Problem with no3ppm_min (SoilN constant)

[ApsimBot]

Legacy Bug ID: 2014 Author:
Date: 2014-12-10 13:43:13 +0000 UTC

Hi

If you have time I would appreciate your advice on a potential APSIMv7.6 bug.

I would like to check with you if there is an explanation I have overlooked for why I simulate (with APSIM v7.6) very large amounts of nitrate leaching (leach_no3) when changing the 'SoilN' constant 'no3ppm_min' from 0.0 to 1.0 ppm.

I have reproduced the problem in two simple simulations (attached) where I used a:

A bare soil

No soil carbon, root_wt or surface residues

Both simulations were set with:

Initial no3 (all layers) = 1 ppm (~22.5kgN/ha)

Initial nh4 (all layers) = 0 ppm

The only difference between the simulations is the value of no3ppm_min. One is set to zero (default) and the other to 1ppm which is equivalent to ~ 22.5 kgNO3-N/ha.

If you wish to rerun the attached sims you will need to make a change to the PlugIn node in the second simulation so it points to the location of the Soil_1ppmMinNO3.xml on your computer.

Here is the resulting figure (Units = kgN/ha):

[ApsimBot]

Author: big032 Date: 2014-12-10 13:45:00 +0000 UTC

Potential APSIMv7.6 Bug.pdf Size: 186211

[ApsimBot]

Author: zur003 Date: 2014-12-12 10:43:00 +0000 UTC

That definitely looks like a problem. I think what is happening is that soilwat is saying: "You have that much N in the profile? I'm going to leach this much away.". SoilN is told the leached amount, but says "I don't care what you want, soilwat, I'm not going to let N drop below 1 ppm". This will happen whenever there's a heavy rain/irrigation event comes along, and there's more leaching. It's really a mass balance problem - soilwat thinks it's removed the N from the system although soiln doesn't actually allow its removal.

It would be interesting to know why this wasn't a problem in 3.6, but that's going back to some pretty old code.

It may be useful if someone would attach the zip archive with the sample simulation referred to in the pdf document.