Subject: nitrogen & nitrogen fixation - general information (fwd)

From: "Lawrence F. London, Jr." <[email-masked]>
Date: Sat, 18 Oct 1997 15:58:25 -0400 (EDT)

Date: Sun, 12 Oct 97 20:43:52
Newsgroups: rec.gardens.ecosystems
From: [email-masked] (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: [email-masked] (Allyn Weaks)
Newsgroups: rec.gardens.ecosystems

In article <[email-masked]>, [email-masked]
(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  [email-masked]
PNW Native Wildlife Gardening:  http://chemwww.chem.washington.edu/natives/
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---

From: [email-masked]
Date: Sun, 19 Oct 1997 12:39:59 -0400 (EDT)

In a message dated 10/18/97 3:59:18 PM, you wrote:
[snip]
>During the growth stages, legumes will
>their nitrogen needs from the soil.
[snip]

Larry there are a few matters to consider here that you haven't included.
 First, whether or not a legume competes for N with other plants depends on
the legume.  Some exude N from their roots.  CF Duke:  Handbook of Legumes of
World Economic Importance, for info.  Legume grass pastures are well known to
be very sustainable and higher yielding than either component alone.

Legumes do take N from the air, but the air in the soil, through the roots.
 I know of no examples where a legume translocates this from the leaves, etc.
 Compacted soil therefore results in minimal nodulation and virtually no
fixation.  Maybe there are exceptions, particularly among those relatively
few nitrogen fixing legumes that tolerate water-saturated soils, such as
joint vetch.  This bears examination, but if the N is translocated, it will
be through special organs comparable to the knees of bald cypress, and not to
a general characteristic.  While most crop legumes fix N, it is important to
not that most legume species, have shown no such symbiotic association with
legumes.  Needless to say, the N fixers are more prevalant because of their
adaptability.

In pastures where c-4 grasses are mixed with legumes, such as white colover
(T repens), the act of cropping the legume STIMULATES N-fixation.  Moreover,
research has shown that N in the form of protein does not depress N-fixation
(though the protein decomposes over the course of the studies) while N in the
form of soluble chemicals does depress N fixation.  This has been studied for
Rhizobia.  I have no idea if it holds true for Frankia such as fix nitrogen
in symbiosis with  various non-legumes such as alders, eleagnus spp, sweet
fern, myrtle, etc.  

We have been having very good success using broadcast legume seed into
untilled ground as part of pasture restoration.  The soil here showed very
little legume activity, which concerned me as legumes are often an essential
source of phosphorous for growing cattle.  As the area was wet (mostly
flooded) when we took title a bit less than a year ago, we used joint vetch
as it thrives in wet areas in warm climates. First I tried mixing some seed
with grain, sticking the small legumes seeds to cracked corn with a touch of
molasses stirred in.  Clovers are often planted in this manner in New England
(USA), where I am from.  The joint vetch germinated but died out due to the
unusually dry summer here and the propensity of manure to wick moisture away
to evaporation.  Next I tried broadcasting seed (mixed with sand and
limestone for an evener spread) and mowing weeds and coarse stems not eaten
by the steer.  The organic debris I left to cover the seed.  I mowed very
low, partially to causes as much damage as possible to buttonbush which is a
serious pasture pest that has been allowed to take over areas here in the
past decade.  (Any uses for buttonbush would be most appreciated.)  We
planted when rain was forcast and were very lucky that it did indeed rain.
 We have an excellent stand that, in places, has been grazed through three
rotations now in addition to grass.  The steer seems to graze the joint vetch
and grass equally.  As conditions dried (water table dropped nearly six feet
during the "rainy season" -- someone is stealing water!), I broadcast
cowpeas, again with mostly lucky timing regarding rain and mosture.  

In the lowest pasture where most of the jointvetch is growing, I selectively
broadcast in places where it was not thickly established.  There I set the mow
er (we use a walk-behind sickle bar mower) to the highest setting to spare
the "vetch>"    The joint vetch recovered nicely and is now in full bloom
with a fresh set of seed forming.  When there is enough mature seed, I'll
return the steer to eat the vetch, seed and all as a means to further spread
the seed.  It should remain dormant until warmer weather, by which time the
manure will have melted back into the soil.  The cowpeas are doing well.
 Grasses continue to grow well and show no sign of competition from the
legumes.

In drier paddocks, which were also cut close for weed control, it took
several rains for the cowpeas to develop.  In one paddock, apparently there
was not enough moisture consistently and we have had poor establishment.  In
another which contains a mix of moisture conditions, the cowpeas established
quickly in the moister areas--enough so that I could allow a few days of
light grazing to top them off, as well as some palatable "weeds" that had
reseeded, and keep them from canopying over the grasses.  This worked nicely
and we are now getting some flowering.

Meanwhile, all of the joint vetch is in blossom simultaneously, regardless of
planting time, which leads me to conclude it is photopheriodic.  Blossoming
commenced at or shortly after the fall equinox.  Seed set is excellent and
I've seen more bees foraging there than I suspected we had around here.  

This last week, I've now broadcast rye grass.  The "wet" paddocks have an
excellent winter (cool weather) grass, but most have bermuda grass which
barely grows at all in the winter here.  As the warm season annual legumes
give it up, presumably the above ground parts being grazed, I expect a
release of nitrogen to give the rye a boost to increase cool-season grazing.
 Overall I expect that grazing will get better, if anything, if we can get at
least modest rainfall.

Control of serious pasture weeds has been excellent, both herbaceous and
woody weeds are obvously much stressed and reduced invigor while grasses are
responding to the mowing and legumes are doing their thing.

Yes, we have an agroforestry component slowly coming into place.  Forage
bamboo is being planted in some areas and timber bamboo in others where shade
limits graziang anyway.  Tree crops are laid out parallel to fencelines so
that as fencing deteriorates we can transfer lines to the trees.  Myrtle in
particular is being encouraged as an indigenous N-fixing shrub that also
gives a great deal of protection from flies.  (The steer has almost no flies
on him when he rests in the myrtle.  Sometimes he just walks through a
thicket to ditch the flies on the other side.  He can recognize the myrtle in
a new paddock and immediately makes use of it..)  We have some jujube genus
and honey locusts planted in the pasture so far.  Wet areas will be heavily
planted to mayhaws, which will be more of a commercial crop for us than beef,
which we will use in the understory.  Deer seem to like to browse the
mayhaws, but I think they will hold up once they are bearing age--meanwhile
we have protected them.  As we recover pastures, there is less cover for deer
and I'm happy to eat all I can get.  We are also allowing a number of the
native oak seedlings to stand for shade and fall forage--even while still a
small calf, the steer liked these acorns.  

The legumes, fixing away nitrogen, are key to accelerating the development of
this new system on worn out land, run as one big pasture for a horse, and not
managed in any useful way after perimeter fencing.  The amount of improvement
we have seen in one season using these techniques is already impressive.

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  Next starts in Oct. 1997. Internships available.
Copyright, 1997, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL 32192 USA
 [email-masked]  

If its not in our food chain, we're not thinking.