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Chapter 1. A Lifetime Perspective on The Chemistry of Soil Organic Matter

Chapter 1. A Lifetime Perspective on The Chemistry of Soil Organic Matter

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M. SCHNITZER



VI.



VII.

VIII.



IX.

X.



XI.

XII.



H. Phosphorus in Soils and SOM

I. Sulfur Compounds in Soils and Humic Substances

Colloid Chemical Characteristics of Humic Acids and Fulvic Acids

A. Surface Tension, Surface Pressure, and Viscosity Measurements on HAs

and FAs

Water Retention by Humic Substances

Reactions of Humic Substances with Metals and Minerals

A. Formation of Water-Soluble Complexes

B. Mixed Ligand Complexes

C. Adsorption and Desorption

D. Dissolution of Minerals

E. Adsorption on External Mineral Surfaces

F. Adsorption in Clay Interlayers

Interactions of Pesticides and Herbicides with Humic Substances

Functions and Uses of Humic Substances

A. Functions in Soils

B. Uses and Potential Uses

Conclusions and Outlook for the Future

Personal Encounters with Outstanding Scientists

References



The author has researched the chemistry of soil organic matter for almost 50

years. In this chapter, he presents a personal account of how soil organic matter

chemistry has evolved during the second half of this century from wet to computational chemistry. The chapter begins with a definition of soil organic matter and

how it relates to humus and humic substances. Problems associated with the extraction of organic matter from soils, separation of the extract into humic substances, and purification of the resulting fractions are then discussed. New experimental approaches to the in situ analysis of organic matter in whole soils to

overcome these problems are described. Investigations on the chemistry of soil organic matter are outlined in terms of (a) an analytical and (b) a structural approach.

The analytical approach involves determinations of the characteristics of humic

substances by chemical methods, infrared, 13C nuclear magnetic resonance, electron spin resonance spectroscopy, and electron microscopy, whereas the structural

approach consists of oxidative and reductive degradations, pyrolysis–field ionization mass spectrometry, and Curie-point pyrolysis–gas chromatography/mass

spectrometry. The author recounts how the results of the analytical and structural

studies led to the formulation of a two-dimensional humic acid model structure and

how the latter was converted with the aid of computational chemistry to a threedimensional humic acid model structure and later to three-dimensional model

structures of soil organic matter and whole soils. The next topics discussed by the

author are advances in the chemistry of N-, P-, and S-containing components of

soil organic matter. Especially noteworthy is progress in the chemistry of N in soil

organic matter, which points to a prominent role of heterocyclic N. As far as colloid-chemical characteristics of humic substances are concerned, the three parameters that control the molecular characteristics (molecular weight, size, and shape)

of humic and fulvic acids are (a) the concentration of the humic substance, (b) the

pH of the system, and (c) the electrolyte concentration of the medium. In the last



A LIFETIME PERSPECTIVE



3



part of the chapter, the author discusses how humic substances interact with water,

metals, minerals, pesticides, and herbicides; lists functions and uses of humic substances; and describes personal encounters with outstanding scientists who influenced his research.

© 2000 Academic Press.



I. INTRODUCTION

I thank Dr. D. L. Sparks for not only inviting me to write this chapter but also

for suggesting the title. After almost 50 years of continuous research on the chemistry of soil organic matter (SOM), I have learned a lot about this complex material, and I am pleased to have the opportunity to communicate some of this knowledge to readers. This chapter is not a literature review but a personal account of

how SOM chemistry has evolved during the second half of this century and what

the prospects for the future are. Over the years, the success of SOM chemists in

dealing with these complex materials depended to a large extent on how well they

could adapt newly developed methods and instruments to SOM. In the late 1940s,

wet chemistry done in beakers, flasks, and test tubes was predominant. The major

instruments that were available to me at that time were pH meters, powered by batteries, and colorimeters requiring filters for changing wavelengths. In the early

1950s, recording ultraviolet (UV) spectrophotometers became available, and in the

mid-1950s, I remember convincing my director to purchase an infrared (IR) spectrophotometer. The early 1960s saw the arrival of gas chromatographs. This was

an important development because it allowed us to separate complex mixtures of

humic acid (HA) and fulvic acid (FA) oxidation products, along with organic soil

extracts containing alkanes, alkenes, fatty acids, and esters. In the mid-1960s we

purchased a mass spectrometer, which we attached to a gas chromatograph. This

allowed us to not only separate complex mixtures of organics but also to identify

the separated compounds. About the same time, we saw the arrival of an electron

spin resonance (ESR) spectrometer, which enabled us to measure concentrations

of free radicals in humic materials and to obtain information on the nature of free

radicals. ESR also helped us throw light on the symmetry and coordination of paramagnetic metals such as Fe3+, Cu2+, and Mn2+ bound to HA and FA. In the early

1980s, we purchased a liquid-state 13C nuclear magnetic resonance (NMR) spectrometer. After we had learned how to use this instrument properly, we realized

that 13C NMR was of great importance to SOM chemists. It showed, for the first

time, that aliphatic C in HAs and FAs was as important as aromatic C and that the

older theories that HAs were almost completely aromatic were no longer valid for

SOM in most agricultural soils. Finally, the mid-1980s saw the arrival of pyrolysis–field ionization mass spectrometry (Py-FIMS), which we applied to the in situ



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M. SCHNITZER



analysis of SOM, i.e., the direct analysis of SOM in whole soils, without extractions, purifications, etc. Another application of mass spectrometry was Curie-point

pyrolysis–gas chromatography/mass spectrometry (GS/MS), which we used in

structural studies on HAs and which resulted in proposing a two-dimensional

structural HA model. We then converted the latter by computational chemistry into

a three-dimensional HA model structure. We similarly generated three-dimensional model structures for SOM and a whole soil with both inorganic and organic constituents. Thus, during the past 50 years, I was fortunate to have participated, along with other scientists, in the evolution of SOM chemistry from wet

chemistry to computational chemistry.

