Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 237

features such as soil type or pH (Tabatabai 1994). Dehydrogenases occur intracellularly
in all living microbial cells and they are linked with microbial respiration processes.
They are considered to play an essential role in the initial stages of the oxidation of soil
organic matter by transferring hydrogen and electrons from the substrates to the accep-
tors (Nannipieri et al. 1990).
The increased sensitivity of DHA to insecticides has been reported in some studies
(Martínez-Toledo et al. 1998; Pandey and Singh 2006). However, the stimulation of enzyme
activity in response to soil amendment with diazinon and other insecticides has also been
observed (Gundi et al. 2007; Singh and Singh 2005). In turn, Cycoń et al. (2010c) found no
effect of diazinon applied at the recommended field rate on DHA in sandy soils in a 28-day
experiment, while the addition of the highest dosage of this insecticide (700 mg/kg soil)
resulted in strong DHA inhibition during the whole experimental period. Application of
higher rates (5-12.5 kg/ha) of two organophosphorus insecticides (monocrotophos and
quinalphos) and two synthetic pyrethroids (cypermethrin and fenvalerate) were found to
be either innocuous or toxic to the activities of the dehydrogenase in the soil (Rangaswamy
et al. 1993). By contrast, Yao et al. (2006) did not find any effect till 14 days or even with
the stimulation of DHA after this time after the addition of the neonicotinoid insecticide
acetamiprid.
Varying impacts of herbicides on DHA are also associated with the variations in the soil
organic matter content, type of herbicide and its dosage, and farming history (Zabaloy
et al. 2008a,b). A positive effect of glyphosate applied up to 200 mg/kg soil on DHA has
been reported by some authors (Accinelli et al. 2002; Araújo et al. 2003). Also, the stimula-
tory effect on DHA has been found for atrazine application at different levels (Moreno et al.
2007). In turn, Grenni et al. (2009) found no changes in DHA between the control and the
soils treated with the phenylurea herbicide linuron at the agricultural rate. However, the
application of linuron at higher dosages (20 and 400 mg/kg soil) resulted in the decrease
of DHA in sandy soils (Cycoń et al. 2010a).
As indicated by the data from various studies, the different levels of some fungicides in
the soil have been found inhibitory to DHA. A strong inhibition of DHA was observed as a
microbial response to soil amendment with all the dosages of two fungicides, mefenoxam
(1-1000 mg/kg soil) and metalaxyl (2-1000 mg/kg soil), during 90-day experimental
period (Monkiedje et al. 2002). Similarly, Chen et al. (2001a) observed a negative effect of
the fungicides benomyl and captan on DHA. However, this effect was found only at the
beginning of the experiment. Inhibitory effect was also found for other fungicides such as
azoxystrobin, tebuconazole, and chlorothalonil (Bending et al. 2007).
A low DHA in the soils treated with pesticides may be associated with the death of a
part of microbial fraction sensitive to pesticides, and the dehydrogenases released from
dead cells do not accumulate in soil since they are rapidly degraded. Free enzymes nor-
mally have a short-lived activity because they can be rapidly denatured, degraded, or irre-
versibly inhibited (Marx et al. 2005). However, a certain proportion of free enzymes may
undergo stabilization through adsorption into humic materials, which, despite affecting
their catalytic potential, may enable enzyme activity to persist in soil (Badine et al. 2001).
8.3.2 Fluorescein Diacetate Hydrolyzing Activity
Fluorescein diacetate hydrolyzing activity (FDHA) is widely accepted as a valuable and
simple method for measuring lipase, protease, and esterase activities in soils (Adam
and Duncan 2001). As FDHA is a sensitive and nonspecific test able to depict the hydro-
lytic activity of soil microorganisms, it is used as a suitable tool for measuring an early