Micro-organisms the biggest part of the puzzle.
November 14, 2007 By Ralph Pearce
More than 10 years have passed since the introduction of genetically modified
crops to North America. Although the consensus among most in the agri-food industry
is that they have provided benefits and enhancements, there is still much to
be learned surrounding what, if any, impact they have on soil health and general
At the 2007 Southwest Agricultural Conference, Dr. Kari Dunfield, an applied
soil ecologist with the Department of Land Resource Science at the University
of Guelph, provided a glimpse into research she has done on the subject since
1999. In particular, her interest pertains to the kinds of choices growers are
making, and any affect, negative or positive, on the activity of micro-organisms.
"Management systems that impair soil quality often result in greater inputs
of water, nutrients and pesticides, and energy for tillage," explains Dunfield,
who did much of her original work at the University of Saskatchewan. "If
you're decreasing the health of your soil, you're going to have to put more
inputs into that soil to get equivalent yields."
According to Dunfield, understanding any impacts of genetically modified crops
requires an understanding of the kinds of activity among micro-organisms and
their 'microbial community' in the soil. Different soil types and structures
contain different types of micro-organisms and it is their combined ability
to break down organic matter, promote plant growth, even ameliorate soils in
damaging situations, that Dunfield is studying. "There are more than 10,000
different types of bacteria or fungi and 100 to 1000 invertebrate species in
a square metre of soil," she says, adding that microbial diversity is a
key component in overall soil health. "But it's also important to know
how active the micro-organisms are. If you have an organism that can break down
organic matter but it's not active, that's important to know."
Dunfield's work in Saskatchewan sought to apply microbial diversity and activity
to genetically modified crops. There had been studies that showed different
plant species had major effects on micro-organisms and microbial communities.
"We wanted to know that if you insert a new gene into a plant, how much
that could affect the microbial communities," she says. In theory, the
genetic insertion enables the expression of that gene or process throughout
the plant continually. "That means it can be released into the soil and
micro-organisms can then come in contact with that plant through root exudation,
and depending on where that gene's inserted, you can have slight changes in
plant structure or genetics and those changes can have some impact on microbial
Using eight different canola varieties, Dunfield's research tried to use DNA
fingerprinting to determine the effect of genetically modified plants versus
conventional. Three Liberty Link varieties, one Roundup Ready, three conventional
and a Pursuit-Smart variety (considered non-GM since its modification is done
via mutagenesis) along with a fallow plot, were grown. Sampling was done in
July and results showed a considerable difference in microbial diversity between
the varieties. "But in comparison to the difference of the effect in the
plant, the difference in the genetic structure is relatively small," says
Dunfield, noting the next step was to determine if changes in soil microbial
activity persisted throughout the growing season.
In that project, differences in the Roundup Ready, conventional and fallow
systems were measured in May, June, July, August, October and April. The findings
indicated a large difference in microbial communities from May to June and June
through August. "In August, those differences decreased and in October,
we saw some differences again," says Dunfield. By April, conditions became
similar to what they had been in May. "While there were differences between
the varieties as well as seasonal variations, the changes in microbial communities
were temporary and dependent on the presence of the plant."
Another factor that Dunfield mentions is the variation in soil type. There
were multiple sites across varying regions in Saskatchewan and although it was
no surprise to see different microbial communities in each soil type, it was
interesting to see that different microbial communities were associated with
different plant varieties. "In general, the effect of plant genetics was
not due to the insertion of the gene into the plant and the transgenic effect
was really minimal," says Dunfield. "There are changes in the microbial
structure of the community, but it didn't persist."
Still a note of caution
In spite of the project's summary, Dunfield insists growers be aware of consequences,
negative or positive, of using genetically modified varieties and hybrids. In
canola, her laboratory found plant growth-promoting rhizobacteria in the soil.
Yet on the negative side, studies by the USDA have shown Roundup Ready soybeans
can increase colonization of fusarium. "That could be because of this changing
of the diversity of micro-organisms," she states.
The study she is currently engaged in with the University of Guelph is taking
much of what she learned in Saskatchewan and applying it to cropping practices
in Ontario, including the use of in-crop glyphosate and lesser degrees of tillage.
Non-crop applications of glyphosate, or burndown, were not part of this study.
In a greenhouse study, Dunfield says initial findings indicate soil microbial
activity, including N-fixing rhizobia and mycorrhizal fungi, actually increased
in soils treated with a single application glyphosate. -30-