The main source of environmental nitrogen is the atmosphere. We are all breathing about 78% nitrogen gas right now. Nitrogen fixation is the process by which gaseous nitrogen changes into nitrate or ammonium, something more usable to grapevines.
The above picture shows vetch, caught in the act of, fixing nitrogen from the atmosphere. The pink nodules on the roots show that nitrogen fixation is active. The amount of fixation depends on many things; cover crop type, soil pH, soil temperature, soil moisture, etc. The incorporation of the vetch into the soil, by disking, can double soil nitrogen content compared to a no-till green manure approach. However, disking is bad for soil health as it causes compaction, so here, in Vinoland, we mow. A significant amount of nitrogen is lost to the atmosphere by volatilisation in a no-till operation, but that's OK, the cycle will happen again next year...and I'll have something to kvetch about once more.