Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 239

However, the authors found that all fungicide dosages significantly inhibited PHOS-OH
over a 90-day incubation period. Also, Chen et al. (2001a) observed a significant decrease
in PHOS-H in silt loam soil after treatment with captan (125 mg/kg soil) or benomyl
(51 mg/kg soil), and this effect was more pronounced with time. Inhibition of the acitivi-
ties of both the phosphatases in sandy loam soil amended with captan during 98 days of
incubation was also ascertained by Piotrowska-Seget et al. (2008). Also, Cycoń et al. (2010d)
found that the fungicidal mixture of mancozeb and dimethomorph negatively affected
both the phosphatases in sandy soils. In addition, the higher the concentration of the fun-
gicides added into the soil, the higher is the decrease in phosphatase activities. However,
the authors observed a greater inhibition of this activity in the soil with lower contents of
clay and organic matter. Being extracellular enzymes, phosphatases are immobilized by
soil colloids, which protect them from degradation in polluted soils (Boyd and Mortland
1990). This fact could explain a greater sensitivity of both the phosphatases to fungicides
in sandy soils than clay soils. Moreover, the higher decrease in alkaline phosphatase than
acid phosphatase in the soils treated with fungicides may have been associated with the
decreased growth of the indigenous fungi that are known as soil producers of alkaline
phosphatase (Nannipieri et al. 1990).
8.3.4 Urease Activity
The activity of urease, an extracellular enzyme that catalyzes the hydrolysis of urea to
ammonia, may also reflect the response of microorganisms to pesticide application.
Depending on the type of herbicide, dosage, application rate, and soil type, the response
of microbial community expressed as urease activity (URE) showed a high variability. For
example, Ingram et al. (2005) found a significant short-term inhibitory effect of diazinon
on the microbial urease-producing community. Similarly, Jung et al. (1995) found that
diazinon decreased URE by 50% at concentrations greatly exceeding those found in soils
during agricultural practices. A great sensitivity of urease to diazinon addition into soils
has also been reported by Cycoń et al. (2010b). However, this effect was transient and URE
in the soil with higher clay and organic matter contents recovered to the level as in the
control after 28 days of incubation. Previously, Lethbridge and Burns (1975) also observed
40%-50% urease inhibition 60 days after applying other insecticides, malathion, accothion,
or thimet, into sandy clay loam. By contrast, the application of the insecticide acetamiprid
did not affect URE over a 35-day incubation period (Yao et al. 2006).
A variable activity of urease in soils treated with different herbicides was observed by
several authors; however, no effect or even stimulation of URE was generally observed.
For example, Moreno et al. (2007) found that application of atrazine into clay loam soil at
a wide range of concentrations (0.2-500 mg/kg soil) had no effect at the beginning of the
experiment or stimulated URE at two highest dosages on day 45. This phenomenon was
also reported by Jorge et al. (2007). Also, Sannino and Gianfreda (2001) observed a signifi-
cant increase in URE in soils treated with other herbicides, paraquat and glyphosate, over
an incubation period. By contrast, Hua et al. (2009) reported that napropamide applied at
all used concentrations (2-80 mg/kg soil) inhibited URE in the soil over a 56-day experi-
mental period, and additionally, this effect was more pronounced with time. Similarly, the
sulfonylurea herbicide metsulfuron-methyl negatively affected URE in soils (Ismail et al.
1998).
Different levels of some fungicides in the soil have also been found inhibitory or
stimulatory to URE. For example, application of the phenylamide fungicide metalaxyl
resulted in the gradual decrease in URE in clay loam soil from 10 to 60 days (Sukul 2006).