Changes in soil microbial population dynamics in response to application of selected pesticides under a sugarcane agro-ecosystem

Thiophanate methyl (TM), chlorantraniliprole (CAP) and chlorpyriphos (CP), off applied as sett and/or soil treatment, are the three most explored and recommended pesticides used in sugarcane cultivation for insect pest and disease management. Despite the widespread use of these pesticides, there have been very limited studies on the toxicological effects of these pesticides on soil-associated microbes, especially under the sugarcane agro-ecosystem. As such a 30 day microcosm experiment was conducted to study the impact of these pesticides on population of major soil microbial groups (bacteria, fungi and actinomycetes) and the beneficial fungus Trichoderma, when applied as sett and soil treatment in sugarcane. The impact of these pesticides on microbial population dynamics was assessed at intervals of 72 h and 30 days post application following serial dilution method and using suitable culture medium. The results clearly revealed that all three pesticides exerted a significant impact on soil microbial population dynamics for a period of at least 30 days post application. All three pesticides significantly reduced bacterial, fungal and Trichoderma populations; however, TM and CAP application was observed to result in a significant stimulation of the actinomycetes population. Among the three pesticides, CAP was observed to most significantly impact the bacterial and fungal populations which showed drastic reduction even at 30 days post application. Overall, the results of our study clearly established that soil application of these pesticides, especially CAP, have a significant negative impact on soil microbial dynamics, at least over a short term period. As such further detailed investigations on the impact of these pesticides on soil microbial population dynamics as well as functional microbial groups over longer time duration are essential to ascertain the environmental safety of their repeated long term use.