Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 173

no difference in microbial community structure between soils cultivated with two differ-
ent lines of Bt maize and corresponding non- Bt maize lines. Similarly, there was no dif-
ference in microbial community structure as determined by PLFA between Bt maize and
non- Bt maize grown in greenhouse pots using soil collected from field plots that had been
cultivated with the same Bt and non- Bt cultivars (Griffiths et al., 2006). When the effects of
eight different Bt maize lines (expressing Cry1Ab) and their corresponding non- Bt maize
isolines on soil microbes were evaluated using PLFA analysis, Griffiths, Heckmann, et
al. (2007) reported that although soil microbial community structure was significantly
affected by the growth stage of the plant, it was not affected by the Bt gene insertion. There
were also no negative effects of the cultivation of Bt cotton on species richness or func-
tional diversity of rhizosphere microbial communities as determined by Biolog assays or
on enzyme activities (urease, phosphatase, dehydrogenase, phenol oxidase, and protease)
when compared to soil cultivated with non- Bt cotton (Shen et al., 2006).
Most field studies have also demonstrated no significant negative effects of Bt crop
cultivation on soil microbes. In a 2-year field experiment, there were no effects of Bt maize
cultivation on microbial activity (N mineralization potential, short-term nitrification
rate, and soil respiration) or bacterial community structure detected by T-RFLP analysis
when compared to soil cultivated with a non- Bt maize isoline (Devare et al., 2004). After 3
years, there were still no significant differences in microbial biomass or microbial activity
detected in the Bt versus non- Bt maize plots, although there were seasonal variations in
microbial biomass and activity (Devare et al., 2007).
In rhizosphere soils collected from Bt maize and three different non- Bt maize culti-
vars over 3 years, there were no differences in rhizosphere bacterial community struc-
ture as determined by polymerase chain reaction (PCR) amplification of 16S rRNA genes
(Miethling-Graff et al., 2010), and there was no difference in bacterial community structure
between rhizosphere soils of Bt maize and two non- Bt maize lines grown for 2 years at
three different European field sites (Denmark, eastern France, southwestern France) as
determined through PLFA analysis and CLPP (Griffiths et al., 2005). There were also no
differences in the culturable aerobic bacteria (including actinomycetes), dehydrogenase
and nitrogenase activities, or ATP (adenosine triphosphate) content, in rhizosphere soils
of Bt maize versus non- Bt maize over a 2-year field study, although there were seasonal
variations as well as significant differences between rhizosphere and bulk soil samples
(Oliveira et al., 2008). When soils were cultivated for 4 years with four different lines of Bt
maize (two lines of Bt 11; Cry1Ab [sweet corn and field corn], Event MON810; Cry1Ab, and
Event MON863; Cry3Bb1) and non- Bt maize, there was no difference in microbial diversity
detected through dilution plating and DGGE or in enzyme activities (arylsulfatases, acid
and alkaline phosphatases, dehydrogenases, and proteases) (Icoz et al., 2008).
Using metabolic profiling and molecular analysis of 16S rRNA genes, Fang et al. (2005)
determined that rhizosphere bacterial diversity was affected more by soil texture than
by cultivation with Bt maize in both greenhouse and field studies. Similarly, in soil cul-
tivated with Bt maize and non- Bt maize, the rhizosphere bacterial community structure
was more affected by environmental factors such as the ontogeny of the plants or hetero-
geneities within the field soil than by the Cry1Ab protein expressed in the Bt maize plants
as determined by SSCP (single-strand conformation polymorphism) of PCR-amplified 16S
rRNA genes (Baumgarte and Tebbe, 2005). When soil was cultivated with Bt cotton, Hu
et al. (2009) found no negative effects of the Bt cultivar on rhizosphere bacteria, and Li et
al. (2011) reported no differences in Azotobacter , denitrifying bacteria, ammonia-oxidizing
bacteria, or microbial diversity between soil cultivated with Bt and non- Bt cotton over a
3-year field study as determined by selective plating and most probable number (MPN)