The nitrogen comes from the air - we're perfectly capable of capturing it using renewables.

It's probably one of the last things to be created that way because it's one of the places where methane is used more efficiently than burning it... But fundamentally there's no issue here except energy availability and a short term supply shock.

The nitrogen in generated fertiliser comes from the air, it’s the hydrogen in the process that comes from natural gas.

You can theoretically get it from water instead, but the energy cost is something like 3-4 times as high. It may be feasible at some point.