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Sinorhizobium meliloti strain L33, on the establishment of rhizobial communities. In a
subsequent study, however, Miethling et al. (2003) found that both soil type and plant
species affected structural diversity in the rhizosphere communities of three legumes,
namely; alfalfa (M. sativa); common bean (Phaseolus vulgaris) and clover (Trifolium
pratense). In the same soil, significant differences were found in the composition of
leguminous rhizosphere communities and plant-specific organisms. Dominant alfalfa
rhizosphere populations differed in two soils with distinct agricultural histories, and
the three leguminous rhizosphere populations could be differentiated.
Anthropogenic disturbance has drastically altered the composition and productivity of
plant communities in the arid land ecosystem of the Colorado (USA) plateau
grasslands. Kuske et al. (2002) made comparisons at different depths of rhizosphere
bacterial communities of the native bunchgrasses Stipa hymenoides and Hilaria
jamesii, the invading annual grass Bromus tectorum and of interspaces colonised by
cyanobacterial soil crusts. A significant difference was found in the total bacterial
population structure and in the Acidobacterium division between the soil crust
interspaces and the plant rhizospheres, with large differences also seen among the
three rhizospheres, particularly in the Acidobacterium analysis. It was shown that soil
depth in plant rhizospheres as well as in the interspaces affected both the total
bacterial community and bacteria from the Acidobacterium division, with different
members of this division occupying specific niches in the grassland soil.
The effects of genetically engineered (GE) crops on agricultural practice, human
health and the environment were studied by Schmalenberger and Tebbe (2003).
Bacterial community diversity in the rhizosphere of a transgenic, herbicide
(glufosinate)-resistant sugar beet (Beta vulgaris) was compared with that of its non-
engineered counterpart. Differences in community composition due to field and
annual variability were evident but there was no detectable effect of transgenic
herbicide resistance on the microbial community. In a similar study, Heuer et al.
(2002) investigated the possibility that GE plants could change rhizosphere bacterial
consortia through transgenic T4 lysozyme release or by a change in root exudate
composition, and thereby change agroecosystems. Bacterial populations from
transgenic potato rhizospheres were compared with those of wild-type plants and non-
lysozyme producing transgenic controls. The authors found environmental factors
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