Definitions

Though they are commonly used in both common and technical usage, neither of the terms, 'composting' or 'humic substances' is well defined. In some cases, the term composting is used to define very general processes, as in:

The biological decomposition of organic matter.

Composting can also be defined operationally and more specifically as:

A controlled biological process which converts organic constituents, usually wastes, into humus-like material suitable for use as a soil amendment or organic fertilizer.

In general, most definitions ofcomposting consider it as a process that is carried out on purpose with the intent of reducing simple plant (and sometimes animal) material to a more stable form similar to humus in soil. Thus, we separate the technical, purposeful, controlled process of composting from the uncontrolled decomposition of organic matter in the environment. Composting can be highly beneficial to society and the environment in three basic ways. First, the cost of disposing of suitable (for composting) organic waste materials by other means such as landfilling or incineration can be avoided. Second, the end product of composting can typically be used as a soil amendment or mulch, and third, some processes of composting and composts can be used to achieve other goals, such as destroying pathogens and organic contaminants, and stabilizing soil contaminants such as trace metals from mining.

Generally, composting can be differentiated from simple breakdown of organic matter by considering composting as a high-energy process that results in elevated temperatures through aerobic decomposition by microorganisms generally referred to as 'thermophiles'. By bringing a relatively large amount of readily decomposable organic material together in one place, and providing optimum moisture and high aerobic conditions, elevated temperatures are rapidly achieved through microbial respiration coupled with insulation against heat loss, and the composting process is carried out by different functional types ofmicroorganisms. For instance, below about 5 °C, there is little or no bacterial processing of organic matter; between 6 and 19 ° C, psychrophilic or slow, moderate decomposition by acti-nomycetes and fungi takes place. This is not considered to be 'composting' by definition because organic matter decomposition is relatively slow. Composting begins between 20 and 44 °C, where mesophylic bacteria are dominant and decomposition rates are relatively rapid, and between 45 and 70 °C, where thermophilic bacteria dominate, with extremely high rates of decomposition and destruction of many kinds of pathogenic organisms, including Giardia. Higher temperatures are not normally desirable, since even thermophilic bacteria can be killed under extremely high temperature conditions. Normally, the composting process is self-regulating at higher temperatures, but sometimes the rate of aeration is controlled, water is added, or the compost pile is turned to mix and reduce extremely high temperature in some spots.

After an initial high-temperature phase, decomposition rates are reduced because the readily decomposable organic substrate is lost, and materials more recalcitrant to further decomposition remain (Figure 1).

During the production of compost, simple organic molecules such as sugars and carbohydrates are rapidly broken down and either incorporated into microbial biomass, evolved as carbon dioxide during respiration, or left as residual organic material. As composting proceeds, organic material of various colors and sizes is converted into relatively complex organic molecules normally of dark color, small particle size, and complex chemistry rich in phenolic, carboxylic, and amine functional groups. After a compost is 'finished', further microbial activity is much slower, and in its final stages, compost resembles soil humic substances. For instance, Figure 2 shows a potential model for soil humus that includes not a specific structure for all humus molecules, but a generalized large organic molecule with characteristic aromatic COOH, aliphatic COOH, phenolic OH, H-bonded phenolic OH, saccharide, quinone, peptide, o

3 50

3 50

Extreme temperature, compost self-sterilizes

Thermophilic bacteria, high rates of decomposition, larger compost piles, well-aerated and with organic material, highly decomposable, destruction of most pathogens

Mesophilic bacteria, moderate rates of decomposition, smaller compost piles, and more recalcitrant organic materials

Psychrophilic (slow) decomposition, actinomycetes and fungi dominate

Little decomposition

20 40 60

Days of composting

Figure 1 Temperature of a municipal refuse compost pile with forced aeration.

20 40 60

Days of composting

Extreme temperature, compost self-sterilizes

Thermophilic bacteria, high rates of decomposition, larger compost piles, well-aerated and with organic material, highly decomposable, destruction of most pathogens

Mesophilic bacteria, moderate rates of decomposition, smaller compost piles, and more recalcitrant organic materials

Psychrophilic (slow) decomposition, actinomycetes and fungi dominate

Little decomposition

Figure 1 Temperature of a municipal refuse compost pile with forced aeration.

Figure 2 A potential general model for soil humus. This is a model with some generalized properties of humus only. Note that humus is a large organic molecule, and any specific molecule may or may not contain the illustrated functional groups, and in different amounts.

and cyclic N units as part of a large organic molecule. This model is to be considered as only giving the 'general' form of humus, and not the specific chemical composition, which is highly variable. In fact, the specific structural unit pictured may not even exist in nature.

