Response of Soil Microbial Respiration to Spirotetramat Insecticide Under Different Soil Field Capacities

Due to the repeated applications of pesticides, the amount of the pesticides and their products by decomposition may accumulate in the soil ecosystems which are affected by abiotic and biotic factors. Soil microorganisms are the important players that are able to decompose and utilize these chemical waste materials as energy sources and regulate them in the cycling in the soil environment. One of the important insecticides for the control of insect pests including aphids is spirotetramat which can provide protection to plant roots from the attack of insects when it was sprayed on the crops. However, effects of high concentrations of spirotetramat on soil microbial respiration under different soil water contents are unknown. Recommended field dose (RFD) and its 5 (RFD x 5) and 10 (RFD x 10) folds of spirotetramat were mixed with a clay soil; these mixtures were humidified at 50% (50FC), 75% (75FC), and 100% (100FC) of field capacity and then incubated at 28 degrees C for 21 days. At the end of the incubation period, (1) in general, soil microbial respiration was significantly increased as soil moisture increased in all treatments (50FC < 75FC < 100FC, P < 0.05); (2) all concentrations of spirotetramat significantly decreased the microbial respiration under 100FC (P < 0.05); (3) only RFD x 5 significantly reduced this activity under 75FC (P < 0.05); (4) no significant differences between control and treatments were found under 50FC. In conclusion, high concentrations of spirotetramat insecticide had low toxic effects on soil microbial respiration while the soil moisture regulated the toxicity effects of this insecticide.