Agroecosystems comprise 30% of Earth's surface (Altieri, 1991). Microbial diversity is one factor that controls agroecosystem productivity and quality. Biological diversity studies generally have been centered around plants, animals, and insect

Table 1 Justification for Investigations of Microbes and Microbial Diversity

Microbes are an important source of knowledge about life strategies and limitations. Microbes are key to understanding evolutionary history.

The untapped diversity of microorganisms is a resource for new genes and organisms of value to biotechnology.

Microbes and microbial diversity patterns can be used as indicators of environmental changes. Microbial communities are key to understanding biological interactions. Microbes play a key role in conservation and restoration biology of higher organisms. Microbes are critical to sustainability.

Source: Microbial Diversity Research Priorities, American Society of Microbiology, Washington, D.C., 1994. With permission.

species with little attention being given to microorganisms. Without microorganisms and their biochemical processes, life on Earth would not be possible (Price, 1988). Microbial diversity can directly influence plant productivity and diversity by influencing plant growth and development, plant competition, and nutrient and water uptake. Microbes provide sources of genetic material; in addition, they can be used as indicators of environmental change (American Society of Microbiology, 1994; Table 1).

Microorganisms are key to understanding biological interactions. They play a role in conservation and restoration of agroecosystems and are essential to quality considerations. Soil organisms constitute a large, dynamic nutrient source in all ecosystems and play a major role in residue decomposition and nutrient cycling (Smith and Paul, 1990). In addition, microorganisms are responsible for chemical changes in the soil, such as the accumulation of soil organic matter, dinitrogen fixation, and other changes in soil properties that may affect plant growth. When a disturbed system begins to recover, it is not only due to plant recolonization, but is also dependent upon the action of microbes in promoting favorable soil conditions and fertility.

We are unaware of the true extent or dimension of the diversity of soil microbes, although molecular investigations suggest that populations in soil are greater than we are able to understand with cultural techniques (Holben and Tiedje, 1988; Torsvik et al., 1990). Microbial diversity is critical to ecosystem functioning as a result of the myriad of processes for which microbes are responsible, such as decomposition and nutrient cycling, soil aggregation, and pathogenicity. It is necessary for us to increase our knowledge of biotic and functional diversity to understand better the desired makeup of microbial communities and the optimum management practices for an agroecosystem.

Anthropogenic influences may affect ecosystem functioning and diversity. Some of the most dramatic examples of ecosystem disturbances are occurring as a result of soil erosion from agroecosystem perturbations. These disturbances may be leading to progressive declines in biological, chemical, and physical stability in ecosystems. Continued disturbance of these areas may cause massive changes in global carbon cycling, resulting from significant organic carbon loss from the terrestrial environment. Microorganisms are highly sensitive to disturbance, such as those introduced by agriculture (Elliott and Lynch, 1994), and they may function as early warning indicators of changes in quality. Fluxes in microbial diversity and functional diversity may contribute greatly to the understanding of soil quality and the development of sustainable agroecosystems (di Castri and Younes, 1990; Hawksworth, 1991a; Thomas and Kevan, 1993). Soil organisms are useful in classifying disturbed or contaminated systems, since diversity can be affected by minute changes in the ecosystem. The use of microorganisms and their functioning for examination of environmental stress and declining biological diversity needs to be exploited for the benefit of agroecosystems (Office of Technology Assessment, 1987). The purpose of this chapter is to explore the issue of microbial diversity in agroecosystems and its influence on the quality of that agroecosystem.

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