Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 234

et al. 2001a; Araújo et al. 2003; Lupwayi et al. 2009a; Cycoń et al. 2010a). The observed changes
depend on the types of pesticides, their spectrum of activities, persistence, and dosages
applied. Moreover, active substances of pesticide preparations may be used as a source of
energy and nutrients by some microorganisms and may be degraded with variable intensi-
ties, which may result in the increase of microbial populations (Johnsen et al. 2001; Cycoń
et al. 2010b). Certain agrochemicals, which are not utilizable by the soil, might be degraded
in the soil by the microorganisms through cometabolism (Dejonghe et al. 2003). Moreover,
metabolites, which are the products of pesticide degradation, may be more toxic than the
parent substances, resulting in the inhibition of the activities of microbial groups responsible
for the degradation of pesticides introduced into the soil (Vonk 1991; Matsushita et al. 2003).
A large number of different assays including measurements of chemical, physical, and
biological soil parameters have been used in the studies on pesticide toxicity to soil eco-
system. Chemical and physical soil properties (e.g., organic matter, nutrient status, and soil
texture) change very slowly, and therefore a long period of time is needed to observe sig-
nificant changes. On the contrary, the microbiological indicators may rapidly reflect even
small changes that occur in the soil, providing accurate data about the health and quality
of the soil (Filip 2002; Schloter et al. 2003). Due to their fast response to contaminants, soil
microorganisms are suitable to act as a “biomarker,” reflecting the negative effects of pes-
ticide treatment and are commonly used in ecotoxicological tests to evaluate the influence
of chemicals on soil systems (Pascual et al. 2000; Filip 2002). Based on the definition of
Domsch et al. (1983), a delay in the restitution of normal microbial population or functions
within 31-60 days can be considered as having “tolerable” effects, while for more than
60 days indicates “critical” effects.
The literature on the effect of pesticides on soil microorganisms and processes is
extremely diverse, ranging from reports of the effects of chemicals on individual species
of microorganisms to those on populations of microorganisms and individual biological
systems, using a great diversity of testing methods in both field and laboratory. The most
common are measurements of global parameters such as microbial respiration, organic
matter turnover, microbial biomass, and more specific indicators based on particular
microbial activities such as nitrogen fixation, nitrification, and denitrification, as well as
soil enzyme activities (Araújo et al. 2003; Chen et al. 2003; Singh and Singh 2005; Cycoń
and Piotrowska-Seget 2009). Since these methods do not give insight into all the microbial
communities, methods based on molecular techniques have been applied for the studies
on the structure of soil microbial communities. These approaches involve the analysis of
nucleic acids (DNA and/or RNA) and fatty acids (phospholipids fatty acid (PLFA) and/
or fatty acid methyl ester (FAME)) isolated directly from environmental samples (Seghers
et al. 2003; Lin et al. 2008; Hua et al. 2009; Zhang et al. 2010a). In this chapter, both culture-
dependent and culture-independent methods and parameters used for the assessment of
ecological risk related to pesticide usage are presented and discussed, with regard to their
potential successful application for the estimation of the response of soil microorganisms
to pesticide treatment.
8.2 Effect of Pesticides on Soil Respiration and Microbial Biomass
The substrate-induced respiration (SIR) method is commonly accepted for the estimation
of potential perturbations in microbe-mediated degradation of organic matter in the soil