How much nitrogen is in the rain?

The answer to that question depends on where you are. It's generally not very much. Fortunately, this isn't something that you need to guess at. There are actual measurements of atmospheric nitrogen deposition.

If you are in the United States, I recommend the National Atmospheric Deposition Program (NADP) annual summaries.These summary reports includes maps such as the one shown here, with a detailed record of N deposition.

The 2021 report shows a range of 0 to 8 kg N/ha, depending…