Microbial Impacts On Agroecosystems

Microbes impact agroecosystems through a large list of functions for which they are responsible (Table 2). Soil humus formation, cycling of nutrients, and building soil tilth and structure (Lynch, 1983; Wood, 1991) are distributed among a large number of different genera and species. Microorganisms are responsible for many transformations in soil related to plant nutrition and health. The majority of soil microbes are beneficial to plant growth, but they need to be managed effectively (Lynch, 1983). Potential harmful effects from soil microorganisms include plant disease, production of plant-suppressive compounds, and loss of plant-available nutrients. Specific microorganisms can be manipulated to produce beneficial effects for agriculture (Lynch, 1983), for example, rhizobia to increase plant available nitrogen (Sprent, 1979), mycorrhizal associations to enhance nutrient uptake (Mohammad et al., 1995), or biological control of plant pests to reduce chemical inputs (Cook and Baker, 1983; Kennedy et al., 1991).

Beneficial soil bacteria can enhance plant performance by an increase in mineral solubilization (Okon, 1982), dinitrogen fixation (Albrecht et al., 1981), the production of hormones (Brown, 1972), and the suppression of harmful pathogens (Chang and Kommendahl, 1968; Cook and Baker, 1983). The symbiotic relationship between bacteria and legumes is one of the most widely studied and applied plant-microbial

Table 2 Several Functions of Soil Microbes

Decomposition of organic residues with release of nutrients

Formation of beneficial soil humus by decomposing organic residues and through synthesis of new compounds Release of plant nutrients from insoluble inorganic forms

Improved plant nutrition through mycorrhizal relationships which are symbiotic relationships between fungi and plant roots Transformation of atmospheric dinitrogen to plant-available N Improvement of soil aggregation, aeration, and water infiltration Antagonistic action against insects, plant pathogens, and weeds (biological control)

interactions (Sprent, 1979). The bacterium Rhizobium forms nodules on the roots of the legume plant, takes dinitrogen from the air, and transforms it to plant-available nitrogen (i.e., NH4+, NO3-). The plant provides nodules and photosynthate for the bacteria, while the bacteria give the plant the nitrogen it needs. Inoculation of legumes with dinitrogen-fixing Rhizobium can add appreciable amounts of nitrogen to the soil. The distribution and diversity of specific strains of dinitrogen-fixing bacteria vary with environmental conditions (Turco and Bezdicek, 1987; Hirsch et al., 1993). The plant community can influence the presence or absence of specific strains of Rhizobium, and thus impact the diversity of this group of microorganisms (Strain et al., 1994). The interaction involving mycorrhizal fungi and rhizobia may further affect the host plant by increasing nitrogen and phosphorus nutrition (Allen, 1992). These interactions are specific (Molina et al., 1992), further illustrating the complexity of the plant-microbe interaction and the changes in diversity of various microbial groups that can affect plant growth or impact other soil features.

Plant-suppressive bacteria reduce seed germination and delay plant development by the production of phytotoxic substances (Woltz, 1978; Suslow and Schroth, 1982; Alstrom, 1987; Schippers et al., 1987). Pathogenic fungi greatly reduce the survival, growth, and reproduction of plants (Shipton, 1977; Bruehl, 1987; Burdon, 1987), while beneficial mycorrhizal fungi can enhance plant growth by increasing nutrient (Fitter, 1977; Hall, 1978; Rovira, 1978; Ocampo, 1986) and water uptake (Tinker, 1976).

Mycorrhizal fungi have been found in a vast majority of plant communities (Allen and Allen, 1990) growing in association with 90% of terrestrial plants examined (Harley and Smith, 1983). Mycorrhizae are nonpathogenic fungi that form symbiotic associations with plant roots. The diversity of this group of organisms is vast, yet is not well studied. These associations are highly specific and indicate the diversity of this group of microorganisms (Molina et al., 1992). Mycorrhizal associations have been shown to be of greatest importance in stressed environments, phosphorus-deficient soils, eroded sites, and acidic or reclaimed lands (Harley and Smith, 1983; Barea, 1991). This association may be key in plant productivity and nutrient cycling (Barea, 1991; Allen, 1992). Mycorrhizal associations are enhanced by crop rotation and management practices favoring minimum disturbance.

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