Soil Microorganisms In Agroecosystems

The soil is full of microorganisms such as bacteria, actinomycetes, fungi, algae, viruses, and protozoa. Microbes are classified in categories based on their utilization of carbon and energy and their nutrient requirements (i.e., phototrophs, chemotrophs, autotrophs, heterotrophs, or lithotrophs). This provides limited information, since many situations overlap and refinement is needed (Alexander, 1977; Walker, 1992; Zak et al., 1994). Bacteria and fungi, as primary decomposers in the cycling of nutrients, occupy a critical position in the soil food web. Of all nutrient cycling, 90 to 95% passes through these two groups of organisms to higher trophic levels. Thus, the function and possibly the diversity of bacteria and fungi will be a large factor in determining the quality of agroecosystems (Lynch, 1983).

Bacteria and actinomycetes are the most numerous of microbial groups in soils, but because of their small size, 1 to 10 |im, they only account for 50% of the total biomass in soil (Alexander, 1977). Bacteria are found in soil at populations of 104 to 109 cells g-1 soil. As a group they are diverse metabolically and use many different sources of energy and carbon. Bacteria play an important role in the breakdown of organic material and nutrient cycling. Most natural and xenobiotic compounds can be broken down by soil microflora with few compounds becoming recalcitrant (Dorn et al., 1974). Some bacteria have the potential for nitrogen fixation (Sprent, 1979) or methane production or utilization (Jones, 1991). Denitrification and sulfate reduction involve a variety of facultative and obligate anaerobic bacteria (Tiedje et al., 1984). Nitrification and sulfur oxidation, on the other hand, are the result of the activity of a limited number of genera of aerobic autotrophic bacteria (Belser and Schmidt, 1978; Bock et al., 1989).

Fungi are less diverse than bacteria and less numerous in soil at 104 to 106 propagules g-1 soil. Fungi however, are responsible for up to 70% of the biomass (Lynch, 1983). Fungi are found in soil, in associations with plant roots, or as saprophytes on detrital material (Swift and Boddy, 1984). Fungi can withstand adverse soil conditions better than other microorganisms, and they survive at lower water potentials than bacteria (Papendick and Campbell, 1975). Hyphal strands allow fungi to overcome the environmental constraints of low moisture and depleted nutrients by translocation of water and nutrients. Also, several fungi excrete organic acids that solubilize otherwise unavailable nutrients (Sollins et al., 1981). Fungi are active decomposers of cellulose, lignin, and other organic materials. The products of decomposition are then released and used by other organisms, especially bacteria. Many fungi are plant pathogens, yet some form beneficial relationships with plant roots (e.g., mycorrhizae).

Algae are most numerous in surface soil and found at populations of 102 to 106 cells g-1 soil. In some agricultural systems, algae contribute to nitrogen cycling by nitrogen fixation or to soil stabilization (Metting and Rayburn, 1983). Protozoa inhabit soil at populations between 103 and 105 cells g-1 soil. These organisms are major predators of bacteria and thus regulate bacterial populations (Opperman et al., 1989).

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