Response of Soil Microbial Communities to Different Doses of Glyphosate and Sulfosulfuron in a Calcareous Soil

To investigate the response of soil microbial populations to different doses of glyphosate and sulfosulfuron, a factorial experiment based on a complete block design was conducted at Shiraz University, Iran. The factors included different herbicides and dose rates (glyphosate at ۰, ۵۴۰, ۱,۰۸۰, and ۴,۳۲۰ g ae ha-۱ and sulfosulfuron at ۰, ۱۲.۵, ۲۵, and ۵۰ g ai ha-۱), and time of measurements (۴, ۱۵, ۴۵, and ۶۵ days after herbicides spray). Microbial respiration, microbial biomass carbon, metabolic quotient, dehydrogenase activity, and aerobic heterotrophic bacteria were measured in soil samples. The results showed that microbial respiration, microbial biomass carbon and metabolic quotient were highest for glyphosate ۱,۰۸۰ g ae ha-۱ at ۴ days after herbicide application. Dehydrogenase activity had a decreasing trend in all herbicide treatments in comparison with the control treatment in all measuring times, except ۴ days after spraying. There was no significant difference in dehydrogenase activity between herbicide treatments. The effect of sulfosulfuron on microbial respiration and metabolic quotient was not significant, whereas time and its interaction with herbicide dose rate affected these two variables significantly. Generally, all the measured indices for sulfosulfuron and glyphosate treatments decreased with time after herbicide application. Sulfosulfuron at ۵۰ g ha-۱ and glyphosate at ۴,۳۲۰ g ha-۱ had the lowest amounts of aerobic heterotrophic bacteria after ۶۵ days, decreased by ۲۳.۷ and ۵۰%, respectively compared with the control. Our results demonstrate that the effects of herbicides on soil microbial communities are strongly related to the herbicide dose and the time after herbicide spray. In conclusions, the herbicides at doses more than the recommended doses showed inhibitory effects on soil microbial communities in the alkaline soil, where the inhibitory effect was more at ۴,۳۲۰ g ae ha-۱ glyphosate.

Keywords:

Aerobic heterotrophic bacteria , Metabolic quotient , Microbial biomass carbon , Microbial respiration.