N&P boundary input in water quality modeling

(Ⅰ) For N:(1) If I have sample data of TKN and NHX, can I use “[TKN] - [NHX]” to get the concentration of “Organic Nitrogen”? But I see the handbook that “[TKN] = [PON] + [NHX]”. So in EE, “[TKN] - [NHX]” is “[PON]”? Why exclude [DON]?(Ⅱ) For P:(1) The 10th Water Quality constituent in EE is: P4D - total phosphate. Does P4D mean “dissolved inorganic phosphate”? That is, P4D is dissolved and inorganic?If so, why does it be named of “total”? I suppose P4D is “PO4” (a salt of phosphoric acid). If it doesn’t contain “particulate” or “organic”, why “total”?(2) Is P4D the same as “Dissolved Reactive P (DRP)”?(Ⅲ) For Organic N&P:Organic constituents for N and P all have three categories: Refractory, Labile, Dissolved. I find that almost all in-site monitorings can not monitor the water sample so fine. Could you give me some advice on how to make input of these organic items when only routine WQ data are available?

More specific about Question (Ⅱ-1): For input of P4D in EE, which method is applied when mesuring this value:(a) water sample is digested before analysis (e.g., heat and acid), then measure the PO4;(b) water sample is through a filter paper + without digestion, then measure the PO4;In my stydy area, method (a)&(b) are both applied. So there are two different values of “PO4”. I need to know which value is thought to be the input of “P4D” in EE. Thanks.

(Qusetion Ⅳ) For “Organic Carbon” (i.e., RPOC, LPOC, DOC), we usually don’t have and measured data about them. Could you also give me some advice? If you cannot answer my question, please let me know. Becasue it is a little anxious to see EE support team answer selectively other’s quetion instead of mine.