Effects of thiobencarb in combinations with molinate and chlorpyrifos on selected soil microbial processes

Abstract

The impact of pesticides, namely thiobencarb (TBC), molinate (MOL) and chlorpyrifos (CPF), on soil microbial processes was studied in two Australian soils. Substrate induced respiration (SIR), substrate induced nitrification (SIN) and phosphatases and chitinase enzymatic activities were assessed during a 30-day microcosm study. The pesticides were applied to soils at recommended rates either alone, or as binary mixtures with TBC. Soil samples were sampled at 5, 15 and 30 days after pesticide treatments. Substrate induced respiration was only transiently affected by pesticides in both soils. In contrast, the process of indigenous nitrification was affected by the presence of pesticides in both soils, especially when the pesticides were applied as binary mixtures. Substrate induced nitrification increased with pesticides in the Griffith soil (except with MOL+TBC after 5 days) whereas SIN values were non-significantly different to the control on the Coleambally soil. The binary mixtures of pesticides with TBC resulted in a decrease in SIN in both soils, but the effects disappeared within 30 days. The enzymatic activities were not consistently affected by pesticides, and varied with the soil and pesticides studied. This study showed that, when applied at recommended application rates, TBC, MOL, and CPF (individually or as binary mixtures), had little or only transitory effects on the functional endpoints studied. However, further investigations are needed to assess the effect on microbial densities and community structure despite the low disturbance to the functions noted in this work.

Keywords:

• Enzyme activities
• microcosms
• nitrification
• respiration
• pesticides
• terrestrial ecotoxicology

Acknowledgments

Financial support by Rural Industries Research and Development Corporation (RIRDC) is gratefully acknowledged. We are particularly thankful to several CSIRO colleagues, namely Ms. Cathy Fiebiger, Ms. Danni Oliver, Ms. Alicia Navidad and Dr. Wendy Quayle for their collaboration and assistance during this project. Mr. Mark Robb and Mr. Arun Tiwari of Coleambally Irrigation kindly helped us collect soil samples for this study. Drs. Kris Broos and David Johnson are also thanked for their very helpful comments during internal review of the manuscript. We are grateful to Ms. Mary Barnes and Dr. Ray Correll for their advice on statistical aspects.

Notes

^a OC = organic carbon.

^b MHWC = maximum water holding capacity.

∗Denotes significant difference from controls (P < 0.05).

^a BDL = Below detection limit.