[thaumasiotes]

> Until people/animals eat it, or it decomposes.

Well, if you want to think about it that way (perfectly reasonable), you'd also want to consider the production of new alfalfa. Figure that at any given time, the world contains X amount of alfalfa, and that amount determines how much carbon is absorbed by the alfalfa industry.

[malfist]

I'm not sure any carbon is absorbed even by this metric. Unless were growing alfalfa and sequestering it below ground.

You should probably also consider inputs to growing that alfalfa too. Even single order inputs like transportation, fertilizer, water, etc would likely have more carbon release than the carbon mass of the alfalfa.

Is alfalfa even one of the plants that will nitrogen fix from the air? Or is it all pulled from the growing medium?