SOIL ORGANIC COLLOIDS

Transcription

1 PART II SOIL ORGANIC COLLOIDS INTRODUCTORY REMARKS To varying extents, humic substances are present in all soils and waters, and are of fundamental importance to the long-term fertility of soils. Soil polysaccharides are present in all soils where microorganisms proliferate, and these substances are important as well because they are considered to have a role in stabilizing and possibly in forming soil aggregates. Microorganisms are, of course, essential to the healthy soil. Without soil microorganisms organic residues would accumulate, and in a short time the world would be buried under plant and animal debris. Traditional farmers have always considered humus and soil organic matter to be of fundamental importance to soil fertility. The maxim of the early agricultural chemists that "Corruption is the Mother of Vegetation" (E.W. Russel, 1950, Soil Conditions and Plant Growth, Longman, London, p.2) had its origins in observations of the influences on plant growth and soil health of farmyard manure, composts, and animal remains. When it became evident that it was the nitrogen and inorganic elements released from decaying organic matter that had the immediate influence on plant growth, the foundations were laid for the development of the fertilizer industry. To a considerable extent, the development of mineral fertilizers has diverted attention from the importance of soil humic and humus substances. Applications of such fertilizers supply the nutrients needed to sustain plant growth, but these do not provide the organic matter essential for the proliferations of soil microorganisms. Thus, where continuous cultivation is practiced, soil microorganisms attack the indigenous soil organic matter, and so the reserves become depleted gradually. This depletion manifests itself in the slow deterioration of soil structure, and in the erosion of the inorganic soil colloids after the soil crumbs are degraded (see Chapters 17 and 18). However, the deleterious effects of such cultivation practices are slow to become manifest in fertile soils, and generous applications of fertilizers and of biocidal chemicals maintain high crop yields for prolonged periods. Unfortunately, the cries of conservationists, and of persons who fear for the food supplies for future generations, are slow to be heeded by those who practice factory farming methods in crop production, and thus the loss of soil organic matter, soil inorganic colloids, and ultimately of soil fertility continues in many parts of the world that provide the 'bread baskets' for numerous millions. 239

2 240 Soil Colloids and their Associations in Aggregates The importance which was attached to the influences of soil organic matter have been reflected in the numerous studies reported in the literature in the past. Early students of composition and structure were hampered because the sophisticated instrumentation needed had not been developed. However, the tools which have become available during the past 10 to 20 years are adequate to allow significant advances to be made, but during that time the numbers of soil scientists interested in humic structures have declined. Too many with power and vested interests hold the view at this time that plausible structures will not be resolved for humic substances, and consider that details of structure are not important. Soil science would not have entered the modem era as a discipline in its own right if similar attitudes guided research on the structures of the inorganic colloidal components of soils. Many of the earlier studies of the structures of soil humic substances were influenced by the work of coal chemists, although that influence has waned during the past thirty years. But, because the importance of humic substances in waters has become recognised in recent times, water chemists and sedimentologists have been able to provide a new and welcome impetus to studies of humic composition and structure. However, until recently, soil, coal, and water scientists communicated largely within their own groups, and there was little exchange of ideas or sharing of techniques between the different groups at the beginning of the present decade. To overcome this, scientists from the three disciplines convened in Denver, Colorado, in September, 1981 and founded the International Humic Substances Society (IHSS). Their hope that the new Society would bridge the gap between the disciplines has been amply rewarded. At the first international meeting of IHSS, held in Estes Park, Colorado, in 1983, soil, coal, and water scientists convened in significant numbers to discuss aspects of the geochemistry, isolation, and characterisation of humic substances from all sources. It was clear from that meeting that there was a lack of awareness in each group of the techniques and ideas found useful by others. However, by the time the second international meeting of IHSS had convened in Birmingham, in July 1984, and discussed structures and interactions of humic substances, it was evident that the different groups had learned much from each other. The publications from the Estes Park and Birmingham meetings (Aiken et az., 1985; Hayes and Swift, 1985; Hayes et az., 1989; MacCarthy et at., 1989; referenced in Chapter 10) have outlined what we know, what we think we know, and what wo do not know about aspects of the geochemistry, isolation, composition, structures and interactions of humic substances from soil, coal, and freshwater environments. These works provide adequate reviews of the techniques and concepts that have guided research in humic substances, and provide a necessary base for the development of the science in the immediate future. In Chapter 10 Dr. Michael H.B. Hayes and Professor Roger S. Swift have provided the definitions as used at this time for humic acids, fulvic acids, and humin, the gross fractions of humic substances. The authors have then progressed to a discussion of genesis and of the isolation of the substances from soil, and of the fractionation of components of the gross fractions on the basis of molecular size, charge, sorption characteristics, and other properties. They have outlined the principles involved in the isolation and fractionation procedures commonly used, and provided enough basic information to allow persons new to the field to make choices based on structural properties of reagents and of substrates.

