Actually, it looks like a separate slot was made to be less water specific... see https://genomicsstandardsconsortium.github.io/mixs/0000530/

I recommend deprecating tot_nitro and using tot_nitro_content

will complete when deprecation protocol is in place

The nuance of what is being measured, concentration vs % of nitrogen in the soil could be lost, if these two were combined.
In order to use a more generic term 'total nitrogen content'. The second part of the definition should include the water/soil details.
The 'new' term could just be: 'total nitrogen'

Reviewing the definitions:
for water:
Term: total nitrogen concentration (tot_nitro)
Definition:
Total nitrogen concentration of water samples, calculated by: total nitrogen = total dissolved nitrogen + particulate nitrogen. Can also be measured without filtering, reported as nitrogen

Note: two forms of Nitrogen are measured

for soil:
Term: total nitrogen content (tot_nitro_content)
Total nitrogen content of the sample

To measure total C and N, subsamples of dried, finely ground soil are weighed into small foil capsules that are combusted in an automated CHN analyzer that measures the amount of released CO2 and N2 by gas chromatography. Final values are expressed as the percentage of carbon or nitrogen in dry soil.