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  • From: <permed@nor.com.au>
  • To: "permaculture" <permaculture@franklin.oit.unc.edu>
  • Subject: Re: Ethylene-oxygen cycle
  • Date: Fri, 14 Dec 01 00:55:39 -0000


>>Reminds me of the great soil research conducted by Dr Alan Smith when he
>>discovered the ethylene-oxygen cycle. His findings weren't what the
>
>Could you describe the ethylene-oxygen cycle with references and
>URL's?

It's quite a process to describe - Smith's conference paper (the one he
delivered before being silenced) was published in one the early
Permaculture Journals and republished in PIJ issue #39. I just ferreted
through my old floppy disks and found the article (I was editor at the
time) and have pasted it below. It's an important soil process that
explains the science of why Fukuoka's sytem is so successful and
questions many mainstream agriculture assumptions about the relationship
between plants, soil microorganisms, pathogens and nutrient availability
- recommend reading it right through.
Robyn


THE LIVING SOIL

No-dig gardeners and no-tillage farmers realise that healthy plants and
good yields can be obtained on a sustained basis from undisturbed soils,
nevertheless for many, lingering doubts remain. Doesn't soil need to be
periodically aerated to stimulate microbial activity and liberate
nutrients to plants?

Soil scientist Alan Smith has been the principal research scientist for
the New South Wales Department of Agriculture. IPJ put the question to
him, his reply, and the article which follows contained some real
surprises!

"I can understand your confusion when trying to interpret the claims and
counterclaims made regarding the value of 'aerating' soil. Obviously, I
am a non-believer in the value of aeration, at least in Australian
conditions. One thing that we must always be wary about are treatments
that may give initial, short-term gains but lead to long-term problems. I
believe the 'aeration' theory is such a treatment. There is no doubt that
ploughing soil does initially increase aeration and does result in
intimate contact between the mineral soil and any organic residues. This
stimulates microbial activity and nutrients immobilised in the organic
reserves are liberated rapidly into the soil. However, unless plants are
growing in the soil to take immediate advantage of these mobilised
nutrients, they are leached or rapidly fixed in unavailable forms.
'Aerating' soil, of course, usually results in the removal of plant
material and so there are no plants (or only a few) left to take
advantage of the released nutrients. If this practice is continued season
after season then it is obvious that a loss of nutrients results. The
'aeration theory' really developed in the northern hemisphere where the
extended cold winters prevent microbial decomposition of organic residues
in soil. In spring it is advantageous to stimulate the decomposition rate
so that plants can obtain nutrients during a relatively short growing
season. In Australia, conditions are generally favourable right
throughout winter for at least some organic matter breakdown. Thus, our
conditions are very different. It is also worth considering just what
problems arise when this 'aeration' is attempted in tropical soils where
conditions are even more favourable for the breakdown of organic matter,
Yes, we all recognise that under those conditions it is a recipe for
disaster."

MICROBIAL INTERACTIONS IN SOIL AND HEALTHY PLANT GROWTH
Microbial interactions in soil play a key role in the biological control
of plant diseases, the turnover of organic matter, and the recycling of
essential plant nutrients. An understanding of the mechanisms involved
may lead to more efficient methods of growing plants, whether they be
food crops in agriculture or plants in gardens.

Before these interactions can be discussed, however, it is essential to
reaffirm the unique position that plants have in any ecosystem. They are
the only living organisms that can directly utilise the energy of the sun
and in the process they transform this energy into forms available to
other living things. The green pigment, chlorophyll, in their leaves
traps the light energy from the sun and an interaction occurs in leaves
with carbon dioxide gas from the atmosphere to produce carbon compounds
then available as energy sources to other living things, including man,
other animals, insects and micro-organisms when they consume plants or
plant remains.

