Well, I know this is conventional wisdom, that the non-symbionts are not
worth the trouble. In my opinion, however, this is an error. The Cubans
have some interesting experiences with Azotobacter achieving higher levels
than this. There is also some work in the tropics (Brazil) that suggest
Azotobacter can be a significant N-fixer under the right conditions.
Also, there is something else to keep in mind: 50kg/ha of N fixed in the
context of a complex soil microbiology and good soil structure can have a
dramatically different effect on the soil and on crop productivity than
does an equivalent or greater amount of soluble N salts or anhydrous
ammonia. In an organic soil it just isn't necessary to add such high
levels of N as are common on some conventional farms, a large percentage of
which ends up in the drainage.
Also in the humid tropics we are beginning to suspect that N-fixation by
bacteria on the leaf surfaces could be important, in which case Mo given as
a foliar fertilizer could be very beneficial. Experience so far seem to
bear this out. I don't know how relevant this source could be in more
temperate or drier areas. A friend of mine doing reserarch in Oaxaca has
found 15 species of bacteria, several of them N fixers, in the exudate from
the adventitious roots of Indian corn varieties. This is not precisely a
symbiosos of the rhizobium type but is probably part of the rhizosphere
microbial populations that feed on carbohydrates the corn plant releases
into the soil for that purpose. The point is there is a lot more to
N-fixation in agriculture than Rhizobium and there is even more to it that
we don't know about. I say don't write off the non-sybionts too fast. They
have a great potential to contribute to more adequate N management in
future agriculture.
I remember reading a few years ago that the nitrogenase enzyme was
remarkably uniform in different organism. It has two metaloproteins, one
with Moly, Iron and Sulfur and the other with an Fe-S group. Its
uniformity, according to what I read, suggests that it evolved only once in
the history of life. There may be more recent research I am unfamiliar
with. I recall that V can substitute for Mo in some metaloproteins but I
thought the enzymatic activity would be considerably lower. I remember some
speculation of possible V-based N-fixation systems and would like to have
any references on that. That would be very interesting.
Finally, we should always remember that N-fixation is only one link in the
nitrogen cycle, albeit a critical one. But proper N management means paying
attention to other important links, to create conditions that retain
organic N in the soil biomass as much as possible.
Regards,
Ronald Nigh
Dana, A.C.
Mexico, D.F. & San Cristóbal de Las Casas, Chiapas
Tel. y FAX 525-666-73-66 (DF)
529-678-72-15 (Chiapas)
danamex@mail.internet.com.mx
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