other human foodstuffs includes yeast (Saccharomyces cerevisiae) in bread making and the cultivation of mushrooms.

Managing microorganisms can mean combating pathogenic or infectious organisms or promoting beneficial organisms or their products. Ancient Chinese and Arab cultures developed a procedure for scraping sores of people infected with mild cases of smallpox and infecting healthy people to ward off a more serious case of the disease. This practice was eventually introduced in Western Europe in the 1800s when Edward Jenner inoculated people with cowpox germs as a prevention against smallpox (Wilson, 1976). Jenner termed the procedure vaccination, from the Latin word vacca, for cow. However, the rational basis for vaccination came from Louis Pasteur's experiments with chicken cholera. Pasteur could have used the phrase he coined, "Chance only favors the prepared mind," to describe another manipulation of organisms that led to the discovery and production of penicillin. The accidental contamination of a culture plate of staphylococci by a mold (Penicillium notatum) led to the discovery and naming of penicillin by Sir Alexander Fleming in 1928 (Wilson, 1976; Forester, 1998). This discovery led to the production of penicillin as an antibacterial drug 12 years later by E. Chain and H. W. Florey. This touched off a worldwide search for soil organisms with antibiotic characteristics and the discovery of streptomycin by Dr. Selman Waksman, a soil microbiologist.

Since agriculture has been practiced for thousands of years, it is no surprise that examples of managing and manipulating microorganisms are abundant. Farming, especially in the uplands of Greece, intensified around 800 BCE, giving rise to erosion and decreased fertility (Encyclopedia Britannica, http://www. search. As time went on, the Romans continued the practice of clearing forests and excessive grazing and cultivation of the land. However, the Romans recognized that to help fertility, the land needed to be fallowed at some time. They also were aware that to boost fertility, they needed to rotate crops and add lime and manure (Hillel, 1992). They found that growing alfalfa and clover added fertility, as did using green manures such as lupines, but they did not know why. Parallel with agriculture development was the practice of composting, in which a soil pit was maintained with human and animal waste, weeds, leaves, and household waste and watered regularly. The decomposed material was used as fertilizer and mulch that improved the physical, chemical, and biological characteristics of the soil. The management of microorganisms extends to plant pathology as well. For centuries it was known that there was an association between barberry and stem rust of grain (Walker, 1950). In the 1600s, farmers sought to adopt a barberry-eradication measure without knowing the cause-and-effect relationship. Likewise with simple observation and intuition, farmers began using salt brine to control wheat bunt.

These few examples depict a human population that has been managing microorganism and their processes for thousands of years. Many of the agricultural, forestry, and rangeland practices we use today actually have deleterious effects upon microorganisms and their processes. Practices including plowing, clear-cutting forests, and overgrazing of rangeland decrease organism populations and promote nutrient loss, with an overall result of decreasing soil quality. Humans as caretakers of the land must reverse land degradation to increase soil quality and ecosystem health to provide food and fiber for a growing world population. We can begin this long journey by developing ways in which to manage organisms and their beneficial processes in soil systems.

changing soil organism populations and processes

Soil organic matter (SOM), a direct result of microbial activity, plays a role in terrestrial ecosystem development and functioning. In both undisturbed and cultivated systems, potential productivity is directly related to SOM concentrations and turnover. The dominant effect that SOM has on ecosystem structure and stability is clear evidence for the need to protect current organic matter levels and to develop management practices that will enhance soils with declining SOM contents (see Chap. 12 for more detail on organic matter). Organic matter contents in soils range from less than 0.2% in desert soils to over 80% in peat soils. In temperate regions, SOM ranges between 0.4 and 10.0%, with soils of humid regions averaging 3-4% and those in semiarid areas 1-3%. Although it is only a small fraction of the soil, components of SOM are the chief binding agents for soil aggregates that, in turn, control air and water relations for root growth and provide resistance to wind and water erosion. With 95% of soil nitrogen (N), 40% of soil phosphorus (P), and 90% of soil sulfur (S) being associated with the SOM fraction, decomposition and turnover can supply most macronutrients needed for plant growth (Smith and Elliott, 1990; Smith et al., 1992). During decomposition, microorganisms assimilate complex organic substrates for energy and carbon (C) and release inorganic nutrients. This process is controlled by temperature, moisture, soil disturbance, and the quality of SOM as a microbial substrate. These factors, together with the size and activity of the microbial population, regulate the rate of decomposition and nutrient release (Smith and Paul, 1990; Smith, 1994).

The two most significant regional ecosystem disturbances that directly decrease SOM are tropical and boreal deforestation and the intensive cropping of the world's prime farmland and forests. Forests and agricultural lands contain 90% of terrestrial C and are responsible for 80% of the yearly primary C production (Smith and Paul, 1990). The decrease in SOM is paralleled by declines in soil productivity and contributes to increasing global CO2 concentrations. In both forest and agricultural ecosystems, the management of plant residues is necessary to prevent SOM decline and possible ecosystem collapse.

Recent years have witnessed increased concern about environmental pollution from synthetic chemicals and disposal of wastes. Problems with toxic waste dumps, garbage landfills, and acid rainfall have focused attention on the soil, the most widely used depository. The soil as a depository is a perturbed system, experiencing gross changes in temperature, moisture, and biological activity. Controlling factors and processes may be drastically altered under these circumstances, causing chemical cycling, ecosystem stability, and system resiliency to shift in unpredictable ways.

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