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The investigation on “Approaches to assess chlorpyrifos degradation in
northern laterite soils of Kasaragod (AEU 11)” was undertaken with the objective to
evaluate the impact of physical, chemical and biological methods on degradation of
chlorpyrifos in laterite soils. The study was carried out during 2019 – 2021 at College
of Agriculture, Padannakkad in two incubation experiments.
The incubation experiment no. 1 was carried out to know the pattern and time
required for degradation of chlorpyrifos in soil. Northern laterite soils (AEU 11)
collected from Pilicode were selected for the study. Pot culture study was conducted in
five pots filled with 10 kg soil and drenched with chlorpyrifos (20 EC) at the
concentration of 2.5 ml/L. Soil was analyzed at weekly intervals and results from the
incubation study showed that chlorpyrifos content was reduced to 34.76% within 60
days. Significant decrease on soil pH (2.29%) was noticed after the application of
chlorpyrifos. Chloride and phosphate ions were increased during the incubation period
due to release of these ions from the chlorpyrifos compound during the degradation
process. Microbial biomass carbon (8.2%) in soil was reduced significantly. Based on
the results and findings from the incubation experiment no.1, duration of incubation
experiment no.2 was decided for 60 days.
The incubation experiment no. 2 was carried out to assess the best method of
degradation of chlorpyrifos in laterite soil. The experiment was laid out in CRD with
12 treatments and three replications. Physical, chemical and biological agents were
applied and evaluated to study their effect on degradation of chlorpyrifos. The treatment
combinations were control (T1), hydrogen peroxide-5% (T2), Fenton reagent -0.5%
(T3), hydrogen peroxide-5% + Fenton reagent -0.5% (T4), Pseudomonas fluorescens
(T5), Trichoderma viride (T6), Pseudomonas fluorescens + Trichoderma viride (T7),
sunlight – 6hrs (T8), ultra violet – 4hrs (T9), sunlight – 6hrs + ultra violet – 4hrs (T10),
soil under saturated condition at 5 cm level of submergence (T11) and soil under
saturated condition at 5 cm level of submergence with azolla (T12)
Results from the incubation study revealed that combination of Pseudomonas
fluorescens + Trichoderma viride showed the highest rate (74.99%) of chlorpyrifos
degradation followed by Pseudomonas fluorescens (69.94 %) and Trichoderma viride
(66.35 %) within 60 days. Effect of chlorpyrifos application on chemical properties of
soil was studied at biweekly intervals. Soil pH was found to be significantly decreased
throughout the incubation period. Highest pH (5.09) was recorded in treatment T10
(sunlight + UV light) whereas lowest in T4 (4.93). Effect of treatments on chloride ions
in soil were found to be non significant, however it was continuously increased
throughout the incubation period. Phosphate ions in soil decreased significantly in the
initial period followed by gradual increase in the phosphate ions in soil. In the 8th week,
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T5 recorded the highest phosphate ions (31.80 mg kg-1
) while T1 recorded the lowest
phosphate ions (28.86 mg kg-1
) in soil.
The effect of treatment application on soil biological properties were studied
and showed that chlorpyrifos has inhibitory effect on microbial biomass carbon,
dehydrogenase, phosphatase and urease activities of the soil immediately after
chlorpyrifos application but later restored the activities. The treatments that received
biological agents were not much affected with respect to the biological properties of
the soil. Biological treatments such as T5 recorded the highest dehydrogenase activity
(11.74 µg TPF g-1
soil day-1
) while the T6 recorded the highest phosphatase activity
(17.06 µg PNP g-1
soil hr-1
) of the soil. Treatment T7 recorded the highest microbial
biomass carbon (99.15 µg g-1
) and the urease activity (36.16 µg NH4
+
-N g-1
soil hr-1
) in
soil.
There was a significant effect with respect to the two treatments maintained
under submergence. The leachate from the chlorpyrifos treated soils were analysed at
biweekly intervals. Treatments showed significant effect on leachate of chlorpyrifos
residue at sixth and eighth week intervals. Treatment T11 (2598.62 µg L-1
) showed the
highest degradation followed by T12 (3318.07 µg L-1
). The growth of azolla was normal
during the initial period, later decaying of azolla was noticed. Growth of azolla was
inhibited under the chlorpyrifos treatment because it could not tolerate the residual
effect of chlorpyrifos.
The results from the investigation revealed that chlorpyrifos degradation using
combination of Pseudomonas fluorescens + Trichoderma viride had the best potential
to remove the residues of chlorpyrifos insecticide present in treated soils. Biological
treatments are recorded as the prominent agents in chlorpyrifos degradation and also
maintains the soil health. On account of these findings, we can recommend the use of
biological agents in combination or alone, as an ideal approach for degradation of
chlorpyrifos in laterite soils