Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 249

(16:1ω7 t , 18:1ω7 t , cy 17:0) regarded as an indicator of GN bacteria and 19:0 and the decrease
in fatty acids (14:0, i 15:0 and a 17:0) known as a specification for GP bacteria (Zhang et al.
2009).
In another study, Zhang et al. (2010a) used PLFA approach to assess the community
structure of microorganisms in soils differing in their fertility and organic matter con-
tents and treated with the 2,4-D butyl ester at dosages of 10, 100, and 1000 μg/g soil. PLFA
profiling showed that biomass expressed as the total PLFA abundance was correlated with
herbicide concentration and soil type. The total bacterial and fungal PLFAs decreased
with the increasing herbicide concentration, reaching the minimum level at 1000 μg/g in
soil with higher organic matter content. In soils of low organic matter amount, the high-
est PLFA concentration was found in the soil treated with 2,4-D butyl ester at dosage of
100 μg/g. The distribution of indicator fatty acids in PLFA profiles revealed that the her-
bicide significantly decreased the amount of GN bacteria and the rate of this decrease
was correlated with the concentration of the herbicide. In turn, a decrease in GP bacteria
biomass was observed only in the soils treated with the highest herbicide concentration
(1000 μg/g soil), indicating that GN bacteria were more sensitive to 2,4-D butyl ester than
GP bacteria (Zhang et al. 2010a). Another herbicide, imazethapyr, applied into two agri-
cultural soils and incubated for 120 days also changed the structure of the soil microbial
community (Zhang et al. 2010b). A decrease in the total amount of PLFAs and lower ratios
of GN/GP and fungi/bacteria in the imazethapyr-treated soils in comparison to the con-
trol were observed. Additionally, the authors calculated the level of stress expressed as the
ratio ( cy 17:0 + cy 19:0)/(16:1ω7 t + 18:1ω7 t ) and observed that the high pesticide input (1 and
10 mg/kg soil) caused the highest stress in soil with low organic matter content and worse
nutrient status. The observed shift in soil microbial community was transient and it recov-
ered after 60 days.
Structural changes in soil microbial communities may be also estimated by the whole-
cell FAME profiles isolated from soil. Using this method, Lancaster et al. (2010) showed
that the successive glyphosate applications altered the bacterial diversity much more than
its single application. They also observed that FAMEs regarded as markers of GN bacteria
were present in higher concentrations following five applications in comparison to 1, 2, 3,
or 4 applications during first 2 weeks of the experiment.
8.6.3 Community Level Physiological Profile
As microbial response to toxicant involves the study of both the structure and the function
of an ecosystem, assessment of microbial communities should consider not only the abun-
dance of bacteria but also the functional diversity and redundancy present in the microbial
community (Kent and Triplett 2002). Recently, the most common method used to establish
the functional diversity of microbial assemblages is the measurement of community-level
physiological profiling (CLPP) based on the ability of the community to utilize a wide
range of carbon substrates (Garland and Mills 1991). This technique has shown its value
in the field of ecotoxicology, for discriminating between pesticide-treated and nontreated
microcosms and field sites (de Lipthay et al. 2004; Mijangos et al. 2009).
The CLPP approach was used by Zabaloy et al. (2008b) for estimating the effect of the
herbicides glyphosate, 2,4-D, and metsulfuron-methyl on the soil functional richness.
They found that the pesticides used only slightly affected the catabolic potential of the
soil microorganisms and this effect was related to the type of soil. Also, no effect of
glyphosate on the catabolic richness in the soil exposed to this herbicide for a long period
was reported by Busse et al. (2001), who measured the catabolic response of microflora