A common definition for humic substances might include the following: A series of relatively high-molecular-weight, yellow- to black-colored organic substances formed by secondary synthesis reactions in soils. The term 'humic substances' is used in a generic sense to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to soil environments in that they are dissimilar to the biopolymers of microorganisms and higher plants (including lignin). Humic substances include operational forms of humic acid, fulvic acid, and humin.

The formation of humic substances during composting and soil formation is one of the least understood aspects of humus chemistry. Many researchers have dedicated their careers to developing an understanding of how organic material is decomposed, changed, and eventually results in the variable but relatively stable material we refer to as humus. Composting and the breakdown of organic matter in general produce humic substances as an end product, but the exact pathway of this breakdown continues to be a source of debate.

Four basic humus-formation models commonly recognized include the following:

1. Humic substances are formed when lignin is incompletely utilized by microorganisms, and the by-products of decomposition become part of soil humus.

2. Lignin decomposition products of microbial transformation, including phenolic aldehydes and acids, undergo enzymatic conversion to quinones. The qui-nones then polymerize in the presence of amino compounds to form large humus-like molecules.

3. Cellulose and other nonlignin organic molecules are decomposed by microorganisms, resulting in synthesis of polyphenols, and eventual enzymatic conversion to quinones. These polymerize in the presence of amino compounds to form large humus-like molecules as in pathway 2.

4. Humus forms from a nonenzymatic 'browning' reaction similar to what happens during the dehydration of some food products.

Several general mechanisms for these models are shown in Figures 3-6. The oldest theory, formalized in 1938, considers humus to form when lignins decompose and are modified to form humic substances resistant to further decomposition.

This theory further defines soil humus and the breakdown of organic matter eventually into humus as follows:

a complex aggregate of brown to dark-colored amorphous substances which have originated during the decomposition of plant and animal residues by microorganisms, under aerobic and anaerobic conditions, usually in soils, composts, peat bogs, and water basins. Chemically, humus consists of various constituents of the original plant material resistant to further decomposition; of substances undergoing decomposition; of complexes resulting from decomposition either by processes of hydrolysis or by oxidation and reduction; and of various compounds synthesized by microorganisms. Humus is a natural body; it is a composite entity, just as are plant, animal, and microbial substances; it is even much more complex chemically, since all these materials contribute to its formation. Humus possesses certain specific physical, chemical, and biological properties which make it distinct from other natural organic bodies.

Lignin

Modified lignins

Humic acids

Fulvic acids

Soil

Demethylation humus oxidation, org-N condensation

Figure 3 Formation of soil humus from lignin.

Plant and / animal residues

Microbial transformation

Phenolic aldehydes, acids

Amino compounds

Quinones

Soil humus

Figure 4 Formation of soil humus from plant and animal residues, with lignin and amino compound degradation pathways.

Plant and / animal residues

Non lignin C Microbial transformation

Polyphenols

Amino compounds

Quinones

Soil humus

Figure 5 Formation of soil humus from plant and animal residues, nonlignin C, and amino compound degradation pathways.

Plant and animal residues

dehydration, 'browning'

Residue

Soil humus

Amino

Microbial transformation compounds

Figure 6 Formation of soil humus from plant and animal residues, with microbial transformation and dehydration pathways.

Humus, in itself or by interaction with certain inorganic constituents of the soil, forms a complex colloidal system, the different constituents of which are held together by surface forces; this system is adaptable to changing conditions of reaction, moisture, and action by electrolytes. The numerous activities of the soil microorganisms take place in this system to a large extent.

Currently, humus investigators consider mechanisms involving the formation of phenolic aldehydes and acids (Figure 4) and polyphenols (Figure 5) first, and subsequent reaction with amino compounds to eventually form soil humic substances.

A fourth mechanism, where humus is formed by none-nzymatic 'browning' of sugars, similar to what happens when sugar is heated in food preparation, is also found in the early literature.

It is almost certain that the formation of humus involves more than one series of processes, including more than one of the above mechanisms. It is also likely that different mechanisms predominate depending on the nature of organic material used in composting (i.e., feed stocks), and under different composting conditions (i.e., 'fast' and 'curing compost stages). Further breakdown of organic matter continues once the compost is added to soils, which have a large variation in their own physical, chemical, and biological properties.

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