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We hypothesized that differences in focus and methodologies among the studies would influence the likelihood of detecting a compositional response. Therefore, for each study we recorded the breadth of the taxonomic group targeted (e.g., bacteria and fungi; methanotrophic bacteria), the habitat, the method used to examine composition, and the disturbance applied, as well as whether composition of the target group changed in response to the disturbance. We also recorded the time after the disturbance was first applied at which composition was assessed, which is relevant for the discussion of resilience below.

Papers from this search included studies that targeted composition broadly such as with phospholipid fatty acid analysis, which targets bacteria and fungi. In contrast, other studies examine narrower functional guilds within the Bacteria by PCR-amplifying a functional gene followed by a fingerprinting technique (such as denaturing gradient gel electrophoresis or terminal restriction fragment length polymorphism). Thus, the breadth of the taxa studied varied widely and is related to the methodology used to evaluate community composition. The methodology itself might also have an influence on whether a change in composition is detected. For instance, sequencing of PCR-amplified clone libraries will reveal finer-scale genetic changes than fingerprinting methods that target the same gene. Finally, because we were interested in compositional changes, our search excluded many studies that measure only total microbial biomass [e.g., those reviewed in Wardle (1992)].

Table 8.2 summarizes the results by the four disturbance types. The majority of these studies demonstrate that composition is sensitive to disturbance. More than 80% of the mineral fertilization (N/P/K), temperature, and C amendment studies found significant effects of disturbance on microbial composition. In contrast, the effect of elevated CO2 was found less often, in only 60% of the studies.

TABLE 8.2 Summary of Studies, Including the Percentage of Studies in Which Microbial Community Composition Was Sensitive to the Disturbance, and the Average Length of Studies That Found Resistant Versus Sensitive Composition

Disturbance

No. of Studies

Percentage of Studies Sensitive

Average Length of Study (years ± SD)

 

Resistant

Sensitive

P value

CO2 increase

20

60

3.3 ± 1.9

3.6 ± 1.9

0.78

N/P/K fertilization

38

84

4.8 ± 4.5

8.2 ± 8.7

0.17

Temperature

11

82

1.1 ± 1.3

3.0 ± 5.0

0.35

C amendments

41

83

0.15 ± 0.09

4.9 ± 12.6

0.03

NOTE: The P value reported is the result of a t test (assuming unequal variances) between the study times for the resistant versus sensitive studies.



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