Approaches to assess chlorpyrifos degradation in northern laterite soils of Kasaragod (AEU 11)

Abstract

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, 106 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