Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 238

detrimental effect of pesticides on soil microbial biomass (Perucci et al. 2000). Depending
on the class of pesticides, application rate, and soil type, a decrease, no consistent changes,
or an increase in FDHA has been reported. For example, Dutta et al. (2010) reported that
the organophosphorus insecticide chlorpyrifos applied at the field rate (0.5 mg/kg soil)
into clay loam soil did not adversely affect FDHA and even a stimulation at the begin-
ning of the experiment was observed. By contrast, there was a sharp decrease in FDHA
in the soil treated with a higher concentration (100 mg/kg soil) over a 90-day incubation
period. A negative effect of chlorpyrifos and endosulfan was also observed in tea garden
soil by Bishnu et al. (2008). As revealed by Zabaloy et al. (2008b), the application of 2,4-D
into sandy clay loam soil reduced FDHA by 11% in comparison with the control, but this
effect was observed only 1 week after herbicide treatment. In turn, this herbicide did not
affect FDHA in sandy loam soil over a 21-day experimental period. Authors also found a
transient reduction in the hydrolyzing of FDA (fluorescein diacetate) after application of
other herbicides, metsulfuron-methyl and glyphosate (Zabaloy et al. 2008b). A transient
inhibition of FDHA at the beginning of the experiment (up to day 15) in two soils was also
observed by Pal et al. (2005), in the case of the phenylurea fungicide pencycuron.
8.3.3 Phosphatase Activity
Phosphatases are the extracellular enzymes that catalyze the hydrolysis of organic phos-
phorus compounds into inorganic phosphorus (Pant and Warman 2000). Generally, it is
stated that acid and alkaline phosphatases (PHOS-H and PHOS-OH) are insensitive to the
toxic effect of pesticides (Boyd and Mortland 1990). For example, Cycoń et al. (2010b) found
no effect of diazinon applied at concentrations of 7, 35, and 700 mg/kg soil on PHOS-OH
in sandy loam soil over a 28-day incubation period. By contrast, the activity of PHOS-H
was significantly stimulated by the applied insecticide during the whole experimental
period. Similarly, Megharaj et al. (1999) revealed that the application of another insecticide
(fenamiphos) into loamy sand soil did not negatively affect PHOS-OH, and even stimula-
tion up to 20% at a concentration of 100 mg/kg soil was observed. By contrast, the inhibi-
tory effect of insecticides on PHOS-OH was found in the case of acetamiprid applied at
higher dosages (Yao et al. 2006).
Sannino and Gianfreda (2001) found that the herbicides glyphosate, paraquat, and atra-
zine strongly affected the phosphatases in 21 soils. The greatest sensitivity of phosphatase
was observed in the glyphosate-treated soils. Authors also suggested that the inhibitory
effect of this herbicide may be associated with the presence of phosphate group in its
molecules. It has been demonstrated that soil phosphatases are strongly inhibited by inor-
ganic phosphate and phosphate fertilizers (Speir and Ross 1978). Also, Perucci et al. (2000)
observed detrimental effects of the sulfonylurea herbicide rimsulfuron and the imidazo-
line herbicide imazethapyr applied at field and 10-fold field rates on both the phospha-
tases. A decrease in the activities of both the phosphatases was also found in the soil
treated with other herbicides, trifluralin (Wyszkowska and Kucharski 2004), glyphosate
and paraquat (Sannino and Gianfreda 2001), or alachlor (Pozo et al. 1994). However, for
alachlor, PHOS-H and PHOS-OH decreased significantly shortly after the introduction to
soil at concentrations of 5.0-10.0 kg/ha soil, but with time, the enzyme activities reached
levels similar to that found in the control (Pozo et al. 1994).
As indicated by the data from various studies, the different levels of some fungicides in
soil have been found as inhibitory or stimulatory to phosphatase activities. For example,
application of the fungicides mefenoxam (1-1000 mg/kg) and metalaxyl (2-2000 mg/kg)
into sandy loam soil resulted in no effect or stimulation of PHOS-H (Monkiedje et al. 2002).