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188
<br />SMITH, REDENTE, and HOOPER
<br />molecular weight. Generally, 15 to 30 percent of the C in humus remains
<br />after 1 year as resistant microbial cells or new humus (37).
<br />Organic residues of plants deposited on or in the soil vary widely in their
<br />susceptibility to and rate of decomposition by microorganisms under aero-
<br />bic conditions (2). Simple carbohydrates and proteins decompose rapidly.
<br />Lignin fats, tannins, and waxes are more resistant to biological decay (4).
<br />Water - soluble residues are the quickest to be digested by soil microorga-
<br />nisms. In addition, succulent substances decompose more rapidly than the
<br />same material in a drier condition. Cellulose and hemicellulose are con-
<br />verted readily to soluble forms and used quickly by soil organisms (69).
<br />Organic residue from animals contains higher levels of protein and fat
<br />than plant tissue. Animal tissue also does not contain lignified tissues as
<br />does plant tissue. Much animal tissue is used by other animals, reaching the
<br />soil through excretion.
<br />Soil organisms affect organic residue decomposition rates. Fresh plant
<br />tissue, deposited on or in the soil , creates favorable conditions for rapid mi-
<br />crobial population increases. Heterotrophic microflora (organic compound
<br />digesters) and, to a lesser extent, microfauna use organic substances for
<br />energy supply. Microfauna, especially earthworms (Lumbricus spp.), speed
<br />up decomposition by directly digesting organic debris and /or soil and leav-
<br />ing trails of castings (81). Earthworms, however, generally do not occur in
<br />arid and semiarid soils, except in agricultural and riparian areas where they
<br />have been introduced (24).
<br />As microorganisms and their activity increase, there is a corresponding
<br />increase in CO emission, which is a direct measure of organic matter de-
<br />composition. Assuming no organic residues are added, the CO: emitted de-
<br />creases with time, but the rate of decomposition will be equal to or greater
<br />than rates during periods when fresh residues are added (69). Therefore, the
<br />amount of organic C evolved as CO: directly relates to the soil's C content.
<br />Temperature, moisture, pH, and 0 level affect the size and composition
<br />of organism communities. Warm temperatures and moist soil conditions
<br />are usually optimal for organism activity (2). But decomposition also occurs
<br />in the northern Rockies and in cold deserts during the winter when soils are
<br />moist (29). Anaerobic conditions also create conditions that totally change
<br />and generally reduce organism activity.
<br />Inhibitory substances present in, or deposited on, soils affect decomposi-
<br />tion rates. For example, excessive salt concentrations in saline or sodic soils
<br />or herbicides can cause a change in the composition and size of organism
<br />communities, thereby affecting the rate and possibly the end products of
<br />decay. Other substances, such as Na, facilitate dispersion of organic matter
<br />on the surface of the soil.
<br />Nature of organic matter. Included in the organic residues contributed
<br />by plant and animal tissues are carbohydrates (cellulose, hemicellulose,
<br />SOIL ORGANIC MATTER
<br />sugars), protein, alcohols, li
<br />glucosides, alkaloids, and pil
<br />residues decompose.
<br />Physical and chemical prop
<br />matter freshly added to the soi
<br />animal residue. For example, 1
<br />and animal litter than do prairi
<br />material is altered physically a
<br />material.
<br />Organic residues constitute a
<br />ually less than 35 percent. Sot
<br />materials as plant roots, anim
<br />and aboveground plant remain:
<br />more to soil organic matter th,
<br />Humus, the primary constit
<br />dark brown to black, and nea
<br />alkali (NaOH or KOH) solutioi
<br />N -rich proteins, the slow -to -dc
<br />complex sugars are a large perc
<br />cement soil aggregates together
<br />N, some 0, and lesser amount
<br />change capacity, on a dry weir
<br />clay colloids (28). The particle c
<br />quartz minerals, ranging from
<br />Humic acid (proteins and ligi
<br />fraction of humus that is solul
<br />(pH 1.0 to 2.0). It is dark bro'
<br />Of the several chemical elen
<br />the most important. While the
<br />levels range from 48 to 58 per
<br />various soil horizons is a usefu
<br />cumulation.
<br />Biological properties. A prc
<br />nisms and small animals that r
<br />for plants and other microor
<br />available, plant species, amoi
<br />plied, soil texture, pH, moistt
<br />the numbers, kinds, and activi
<br />of the survival of the fittest. T
<br />lar microenvironment are bes,
<br />sent the most successful comp(
<br />actinomycetes, fungi, algae pi
<br />mals.
<br />The chief role of soil fauna
<br />
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