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VII. Speculation: Are We Measuring and Averaging at Consistent Scales?

VII. Speculation: Are We Measuring and Averaging at Consistent Scales?

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germplasm which has a very low variance for each phasic stage, and mixing

and manipulating the soil surface to remove microtopographic irregularities

which would interfere with this uniform plant development. Concern for the

influence of soil variability on crop yield prompted literally thousands of

uniformity trials by breeders and agronomists some decades ago which were

the basis to well-established criteria for trial design and size (Cochran and

Cox, 1957). Uniformity trials established that spatial heterogeneity of crop

response was not distributed randomly: contiguous plots were more likely to

have similar yields than those further apart. Now, although plot size is

constrained on the one hand by the limitations imposed by row-spacing

requirements and edge effects and on the other by field size and the number of

treatments, within a range of, say, 1 to 240 m2,CVs for yields from uniformity

trials over a wide range of crops and soil types are seldom greater than 15 %

and usually around 10 % (Frey and Baten, 1953; Hallauer, 1964; Elliott et al.,

1952). This is remarkably low when we recall the very large CVs mentioned

for soil hydraulic properties in Section IV,B,l. However, the crop is as much a

product of its aerial environment as its soil environment, and relatively little

spatial heterogeneity occurs in the atmosphere at a field scale of 100 ha by

1 m in height, with a uniform crop canopy. In addition, it may be that there is

a periodicity in the autocorrelation interval of soil properties. In other words,

samples which are correlated at small spatial scales may be uncorrelated at

intermediary scales but are again correlated at larger scales. Finally, it must

be remembered that the crop is integrating the environmental effects through

time. Temporal variations observed at hourly or daily scales may be

smoothed out over a season just as averaging procedures themselves may

obliterate critical extreme values which set the limits to growth processes.

Gardner and Gardner (1983) drew attention to this type of variation of

response from one scale to another when they compared the near perfect

linearity which exists between evapotranspiration (water use) and dry matter

production measured on uniform experimental plots with the parabolic

relationship which exists for the same parameters measured on a large field

scale. The departure from linearity was ascribed to nonuniformity of available soil water. In the same paper they demonstrated that nonuniformity of

water distribution lowers yields in crop ecosystems which are relatively well

supplied with water, but where precipitation is less than about 25% of the

optimum, nonuniformity of soil-water distribution may enhance the yield

averaged over large areas, allowing some parts of the crop to achieve nearer

their potential at the expense of others.

It may seem faint hearted to close on so intriguing an enigma. I hope that

the moral to be gained from it, and from other unexpected results which have

been mentioned in this paper, provide us with the stimulus to anticipate

nonuniformity as the norm, not the exception, and to adapt our thinking and



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151



our methods to the heterogeneity of the real world. There is no question that

sufficient field data now exist to convince us of the profound influence that

the soil-pore system has on water and root growth and of the great

importance of maintaining or creating sufficient transmission porosity for

both to function effectively. In many environments we now have the cultural

practices to achieve better soil structures. An exciting challenge exists to

identify and select crop varieties which can best use the water regimes

imposed on these ameliorated soils.



VIII. SUMMARY

Soil structure is defined in terms of the pore system. The transmission

porosity ( - > 50 pm) is responsible for rapid water movement and provides

preexisting channels for root growth. Disruption, or lack of, transmission

pores is common in cultivated soil. This leads to impeded water flow and root

growth within the profile. Recent field studies on soil hydraulic properties

consistently emphasize the role of longitudinally continuous transmission

pores (also termed biopores and macropores).

Structural instability is frequently encountered at the surface of tilled soils.

It gives rise to crusting and translocation of clay into the subsoil. Crusting

can reduce the infiltration rate by up to two orders of magnitude. Organic

matter accumulation at the soil surface, by conservation tillage practices,

provides the most cost-effective remedy. Predictions of time to ponding and

runoff are still seriously hampered by our inability to estimate the instability

potential of many soils.

Spatial and temporal heterogeneity are normal in most soils. High

coefficients of variation occur in soil physical properties, which also have

nonnormally distributed values. Appreciation of these factors has led to the

adoption of geostatistical techniques and scaling methods to measure and

analyze hydraulic properties in the field. These approaches have not yet

extended to root-water uptake studies.

Crop roots have been studied less than crop canopies, but it appears that

considerable inter- and intraspecific variation exists in such properties as

maximum rooting depth, lateral branch production, xylem vessel diameter,

and root hair length. These may be useful for selecting crops of higher wateruse efficiency, but as yet there is a serious lack of unambiguous evidence on

the age and proportion of the root system taking up water, especially during

maturation phases.

Mechanical impedance and transient waterlogging are structurally related

causes of restriction to root development in arable soils. There is evidence

that loosening of zones of high strength improves crop-water relations. The



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effects of nonuniform root distribution on water uptake have not been

extensively researched.

