The Haber-Bosch reaction is what turns atmospheric dinitrogen (N2) into ammonia. Ammonia can be oxidized to make nitric acid and nitrate fertilizers, combined with carbon dioxide to make urea, or used as-is for fertilizer. The inputs to the Haber-Bosch reaction are hydrogen and nitrogen gases:

https://en.wikipedia.org/wiki/Haber_process

The nitrogen is easily separated from air. The hydrogen is the energetically expensive input. Most hydrogen is presently made by steam reforming fossil fuels. Hydrogen can also be produced from water and electricity via electrolysis:

https://en.wikipedia.org/wiki/Electrolysis_of_water

In the 20th century, surplus hydroelectric power from dams was used to electrolyze water for making ammonia. More direct demand for electricity and cheaper processes for making hydrogen from fossil fuels gradually eliminated it. Now people are planning to bring back large scale electrolytic hydrogen for fossil-free hydrogen from renewables:

https://www.cnbc.com/2021/01/19/canada-is-set-to-have-one-th...

https://www.chemengonline.com/uniper-unveils-plans-for-germa...

So the process can hypothetically run from renewable energy in the future, but the primary energy source and primary source of hydrogen are probably both methane right now.

Methane can be and sometimes is produced from biomass.

And there are a few plants in operation that produce without any fossil inputs, but most of them aren't switched over yet, mostly because green hydrogen production isn't at the necessary scale yet and will probably be used for steel making first.

Perhaps a simple explanation would be to quantify the inputs and outputs, maybe from year to year. For instance the carbon footprint of farming should be shrinking if all of these green measures are coming online.