In a study of the diversity of prairie and cultivated soils, diversity indexes were greater in disturbed or cultivated systems when compared with grassland (Kennedy and Smith, 1995). The increase in diversity with disturbance indicates a change in the microbial community to one that exhibited a greater range of substrate utilization and stress resistance. Soil microorganisms may affect plant growth and may influence plant competition. In turn, plants may act as a selective force for rhizosphere microbial populations through their influences on soil nutrients.
The ecology of root-microbe interactions after minimum tillage practices is vastly different from that after extensive plowing to prepare the seedbed. The changes in the physical and chemical properties of the soil resulting from tillage greatly alter the matrix-supporting growth of the microbial population. Within a given soil, there is considerable variation in the composition of the microbial community and diversity with depth in the profile. In no-till agricultural systems, microbial activities differed drastically with depth, with the greatest microbial activity occurring near the no-till surface; in the tilled system, activities were more evenly distributed throughout the plow layer (Doran, 1980). The composition of the microbial community influenced the rate of residue decomposition and nutrient cycling in both no-till and conventionally tilled systems (Beare et al., 1993). Decomposition in the no-till system was dominated by fungi, while the bacterial component was found to be responsible for a greater portion of the decomposition of residue in conventionally tilled systems. These studies illustrate the alteration of the makeup of the microbial communities and possibly the diversity of basic microbial groups with changes in management systems.
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