Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 243

indicators of both short- and long-term changes in soil quality. That is the reason why the
cultivation-dependent approach is still a useful method for the detection of variable micro-
bial response to toxic substances.
Impact of pesticides on soil microflora is changeable and results not only from the reac-
tion of microorganisms to active substances and formulation additives but also from the
development of specific groups of microorganisms. The ability of some microorganisms
to grow in the presence of pesticides may result in the compensation of their adverse
effects by the increased activity of the remaining part of the soil microbial community.
Microorganisms not sensitive to pesticides utilize released nutrients from dead cells,
which may result in the increase in their numbers in the soil (Das and Mukherjee 2000).
8.5.1 Heterotrophic Bacteria
It has been reported that some pesticides affect the numbers of soil microflora. For exam-
ple, Das and Mukherjee (2000) found that the population sizes of bacteria were greatest
30 days following the application of hexachlorocyclohexane (HCH), phorate, and fenval-
erate. Earlier, an increase in the numbers of soil bacteria was revealed during the bio-
degradation of chlorinated hydrocarbons, organophosphates, and synthetic pyrethroid
insecticides (Rache and Coats 1988; Das et al. 1995). Significant increases in the numbers of
culturable bacteria in the soil amended with diazinon was found by Singh and Singh (2005)
and Cycoń et al. (2010b). Das et al. (2005) also found increased numbers of culturable bac-
teria in the phorate-treated soil. Also, for λ-cyhalothrin applied at different concentrations,
the stimulatory effect on bacterial populations in soil was observed by Cycoń et al. (2006).
Generally, insecticides are not toxic to bacteria, as these organisms do not have sensitive
targets (Topp et al. 1997), and, in addition, these chemicals have been degraded by several
bacterial species (Finkelstein et al. 2001; Grant et al. 2002; Cycoń et al. 2009). However, in
some studies, decrease in the total numbers of heterotrophic bacteria in the soil treated
with insecticides was observed. For example, Cycoń and Piotrowska-Seget (2009) found
that fenitrothion significantly decreased the numbers of these bacteria in loamy sand soil
after 1 day of experiment at all used dosages (2, 10, and 200 mg/kg soil). However, this
effect in lower treatments was transient; the highest dosage negatively affected the num-
bers of bacteria over a 28-day experimental period. This effect was probably caused by
high toxicity of the intermediate metabolites of fenitrothion biodegradation, supporting
the results by Matsushita et al. (2003). Also, Das et al. (2005) found that the total bacterial
population was reduced due the application of the carbamate insecticide carbofuran at its
recommended field rate (1 kg/ha soil).
Studies on the effect of various herbicides on the microbial numbers in soils revealed
that these compounds are generally well tolerated by soil bacteria. Cycoń and Piotrowska-
Seget (2009) found that after the application of diuron into loamy sand soil, higher counts
of heterotrophic bacteria were enumerated. A similar stimulatory effect on the total num-
bers of these bacteria in soil was also observed by Cycoń et al. (2010a) after the appli-
cation of linuron, representing the same class of phenylurea herbicides. Generally, these
compounds can be degraded by different bacteria and might be used as a source of car-
bon and nitrogen, resulting in a significant increase in bacterial counts (Dejonghe et al.
2003). Recently, Grenni et al. (2009) indicated the lack of linuron effect on the numbers of
viable bacteria in the treated soil because of their adaptation to the presence of the her-
bicide. Zabaloy et al. (2008b) also observed increased numbers of culturable bacteria in
different soils treated with metsulfuron-methyl and glyphosate. In addition, Araújo et al.
(2003) reported increased counts of soil heterotrophic bacteria in response to the applied