Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 235

treated with pesticides (Domsch et al. 1983). Responses of soil microorganisms to pesticide
application measured by SIR and microbial biomass amount are contrasting. This is largely
because of the differences in the soil type and the class and dosage of the pesticide, which
results in effects on the cellular metabolism of soil microorganisms. For example, Moreno
et al. (2007) found that atrazine applied at concentrations ranging from 0.2 to 250 mg/kg
soil did not affect the soil microbial respiration (SMR), whereas when applied at higher
dosages (500 and 1000 mg/kg soil), it significantly increased the SMR values. At higher
treatments, authors also observed an increase in the microbial biomass with the incuba-
tion time; however, at the end of the experiment (45 days), the biomass was lower as com-
pared with that on day 16. To study the side effects of atrazine on microbial activity, they
calculated the metabolic quotient ( q CO 2 ), which represents the amount of CO 2 -C evolved
per unit of microbial carbon per hour. In their study, the values of q CO 2 were significantly
higher in the soils treated with the highest herbicide dosages than those in the control soil
at each sampling time (Moreno et al. 2007). Increase in the q CO 2 values was also observed
by Jones and Ananyeva (2001) as a result of metalaxyl application, showing that fungicide
treatment led to a change in the ecophysiological status of the soil microbial community.
Metabolic quotient is a parameter used to evaluate the microbial stress in the soil and
its increase reflects the harmful effect of a pesticide on microorganisms, which have to
use a great part of their energy to survive under unfavorable conditions, resulting in less
organic C incorporation into the microbial biomass (Chander and Brookes 1991).
Dose-dependent changes in the values of the basal soil respiration (BSR) in response
to napropamide were observed by Hua et al. (2009). In the soil treated with a herbicide at
2-80 mg/kg soil, the increase in BSR was observed 7 days after napropamide application.
During the experiment, the values of BSR decreased, and after 56 days in the soils treated
with higher dosages (20-80 mg/kg soil), BSR was significantly lower in comparison to the
control. This phenomenon was also observed for microbial biomass. On day 56, the high
herbicide input (40-80 mg/kg soil) significantly decreased the microbial biomass, whereas
the low napropramide dosages (2-20 mg/kg soil) stimulated it (Hua et al. 2009). Dinelli
et al. (1998) observed a similar transient increase in SIR in the soils amended with other
herbicides, for example, triasulfuron, primisulfuron methyl, and rimsulfuron. Accinelli
et al. (2002), studying the short-term effects of six agrochemicals on the microbial activ-
ity found that the sulfonylurea herbicides applied up to 20 μg/g soil stimulated soil res-
piration, whereas at the agricultural rate it did not exert any significant impact on the
soil microbial activity. Soil respiration was also stimulated by hexazinone over a 3-week
period, when it was applied at a level up to 100 times higher than the recommended field
rate (Vienneau et al. 2004).
Changes in the SIR value caused by other herbicides, metazachlor and dinoterb, were
observed by Buelke and Malkomes (2001). They found that dinoterb more strongly inhib-
ited SIR as compared with metazachlor and the side effect of dinoterb was more pro-
nounced at 30°C. Busse et al. (2001) observed that glyphosate had no significant effect on
microbial respiration in the soil treated with the expected concentrations following field
application (5-50 mg/kg soil). However, soil respiration was strongly stimulated by the
addition of glyphosate at concentrations of 500 and 5000 mg/kg soil. This herbicide dis-
played stimulatory effect on the microbial respiration also in forest acidic soils (Stratton
and Steward 1992) and agricultural soils, with herbicide application for several years and
without history of glyphosate treatment (Araújo et al. 2003). Similarly, methamidophos at
both low (0.5 mg/g soil) and high (5.0 mg/g soil) inputs did not display negative effects
on soil respiration rate, but in contrast, the respiration rate of the soil microbial biomass
C (μg CO 2 /g C mic h) was significantly higher (Wang et al. 2008). No effect on the microbial