3 Introduction to Part II 241 In their short treatment of genesis, the authors have referred to some of the numerous processes which can lead to the formation of humic substances. Their ouline describes some of the theories of genesis which have been put forward over the past 100 years, and it is clear that some of the theories had more adherents than they deserved. Some theories have also influenced thinking for longer than they should because appropriate though often simple instrumentation was not always available to compare naturally occurring with the synthetic humic substances. It is possible too that the 'lifetime' of the less sound theories would have been shortened significantly if scientists from the several disciplines with interests in humic substances had available the necessary forum for convention, and for peer scrutiny of concepts and products. Although techniques and procedures such as spectroscopy and titration have helped greatly our understanding of many of the functional groups in humic substances, they have not helped our knowledge of the distributions of the groups, and of their juxtapositions in the macromolecules, and they have not given indications of the structures to which the functional groups are attached. It may never be possible to establish accurately the exact juxtapositions of the functional groups, but it is highly probable that there will evolve sufficient awareness of the positioning to allow to evolve an appropriate concept of the complexes which are characteristic of soil humic substances. Hayes and Swift have outlined in Chapter 10 the difficulties inherent in establishing the structural units to which the functional groups are attached, or in other words in determining the 'building blocks' of the 'core' or 'backbone' structures of humic macromolecules. The 'core' or 'backbone' is considered to consist of the components of the macromolecules which persist when the hydro1ysab1e moieties are removed. These authors have outlined how the high energy inputs required to cleave the 'backbone' alter the structures of the component units, and they have stressed the need to understand fully the mechanisms of the degradation reactions in order to be able to make plausible predictions of the components in the macromolecules which give rise to the compounds identified in the degradation digests. As yet, no procedure has been used which can degrade all of the humic macromolecules to compounds that are identifiable and provide meaningful indications of the component molecules in the macromolecular structures. It will not be possible to make comprehensive predictions of all of the component molecules until each of the structural units, or derivatives of such units in the macromolecules are freed and identified. It is folly, on the basis on the instrumentation available, to predict that spectroscopic procedures will provide such details of structures in the near future. Studies of the behaviour of humic macromolecules in solution have made it possible to make useful predictions about the sizes and shapes of the macromolecules. The data emphasise the extreme po1ydispersity in the sizes and charge characteristics of the macromolecules. It would appear, however, that the shapes of the macromolecules (in solution) are relatively simple and may be described by random coil conformations, and there is evidence for some branching in the cases of the larger molecules. A knowledge of the solution conformations and of the composition of the macromolecules allows predictions of shapes and associations in the solid and gel states. Such information is very useful for considerations of the behaviour of humic substances in the soil environment.

4 242 Soil Colloids and their Associations in Aggregates The importance of polysaccharides in soils was recognised about 50 years ago, but there is not yeat a full acceptance by soil scientists of the important role which we consider that these play in stabilizing, and possibly also in forming soil aggregates. In Chapter 11, Dr. Martin V. Cheshire and Dr. Michael H.B. Hayes have outlined the evolution of awareness of the role and composition of soil polysaccharides. They have referred to the origins of the awareness, and then discussed procedures for the extraction of polysaccharides from soil, and for the fractionation of these extracts into components that are relatively homogeneous with respect to size and charge density. It can be stated with conviction that there is biological control of the synthesis of soil polysaccharides. (Certainly there can be no comparable conviction for considerations of the origins of soil humic substances.) Such control of synthesis, and the lability of the glycosidic linkages of polysaccharides to hydrolysis should make determinations of composition and structure relatively simple. However, the evidence we have suggests that soil microorganisms are predominantly responsible for the synthesis of soil polysaccharides. Thus, because of the enormous variety of microorganisms in the healthy soil environment, there is likely to be a highly complicated variety of polysaccharide structures in every soil where such organisms proliferate. Hence it is very difficult to isolate pure polysaccharide substances from soil. In fact, there has not, as yet, been any pure polysaccharide substance obtained from soil, although mixtures have been isolated which were relatively homogeneous with respect to molecular weight and charge density properties. There is some evidence to suggest that the nature of the glycosidic linkages, at least in so far as the a- and ~ - c o n f i g uare r concerned, a t i o n s influence the interactions of the polymers with clays, and possibly with other inorganic soil colloids as well. Sufficient use has not been made of procedures which isolate from clays and oxyhydroxides polysaccharides that were sorbed to these from mixtures that were relatively homogeneous with regard to size and to charge characteristics. Such experimentation could provide useful supplements to the more conventional purification and fractionation techniques, and it could focus attention on the structures which may well be most important for aggregation of the inorganic soil colloids and for the stabilization of soil aggregates. It seems worthwhile also to consider the isolation of polysaccharides from the clay-sized fraction of soils, rather than from whole soils. In Chapter 12, Dr. Richard Bums brings into focus the importance of soil microorganisms and enzymes at soil colloid surfaces. Soil chemists tend to dismiss the influence of microorganisms, and often use blanket terms which attribute to the microbial population processes and reactions which cannot be explained by the chemistry of soil colloids and of the soil solution. Dr. Bums has left us in no doubt about the influences of the 'teeming millions' of microorganisms (l x 10 7 to 1 X 1010 per gram of dry soil) in fertile soils, and those who design and construct models for predictions of soil performance should have an awareness of details of the roles which the biota play in soil, if their models are to have relevance. We tend to forget the different habitats which soil provides for microorganisms, and that the environment of the microbe at the centre of a 2 mm soil aggregate is 'a world apart' from that of the organism at the aggregate surface, in so far as protection from predators, and from fluctuating moisture stresses is concerned. Dr. Bums has stressed the common pitfalls which attract many who sample soil microorganisms. From the discussion it appears to us that taking soils from the

