ing of compositional assessment varied widely, from just a few hours to decades. For C amendments, the studies in which microbial community composition changed were significantly longer than studies that did not detect a change (Table 8.2). This result implies that there is a lag in the response of microbial communities to C additions and does not support the idea that these communities are resilient. For elevated CO2, mineral fertilization, and temperature, all studies were equally likely to find shifts in community composition, regardless of time since disturbance. On average, the reviewed studies examined composition after several years of the disturbance application. Thus, as a conservative boundary, microbial composition is often not resilient within a few years.
Certainly, the strength of the disturbance and how often it is applied will have an effect on the resilience of microbial composition. Most of the studies we reviewed continued to apply the disturbance throughout the study (as occurs for most global change disturbances), rather than a one-time application at the beginning of the experiment. For instance, Enwall et al. (2007) compared fertilized and unfertilized soil plots that have been maintained since 1956. The composition of the general bacteria and ammonia-oxidizing bacteria still differs between the plot types. In contrast, Stark et al. (2007) applied organic and inorganic forms of N to soil samples and compared the composition of Actinomycetes, alpha-Proteobacteria, and Pseudomonads. After 10 days, composition differed between the soil treatments, but after 91 days composition differed only among the Pseudomonads. Conversely, some of the studies that found no effect of disturbance on composition might have found an effect if the study was carried out longer.
Our literature survey clearly indicates that microbial communities are sensitive to disturbance and often do not rapidly recover to their original state. These responses beg the question of whether compositional shifts will affect ecosystem processes—will the disturbed community be functionally similar to the original community? There are two reasons why changes in microbial composition might not affect ecosystem process rates. First, the new community might contain taxa that are functionally redundant with the taxa in the old community. Second, taxa in the new community may function differently but result in the same process rate when combined at the community level.
What do we know about functional redundancy in microbial communities? Few studies compare the degree of redundancy within different microbial functional groups [but see, for example, Wohl et al. (2004) and Setälä and McLean (2004)]. Functional redundancy is difficult to estab-