3.1. Degradation of chlorpyrifos under natural conditions
The first degradation experiment was carried out by application of chlorpyrifos in coarse textured laterite soils under natural conditions. The experimental soil was initially analyzed for chlorpyrifos content and was found to be negligible. The data showed that the chlorpyrifos content was 76.94 mg kg− 1on the 5th day which reduced to 50.1 mg kg− 1 on the 60th day (Fig. 1). The chlorpyrifos content was reduced throughout the period of incubation. There was 34.76% reduction in chlorpyrifos content on 60th day on comparison with 5th day of incubation study. The initial soil pH before the start of the degradation experiment was 5.26 (Table 1). After addition of chlorpyrifos, soil pH (Table 2) decreased from 5.14 on the 5th day to 5.02 on the 60th day during the natural process of degradation. The major products of chlorpyrifos degradation namely phosphate, sulphate and nitrate ions were slightly increased (Table 2) during the incubation period. Chlorides increased from 14.2 on the 5th day to 22.72 mg kg− 1 on 60th day. The initial soil analysis of experimental soil revealed that chloride content was 14.12 mgkg− 1, while phosphates, sulphate and nitrate were found to be 29.32, 0.012 and 0.035 mg kg− 1 respectively. Application of chlorpyrifos showed significant effect on microbial biomass carbon in soil and showed a decrease from 90.14 µg g− 1 on 5th day to 84.38 µg g− 1 on 60th day of incubation. Based on the results from the degradation experiment under natural conditions, the duration of the chlorpyrifos degradation experiment aided with different treatments was scheduled for 60 days of incubation period.
3.2 Effect of treatments on chlorpyrifos degradation
The data showed that the physical, chemical and biological (microbial inoculants) agents imposed as treatments could significantly increase chlorpyrifos degradation during the 60 days incubation period. Chlorpyrifos content of soil treated with different agents decreased to varying extent during the period of incubation. Biological treatment consisting of microbial inoculants applied as combination of Pseudomonas fluorescens and Trichoderma viride recorded significantly highest degradation (18.42 mg kg− 1), followed by Pseudomonas fluorescens (22.13 mg kg− 1) and Trichoderma viride (24.77 mg kg− 1) within 60 days of incubation period (Fig. 2). Lowest degradation (48.72 mg kg− 1) was observed in control, T1.
Percentage reduction was 74.99 in treatment with Pseudomonas fluorescens and Trichoderma viride while control recorded only 33.82% (Fig. 3). The treatments with Fenton reagent − 0.5% (T3) recorded 58.3% reduction in chlorpyrifos whereas combination of sunlight plus ultra violet light (T10) recorded 47.7% reduction of chlorpyrifos on 60th day of degradation (Fig. 3).
3.3 Effect of treatments on microbial biomass carbon and soil enzymes
The soil microbial biomass carbon increased to 99.16 µg g− 1 in the treatment with a combination of Pseudomonas fluorescens + Trichoderma viride (Fig. 4). Treatment with chemicals, Fenton (79.63 µg g− 1) and combination of H2O2 and Fenton (78.23 µg g− 1) showed lower microbial biomass carbon which was lower than the control (83.77 µg g− 1).
Effect of various treatments on soil enzymes such as dehydrogenase, phosphatase and urease activity of the experimental soil was assessed and presented in Table 3. Values prior to application of treatments were 11.78 µg TPF g− 1 soil day− 1, 13.62 µg PNP g− 1 soil h− 1, 33.25 µg NH4+-N g− 1 soil h− 1 for dehydrogenase, phosphatase and urease enzymes (Table 1) respectively.
The effect of treatment application on dehydrogenase activity of the soil was found to be significant. Treatment with microbes resulted only a slight reduction in dehydrogenase activity whereas chemical and physical treatments showed higher reduction when compared with initial dehydrogenase activity of the soil. Highest dehydrogenase activity was observed in treatment with Pseudomonas fluorescens (11.74 µg TPF g− 1 soil day− 1). Treatment with combination of Pseudomonas fluorescens + Trichoderma viride recorded 11.37µg TPF g− 1 soil day− 1 and the lowest activity was recorded in control (7.83 µg TPF g− 1 soil day− 1) at 60th day of incubation.
Maximum phosphatase activity was recorded with Trichoderma viride (17.06 µg PNP g− 1 soil h− 1) inoculation. Treatment with combination of Pseudomonas fluorescens + Trichoderma viride recorded 16.53µg PNP g− 1 soil h− 1 and the lowest activity was recorded in control (12.04 µg PNP g− 1 soil h− 1) on 60th day of incubation. The phosphatase activity recorded was lower in physical and chemical treatments.
Treatments with microbial inoculants showed an increase in the urease activity in comparison with initial values. At the same time chemical and physical treatments showed a decrease in the urease activity when compared with urease activity recorded at start of the experiment. Treatment with combination of Pseudomonas fluorescens + Trichoderma viride (36.16 µg NH4+-N g− 1 soil h− 1) showed highest urease activity among the treatments whereas lowest activity was observed in control (28.58 µg NH4+-N g− 1 soil h− 1) on 60th day of incubation.
3.4 Effect of treatments on chemical parameters
Application of different treatments had a significant impact on various chemical parameters such as soil pH, phosphate, chloride, sulphate and nitrate (Table 4). It was observed that there was a significant decrease in the soil pH during the degradation of chlorpyrifos. The lowest pH was recorded in treatment with combination of chemicals T4 (4.93) whereas biological and physical treatments showed higher values of soil pH compared to control. Treatment with combination of Pseudomonas fluorescens + Trichoderma viride recorded pH values of 5.06 at 60th day of incubation.
With respect to phosphate ions, an increase in content in the biological and chemical treatments but there was a reduction observed in UV light treatment and control in comparison with initial soil values (Table 1). Soil treated with Pseudomonas fluorescens recorded the maximum values of phosphate ions (31.80 mg kg− 1) while treatment with combination of Pseudomonas fluorescens + Trichoderma viride recorded 31.30 mg kg− 1. The control (28.86 mg kg− 1) and UV light (29.10 mg kg− 1) showed lower phosphate ions in soil. Chloride, sulphate and nitrate ions in soil were found to be non-significant with respect to treatment effects. Treatment with combination of Pseudomonas fluorescens + Trichoderma viride recorded 30.76, 0.046 and 0.073 mgkg− 1 for chlorides, sulphates and nitrates, respectively while the corresponding values in control was 21.3, 0.035 and 0.058 mg kg− 1.