We have known how to do it for a long time, the problem is the energy cost. It takes 10x as much electrical power to produce fertilizer without fossil fuels as

We have known how to do it for a long time, the problem is the energy cost. It takes 10x as much electrical power to produce fertilizer without fossil fuels as an input, and fertilizer production already consumes 1% of total world energy output. So to decarbonize it we need to increase worldwide electrical output by atleast 10% to cover the energy requirements, and it has to be all done with clean energy. Because otherwise you lose 70% of that energy in the fuel-electricity conversion, while the direct fossil fuel to fertilizer conversion is much more efficient. It does need to be done, but to me it seems like it should be a second priority after not just cleaning up electrical production but increasing electrical production overall. There are probably some efficiency gains to be made in synthesizing fertilizer from the air, but as far as I can tell there is limited room for improvements because creating nitrogen from air is basically the same thing as creating fuel from the air and that energy has to come from somewhere.