In addition to studies on the chemical structure of humic substances, I also

worked on determining colloid chemical properties of these materials, mechanisms of water retention, reactions with metals and minerals, and with pesticides

and herbicides. Thus, the overall objective of my research was to investigate the

chemical structure and reactions of humic substances. It was and still is my hope

that the results of this research will assist soil scientists, agronomists, and farmers

in the development of more efficient management and production systems so that

they can grow sufficient food for an increasing population.

At the end of the chapter, I describe personal encounters with some outstanding

scientists of the past 50 years.



II. SOIL ORGANIC MATTER (SOM)

A. DEFINITIONS

The term “soil organic matter,” as used in this chapter, refers to the sum total of

all organic carbon-containing substances in the soil. SOM consists of a mixture of

plant and animal residues in various stages of decomposition, substances synthesized microbiologically and/or chemically from the breakdown products, and the

bodies of live and dead microorganisms and their decomposing remains (Schnitzer

and Khan, 1978). Solid-state 13C NMR spectra of whole soils show the presence

of paraffinic C, OCH3-C, amino acid-C, C in carbohydrates and aliphatic structures bearing OH groups, aromatic C, phenolic C, and C in CO2H groups (Arshad

et al., 1988). From the 13C NMR spectrum, aromaticity and aliphaticity of SOM

can be calculated. Resulting data show that the C aromaticity of SOM seldom exceeds 55% and that the aliphaticity of SOM is often greater than its aromaticity

(Schnitzer and Preston, 1986). Similarly, Py-FIMS of SOM in whole soils indicates the presence of carbohydrates, phenols, lignin monomers, lignin dimers,

alkanes, fatty acids, n-alkyl mono,di, and tri esters, n-alkylbenzenes, methylnaphthalenes, methylphenanthrenes, and N compounds (Schnitzer and Schulten, 1992).

Carbohydrates, proteinaceous materials (amino acids, peptides, proteins), and



A LIFETIME PERSPECTIVE



5



lipids (alkanes, alkenes, saturated and unsaturated fatty acids, alkyl mono, di, and

tri esters) in SOM appear to be strongly retained by the aromatic SOM components and can only be separated from them with great difficulty. For example, even

after exhaustive extractions with n-hexane, followed by chloroform, Schnitzer and

Schuppli (1989) could remove only 10% of the total lipids from three agricultural soils sampled in western Canada. The separation of carbohydrates and proteinaceous materials from SOM requires prolonged hydrolyses with relatively

strong acids under reflux. Thus, the different chemical components of SOM are

closely associated to form a complex structure.



B. RELATIONSHIP AMONG SOM, HUMUS,

AND HUMIC SUBSTANCES

There is some confusion among soil chemists about the meanings of SOM, humus, and humic substances. Do these terms depict different materials? According

to Stevenson (1994), SOM is synonymous with humus. In my opinion, the term

total humic substance is also synonymous with SOM and humus as long as losses

occurring during the extraction and separation procedures are held to a minimum.

My definition of humic substances is that it is the sum of humic acid ϩ fulvic acid

ϩ humin. While essentially each of the three terms can be used, I personally, as a

SOM chemist, prefer use of the term SOM.



C. PROBLEMS ASSOCIATED WITH EXTRACTION

OF SOM FROM SOILS

The SOM content of agricultural soils usually ranges between 1 and 4% (w/w),

with most soils containing between 2 and 3% SOM. In the soil, because SOM and

inorganic soil constituents are closely associated, it is necessary to separate the two

before either can be examined in greater detail. This separation is usually achieved

by extracting the SOM with either dilute base (0.1–0.5 M NaOH solution) or by a

neutral salt solution such as aqueous 0.1 M Na4P2O7. Extraction of SOM with a

dilute base works reasonably well and was originated by Archard in 1786. Separation of the alkaline extract into HA, FA, and humin was first carried out by Sprengel (1826). The three fractions into which the alkaline SOM extract is partitioned

are (1) HA, which is that fraction of SOM that coagulates when the alkaline extract is acidified; (2) FA, which is the SOM fraction that remains in solution when

the extract is acidified, i.e., it is soluble in both acid and alkali; and (3) humin,

which is that SOM fraction that remains with the soil, i.e., it is insoluble in both

alkali and acid. Over the years, many objections have been raised against the use

of alkaline solutions, which are still the most efficient SOM extractants today.

Stevenson (1994) lists the following objections: (1) silica is dissolved from the



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