permaculture

["Lawrence F. London, Jr." <london@sunsite.unc.edu>]

       Date: Sun, 12 Oct 97 20:43:52
Newsgroups: rec.gardens.ecosystems
From: kjohnsto@mesastate.edu (karen johnstone)
Subject: Re: Nitrogen fixing beans

I'm not a botanist, and don't have expert information, but somehow the
thread still seems to lack information.  I recently did a modeling
project on nitrogen cycling with cowpeas/corn.  Had to find a lot of
specific numbers.  Conclusions I came up with from my research:

  The nitrogen fixed by the rhyzobial bacteria  is only a part of the total
nitrogen returned to the soil by a legume crop.  In the case of large
nitrogen contributors, like cowpeas and fava beans, the decay of the top
growth is the major contributor.  During the growth stages, legumes will
deplete nitrogen in competition with other crops in an intercropped system. 
In soils where nitrogen is deficient, the contribution is most significant. 
Given the figures that I have seen, they must pull some nitrogen from the
air, since the organic nitrogen present in the above ground part of the
plants far exeeds the available nitrogen from the soil at the time of
planting.   If surplus nitrogen is present, the legumes will take all of
their nitrogen needs from the soil.  I can only assume, until someone with
expert information tells me otherwise, that in the poor-soil situation, the
legumes have the ability to pull nitrogen from the air.  (Which should be
easy for a plant, which is so very good at pulling a much less plentiful
gas, CO2, from the air).  The nitrogen must come from somewhere, since a
rotation of legumes will leave a field with a higher nitrogen level than it
started with.  


Subject: Re: Nitrogen fixing beans
Date: Mon, 13 Oct 97 01:49:51
From: allyn@u.washington.edu (Allyn Weaks)
Newsgroups: rec.gardens.ecosystems

In article <MOD$971012.735@rec.gardens.ecosystems>, kjohnsto@mesastate.edu
(karen johnstone) wrote:

>   The nitrogen fixed by the rhyzobial bacteria  is only a part of the total
> nitrogen returned to the soil by a legume crop.  In the case of large
> nitrogen contributors, like cowpeas and fava beans, the decay of the top
> growth is the major contributor.

The legume is often able to put on so much top growth only because of the
nitrogen fixing bacteria...the nitrogen they fix doesn't all stay in the
roots, but is used by the plant to build the proteins that make up it's
body.

> Given the figures that I have seen, they must pull some nitrogen from the
> air, since the organic nitrogen present in the above ground part of the
> plants far exeeds the available nitrogen from the soil at the time of
> planting.   If surplus nitrogen is present, the legumes will take all of
> their nitrogen needs from the soil.  I can only assume, until someone with
> expert information tells me otherwise, that in the poor-soil situation, the
> legumes have the ability to pull nitrogen from the air.  (Which should be
> easy for a plant, which is so very good at pulling a much less plentiful
> gas, CO2, from the air).

Any nitrogen used in excess of what's already available from the soil does
come from the air, but it has to come by means of the nitrogen fixing
bacteria.  Plants can't do it on their own.  It isn't that they can't
absorb nitrogen molecules from the atmosphere, but they can't _use_ those
molecules in that form.  The chemical structure surrounding an element is
all-important, and that easily gets lost in the common conversational
shorthand of just saying 'nitrogen' in a variety of contexts, when what we
really mean is more complex, and differently complex in different contexts. 
The 'nitrogen' found in soil that's usable by plants is usually nitrate
(NO3-), nitrite (NO2-), urea (CO(NH2)2) and ammonium (NH4+).  Atmospheric
nitrogen is in the form of N2; the bond between those two nitrogen atoms is
quite strong and hard to break apart, and plants haven't learned the trick. 
There's a lot of chemistry involved in getting from N2 to NO3- or any of
the other common forms, and it's pretty expensive chemistry--it usually
takes a fair bit of energy to do it.  It's that entire chemical process
that we mean when we say that the nitrogen is 'fixed'.  Because plants
don't have the right kind of cellular factories to do the fixing
themselves, they need to make do with what's already in the soil (most
plants), hire bacteria to produce some extra primarily for them (legumes,
alders), capture and eat insects (sundews, fly traps), or capture and eat
large mammals (trifids, slaver sunflowers, and a few victorian orchids).

>From Ken Killham's _Soil Ecology_, the organisms that can convert
atmospheric nitrogen to more complex and more useable nitrogen compounds
are:

) free living bacteria such as Bacillus, Klebsiella and Clostridium
) bacteria of genus Rhizobium, mainly in legume root nodules
) actinomycetes of the genus Frankia, mainly in non-legume root nodules
    (such as alders)
) free living cyanobacteria (formerly blue-green algae) on the soil surface
    (genera Nostoc and Anabaena)
) symbiotic cyanobacteria such as are found in some lichens
    [this is an important source of N in pacific NW forests]
) bacteria loosely associated with the roots of certain plants,
    sometimes called 'rhizocoenoses'; includes Azobacter, Beijerinckia
    and Azospirillum
) some soil animals may be able to fix nitrogen without relying on
    bacteria in their gut, but this isn't certain, and in any case
    would be ecologically insignificant.

Other sources of fixed nitrogen in soil are lightning, synthetic
fertilizers, and some comes from air pollution such as nitrates in car
exhaust (about 40 kg per hectare per year in Europe).  (Air pollution
nitrates are yet another threat to native plant populations which are well
adapted to N-poor soils but are easily overrun by weedy exotics when the
nitrogen level increases.  'Good' soil isn't always a good thing!)
-- 
Allyn Weaks  allyn@u.washington.edu
PNW Native Wildlife Gardening:  http://chemwww.chem.washington.edu/natives/
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