5 Introduction to Part II 243 different horizons in a profile, mixing the soils, and sampling the mixture has as much relevance, with regard to the microbial population in the soil, as mixing all the people and animals of the world, taking a sample of these at random, and deciding that the sample provides a valid representation of the distribution of the different mammals anywhere in the world. The microbial populations in the various horizons in the soil profile are as different as are the distributions of mammalian species throughout the different climatic and vegetative regions of the world. The discussion by Dr. Bums of the soil microenvironment is of great relevance to the topics of this book, and to those in the second in the series which discusses interactions at the soil colloid - soil solution interface. Because the soil microenvironment is dominated by the soil colloids, the organic substances for microorganisms tend to accumulate at the surfaces of the colloids, and the soil microorganisms tend to gather there too because of the enrichment of food and water. The soil gas is influenced as well by the environment, and the composition of the gas, in solution and free, is greatly different from that above the soil. Nowadays, immobilized enzymes are important in industry. Few are aware though that soil colloids have always been repositories of such enzymes. Dr. Bums has provided a lucid account of the possible ways in which soil humic substances, and humic acids in particular, preserve enzymatic activity against the ravages of heat, drought, and predator organisms. The protection mechanisms are relevant also to discussions in Sections 10.7 and 10.8, and these could involve steric shielding by the humic macromolecules, as well as associations, through covalent linkages and van der Waals forces between the enzymes and the humic 'stationary' phases. The fact that enzymatic activity is preserved, though decreased, is relevant to experiments which have shown that 'copolymer' synthetic humic acids are formed from reactions of polymerizing compounds, such as p-benzoquinone, with enzymes. Enzymatic activity was also preserved when heating and drying conditions were introduced. There was, however, no protection of activity provided when enzymes were added to systems containing 'humic acids' synthesized in the absence of the enzymes. In fact, enzymes added to such 'humic acids' were readily desorbed by washing with water or dilute buffers. The foregoing observation raises a question with regard to the time of incorporation of enzymes into soil humic acids. It is possible, of course, that microbial enzymes reacted with functional groups on the 'backbone' structures during the formation of the humic acids, and in this way became integrated early in the macromolecular substances (see Sections 10.3, 10.7, and 10.3). An appropriate experiment might add enzymes to naturally occurring humic acids in solution and then cause the loosely coiled macromolecular structures (Section 10.6) to collapse by adjusting the ph downwards, or by adding divalent or polyvalent cations to the medium. Applications of heat to the humic mixture in the gel state would accelerate chemical interactions between the trapped enzyme and the humic acid, and such would also test the protection afforded by the humic acids to the active sites in the enzyme. Desorption experiments would require expansion of the macromolecules, in alkaline buffers or dilute alkali solutions, and the use of gel filtration procedures. Dr. Burns has summarised well the nature of associations of microorganisms with clays. Because most microorganisms are negatively charged in the normal range of soil ph values, the natural tendency is for clays to repel them. There are, however,

6 244 Soil Colloids and their Associations in Aggregates several mechanisms by which the organisms can counter the repelling effects, and these include uses of the divalent and polyvalent metals which bridge the negative charges on the different surfaces, and the uses of sticky extracellular macromolecules, such as polysaccharides. The involvement of polysaccharides caused us to suggest earlier that extractions of these polymers from the clay fraction might provide the polysaccharide components which are relevant to soil aggregate formation and stabilization.