To get even more "scientific" about nitrogen:
Total nitrogen = Total Kjeldahl nitrogen (or TKN) + Nitrate and nitrite nitrogen.
The TKN is the sum of nitrogen in the inorganic compounds ammonia and ammonium (NH3/NH4+) plus nitrogen in organic compounds, including proteins in the tissues of plants (and manure, etc.) but also including urea. Much of the organic nitrogen is released slowly through decomposition of organic matter, but urea nitrogen is available more quickly. The ammoniacal forms of nitrogen are inorganic forms, available for plant uptake, and will tend to make soil more acid (lower pH); they are also more volatile, with losses to the atmosphere.
The nitrate and nitrite N forms are inorganic nitrogen, also plant available, and tend to raise soil pH (make the soil more basic). Nitrate and nitrite are the most water-soluble and are readily washed through soil with excess rainfall. Denitrification (through bacterial processes) can also lead to loss of N to the atmosphere.
Biological processes can make nitrogen convert from one form of nitrogen to another. Look up "nitrogen cycle" and you will get a good idea of what the relationships are between different forms of nitrogen, plants, soil, and the atmosphere.
If you use more fertilizer in the organic forms, you will lose less nitrogen to the air or to groundwater. Inorganic forms have their place, if you need a more rapid response to fertilizer application.