Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 250

by comparison of SIR rates following the addition of individual C compounds. Similarly,
Weaver et al. (2007) reported that glyphosate had only small and transient effects on the
soil microbial community even when applied at a dosage much higher than the recom-
mended rate. Mijangos et al. (2009) used the Biolog Ecoplates system for estimating the
catabolic potential of the rhizosphere microbial communities of triticale or triticale and
pea under glyphosate and 2,4-D exposures. They observed significant differences in the
carbon substrate utilization patterns between the rhizosphere microbial communities in
the glyphosate-treated (50 and 500 mg/kg soil) and control soil samples on the 15th day
after the pesticide application. On the same day, herbicide treatment resulted in a stimu-
latory effect on the ability of the rhizospheric communities to metabolize the substrates
such as phenylethylamine,d-galacturonic acid, Tween 40, Tween 80, and hydroxybutyric
acid. By contrast, after 30 days of glyphosate treatment, the microbial assemblages were
characterized by a lower capacity of usage of a considerable percentage of the carbon sub-
strates included in the Ecoplates. Interestingly, the response of the microbial communities
was different in triticale versus triticale and pea combination. In contrast to glyphosate,
the application of 2,4-D at a dosage of 5 mg/kg soil increased the functional richness of
the microorganisms (Mijangos et al. 2009). A structural shift in the microbial community
structure under 2,4-D exposure was earlier proved by Macur et al. (2007). They found that
in the soil treated with agriculturally relevant doses of 2,4-D dominated metabolically
versatile genera such as Burkholderia , Bradyrhizobium , Variovorax, and Arthrobacter , whereas
in the control soil only Variovorax -like isolates were identified. In turn, the addition of
metsulfuron-methyl caused transient changes in the percentage of the carbon substrates
utilized by the microbial community. A slight increase in the functional richness in the
pesticide-treated soil was observed only a week after herbicide application.
Based on the results of the field studies with other herbicides such as metribuzin, ima-
zamox/imazethapyr, glufosinate ammonium metribuzin, triasulfuron, and metsulfuron-
methyl, Lupwayi et al. (2004) concluded that the herbicides applied once at recommended
rates did not have significant or consistent effects on the microbial functional diversity. The
CLPP method was also used to establish changes in the catabolic potential of the microbial
communities in soils treated with atrazine at concentrations of 1, 10, 100, and 1000 mg/kg
soil (Ros et al. 2006). The authors found that higher pesticide concentrations negatively
affected the potential catabolic activity. Values of the diversity index (H') calculated for
each class of carbon compounds showed that some carbon groups in the high atrazine-
treated soils were degraded at lower rates as compared with the control. Interestingly, a
negative effect was also observed after 45 days of atrazine application when the pesticide
was totally degraded (Ros et al. 2006).
In turn, Wang et al. (2008) showed in the pot experiments that the organophosphorus
insecticide methamidophos markedly changed the functional and genetic diversity of
the soil microorganisms. Using the polyphasic approaches including ARDRA (amplified
rDNA restriction analysis), PLFA, and CLPP, they revealed that methamidophos decreased
the genetic biodiversity and differentially affected the components of the soil microbial
community. High input of pesticides (64.48 mg/g soil) decreased the fungal biomass but
increased the biomass of the GN bacteria and enhanced their catabolic activity (Wang et al.
2008). By contrast, a minimal impact of the pyrethroid insecticide cypermethrin applied at
a concentration of 10 mg/kg soil on the functional diversity was reported by Xie et al. (2009).
Similarly, no significant effects of another pyrethroid insecticide (λ-cyhalothrin) on the soil
functional bacterial diversity were observed; however, the microbial catabolic abilities in
the insecticide-treated soil were altered (Lupwayi et al. 2009b). A transient effect on the soil
microbial functional diversity was caused by chlorpyrifos applied at concentrations of 4, 8,