[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.