You tried going to https://geog.ubc.ca/courses/geob370/students/class07/landuse_Salmon_River/documents/criticalnutrient.pdf, and it doesn't exist. All is not lost! You can search for what you're looking for.
https://geog.ubc.ca/courses/geob370/students/class07/landuse_Salmon_River/documents/criticalnutrient.pdf