Agriculture Reference
In-Depth Information
on nontarget soil organisms
Soil organisms, including bacteria, fungi, protozoa, nematodes, earthworms, and microar-
thropods, have a tremendously important role in maintaining plant health and soil fertility
through the decomposition of organic matter, nutrient mineralization, provision of protec-
tion against disease, and improved soil structure. Moreover, symbiotic soil organisms, such
as nitrogen-fixing bacteria and arbuscular mycorrhizal fungi (AMF), provide nutritional ben-
efits to plants in exchange for carbon resources and protection by the host plant. Although
ubiquitous, many soil organisms are sensitive to a variety of agricultural practices, includ-
ing pesticide applications, tilling, cultivation practices (e.g., monocultures vs. intercropping,
compost vs. chemical fertilizer), and even the type of plant grown. Because of their close
associations with plant roots, some soil organisms, such as AMF or nitrogen-fixing bacteria,
may be more sensitive to changes in the physiology of the host plant or in the composition of
root exudates as a result of genetic engineering than their free-living counterparts in the soil.
Bacteria are by far the most abundant organisms in soil and are important for nutrient
mineralization, decomposition of organic matter, protection against plant pathogens, deg-
radation of chemicals/toxins in the environment, and nutrient cycling. In both natural and
agroecosystems, bacterial abundance is highest in the rhizosphere (the narrow area of soil
directly surrounding and influenced by plant roots). Plants support the development of
microbial communities in the rhizosphere by producing root exudates that contain car-
bon-rich nutrients such as carbohydrates and proteins (Grayston et al., 1996; Morgan et
al., 2005). While soil organisms take advantage of these carbon resources, the plants also
benefit via increased nutrient availability, improved mineral uptake, and enhanced soil
fertility provided by the soil microbial community (Smith and Gianinazzi-Pearson, 1988;
Morgan et al., 2005; Smith and Read, 2008).
Because of their close association with plant roots and their importance to biotic soil
processes, some of the earliest environmental risk assessment research was aimed at deter-
mining the nontarget effects of
Bt
crops on soil bacteria (summarized in
TableĀ 8.3
). It was
thought that a change in the composition or quality of root exudates from
Bt
crops or accu-
mulation of
Bt
toxin in the rhizosphere might modify the composition or activity of soil
microbes, ultimately influencing biotically driven ecosystem processes and affecting plant
growth and health. To date, however, only a few studies have reported any significant
effects (either positive or negative) on soil bacteria as a result of the addition of purified
Bt
proteins to soil, amendment of soil with
Bt
plant material, or cultivation with
Bt
crops
(e.g., Donegan et al., 1995; Wu et al., 2004a, 2004b; Castaldini et al., 2005; Rui et al., 2005; Xue
et al., 2005; Fang et al., 2007; Sun et al., 2007; Chen et al., 2011). The reported effects of the
addition of
Bt
biomass or protein to the soil, as well as the effects of
Bt
crop cultivation on
soil bacteria, are summarized here.
In one of the first risk assessment studies to examine the effects of
Bt
plants on soil
bacteria, Donegan et al. (1995) reported that soils amended with leaves of different lines
of transgenic
Bt
cotton resulted in a statistically significant, but transient, increase in cul-
turable bacteria. However, the plant line-specific response and the lack of effects of the
addition of purified
Bt
protein (Cry1Ac) on soil bacteria suggest that these effects may
not have resulted from the Cry1Ac protein but rather from a pleiotropic effect (change
in a single gene that affects multiple phenotypic traits) of the genetic manipulation. In a
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