Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 242

By contrast, being suitable substrates for some microorganisms, these mancozeb inter-
mediates may have stimulated ammonification (Černohlávková et al. 2009). The study
performed by Man and Zucong (2009) revealed that the field application rate of chloro-
thalonil (5.5 mg/kg soil) inhibited the nitrification activity in two soils to a less degree but
had no effect on the other four soils. By contrast, chlorothalonil, at the levels of 110 and
220 mg/kg soil, almost completely inhibited nitrification in soils throughout the 14-day
incubation period. Also, Monkiedje et al. (2002) observed that the fungicides metalaxyl
and mefenoxam at a concentration of 1000 μg/g soil severely inhibited the rate of nitri-
fication in sandy loam soil over a 90-day experimental period. In turn, the ammonifica-
tion process was strongly stimulated by the applied fungicides. Similar findings were
also observed by Burrows and Edwards (2002), who studied the impact of the fungicide
carbendazim on the nitrogen transformation processes in soil. They found a significant
increase in ammonium concentration in the soil samples treated with the tested pesticide,
whereas in the case of nitrification process, a reverse effect was observed. By contrast,
Man and Zucong (2009) observed that the inhibitory effect of carbendazim applied even
at the highest concentration (220 mg/kg soil) on nitrification was very weak or absent.
Also, the study performed by Cycoń et al. (2006) showed that tebuconazole had a marked
effect on the nitrogen transformation. This fungicide applied at the highest dosage (270
mg/kg soil) decreased the nitrate concentration in sandy loam soil on day 1, whereas a
stimulating effect of the fungicide on nitrate production was found with time. The data
obtained for various fungicides could partially be interpreted as the result of mineraliza-
tion of the fungi killed by these compounds. However, fungal biomass is known to be a
poor source of nitrogen as compared with bacteria. Therefore, other chemicals present
in the fungicide formulation could stimulate the ammonifying bacteria, resulting in an
enhanced production of ammonium. An increase in the ammonium concentrations in
the soils treated with benomyl or captan has been reported in some studies (Chen et al.
2001a; Piotrowska-Seget et al. 2008). By contrast, the application of other fungicides to
soils has stimulated nitrification as indicated by increased nitrate contents (Chen et al.
2001b; Monkiedje and Spiteller 2002). However, it is difficult to explain why some fungi-
cides could be tolerated by nitrifiers or could even stimulate them. Presumably, soils dif-
fer in their nitrifying capacity and those that can maintain a complex population of active
nitrifiers are less sensitive to pesticides (Hicks et al. 1990).
Results of some studies have shown that some pesticides indirectly stimulate ammoni-
fication probably by killing a part of the microorganisms, which is mineralized, resulting
in increased concentrations of ammonium (Das and Mukherjee 2000). Generally, nitrifica-
tion is more sensitive to pesticides than ammonification, which is conducted by a diverse
microbial population (Hicks et al. 1990).
8.5 Effect of Pesticides on Soil Microbial Numbers
The culturable bacteria usually represent an ecologically important fraction of the soil bac-
terial community. Although only a small fraction of the total bacterial population (0.1%-
10% of microorganisms occurring in soil) can grow on agar plates (Littlefield-Wyer et al.
2008), they constitute a large proportion of the total bacterial biomass and play the most
important role in nutrient cycles (Ellis et al. 2003). Therefore, quantitative changes in the
composition of soil-culturable bacterial communities may serve as important and sensitive