No NO3 in soil during beans years...??

[Daniel Perkins]

I am hoping someone has insight into why my model (simulating corn and bean rotation) does not appear to be nitrifying during bean years (no N fertilizer).  Even though beans don't need N as a fertilizer, and thus I'm not adding any during those years, I still think that under these conditions there should be some NO3 generated from nitrification of existing OM in the soil.  I looked at N percolated and it appears negligible.

Is there an OM parameter that exists that I have set too low?

Other ideas?

[Luca Doro]

Have you printed the output variable NITR? Does it show low values during bean years?

In my opinion, nitrification shouldn't be affected by the plant that is growing, but many interactions are simulated by the model.

Let's see if we can help to understand what's happening in your simulation.

Luca

[Daniel Perkins]

Thanks for the reply, Luca. Yes, I did and can verify that it is negligible.  I am attaching a graph that may point to a different direction than I previously thought... It looks like some percolation occurs BUT plant uptake looks like it takes almost all N out of the soil (looking at the corn year when fertilizer was applied).  I'm also noticing that beans are taking MUCH more N out of soil (somehow? I don't see any N in the soil (ZNO3) in the bean year...). Note, I've also plotted Denitrification, Tile Drainage, and N Percolation to show that N isn't leaving via vertical drainage or subsurface anaerobic conditions.

Fertilizer inputs for the corn year are: 

87.7 kg/ha (OPV1 value is 87.7) of 30%UAN (4/8/2020) where tillage code: 264 OPV:48

678.8 kg/ha (OPV1 value is 678.8) of 32% UAN (5/3/2020) where tillage code: 264 OPV:55

[Daniel Perkins]

I am also plotting NITR in this graph... it's just so low that it doesn't register in the view. Thanks!

[Luca Doro]

Can you run the model and share the OUT output file? Maybe I can see something that looks odd.

Luca

[Daniel Perkins]

I'm including both the SAD and OUT files here.  I appreciate an additional eye on this!

[Luca Doro]

Dear Daniel,

The first thing I can suggest is to switch from the Phoenix to the Century approach to simulate the C-N dynamics. If you want to use the Phoenix approach, we can check the specific parameters used in Phoenix.

You can switch from Phoenix to Century by changing the value of variable ICP in the control table (APEXCONT.DAT) from zero to one.

Let me know if you need more information and if the modification helps with the nitrification.

Luca

[Daniel Perkins]

Thanks Luca, I am fine with CENTURY for C-N. I switched it over and re-ran.  I noticed that some of the parameters for the C-N routine were far from default ranges, so I changed those according to the manual to default values.  I am now seeing some NO3 in the soil profile, but still not as much as I thought might happen.  What is really confusing me is that the plant uptake does not appear to be following what I think I'm doing in the OPS file, see the time series graph and corresponding screen shot of the ops inputs.  It looks like plant uptake is occurring at strange intervals and I suspect that the N in the soil profile is suffering because of that... I'm attaching some details here (OPS, OUT, SAD files). 

Can you see if I'm doing somethings strange in the OPS crop definitions maybe?

[Luca Doro]

Daniel,

I don't see anything strange in the management file. I notice something suspect in the OUT file. In the OUT file, if you search the keyword PURK, you will see that it is printed on several lines, with each line referring to a day of the simulation (the date is printed on the same line). On the same line, the model prints (in this order) the soil water content at the beginning (+) and at the end (-) of the day, rainfall infiltration (+), lateral subsurface flow (-), quick return flow (-), percolation below the root zone (-), and the difference in the water balance. The signs next to each variable name is the sign those variable have in the water balance.

The model prints the line with PURK if the difference in the water balance is greater than a threshold. This let me think that something might be wrong in the water dynamic.

Try to revise the soil information and some parameters related to water percolation.

Luca

[Daniel Perkins]

Thanks for the insight and reply.  I am not sure what the threshold is that would trigger a PURK, but I plotted it and didn't see anything that jumped out at me.  I changed from Hargraves to Penman for ET because it looked like ET was much larger than transpiration (EP). My basic understanding of ET is that Transpiration is 70% of all ET during the active growing season, but my data seems to be the other way around, where Transpiration (EP) is less than half of total ET (see graph of cumulative, annual PRCP, PRK, ET, EP, and QDRN).

I looked through the manual to see what would adjust ET down and Transpiration up, but am not sure. In practice, reference ET has a coefficient that is often crop-specific. Is that adjustable in APEX?  Total ET seems fine in terms of magnitude, but it looks like transpiration is 'giving up' much sooner than KILL (451) in the OPS as seen by the flattening of cumulative EP before harvest (often in the middle of summer).

Is it possible that PRK is taking water out of the system before plants can access it on any give day?  

Best,
Dan

[Luca Doro]

Dan,

Sorry if I ask this, but I don't remember (dealing with several users can be confusing...). Have you shared your input files with me? I would like to run your simulation on my machine to analyze what is triggering the PURK alert.

Please, let me know if you have the possibility to share the input files with me.

Best,

Luca

[Daniel Perkins]

Luca, I am attaching the input files here, thanks!

[Luca Doro]

I have downloaded the input files, but some were missing. For some of them, I was able to reuse generic input files from other project, but I need the site file (.SIT) of your project.

Thanks,

Luca

[Daniel Perkins]

Apologies, here it is!
Dan