellers and field scientists to devise and operate a field experiment in which modellers have stipulated a priori the data needed to validate their models of N transformations and N₂O emissions and the field scientists have tested whether and how well they can measure all the required data.  To keep the experiment uncomplicated, it was conducted on bare soil after the application of urea fertiliser and only one model, APSIM  (an Australian Agricultural Production Systems Model due to  Keating et al., 2003), was tested.  The absence of plants avoided the need to account for their influence on the amount and distribution of water and N in the soil profile.

The experimental field remained bare during the test.  Urea was applied to a circular plot of 50 m diameter at the centre of a larger field at a rate of 200 kg N ha^-1 and incorporated in the top 10 cm of soil at the commencement of the test.  Gas emissions from the plot were measured with a 2 micrometeorological techniques: an integrated horizontal flux technique and a backward Lagrangian stochastic dispersion technique. Both required measurements of wind speed and gas concentration at 5 heights up to 4.8 m.   These, plus soil temperatures, soil moisture contents and evaporation rates at the centre of the plot were measured continuously for 2 months.  Concentrations of N₂O were measured with closed-path FTIR spectroscopy.

Simultaneous emissions of other soil generated gases were also measured, but here we examine agreement between the APSIM model and N₂O emission.