[OJFord]

Regarding nitrogen fixing, my understanding is that 'green manure' & cover crops take from the air, leave in the soil - but microorganisms sounds like it's more in the context of composting, where whatever waste material is 'broken down'. So not 'soil' in the sense of 'it was already there anyway', but rather.. 'in the ground but needing to be made smaller and more available'.

[shagie]

Cover crops have a symbiotic relationship with nitrogen fixing bacteria that they maintain in their roots. When the crop is harvested (and the roots die), the nitrogen that was fixed by the bacteria in the roots remains in the soil.

Free living, nitrogen fixing bacteria are free living and have a protein that allows them to fix nitrogen to allow faster growth than the bacteria that need to get their nitrogen through other processes. They are often anaerobic (or functionally anaerobic) and so flourish in areas that are oxygen poor (like soil and decomposing organic matter) and by fixing the nitrogen present they enable other organisms to live there (their nitrogen fixing allows fungus to become established in the decomposing organic matter - the bacteria themselves aren't doing the decomposition). https://en.wikipedia.org/wiki/Paenibacillus_polymyxa is one such species of nitrogen fixing bacteria that forms a biofilm on plant roots, fixes nitrogen, and produces a substance that makes the plant roots more resistant to other pathogens.

[mountaintimefrm]

Thanks for that explanation. I assume by "free living" you mean they can exist by themselves in soil apart from plant roots (or at least for part of their lifecycle)? I'm more familiar with the ones that are symbiotic with plants, especially the ones for woody plants (Frankia, etc), which I think are largely of a different type than the ones that live on cover crop plants like clover.

Soil and decomposing organic matter aren't necessarily oxygen poor environments, and in ideal conditions they aren't at all. Depending on factors like soil porosity, and rate/frequency of precipitation, there can actually be quite a lot of gas mechanically exchanged between the soil and the atmosphere (water fills up soil pore spaces and pushes out air, then water drains out of soil pore spaces, pulling air back in). Plant roots can respire atmospheric gases into soil as well. Part of the reason why compacted (ie minimal pore space) soil is harder to grow in than the same soil made friable, is because the lack of porosity causes the soil to go anaerobic, which is inhospitable to a lot of beneficial soil microorganisms.

[quixoticelixer-]

Not all cover crops are nitrogen fixing.