Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 171

greenhouse study, Castaldini et al. (2005) observed differences in soil bacteria isolated
from soil amended with Bt versus non- Bt maize residue, with members of a subgroup of
the genus Bacillus isolated from soil amended with Bt biomass, while growth-promoting
rhizobacteria were isolated from soil amended with non- Bt maize biomass. When plant
residues of Bt and non- Bt maize were kept mixed with soil for up to 4 months, soil res-
piration was reduced by 10% in the Bt maize treatments (Castaldini et al., 2005). Fang
et al. (2007) reported that soil amended with Bt maize biomass had a significantly dif-
ferent microbial community structure than soil amended with non- Bt maize biomass as
determined by substrate utilization profiles and denaturing gradient gel electrophoresis
(DGGE) patterns and linked the alterations in the structure of soil microbial communities
to the higher lignin content detected in the Bt maize plants. In soil amended with Bt rice
straw versus non- Bt rice straw, differences in biological activities (dehydrogenase activ-
ity, methanogenesis, hydrogen production, and anaerobic respiration) were detected, but
there was no direct toxic effect of Cry1Ab protein (Wu et al., 2004a, 2004b). In a decompo-
sition study in which litterbags containing Bt rice and non- Bt rice roots or Bt and non- Bt
rice straw were buried in the field and sampled over 2 years, Wu et al. (2009) found that
Bt rice roots decomposed relatively faster than non- Bt rice roots in the first 200 days but
found no difference in microbial decomposition rates between Bt and non- Bt rice straw as
determined by changes in ash-free mass remaining and changes in total carbon and total
nitrogen content after decomposition. The incorporation of Bt cotton stems and leaves into
soil microcosms had a positive effect on soil urease, acid phosphomonoesterase, invertase,
and cellulose activities but a negative effect on arylsulfatase activity (Sun et al., 2007). The
addition of cotton tissue to soil most likely stimulated microbial activity (and thus influ-
enced soil enzyme activity), potentially masking any negative effect of Bt protein on soil
microbial or soil enzyme activity (Sun et al., 2007).
When soil was cultivated with Bt maize, Xue et al. (2005) reported a lower ratio of
gram-positive to gram-negative bacteria compared to soil cultivated with non- Bt maize.
In microcosm and greenhouse experiments, Castaldini et al. (2005) reported differences in
rhizospheric eubacterial communities and in culturable rhizospheric heterotrophic bacte-
ria in soil cultivated with Bt maize versus non- Bt maize as determined by DGGE analysis
of 16S ribosomal RNA (rRNA) genes and culturing methods. In soil cultivated with Bt and
non- Bt cotton, Rui et al. (2005) found that the numbers of colony-forming units (CFUs) of
three different bacterial functional groups (nitrogen fixing, inorganic phosphate dissolv-
ing, and potassium dissolving) were lower in soil collected from the rhizosphere of Bt
cotton at the early and middle stages of plant growth than in soil cultivated with non- Bt
cotton. However, addition of purified Cry1Ac protein directly to soil had no effect on the
CFU number of any of the three bacterial functional groups (except for a decrease in the
nitrogen-fixing bacteria when concentrations of Bt protein were greater than 500 ng/g)
(Rui et al., 2005). In soil consecutively cultivated for 4 years with Bt cotton , Bt and cowpea
trypsin inhibitor ( Bt + CpTI ) cotton, and non- Bt cotton lines, there was a decrease in micro-
bial biomass carbon (MBC), soil microbial activity as determined by catalase activity and
fluorescein diacetate hydrolysis, and some enzyme activities (nitrate reductase, acid phos-
phomonoesterase, arylsulfatase, b-glucosidase, and protease) in the Bt and Bt plus CpTI
cotton lines compared to soil cultivated with non- Bt cotton in greenhouse trials (Chen et
al., 2011). The decreases in MBC, microbial activity, and enzyme activities were correlated
with increasing CryAc protein content (Chen et al., 2011).
Most studies, however, have reported no negative effects of purified Bt proteins, Bt
plant biomass, or the cultivation of Bt crops on soil bacteria (Donegan et al., 1995; Escher
et al., 2000; Saxena and Stotzky, 2001a; Koskella and Stotzky, 2002; Ferreira et al., 2003;