Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 246

in treated soil (Martínez-Toledo et al. 1998; Cycoń et al. 2006). For example, Cycoń et al.
(2010a) found that the numbers of denitrifying bacteria were not affected by linuron in
sandy soils over a 28-day experiment. By contrast, the application of glyphosate to grass
resulted in a 20-30-fold increase in denitrification compared with untreated soil (Tenuta
and Beauchamp 1996). Also, Yeomans and Bremner (1985) found that denitrifying bacteria
were either unaffected or even stimulated in the soils treated with different insecticides
(lindane, fenitrothion, fonofos, malathion, phorate, terbufos, and carbofuran) and fungi-
cides (mancozeb, maneb, thiram, benomyl, captan, and terrazole). A transient decrease
in the numbers of denitrifying bacteria has also been reported in the soil amended with
pyrethroid λ-cyhalothrin (Cycoń et al. 2006).
8.6 Effect of Pesticides on Structural and Functional
Diversity of Soil Microbial Communities
A negative effect of pesticide application may be also related to the changes in the bio-
diversity of microbial communities. These communities may be significantly changed
even if the total numbers of microorganisms and cycling of nutrient elements such as
carbon, nitrogen, sulfur, and phosphoros appear to be unaffected. It happens when the
growth of some microorganisms is inhibited and others proliferate in the vacant eco-
logical niches. This replacement may finally lead to altered biological activities of soils
at a later point in time. Therefore, the pesticide risk assessment should also involve an
examination of the microbial diversity, reflecting both the changes caused by a direct
toxicity and the long-term effect resulting from the successions in the microbial com-
munity (Johnsen et al. 2001).
The culturable microbial community can be characterized in ecophysiological terms
by the r/K strategy concept derived from evolutionary ecology, indicating that there
are genetic differences between organisms in their ability to exploit and survive in dif-
ferent environments (De Leij et al. 1993). In soil microbiology, an interpretation of this
concept is that the r-fraction represents active, viable bacteria, whereas the K-fraction
corresponds to dormant and slow-growing bacteria. Typically, the r-fraction constitutes
5%-20% of SIR in the rhizosphere soil (Stenstróm et al. 2001). In some studies, the analy-
sis of the bacterial growth dynamics revealed that the K-strategists generally dominated
in the soils after treatment with pesticides (e.g., diuron, diazinon, thiram) in compari-
son with the controls. In addition, the application of pesticides, especially at the highest
dosages, resulted in an uneven distribution of the bacterial ecotypes with the domina-
tion of one class representing the K-strategists, which was indicated by the decreased
ecophysiological index. However, higher abundance of r-strategists was ascertained in
the treated soils at the end of the incubation period (Cycoń and Piotrowska-Seget 2009;
Cycoń et al. 2010b).
Since the culturable fraction of soil bacteria represent a small fraction of soil microor-
ganisms, the determination of quantitative changes in the composition of the entire micro-
bial communities as a consequence of pesticide application needs the culture-independent
methods. In recent years, various methods based on cell molecular signature markers such
as DNA or/and fatty acids isolated directly from the soil samples have been shown to be
useful for detecting pesticide-induced changes in the compositions of the microbial com-
munities (Ratcliff et al. 2006; Zhang et al. 2009; Crouzet et al. 2010).