Soil Organic Matter PDF
Preview Soil Organic Matter
Developments in Soil Science 8 SOIL ORGANIC MATTER Edited by M. SCHNITZER Soil Research Institute Agr ic u 1 t u re Canada Ottawa, Ont., Canada and S.U. KHAN Chemistry and Biology Research Institute Agriculture Canada Ottawa, Ont., Canada ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam Oxford New York 1978 ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U.S.A. First edition 1978 Second impression 1983 Third impression 1985 Fourth impression 1989 ISBN 0-444-4 16 10-2 (Vol. 8) ISBN 0-444-40882-7 (Series) 0 Elsevier Science Publishers B.V., 1978 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences & Engineering Division, P.O. Box 330, lo00 AH Amsterdam, The Netherlands. - Special regulations for readers in the USA This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any meth- ods, products, instructions or ideas contained in the material herein. Printed in The Netherlands List of Contributors V.O. BIEDERBECK Research Station, Agriculture Canada Swift Current, Sask., Canada C.A. CAMPBELL Research Station, Agriculture Canada Swift Current, Sask., Canada S.U. KHAN Chemistry and Biology Research Institute Agriculture Canada Ottawa, Ont., Canada C.G. KOWALENKO Soil Research Institute, Agriculture Canada Central Experimental Farm Ottawa, Ont., Canada L.E. LOWE The University of British Columbia Department of Soil Science Vancouver, B.C., Canada M. SCHNITZER Soil Research Institute, Agriculture Canada Central Experimental Farm Ottawa, Ont., Canada PREFACE Soil organic matter, a key component of soils, affects many reactions that occur in these systems. In spite of this, soil organic matter remains a neg- lected field in soil science and receives but scant attention in soil science courses. One of the purposes of this book is to remedy this situation and to provide researchers, teachers and students with an up-to-date account of the current state of knowledge in this field. The first three chapters of the book deal with the principal components of soil organic matter, that is, humic substances, carbohydrates and organic nitrogen-, phosphorus- and sulfur-containing compounds. In Chapter 4 reac- tions between soil organic matter and pesticides are discussed, whereas Chapters 5 and 6 are concerned with the more practical aspects of soil organic matter. The author of each chapter is an active researcher in the field about which he is writing. We were hoping that the direct involvement that each author has with his subject would result in a more adequate and relevant book. Hopefully, the book will be of interest not only to soil scientists and agronomists but also to oceanographers, water scientists, geochemists, environmentalists, biologists and chemists who are concerned with the role of organic matter in terrestrial and aquatic systems. Ottawa, April 1977 M. Schnitzer S.U. Khan Chapter 1 HUMIC SUBSTANCES: CHEMISTRY AND REACTIONS M. SCHNITZER INTRODUCTION Humic substances, the major organic constituents of soils and sediments are widely distributed over the earth’s surface, occurring in almost all terres- trial and aquatic environments. According to recent estimates of Bohn - (1976), the mass of soil organic C (30.0 lOI4 kg) more than equals those of - other .surface C reservoirs combined (atmospheric CO, = 7.0 lOI4 kg, bio- - - mass C = 4.8 lOI4 kg, fresh water C = 2.5 lOI4 kg, and marine C = 5.0- 8.0 * lOI4 kg). Because between approximately 60-70% of the total soil-C occurs in humic materials (Griffith and Schnitzer, 1975a), the role of humic substances in the C cycle as a major source of CO, and as a C reservoir that is sensitive to changes in climate and atmospheric CO, concentrations has cer- tainly been underestimated. According to Bohn (1976), the decay of soil organic matter provides the largest CO, input into the atmosphere. It is true that deeper C deposits in the form of marine organic detritus, coal and pe- troleum, deep sea solute C and C in sediments are much larger, but these are physically separated from active interchange with surface C reservoirs (Bohn, 1976). Humic substances arise from the chemical and biological degradation of plant and animal residues and from synthetic activities of microorganisms. The products so formed tend to associate into complex chemical structures that are more stable than the starting materials. Important characteristics of humic substances are their ability to form water-soluble and water-insoluble complexes with metal ions and hydrous oxides and to interact with clay min- erals and organic compounds such as alkanes, fatty acids, dialkyl phthalates, pesticides, etc. Of special concern is the formation of water-soluble com- plexes of fulvic acids (FA’S) with