Although plants have this unique ability to trap the energy of the sun
and transform it into the chemical energy they need to grow, metabolise
and reproduce, they also require other materials that they are unable to
produce directly. For example, they require various elements, including
nitrogen, phosphorus, sulphur, calcium, magnesium, potassium and trace
elements. The soil is the reservoir of these elements, but to obtain
adequate supplies plants must alter the environment around their roots to
mobilise these nutrients. One important way the plant achieves this is by
stimulating the activity of micro-organisms in soil around their roots
and the microbes then enhance nutrient mobilisation.The plant stimulates
microbial activity in soil by supplying chemical energy in the form of
root exudates and litter. Thus, an intimate relationship exists between
the plants and soil microbes. Unfortunately, in many of the conventional
methods used in agriculture this relationship is impaired, resulting in
problems of nutrient supply to the plant and an increase in the incidence
of disease.

The latest research indicates that during the life of the plant up to 25
per cent of the chemical energy in the form of carbon compounds that is
manufactured in the leaves is lost by the plant into the soil directly
adjacent to the root. This material is lost either as root exudates or as
dead, sloughed plant cells. On a first examination this seems to be a
highly inefficient, wasteful mechanism. The plant goes to considerable
trouble to trap the energy of the sun and convert it to chemical energy,
but then loses almost a quarter of the energy into the soil! One view is
that nothing in nature is perfect and 'leaky' roots are inevitable. I
certainly do not subscribe to this view. I firmly believe that if some
living system is apparently wasting a quarter of the energy that is goes
to the trouble to manufacture, then this loss must ultimately benefit the
organisms directly. IF this is not the case, then evolution should have
resulted in the selection of plants that lost less of their energy.

How does this loss of carbon compounds into the soil benefit the plant?
Most importantly, these compounds are energy sources for the soil
micro-organisms which proliferate in the rhizosphere, i.e. the soil zone
directly adjacent to the plant root. These micro-organisms multiply so
rapidly that they deplete the soil of oxygen at numerous microsites in
the rhizosphere. Thus, oxygen-free anaerobic microsites are formed. The
formation of these anaerobic microsites plays an important role in
ensuring the health and vigor of plants.

ETHYLENE PRODUCTION IN SOIL
Our research shows that ethylene, a simple gaseous compound, is produced
in these anaerobic microsites. Furthermore, this ethylene is a critical
regulator of the activity of soil micro-organisms and, as such, affects
the rate of turnover of organic matter, the recycling of plant nutrients
and the incidence of soil-borne plant diseases. Concentrations of
ethylene in the soil atmosphere rarely exceed 1 to 2 parts per million.
Ethylene does not act by killing soil micro-organisms, but simply by
temporarily inactivating them - when concentrations of ethylene in coil
fall, microbial activity recommences.

Soil ethylene is produced in what we call the OXYGEN-ETHYLENE CYCLE.
Initially, the soil micro-organisms proliferate on the plant root
exudates and deplete the soil of oxygen at microsites. Ethylene is them
produced in these microsites and diffuses out, inactivating without
killing the soil micro-organisms. When this occurs the demand for oxygen
diminishes and oxygen diffuses back into the microsites. This stops or
greatly reduces ethylene production, which enables the soil
micro-organisms to recommence activity. Favourable conditions are then
recreated for ethylene production and the cycle is continuously repeated.

In undisturbed soils, such as found under forest and grasslands, ethylene
can be continually detected in the soil atmosphere, indication that the
oxygen-ethylene cycle is operation efficiently. Conversely, in most
agricultural soils, ethylene concentrations are extremely low or
non-existent. This is to be expected if ethylene plays an important role
in regulation microbial activity in soil. It is well established that in
undisturbed ecosystems where there is a slow, balanced turnover of
organic matter, efficient recycling of plant nutrients and soil-borne
plant diseases are unimportant. When these ecosystems are disturbed for
agricultural of forestry usage the situation changes dramatically, There
is an alarming decline in the amount of soil organic matter, deficiencies
of plant nutrients become commonplace and the incidence of plant disease
increases dramatically. We attempt to overcome these problems by
additions of inorganic fertilisers and by the use of pesticides, which
increase our production costs considerably. It is also generally true
that the longer we farm soil, more and more of these inputs are necessary
to maintain our yields.

