Impact assessment of thiamethoxam on microbial and enzymatic activity in mango rhizosphere

Thiamethoxam is a globally used neonicotinoid insecticide in mango ecosystem which can cause hazardous impact on soil environment. The current study was carried out to assess the impact of 0.2 (T₁/treament) and 2.0 g kg⁻¹ (T₂) concentrations of thiamethoxam on mango rhizospheric microbial community, enzyme activity along with its degradation. Indigenous microflora degraded thiamethoxam from 5.62 to 41.17 mg kg⁻¹ at 0 day to 0.60 and 2.10 mg kg⁻¹ in soil after 60 days of application of 0.2 and 2.0 g kg⁻¹ doses, respectively. Thiamethoxam dissipated via pseudo first-order kinetics with t_1/2 (half-life) values (23.00 and 14.00) days and K values (1.99–17.14) and (21.42–183.6) at T₁ and T₂, respectively. At bacterial count of 5.5 × 10⁷ in soil, fluorescein diacetate (FDA) and dehydrogenase (DHA) showed that there was no change occurred in control soil after 60 days, while in thiamethoxam treated soil DHA was marginally increased from 0.164 to 0.168 µg g⁻¹ and from 0.155 to 0.157 µg g⁻¹ at lower and higher doses, respectively. While, FDA values were slightly decreased from 0.761 to 0.717 µg g⁻¹ at T₁ but no significant change was observed in T₂. However, changes in fungal population were not significant but increase in bacterial population with the degradation of thiamethoxam was observed, which indicated the presence of thiamethoxam degrading beneficial bacteria in mango rhizosphere soil.