Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 245

fungal population size was observed in the case of captan and benomyl (Martínez-Toledo
et al. 1998; Busse et al. 2001; Piotrowska-Seget et al. 2008). The numbers of fungi also
decreased significantly in the sandy soils treated with the fungicidal mixture (mancozeb
and dimethomorph), especially applied at the 100-fold recommended field rate, during a
28-day incubation period (Cycoń et al. 2010d). The results obtained by Cycoń et al. (2006)
also indicated that the negative impact of another fungicide (tebuconazole) on the soil
fungi was transient (14 days), though the fungicide, with a soil half-life of 49 days, has been
suggested as a persistent agrochemical (Strickland et al. 2004).
8.5.3 Bacteria Involved in the Nitrogen Cycling
Among the numerous groups of soil microorganisms, the bacteria involved in the nitro-
gen transformation processes such as ammonification, nitrification, denitrification, and
N 2 -ixation are very important. Due to the significant role of these bacteria in soil fertility
and total nitrogen cycling, the evaluation of changes in their numbers was often performed
in many ecotoxicological studies. For example, Cycoń et al. (2010a) found that the nitrify-
ing bacterial counts decreased in the sandy soils treated with linuron at 400 mg/kg soil;
however, this effect was found only on days 1 and 14. Similarly, a negative effect of another
herbicide (simazine) applied at 50-300 μg/g soil on the nitrifying bacteria was observed by
Martínez-Toledo et al. (1996). In addition, the synergistic action of two herbicides (terbutryn
and terbuthylazine) inhibited the activity of the nitrifying bacteria in acidic and neutral
soils (Kara et al. 2004). Also, Martínez-Toledo et al. (1998) found that the numbers of this
bacterial group significantly decreased in agricultural soils after captan treatment at dose
rates of 2-10 kg/ha soil. Application of diazinon at concentrations of 35 and 700 mg/kg
soil also resulted in the decline in the numbers of nitrifying bacteria in loamy sand and
sandy loam soils over a 28-day incubation period (Cycoń et al. 2010b). A strong inhibition
of the nitrifying bacterial activity was also observed by Cycoń et al. (2010d) in the sandy
soils treated with the fungicidal mixture of mancozeb and dimethomorph, even at the rec-
ommended field rate (15 mg/kg soil). Conversely, the nitrifying bacteria were not affected
upon addition of the herbicide alachlor to an agricultural soil (Pozo et al. 1994). Also, Cycoń
et al. (2006) found that the application of the fungicide tebuconazole, even at the highest
concentration (270 mg/kg soil), resulted in higher numbers of nitrifiers in loamy sand soil
on day 28. However, these dosages negatively affected the activity of the nitrifying bacte-
ria immediately after fungicide treatment. A decrease in the numbers of nitrogen-fixing
bacteria has often been found in the pesticide-treated soils (Martínez-Toledo et al. 1998;
Cycoń et al. 2006; Lin et al. 2008). This bacterial group is regarded as a sensitive microbial
indicator of environmental hazard in soils exposed to elevated concentrations of pesticides
and other chemicals; however, the effects of these compounds on nitrogen fixation appear
to be species specific (Hicks et al. 1990). Gadagi et al. (2004) reported that some insecti-
cides (chlorpyrifos, fenvalerate, and quinalphos) may inhibit the growth and nitrogenase
activity of Azospirillum sp. at all concentrations used, whereas other insecticides (endo-
sulfan and monocrotophos) may be tolerated only at the recommended dose. By contrast,
a stimulation of proliferation of nonsymbiotic N 2 -ixing bacteria was found in a laterite
soil amended with carbofuran and phorate (Das and Mukherjee 2000). A decrease in the
numbers of N 2 -ixing bacteria exposed to fungicides such as captan (Martínez-Toledo
et al. 1998) and tebuconazole (Cycoń et al. 2006) has also been found. As indicated by sev-
eral studies, denitrifiers may be regarded as the group more tolerant toward pesticides
compared with N 2 -ixing bacteria and nitrifiers. It has been reported that different pesti-
cides did not negatively affect the soil denitrifiers and even their growth was stimulated