toxic metals and organics which can in- crease the concentrations of these constituents in soil solutions and in natu- ral waters to levels that are far in excess of their normal solubilities. Chemical investigations on humic substances go back more than 200 years~ (Kononova, 1966; Schnitzer and Khan, 1972). The capacity of humic sub- stances to adsorb water and plant nutrients was one of the first observations. Humic substances were thought to arise from the prolonged rotting of ani- mal and plant bodies. Since that time several thousand scientific papers have been written on humic materials, yet much remains to be learned about their 2 HUMIC SUBSTANCES: CHEMISTRY AND REACTIONS origin, synthesis, chemical structure and reactions and their functions in terrestrial and aquatic environments. Soils and sediments contain a large variety of organic materials that can be grouped into humic and non-humic substances. The latter include those whose physical and chemical characteristics are still recognizable, such as car- bohydrates, proteins, peptides, amino acids, fats, waxes, and low-molecular weight organic acids. Most of these compounds are attacked relatively readily by microorganisms and have usually only a short life span in soils and sedi- ments. By contrast, humic substances exhibit no longer specific physical and chemical characteristics (such as a sharp melting point, exact refractive index and elementary composition, definite IR spectrum, etc.) normally associated with well-defined organic compounds. Humic substances are dark-coloured, acidic, predominantly aromatic, hydrophilic, chemically complex, poly- electrolyte-like materials that range in molecular weights from a few hundred to several thousand. These materials are usually partitioned into the follow- ing three main fractions: (a) humic acid (HA), which is soluble in dilute alkali but is precipitated on acidification of the alkaline extract; (b) fulvic acid (FA), which is that humic fraction which remains in solution when the alkaline extract is acidified; that is, it is soluble in both dilute alkali and acid; (c) humin, which is that humic fraction that cannot be extracted from the soil or sediment by dilute base or acid. From analytical data published in the literature (Schnitzer and Khan, 1972) it appears that structurally the three humic fractions are similar, but that they differ in molecular weight, ulti- mate analysis and functional group content, with FA having a lower molec- ular weight, containing more oxygen but less carbon and nitrogen, and having a higher content of oxygen-containing functional groups (CO,H,OH, C = 0) per unit weight than the other two humic fractions. The chemical structure and properties of the humin fraction appear to be similar to those of HA. The insolubility of humin seems to arise from it being firmly adsorbed on or bonded to inorganic soil and sediment constituents. The observed resis- tance to microbial degradation of humic materials appears to a significant extent also to be due to the formation of stable metal and/or clay-organic complexes. SYNTHESIS OF HUMIC SUBSTANCES The synthesis of humic substances has been the subject of much specula- tion. Felbeck (1971) lists the following four hypotheses for the formation of these materials. (a) The plant alteration hypothesis. Fractions of plant tissues which are re- sistant to microbial degradation, such as lignified tissues, are altered only superficially in the soil to form humic substances. The natare of the humic substance formed is strongly influenced by the nature of the original plant EXTRACTION OF HUMIC SUBSTANCES 3 material. During the first stages of humification high-molecular weight HA’s and humins are formed. These are subsequently degraded into FA’S and ulti- mately to COz and HzO. (6) The chemical polymerization hypothesis. Plant materials are degraded by microbes to small molecules which are then used by microbes as carbon and energy sources. The microbes synthesize phenols and amino acids, which are secreted into the surrounding environment where they are oxidized and polymerized to humic substances. The nature of the original plant material has no effect on the type of humic substance that is formed. (c) The cell autolysis hypothesis. Humic substances are products of the autolysis of plant and microbial cells after their death. The resulting cellular debris (sugars, amino acids, phenols, and other aromatic compounds) con- denses and polymerizes via free radicals. (d) The microbial synthesis hypothesis. Microbes use plant tissue as car- bon and energy sources to synthesize intercellularly high-molecular weight humic materials. After the microbes die, these substances are released into the