Microbes constraints

Despite its importance in ecosystem C fluxes, soil respiration has limitations as a constraint on SOM turnover, for two main reasons. First, it is difficult to partition soil respiration into its two sources (1) decomposition of SOM by microbes (heterotrophic respiration) and (2) respiration from live plant roots (autotrophic respiration) (Kuzyakov, 2006). As a result, an increase in soil respiration may indicate not only an increase in SOM decomposition but also an increase in root respiration. Second, it is likely that in most soils only a small fraction of total SOM contributes to heterotrophic respiration. As a result, respiration measurements provide information about the dynamic fraction of SOM (particularly when combined with 14C measurements of respiration) but do not provide information about the large, stable pools unless they are destabilized and contribute to respiration (detectable with 14C02 respiration measurements). Attributing the sources of respiration from different SOM reservoirs, which may respond differently to climatic variables, is not... 

Physical constraints may be considered in terms of temperature or moisture levels in a soil environment or, alternatively, entrapment of contaminants in a location where the microbes are unable to gain access to them, such as in soil micropores. Furthermore, contaminants may bind to soil, thereby reducing their bioavailability (Hatzinger and Alexander, 1995 Allard and Neilson, 1997 ... 

What evidence exists that the niche affects microbial distributions We surveyed the literature for studies examining either niche or distribution for organisms with propagules < 1-2 mm. While many studies report ecological differences between species, we focused our search on those studies of quantitative niche characteristics that cause spatial segregation between species or result in apparent distributional boundaries at some scale. All studies we found consistently reported niche differences or local adaptation at intra- or interspecific levels, consistent with the fundamental niche in all cases and potentially related to the realised niche in a few cases (Table 15.1). Given that niche constraints on local persistence/occurrence have been observed for microbes, it is reasonable to expect that niche affects distribution of multiple microbial species, consistent with the environment selects portion of the EiE claim (and with much of evolutionary ecology). Tests for source-sink dynamics or dispersal limitation as alternative explanations of microbial niche-distribution relationships will require that the fundamental niche for a species is already... 

Evidence exists for fundamental niche constraints in microbes, plus some evidence for realised niche constraints. Niche-distribution relationships that are consistent with the EiE claim await more extensive and intensive sampling to fully characterise the role of niche in affecting microbial distributions. As for macrobes, we expect niche-distribution relationships will be found to constrain some microbes to distributions that are less than cosmopolitan. 

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