Nitrogen fertilisation expressed as nitrogen equivalent In a plant pathology paper I found levels of fertilisation applied in the experiment expressed as nitrogen equivalent.
Reporting the exact phrase:


  Nitrogen fertilisation with 8.6 g of nitrogen equivalent …


I found that equivalent weight for an element would be its atomic weight divided by its valency so, if I'm not wrong, equivalent weight for N would be ~4.67 g

What does this mean? 
How much fertiliser was applied e.g. using ammonium sulfate?
This is referring only to the mass of elemental nitrogen available, rather than the total mass of fertilizer. In your case, ammonium sulfate is 21% nitrogen (see the table here for other fertilizers and here for more information on fertilizer composition), which gives ~40.95g ammonium sulfate.