We argue that the trend could be reversed, at least partially, if we
could create favourable conditions for ethylene production in these
disturbed soils. We now know that one of the major reasons why disturbed,
agricultural soils fail to produce ethylene is because our techniques
cause a change in the form of nitrogen in soil. In undisturbed soils,
such as under forests or grasslands, virtually all the nitrogen present
is in the ammonium form with just a trace of nitrate nitrogen present.
When these ecosystems are disturbed for agricultural usage, virtually all
the soil nitrogen occurs in the nitrate form. This change in form of
nitrogen occurs because the disturbance associated with agricultural
operations stimulates activity of a specific group of bacteria which
convert ammonium nitrogen to nitrate nitrogen. Plants and micro-organisms
can use either form of nitrogen, but our research has conclusively shown
that ethylene production in soil in inhibited whenever the nitrate form
is present at more than trace amounts. Ammonium nitrogen has no such
inhibitory effect on ethylene production.

Nitrate nitrogen stops ethylene production because it interferes with the
formation of the anaerobic microsites. When all the oxygen is consumed in
the microsite a series of complex chemical changes then occur. One of the
most important changes that occurs is that iron goes form the oxidised or
ferric form to the reduced or ferrous form. Iron is one of the major
constituents of soil, making up somewhere between 2 and 12% of its
weight. In adequately aerated soil virtually all the iron exists as
minute crystals of iron oxide and in this oxidised or ferric form is
immobile in soil. If oxygen is completely consumed in microsites in soil,
and reducing conditions exist, these minute crystals break down and iron
is then transformed into the highly mobile ferrous or reduced form. Again
our research has shown that ethylene production occurs is soil only when
iron is in the reduced or ferrous form. In other words, ferrous iron is a
specific trigger for ethylene production. If there is no oxygen in the
microsites, but nitrate nitrogen is presents. then the complex chemical
changes leading to the reduction of iron form the ferric to the ferrous
form are inhibited. This is how nitrate nitrogen stops ethylene
production.

How does ferrous iron trigger the release of soil ethylene? This form of
iron reacts with a precursor of ethylene that is already present in the
soil and a reaction occurs that results in the release of ethylene. Our
work has established that this precursor originates from plants and, more
importantly, it accumulates to appreciable amounts only in old, senescent
plant leaves. When these old leaves fall to the ground and decompose, the
precursor accumulates the soil. Then, when conditions become favourable
for mobilisation of ferrous iron, ethylene is produced.

We have also show that different plant species vary markedly in the
quantities of precursor that accumulate in their old leaves. This is
important to know when selection plant species to use as cover crops to
increase the ability of agricultural soils to produce ethylene. A few of
the plant species that produce high concentrations of precursor are rice,
phalaris, chrysanthemum, avocado, bullrush and Pinus radiata. Some of the
low producers include Dolichos, paspalum, lucerne and bracken fern.

In retrospect it should not be too surprising that the ethylene precursor
accumulates appreciably only in old, dead plant leaves. After all, in
natural communities of plants old dead leaves comprise the bulk of the
litter that falls on to soil. Also, it is equally clear that in
agricultural situation most of the old plant leaves are removed either
during harvest or by grazing or by burning crop residues. Thus,
agricultural soils are usually deficient in precursor.

It is now possible to specify the soil conditions necessary for ethylene
production - (1) there must initially be intense aerobic microbial
activity, at least in the rhizosphere, to ensure that oxygen-free,
anaerobic microsites form; (2) conditions in the microsites must become
sufficiently reduced to mobilise ferrous iron to trigger ethylene
release; (3) concentrations of nitrate nitrogen in soil must be kept to
trace amounts, otherwise ferrous iron will not be mobilised; (4) there
must be adequate reserves of the ethylene precursor in soil.

MOBILISATION OF ESSENTIAL PLANT NUTRIENTS
A major limitation to plant growth in most agricultural soils is an
inadequate supply of essential plant nutrients. This occurs even though
there are adequate reserves of these nutrients in soil, but they are held
in highly insoluble forms. Their high degree of insolubility prevents
loss from the soil by leaching, but since they are only available to the
plant in the soluble form, problems of supply rate to plants are created.
Formation of anaerobic microsites in the rhizosphere of plants, which is
of such paramount importance to ethylene production, can play a critical
role in the mobilisation and thus supply rate of these essential
nutrients to plants.