soil. Thus, high-molecular weight substances represent the first stages of humification, followed by extracellular microbial degradation to HA, FA and ultimately to COz and HzO. It is difficult to decide at this time which hypothesis is the most valid one. It is likely that all four processes occur simultaneously, although under cer- tain conditions one or the other could dominate. However, what all four hy- potheses suggest is that the more complex, high-molecular weight humic ma- terials are formed first and that these are then degraded, most likely oxida- tively, into lower molecular weight materials. Thus, the sequence of events -, appears to be HA FA. EXTRACTION OF HUMIC SUBSTANCES The organic matter content of soils may range from less than 0.1% in desert soils to close to 100.0% in organic soils. In inorganic soils, organic and inorganic components are so closely associated that it is necessary to first separate the two before either component can be studied in greater detail. Thus, extraction of the organic matter is generally the first major operation that needs to be done. The most efficient and most widely used extractant for humic substances from soils is dilute aqueous NaOH (either 0.1 N or 0.5 N) solution. While the use of alkaline solutions has been criticized, there seems to be little evidence to show that dilute alkali under an atmosphere of N2 damages or modifies the chemical structure and properties of humic materials. Thus, a HA extracted with 0.5% NaOH solution had similar light absorbance characteristics as the same HA extracted with 1%N aF solution (Scheffer and Welte, 1950; Welte, 1952). Other workers (Rydalevskaya and Skorokhod, 1951) found no substantial differences in elementary composi- 4 HUMIC SUBSTANCES: CHEMISTRY AND REACTIONS tion and C02H content between HA’S extracted by 1%N aF and 0.4% NaOH solutions from soils and peats. Similarly, Smith and Lorimer (1964) report that HA’s extracted with dilute Na4P2O7f rom peat soils resembled in all re- spects HA’s extracted with dilute NaOH solution. Schnitzer and Skinner (1968a) extracted FA from a Spodosol Bh horizon under Nz with 0.5 N NaOH and with 0.1 N HCl. Following purification, each extract was charac- terized by chemical and spectrophotometric methods and by gel filtration. The elementary composition of the two materials was very similar and oxy- gencontaining functional groups were of same order of magnitude. Also, IR spectra of both preparations and their fractionation behaviour on Sephadex gels were practically identical. The concentration of the NaOH solution affects the yield of the humic material extracted as well as its ash content. Ponomarova and Plotnikova (1968) and Levesque and Schnitzer (1966) found 0.1 N NaOH to be more efficient than higher NaOH concentrations. However, the most suitable ex- tractant for isolating humic materials low in ash was either 0.4 N or 0.5 N NaOH solution (Levesque and Schnitzer, 1966). Neutral salts of mineral and organic acids have been used for the extrac- tion of humic substances, but yields are usually low. Bremner and Lees (1949) suggested the use of 0.1 M Na-pyrophosphate solution at pH 7 as the most efficient extractant. The action of the neutral salt was thought to depend on the ability of the anion to interact with polyvalent cations bound to humic materials to form either insoluble precipitates or soluble metal complexes, and the formation of a soluble salt of the humic material by re- acting with the cation of the extractant as illustrated by the following reac- tion : R(C00)4 Caz + Na4P20 , + R(COONa), + Ca2Pz0 , (1) According to Alexandrova (1960), Na,P20, solution extracts not only humic substances but also organo-mineral complexes without destroying non- silicate forms of sesquioxides. The efficiency of extraction can be improved by raising the pH from 7.0 to 9.0 (Kononova, 1966) and increasing the temper- ature (Livingston and Moe, 1969; Lefleur, 1969). Kononova and Bel’chikova (1961) recommend the use of a combination of 0.1 M’Na4P207+ 0.1 N NaOH (pH -13). Use of this mixture also avoids decalcification of soils with high pH prior to extraction. Humic materials extracted by the mixture are low in N, (Donnaar, 1972; Vila et al., 1974) and show lower molecular weights and E4/E6 ratios, different electrophoretic patterns and behavior on gel filtration than do humic materials extracted from similar soils with 0.1 N NaOH (Vila et al., 1974). Schnitzer et al. (1958) showed that pyrophos- phate was difficult to remove from humic materials during purification. Other approaches that have been employed for the extraction of or- ganic matter from soils involve treatment with chelating resins (Levesque FRACTIONATION AND PURIFICATION 5 and