Effect of synthetic pyrethroid insecticides on N2-fixation and its mineralization in tea soil

• Pyrethroid insecticides mostly suppressed microbial growth and activities.
• Cypermethrin had a neutral effect on microorganisms and N2-fixation.
• Deltamethrin reduced 11.2% exchangeable NH4+.
• Fenvalerate increased 21.4% N2-fixers but reduced 15.3% nitrifying bacteria.
• Permethrin decreased 9.6% ammonifying bacteria but increased 7.8% N2-fixation.

Microorganisms degrade a great variety of chemical substances including pesticide residues in soil. By virtue of low persistence and broad-spectrum efficacy, pyrethroid insecticides are sometimes used in tea garden to combat insect pests but the residual effects of these chemicals on microbial activities in soil have rarely been studied. The present experiment has been conducted under laboratory conditions to investigate the effect of four synthetic pyrethroid insecticides, viz. cypermethrin, deltamethrin, fenvalerate and permethrin at their field application rates (225, 200, 225, 210 g a.i. ha−1, respectively), on growth and activities of microorganisms in relation to non-symbiotic N2-fixation and N mineralization in a tea garden soil of the Himalayan terai (at the lower base) region of West Bengal, India. Application of insecticides, in general, decreased growth and activities of ammonifying and nitrifying bacteria but increased proliferations of non-symbiotic N2-fixing bacteria, resulting in greater fixation of atmospheric N2, more so with permethrin (7.8%) followed by cypermethrin (5.2%). Most of the insecticides had a deleterious effect on the accumulation of oxidizable organic C and total N, more pronounced with fenvalerate and permethrin. The availability of exchangeable NH4+ was also significantly reduced, more prominently with fenvalerate (15.6%) followed by deltamethrin (11.2%), while permethrin followed by cypermethrin accentuated greater accumulation of soluble NO3− in soil. Therefore, the effects of synthetic pyrethroid insecticides on microbial activities in relation to non-symbiotic N2-fixation and its mineralization in tea soil of West Bengal can not be generalized, rather in most cases the effects were inclined more towards detrimental than a few stimulative one.

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