Nitrogen cycling

Exercise 3: Nitrogen cycling

In this exercise you will observe the fate of fertiliser nitrogen in a fallow situation: Urea to ammonium to nitrate and the loss of soil nitrate via denitrification.

This simulation will introduce us to editing a simple Manager rule and to more advanced features of graphing simulation results. Firstly we need to set up our weather and soil data. The simulation is on Brookstead,Anchorfield soil in the Dalby area.

The examples assume you have read and walked through the previous document: How to Build, Run and Graph a Simulation

1. Start with a new simulation based on Continuous Wheat Simulation
2. Change simulation name to Tipton N Fallow
3. Save the file as Tipton N Fallow.apsim
4. Choose Dalby weather. (C:\Program Files\Apsim73-r1387\Examples\MetFiles)
5. Starting date 1/1/1989 Ending date: 31/12/1989
6. Choose "Black Vertosol-Bongeen (Tipton No116)" soil. (Soils->Australia->Queensland->Darling Downs) (remember to rename it)
7. Set the Starting water to 50% full. Set it as evenly distributed.
8. Set the Starting nitrogen to 19 kg/ha NO₃ and 0 kg/ha NH₄, evenly distributed.
9. Set the initial surface organic matter to 1000 kg/ha wheat.
10. Remove all manager rules under the Manager folder but leave the folder itself.
11. Drag a Fertilise on fixed date to your Manager folder component. (Standard toolbox->Management->Manager (common tasks) )
12. Change the fertiliser management parameters to apply 100 kg/ha of urea_N on 10-Jan. (leave the "Don't add fertiliser if N in top 2 layers exceeds (kg/ha)" property, and set the "Module used to apply the fertiliser" to "fertiliser")
13. Make sure your simulation contains a Fertiliser component in your paddock. Even though it doesn't have any changeable properties it is still necessary when fertiliser is to be applied.
14. Choose these variables to report:

    Component	Variable name
    	dd/mm/yyyy as Date
    Clock	Day
    	Year
    Met	Rain
    Soil (whatever you renamed it to)	Depth - layered (mm) (NB. Certain variables in the output list are Array variables. These are variables that don't have a single value but instead have a value for every layer of the soil. The Array? column in the output list tells you if a variable is an Array variable or not. When we specify layered in these notes, we mean just drop the array variable as is onto the variable list. This will create a separate column in the output file for each layer in the soil). (NB. The array variable for depth of each layer is dlayer)
    	Drainage (mm) (drain)
    	Extractable Soil Water (mm)
    	NO3 sum over profile and change alias to NO3Total. (NB. To sum over the profile, there are methods to sum various numbers of layers of an Array variable to get just a single value [click "?" button next to variable list for more info].) (NB. To alias any variable use the "as" keyword. eg. no3() as NO3Total. [Once again click "?" button next to variable list for more info])
    	NH4 sum over profile and change alias to NH4Total
    	NO3 layered
    	NH4 layered
    	DNIT sum over profile
    	UREA sum over profile

15. Change reporting frequency to end_day.
16. Run simulation
17. Create a graph of day vs urea, total ammonium and total nitrate. Drag an XY graph component onto the simulation. On the Plot node set "Point type" to "None" and leave "Type" as "Solid line".
    

Question: Why does the above graph look the way it does?

Illustrating the extent and conditions required for denitrification losses

Create a new chart of Day vs Rain, DNIT (on Right Hand Axis), ESW and NO3Total.

From this chart you can see that significant nitrogen is lost via denitrification when large amounts of nitrate is available in saturated soil conditions.

Exploring vertical movement of nitrate, after fertilisation, through the soil profile

Let's look at the distribution of nitrate through the soil profile at 21 days after fertilisation, and again at 5 months.

1. Create a depth graph of dlayer vs no3 for 31/01/1989 and 10/06/1989.
   

   NB. Depth plots can only be done when the simulation has dlayer in the output file along with at least one other layered Array variable. This is why we included no3 and nh4 as layered variables in the output file and not just include NO3Total and NO4Total.
   

   Drag a Depth component onto the simulation. Expand the Plot node to get the Depth node. Tick the dates mentioned above. In the Plot node, add "no3" and "nh4" as the X variable and leave the Y variable as "Depth" Now below the graph untick the checkboxes for the "nh4" lines.

   

   From this chart you can see the distribution of nitrate in the soil profile just 21 days after the addition of fertiliser and at 5 months.