Environmental Engineering Reference
In-Depth Information
plants and aid in nutrient uptake. These fungi can also be damaged by herbicides in the
soil. One study found that oryzalin and trifluralin both inhibited the growth of certain
species of mycorrhizal fungi (Kelley and South 1978). Roundup has been shown to be
toxic to mycorrhizal fungi in laboratory studies, and some damaging effects were seen
at concentrations lower than those found in soil following typical applications (Estok
et al. 1989; Chakravarty and Sidhu 1987). Triclopyr was also found to be toxic to several
species of mycorrhizal fungi (Moorman 1989), and oxadiazon reduced the number of
mycorrhizal fungal spores.
In South Africa, the feeding activity of soil organisms was higher in soil from organic
vineyards than from conventionally treated sites (Reinecke et al. 2008). The number of
earthworms was 1.3-3.2 times higher in organic compared to conventional plots, and the
length of plant roots colonized by mycorrhizae was 40% higher in organic than in conven-
tional systems (Mäder et al. 2002). Triclopyr, a herbicide, caused a major reduction in the
growth of mycorrhizae at elevated soil levels (Chakravarty 1987).
The sulfonylurea herbicides metsulfuron and (to a lesser extent) chlorsulfuron caused
a reduction in the growth of Pseudomonas soil bacteria (Boldt and Jacobsen 2006). In
laboratory tests, a combination of two sulfonylurea herbicides, bensulfuron-methyl (B)
and metsulfuron-methyl, caused a considerable reduction in soil microbial biomass
over the first 15 days (El-Ghamry et al. 2001). In bacterial communities in soil, bro-
moxynil (a nitrile herbicide) caused major changes in species composition and diversity.
Bromoxynil inhibited the growth of bacteria capable of degrading chemicals in the soil
(Baxter and Cummings 2008). Also, captan (a fungicide) and the herbicide glyphosate
caused a shift among species in bacterial communities in the soil (Widenfalk et al. 2008).
Certain organophosphate insecticides (e.g., dimethoate) can decrease the activity and
biomass of soil microorganisms, while others (such as fosthiazate) may actually result
in an increase in microbial biomass (Eisenhauer et al. 2009). How pesticides affect long-
term soil fertility is not well understood as this depends on many factors.
Although pesticides are designed to control pest species, the extent of their selectivity
for pests in some cases is not great, and other organisms are injured, including soil micro-
organisms. Inhibitions of microbial activity are most pronounced from fungicides and
fumigants and the suppression may remain for long periods. The impact may be so great
that the natural balance among the resident soil microbial populations is upset and new
organisms, such as plant disease vectors, become prominent. Moreover, certain nutrient
cycles regulated by microorganisms are inhibited by fungicides and fumigants in such
a way that significant adverse effects on plant growth and nutrition become evident. The
lack of widespread concern for these antimicrobial agents is not because of their lack of
toxicity but rather because they are not as widely used as the other two major classes of
pesticides (Alexander 1981).
Insecticides have received a lot of attention in the past. These compounds may be
applied directly to soil for the control of soil-borne insects, or they may reach the soil
from drifting sprays or when treated plant remains fall to the ground or are mixed with
the soil during normal farming practices. Inhibition of some microbial processes or sup-
pressions of individual populations of bacteria, fungi, or actinomycetes occur. On the
other hand, the toxicity is generally not marked, and the beneficial effects of the insec-
ticides in controlling insect pests argue for their use. The US regulatory agencies have
not acted on the basis of possible long-term harm insecticides might have on microbial
processes, but a few instances of major suppression of microbial activities in the field
have been noted, so that a change in policy in regard to their use does not appear war-
ranted (Alexander 1980).
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