Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 236

biomass was found in the soil treated with the fungicide vinclozolin and the insecticide
λ-cyhalothrin (Lupwayi et al. 2009b). In turn, another insecticide, chlorpyrifos, signifi-
cantly increased the microbial biomass in the soils treated with both low (0.5 mg/g soil)
and high (50 mg/g soil) inputs. Moreover, chlorpyrifos applied at the field rate had no
harmful effect on the basal microbial respiration (Dutta et al. 2010).
Smith et al. (2000) studying the impact of the fungicide benomyl on the microorganism
activity observed marked increase in the respiration rate, while in the case of the highest
dosage of the fungicide, they found a reverse effect. In turn, Chen et al. (2001b) observed
that fungicides benomyl, captan, and chlorothalonil suppressed the peak soil respira-
tion by 30%-50%. On the other hand, the studies of Martikainen et al. (1998) showed that
the pesticides dimethoate and benomyl had no effect on the soil respiration intensity.
Černohlávková et al. (2009) also showed that mancozeb increased the soil respiration at
concentrations of 25.6 and 256 mg/kg soil. As indicated by many studies, the changes
in the microbial respiration in the soils treated with fungicides (e.g., benomyl, captan, or
tebuconazole) have been closely related to the dosages of the pesticides used (Martikainen
et al. 1998; Smith et al. 2000; Cycoń et al. 2006).
Many studies have been carried out to evaluate the relationship between microbial bio-
mass, microbial respiration, and degradation rate of the pesticides in different soils. For
example, Bolan and Baskaran (1996) and Voos and Groffman (1997) reported the positive
correlation between the microbial biomass and the degradation rate of 2,4-dichlorophen-
oxyacetic acid (2,4-D) and dicamba. Regression analysis of the pesticide transformation rate
and soil respiration activity revealed a positive correlation between SIR and the transfor-
mation rate constant for metalaxyl and propachlor (Jones and Ananyeva 2001). By contrast,
Entry et al. (1994) did not find such correlation studying 2,4-D and atrazine degradation
in pasture and forest soils. Similarly, no apparent relationship between the soil microbial
biomass and the degradation rates of carbofuran was observed by Karpouzas et al. (2001).
8.3 Effect of Pesticides on Soil Enzyme Activities
Enzyme activities are often proposed as an early and sensitive indicator of microbial
response to stress in both natural ecosystems and agroecosystems (Sannino and Gianfreda
2001; Gil-Sotres et al. 2005). Studies on enzyme activities in soil are important as they indi-
cate the potential of soil to support the biochemical processes, which are essential to the
maintenance of soil fertility. Any management practice that influences microbial commu-
nities and their biochemical activities in soil may be expected to generate changes in the
soil enzyme activity level. Kandeler et al. (1996) have also emphasized that the composi-
tion of a microbial community determines the potential of that community for enzyme
synthesis, and thus, any modification of the microbial community due to environmental
factors should be reflected on the level of the soil enzymatic activities.
8.3.1 Dehydrogenase Activity
Since dehydrogenase activity (DHA) reflects the physiological state of microorganisms,
DHA has been proposed as the accurate measurement of the potential microbial activ-
ity in the soil treated with agrochemicals (Rossel and Tarradellas 1991). However, some
authors have criticized this approach as the enzyme is also affected by different soil