The diversity of the microbial community, as well as the functions within the community, affect the stability and resilience of the soil system. Neither a higher nor lower degree of diversity in a system can be said to be better or worse; however, changes in the activity or community structure may influence the quality of the soil. The microbial biomass has recently been used as a sensitive indicator of management-induced changes (Doran, 1987; Powlson et al., 1987; Visser and Parkinson, 1992). Researchers have found discrepancies in microbial biomass responses to perturbations such as tillage. Kennedy and Smith (1995) found increased diversity with tillage. Utilizing microbial characteristics may better forecast change in soils since they respond to perturbations more rapidly than other indicators (Kennedy and Papendick, 1995).
Soil quality measures have recently become important in system comparisons of management options. Integral to soil quality assessment are measures of the microbial community. Microbiological properties can identify changes in the overall soil quality before changes occur in certain physical and chemical parameters that eventually impact overall crop and soil viability (Visser and Parkinson, 1992). Activity levels, biomass numbers, and community shifts can reflect the stability of a system with respect to the level of nutrient cycling, the amount of carbon utilized in a system, and the overall community structure and function in a soil system. Since microbial diversity of agricultural soils is important to the maintenance of soil formation, toxin removal, and elemental cycling (Borneman et al., 1996), diversity measures will most likely become increasingly important in evaluations of soil quality. Examples of rapid shifts in community structure such as these may serve as early indicators of changes in soil quality (Turco et al., 1994).
Soil resilience is defined as the ability of the soil to recover after disturbance (Elliott and Lynch, 1994). Soil resilience can be restored and biodiversity protected by, among other things, reducing tillage and increasing crop rotations (Elliott and Lynch, 1994). Studies have shown that by reducing the impact of disturbances, such as tillage, microbial biomass levels and soil resilience will improve (Elliot and Lynch, 1994). In addition, crop rotations of at least 3 years can decrease disease-related problems (Rovira et al., 1990), which can also increase crop resilience. However, soil resilience may be different depending on the disturbance; disturbances such as tillage may affect the general biological status, while pesticides or other stressors may only affect individual functional groups (Swift, 1994). By further establishing the relationships between the microbial biodiversity and soil resilience, a greater understanding of microbial indicators can be achieved.
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