Ethylene-oxygen cycle

permed at nor.com.au permed at nor.com.au
Thu Dec 13 19:55:39 EST 2001


>>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 at nor.com.au  www.earthwise.org.au




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