>And of course the exact same nitrogenase (only one has eolved) catalyzes
>free-living N fization as well.
There are some fairly good indications that *vanadium* is a facultative
(and perhaps obligate) substitution for molydenum in several genera of
non-symbiotic nitrogen fixers. I'm trying to find the citations, but
so far without success.
Some of the non-symbiotic microbes also run with an Fe-S flavour of
nitrogenase.
Most non-symbiotes would be doing very well to fix 50 kg/ha nitrogen.
Except in fairly anaerobic environments -- rice, for example -- I
wouldn't go out of my way to encourage them, to say nothing of *add*
them to the soil. They are, however, a delightlfully fascinating
component of creation.
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