Response of Soil Microflora to Pesticides - Pesticides: Evaluation of Environmental Pollution - page 241

8.4 Effect of Pesticides on Soil Nitrogen Transformation Rates
With regard to the great importance of nitrogen cycling for soil fertility and plant develop-
ment, nitrification and ammonification processes are commonly used as the indicators of
microbial activity in soils amended with pesticides (Monkiedje et al. 2002; Kara et al. 2004).
Generally, insecticides have been reported as compounds stimulating N mineralization
and nitrification processes after incorporation into soil. For example, Das and Mukherjee
(1994) found that insecticides such as 1,2,3,4,5,6-hexachlorocyclohexane (BHC), phorate,
carbofuran, and fenvalerate, at their recommended doses, affected nitrogen turnover by
changing the availability of ammonium and nitrogen fixation in the rhizosphere of rice.
The application of BHC and phorate resulted in the lowest retention of total N, compared
with other insecticides. The greater mineralization of N following the insecticide applica-
tion brought a significant increase in ammonium and nitrate concentrations in the soil,
and this remained more prominent up to the 30th day, followed by a steady decline up to
the 60th day, except with BHC and phorate, both of which caused a progressive increase
in nitrate concentration until the 45th day. A similar phenomenon was also confirmed by
Jana et al. (1998). In turn, Cycoń et al. (2006) found slight negative effects (on days 1 and 7)
of two higher treatments (1 and 20 mg/kg soil) of the insecticide λ-cyhalothrin on the
nitrate concentrations in sandy loam soil. By contrast, this insecticide did not have an
effect on ammonium concentrations as compared with the control. Nitrate concentrations
also decreased in the soil samples treated with diazinon; however, this effect was espe-
cially seen for a dosage of 700 mg/kg soil (Cycoń et al. 2010b).
Marsh and Davies (1981) observed the long-term strong inhibition of nitrification in
dichlorprop- and mecoprop-treated soils. Moreover, nitrification was suppressed signifi-
cantly by bensulfuron-methyl applied at concentrations of 0.1 and 1.0 μg/g soil (El-Ghamry
et al. 2002). Cycoń et al. (2010a) also found that the phenylurea herbicide linuron at the
highest dosage (400 mg/kg soil) caused a significant decrease in nitrate concentrations in
sandy soils; however, this effect was observed shortly after its application. Kara et al. (2004)
reported that the herbicide topogard significantly stimulated ammonification in neutral
and alkaline soils, whereas acid soils showed significantly lower ammonium contents in
comparison with the controls. Ammonification was also initially inhibited by the triazine
herbicide hexazinone; however, ammonium contents were similar or greater in all treat-
ments compared with the controls after 4 weeks (Vienneau et al. 2004). Studies on the
successive applications of different pesticides in field conditions over 2 years, carried out
by Schuster and Schröder (1990), showed that herbicides had only slight and transient side
effects on the ammonification process.
As indicated by the data from some studies, the different levels of some fungicides in
soil have been found inhibitory to nitrification. Evidently, the chemical nature of fun-
gicides has decided their effects on this process, albeit soil type has also been of great
value (Monkiedje and Spiteller 2002; Kinney et al. 2005). Martens and Bremner (1997)
ascertained up to 90% inhibition of nitrification in the soil amended with 50 mg/kg soil
of mancozeb. Similarly, Černohlávková et al. (2009) revealed that the nitrification process
was very sensitive to mancozeb and its application caused significant decrease in nitrifica-
tion to 11.2% and 5.6% in arable soil and grassland soil, respectively. An inhibitory effect
of mancozeb on the nitrification process was also observed by other authors (Pozo et al.
1994; Kinney et al. 2005; Cycoń et al. 2010d) and may be linked with its fate in soil. The
fungicide transformation products such as ethylenethiourea and ethyleneurea, persist-
ing for weeks in soil, were reported as strong inhibitors of soil nitrification (Vonk 1991).