Nutrient load and Retention efficiency : Load_n and eff_n

Helo community
I am just trying to understand the input datasets used in NDR.
I have Nitrogen Application (kg/ha/yr) data for my Land Use class, say 25 kg/ha/yr.
The nitrogen retention efficieny of the same Land cover is 0.5.
Shall I construct the biophysical table as given below?

load_n = 25 & eff_n = 0.5.

Or,
Is it wrong according to InVEST userguide (NDR: Nutrient Delivery Ratio — InVEST® documentation)

load_n = 12.5 & eff_n = 0.5.

I appreciate any clarifications in this regards

regards
GK

Hi @Kantharajan , great question. I just read this note from the User’s Guide, and it seems helpful:

Data sources may provide loading values as either the amount of applied nutrient (e.g. fertilizer, livestock waste, atmospheric deposition); or as “extensive” measures of contaminants, which are empirical values representing the contribution of a parcel to the nutrient budget (e.g. nutrient export running off urban areas, crops, etc.) In the case of having applied nutrient values, they should be corrected for the nutrient retention provided by the pixel itself, using the application rate and retention efficiency value (eff_n or eff_p ) for that land cover type. For example, if the nitrogen application rate for an agricultural LULC class is 10 kg/ha/year, and the retention efficiency is 0.4, you should enter a value of 4.0 into the n_load column of the biophysical table. If you have “extensive”/nutrient export values, then you may use them directly in the biophysical table without correction.

So it sounds like “applied” nutrient loading rates should be adjusted (to 12.5 in your case) so that the n_load values in the table represent “extensive” values. The important thing is to be sure whether your data source for these values is measuring application rates or “extensive” values.

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I should add that the definitive answer to this question (as of this date) can be found here: Interpreting the n_load parameter - #11 by swolny

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Hi Dave
Thanks for your clear explanation. I have ‘Applied nutrient loading’ and will convert the values to represent ‘extensive’ values.
Between is there any typo (0.6 instead of 0.4) in your reponse (same pasted below)?

…For example, if the nitrogen application rate for an agricultural LULC class is 10 kg/ha/year, and the retention efficiency is 0.4, you should enter a value of 4.0 into the n_load column of the biophysical table…

Thanks again
GK

@Kantharajan , yes, you’re right. My excerpt from the User’s Guide above is now outdated. The correct math is now included in the latest version of the UG, found in this section: NDR: Nutrient Delivery Ratio — InVEST® documentation

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