Most water uptake models assume uniform root distribution and uptake

proportional to root length. These assumptions are questioned and examples

are given which show that soil-water status, rather than root length, may

control specific uptake rates in soils of moderate to high hydraulic conductivity. Much conflicting evidence on the existence and magnitude of various

resistances to water uptake in the soil-plant continuum may derive from

assumptions on uniformity of water and root distributions, and consequently

inappropriate sampling and statistical procedures.

ACKNOWLEDGMENTS



I wish to express my appreciation particularly to Dr. D. J. Greenland for his advice on the

paper and for discussion over many years on the subject. My thanks also go to the Western

Australian Department of Agriculture for making the time available and to my colleagues, Dr.

W. B. Bowden, Dr. D. Tennant, Mr. M. W. Perry, and Dr. J. Hamblin for comments on the

paper. I am grateful to Dr. J. B. Passioura and Dr. 1. White of CSIRO for critical discussion on

some sections.

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ADVANCES IN AGRONOMY. VOL 38



GASEOUS HYDROCARBONS

IN SOIL

0. Van Cleemput and A. S. El-Sebaay

Faculty of Agriculture, University of Ghent. Ghent, Belgium



I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Formation, Transformation, and Importance of Gaseous Hydrocarbons . . . . . .

A. Methane.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Ethylene.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . .

C. Ethane and Propane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

111. Environmental Factors Affecting the Evolution of Gaseous Hydrocarbons in Soil.

A. Moisture Content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Soil Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. RedoxPotential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Addition of Chemicals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Gases: H,, O,, C O , . . . . . . . . . . . . . . . . . . . . . . . . . . . . .,. . . . .

G. Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

H. Influence of Plants . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . .

I. Additional Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IV. Sampling and Analysis of the Gaseous Hydrocarbons . . . . . . . . . . . . . . . . .

V. Some Physical and Chemical Properties of the Gaseous Hydrocarbons. . . . . . .

VI. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References. . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



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171

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175

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I. INTRODUCTION

Soil aeration, in general, and the composition of the soil gas phase, in

particular, influence crop growth (Erickson and Van Doren, 1960; Wiersum,

1960; Fulton and Erickson, 1964; Grable, 1966; Grable and Siemer, 1968;

Dasberg and Bakker, 1970; Smith and Robertson, 1971; Cornforth and

Stevens, 1973; Gilman et a/., 1982; and many others). This influence can be

through direct action on plant metabolism, i.e., by reducing energy efficiency,

or through indirect action, by affecting the biological processes occurring in

the soil that are related to plant growth, i.e., the formation of toxic products.

The effect of aeration on certain biological processes occurring, in the soil

has been intensively studied. With good aeration aerobic, oxidative processes

flourish. When aeration is poor anaerobic, reductive processes prevail.

Important examples of soil oxidative processes are the decomposition of

159



Copyright C1 1985 by Academic Press, Inc.

All rights or reproduction in any form reserved.



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0. VAN CLEEMPUT AND A. S. EL-SEBAAY



organic substances, the oxidation of ammonium salts to nitrite and nitrate,

the oxidation of reduced forms of manganese and iron into oxidized forms,

the oxidation of sulfur and sulfur compounds to sulfates, and the oxidation of

hydrogen, methane, and other organic substances formed in the soil. These

oxidation processes are generally beneficial to the soil-plant system, in

contrast to reductive processes, which are often injurious to plant growth.

In flooded or waterlogged soils and in soils with poor internal drainage,

micro-aerobic or anaerobic environments are created. Such conditions

induce a series of reduction reactions, both chemical and biochemical.

Included among these reactions are denitrification, manganese reduction,

iron reduction, and sulfate reduction. In addition to this process, organic

substances can also anaerobically degrade. Some of the decomposition

compounds are methane, ethane, propane, butane, ethylene, butene, propylene, fatty acids, hydroxy and dicarboxylic acids, unsaturated acids, aldehydes, ketones, alcohols, monoamines, diamines, mercaptans, and

heterocyclic compounds (Ponnamperuma, 1965; Smith and Scott Russell,

1969; Smith and Restall, 1971; Smith and Dowdell, 1973; EI-Sebaay, 1981;

Van Cleemput et al., 1981, 1983a,b). Some of these numerous products are

toxic to plants ( e g , ferrous sulfide, nitrite, and ethylene). Of the gaseous

species, methane, ethane, propane, butane, and ethylene are the most

important ones.

Ethylene, when produced in sufficient quantities, causes crop damage

(Pratt and Goeschl, 1969; Smith and Scott Russell, 1969; Smith and Restall,

1971; Osborne, 1968; Rovira and Vendrell, 1972).

The origin, formation, and transformation of the gaseous hydrocarbons

methane (CH,), ethane (C,H,), propane (C,H,), and ethylene (C,H,) in the

soil together with their importance as environmental pollution agents are

given below.



II. FORMATION, TRANSFORMATION, AND IMPORTANCE OF

GASEOUS HYDROCARBONS

A. METHANE

1. Formation



The main gas evolved by aerobic decomposition of organic matter in the

soil is CO,. Under anaerobic conditions, an appreciable amount of CH, is

released. This is due to microbial decomposition of different organic sub-



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