Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 244

glyphosate. Ratcliff et al. (2006) did not find changes in the culturable bacterial numbers
between the control and the glyphosate-treated soil, when the herbicide was used at a dos-
age of 50 mg/kg soil, whereas a 100-fold higher dosage increased both the heterotrophic
bacterial counts and the bacteria:fungi ratio. On the contrary, Busse et al. (2001) reported
that glyphosate application decreased the total number of heterotrophic bacteria.
Increased viable bacterial counts in fungicide-treated soil have been found in many stud-
ies. For example, Martínez-Toledo et al. (1998) found that after treatment with captan at
concentrations of 2, 3.5, and 5 kg/ha soil, bacterial populations did not increase significantly
until 14 days, whereas at concentration of 10 kg/ha soil, the populations were significantly
higher than that in the control for days 7, 14, and 30. A similar stimulatory effect on the
numbers of bacteria was also observed in soils after the application of mefenoxam, metal-
axyl (Monkiedje et al. 2002), and tebuconazole (Cycoń et al. 2006), as well as mancozeb and
dimethomorph (Cycoń et al. 2010d). Increased bacterial counts in the fungicide-treated soils
may be associated with increased levels of nutrients and energy sources released from dead
fungal hyphae. Furthermore, soil bacteria can be relieved from competition with indigenous
fungi or antagonistic inhibition by the metabolites synthesized by them (Chen et al. 2001b).
8.5.2 Fungal Populations
Apart from bacteria, fungi are responsible for the mineralization of organic matter in soil
and the release of available carbon; hence, any disturbance in their counts due to vari-
ous pesticides may have a harmful impact on soil quality. It has been reported that not
all insecticides negatively affect the soil fungi, and even their growth was stimulated in
treated soils. For example, Martínez-Toledo et al. (1993) found that the fungal population
in an agricultural loam soil was not inhibited in the presence of the chlorinated hydro-
carbon insecticide lindane at concentrations of 3.5-15 kg/ha soil. In turn, Das et al. (2005)
found a marked increase in the total numbers of fungi in the soil treated with phorate
and carbofuran. Similarly, Cycoń et al. (2006) revealed that the addition of λ-cyhalothrin
into loamy sand soil at concentrations of 1 and 20 mg/kg soil resulted in higher num-
bers of fungi compared with that in the control on day 14, but this effect was not seen
thereafter. However, some studies reported that organophosphorus insecticides profeno-
fos (Abdell-Malek et al. 1994) as well as chlorpyrifos and quinalphos (Pandey and Singh
2004) decreased the fungal counts in soil. Also, Abd El-Mongy and Abd El-Ghany (2009)
indicated that the insecticide chlorpyrifos-methyl affected the numbers of fungi in treated
soil; the numbers of colonies decreased after 1-7 days of insecticide application but then
increased over time.
Several studies indicated that herbicides appear to be pesticides affecting indigenous
soil fungi to a little extent. For example, linuron treatment, even at the highest concentra-
tion (400 mg/kg soil), did not significantly change the fungal numbers in sandy soils as
compared with the corresponding controls (Cycoń et al. 2010a). It would appear that cultur-
able fungi can tolerate linuron concentrations in soils. Several soil fungi, including differ-
ent species of Cunninghamella , Mortierella , Talaromyces , Rhizopus , Rhizoctonia , and Aspergillus
are able to degrade linuron and other phenylurea herbicides (Sørensen et al. 2003). Also,
Araújo et al. (2003) found that the numbers of fungi have increased significantly in the
soils treated with glyphosate (2.16 mg/kg soil) after 32 days of incubation. The pronounced
effect of glyphosate as well as other herbicides (2,4-D and picloram) on the fungal activity
was also confirmed in a previous study (Wardle and Parkinson 1990).
Among the numerous applied compounds, fungicides appear to be pesticides affect-
ing indigenous soil fungi to the most extent. A negative impact of the fungicides on the