This mechanism revolves around the importance of iron in soil. As already
discussed, under normal conditions in soil most of the iron occurs as
minute crystals of iron oxide. These crystals have a large surface area
and are highly charged. As a result plant nutrients such as phosphate,
sulphate and trace elements are tightly bound to the surfaces of these
crystals. In this form they are virtually unavailable to plants. If,
however, anaerobic microsites develop, these crystals break down and the
bound nutrients are released for uptake by the plant.

At the same time high concentrations of ferrous (reduced and mobile form)
iron are released into the soil solution in the microsite. The other
essential plant nutrients, including calcium, potassium, magnesium and
ammonium, are held on the surfaces of clay and organic matter. When
concentrations of ferrous iron increase so much, these nutrients are
displaced by the ferrous iron into the soil solution, where they too are
now available for uptake by plant roots. Since anaerobic microsites are
most likely to form in the rhizosphere of plants, the nutrients are
mobilised exactly where they are required by the plant. An additional
advantage of this mechanism is that if the released nutrients are not
utilised by plant roots they cannot be leached in the soil. As soon as
they migrate to the edge of the anaerobic microsite, reoxidation of the
iron occurs with recrystallisation of iron oxide. These crystals then
rebind the nutrients and prevent their loss by leaching.

The soil conditions necessary for this mechanism to operate are identical
with those required for ethylene production. Thus in agricultural soils,
where ethylene production is inhibited or impaired, this mechanism of
nutrient mobilisation is also restricted. Again, under these conditions,
the elevated concentrations of nitrate nitrogen that occur in
agricultural soils are a major inhibitor of efficient nutrient
mobilisation.

Successful management of soils to increase the likeihood of anaerobic
microsite formation, which will help ensure a balanced oxygen-ethylene
cycle and enhance mobilisation of essential plant nutrients, will demand
alterations to some of the established practices in agriculture. For
example, techniques aimed at increasing aeration and the oxidation states
of soil, which give short-term increases in plant growth but rapidly
create lone-term problems of nutrient depletion and increased plant
disease incidence, will require modification. Treatments which stimulate
rates of nitrification (transformation of ammonium nitrogen to nitrate
nitrogen), such as excessive use of nitrogenous fertilisers, overuse of
legume dominant pastures, or excessive removal of plants by overgrazing
or forestry operations, require re-examination.

Some practical guidelines for successful management of soils include:-
(1) It is essential that organic residues be returned continually to the
soil. Organic residues contain essential plant nutrients for recycling,
stimulate microbial activity in soil, supply ethylene precursor, and
restrict the rate of nitrification in soil. It is best to use mature
plants as a source of organic amendments and it is better to return the
residues to the soil surface rather than incorporate the into the soil.

(2) Techniques of minimum tillage should be utilised wherever practical.
This ensures that plants are growing in soil virtually continually, that
there is a minimum of disturbance to the soil and increases the amount of
organic matter that is returned to the soil. Again, nitrification is
restricted when these techniques are used.

(3) Whenever soil is amended with nitrogenous fertiliser it is best to
apply the nitrogen in the ammonium form and to apply it in several small
applications rather than one or two heavy dressings. This again restricts
the chance of nitrification.

(4) In some situations it will be advisable to add chemical inhibitors of
nitrification (e.g. N-Serve or Terrazole) to soil with the nitrogenous
amendments to further ensure that nitrification is restricted.

This article first appeared in 'Australian Plants' Vol. 9 No. 73, 1977,
then in issue #7 of the International Permaculture Journal in March 1981.

Djanbung Gardens Permaculture Education Centre
home to:
Permaculture Education
ERDA Institute Trust
Nimbin Eco-Village Project Office
Robyn Francis - permaculture designer & educator
PO Box 379, Nimbin NSW 2480 Australia
Ph 02-6689 1755 Fax 02-6689 1225
permed@nor.com.au www.earthwise.org.au





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