Schnitzer, 1967; Dormaar, 1972; De Serra and Schnitzer, 1972). Humic materials extracted with the aid of a chelating resin were more polymer- ized than those extracted by dilute alkali. Another technique that has been used by a number of workers is ultrasonic dispersion (Edwards and Bremner, 1967; Leenheer and Moe, 1969; Watson and Parsons, 1974; Ander- son et al., 1974). Several attempts have been made to extract humic substances with organic solvents. Martin and Reeve (1957a, b) found that acetyl acetone was an effective extractant for organic matter from Spodosol Bh horizons. Porter (1967) used an acetone-water-HC1 system, while Parsons and Tinsley (1960) employed anhydrous formic acid + 10% acetyl acetone to extract organic matter from a calcareous meadow soil. Hayes et al. (1975) compared humic materials extracted from an organic soil by thirteen extractants, which included dipolar aprotic solvents, pyridine, ethylenediamine, organic chelating agents, ion exchange resins, Na4PZ0, and NaOH. Of the two re- agents that were most efficient, ethylenediamine was found by Electron Spin Resonance Spectrometry and elementary analyses to alter the chemical na- ture and composition of the extract while dilute NaOH solution was regard- ed as the more reliable extractant. The danger with using organic solvents containing C and N for extracting organic matter is that under these condi- tions C and N may be added irreversibly to the humic materials and so alter their composition and properties. A number of workers have extracted humic substances by sequential ex- traction, using different reagents (Duchaufour and Jacquin, 1963; Smith and Lorimer, 1964; Gascho and Stevenson, 1968; Goh, 1970). Felbeck (1971) suggests the following sequence: (a) benzene-methanol; (b) 0.1 N HCl; (c) 0.1 M Na4P, Q; (d) 6 N HC1 at 90°C; (e) 5 : 1 chloroform-methanol; and (f) 0.5 N NaOH. By using a sequence of solvents rather than one solvent, a se- ries of fractions can be obtained which may be more homogeneous than the material extracted by one extractant only. FRACTIONATION AND PURIFICATION The classical method of fractionation of humic substances is based on dif- ferences in solubility in aqueous solutions at widely differing pH levels, in alcohol and in the presence of different electrolyte concentrations (Fig. 1). The major humic fractions are HA, FA and humin. Fractionation of HA into hymatomelanic acid or into gray HA and brown HA is not done very often. One may wonder how useful such separations are. Additional methods of fractionation of humic substances that have been tried over the years include treatment with tetrahydrofuran, containing in- creasing percentages of water (Salfeld, 1964; Martin et al., 1963), mixtures of dimethylformamide and water (Otsuki and Hanya, 1966), salting out with 6 HUMIC SUBSTANCES: CHEMISTRY AND REACTIONS soil or sediment I exi ract in sol uble zsotuble HUMIN I precipitate soluble I I IFA HA extract with alcohol I soiuble (hyrnatornelanic acid) redissblve in base and add elect- rolyte precipitated not precipitated (gray HA) (brown HA) Fig. 1. Fractionation of humic substances. ammonium sulfate (Theng et al., 1968), varying the ionic strength and pH of pyrophosphate and sodium hydroxide extracting solutions (Lindqvist, 1968), addition of increasing amounts of metal ions such as PbZ+,B aZ+a nd Cu2+ (Sowden and Deuel, 1961) and adding increasing volumes of ethanol to alka- line solutions containing HA’s (Kyuma, 1964). Freezing methods have also been used (Karpenko and Karavayev, 1966; Archegova, 1967) for this purpose. In recent years, gel filtration has been widely used for the fractionation of soil humic materials. This technique has also been employed for the separa- tion of aquatic humus (Gjessing, 1976). Schnitzer and Skinner (196813) pre- pared seven fractions from a FA by carrying out a series of sequential col- umn chromatographic separations using different Sephadex gels. The frac- tions differed in elementary analysis and functional group content, number- average molecular weights and IR and NMR spectra. Swift and Posner (1971) studied the behavior of HA’s on Sephadex and a number of other gels with a variety of eluants. They found that fractionation based solely on molecular weight differences could be achieved by using alkaline buffers containing large amino cations. They warn that in cases where gel-solute interactions could occur, fractionations based on differences in molecular weights would not be possible. Column chromatography on activated charcoal has been used by Forsyth (1947) for the separation of HA’s. Other workers (Dragunov and Murzakov, 1970) have employed Al2O3i n addition to charcoal.
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