APEX1501 N and P Budget Equations

[Maya Struzak]

Hello,

I am trying to reconstruct the N and P balance equations from APEX output files to better understand nutrient cycling in my simulations. Since variable names differ between source code, output files (.AWP, .MSA), the .OUT balance summaries, and the manual, I'm having trouble matching everything up.

However, my calculated residuals are significantly higher than APEX's reported values (e.g., APEX DF = -2 kg/ha, mine = 19.8 kg/ha), suggesting I'm missing some terms or using incorrect variable definitions. I'm primarily using .AWP and .MSA files for annual totals, with some variables from the .OUT file balance summary. 

Here are the equations I've constructed:

Nitrogen:

Beginning TN (BTN) + Rainfall N (RFN) + Nitrate N fertilizer (FNMN) + Ammonia N Fertilizer (FNMA) + Organic N fertilizer (FNO) + N fixation by legumes (NFIX) + N from point source (PSON) - Final TN (FTN) - N loss with sediment (YN) - N due to wind erosion (YWND) - N loss in crop yield (YLN) - Volatilization (AVOL) - Denitrification (DN) - N loss in groundwater (DPKN) - N loss from burning (BURN) - N loss in runoff (WYLN=QN+RSFN+QRFN+SSFN)

I could not find definitions for the following .OUT file variables: TRMN, TLMN, TSMN, TRON, RSDN (although they are all 0 in my runs).

Phosphorus:

Beginning TP (BTP) + Organic P fertilizer (FPO) + labile P fertilizer (FPL) + point source P (PSOP) - P loss in sediment (YP) - P loss in runoff (QP) - soluble P leaching loss (PRKP) - P loss due to wind erosion (YPWN) - P loss in crop yield (YLP) - final TP (FTP)

I was also not able to find some of the .OUT P balance variables in the .AWP or .MSA files, such as DEP and SPOU. 

Would anyone be able to confirm the equations I'm using or have more accurate ones to share? Any insights on variable definitions, missing terms, or recommended approaches would be greatly appreciated!

Thanks,

Maya