HI @sarahhalperin -
As @dcdenu4 noted, the way that we usually quantify nutrient “retention” is to take the difference between a baseline landscape/management practices and a scenario. The model itself doesn’t really quantify the total amount of nutrient retained on each pixel from what comes at it from downslope + what is generated on the pixel.
Check out this forum post, which has a discussion about nutrient retention that you might find interesting. It’s for NDR 3.7.0, but I believe that the same is still true about 3.9.0.
~ Stacie