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In recent years, intensive use of herbicides has increasingly become a matter of environmental concern partially because of the effects of these chemicals on soil microorganisms. Glyphosate [N- (phosphonomethyl) glycine] (GP) is a broad-spectrum, non-selective, post emergence herbicide that is widely used in agriculture. In this study, glyphosate effects as N, P and C nutrient sources on microbial population and the effect of different concentration of it on dehydrogenease activity and soil respiration were investigated. The results show that in a soil with a long historical use of glyphosate (soil 1), the hetrotrophic bacterial population was significantly (p<0.05) increased. Also, by increase in the bacterial population, the herbicide existence as the possible nutrient source is enhanced. According to results, bacterial populations in the presence of glyphosate as P source was significantly (p<0.01) higher than N and C sources. The application of GP to the soil led to a significant increase in dehydrogenase activity with respect to untreated control soil samples. Also, respiration rates increased with increasing glyphosate application up to 50 mM but in 500 mM the inhibitory effect of glyphosate was observed. It can be concluded that glyphosate application may alter (increase) soil microbial activity and population. Increased microbial activity may be beneficial or detrimental toward plant growth, soil microbial ecology, and soil quality.
Key words: Glyphosate, bacterial population, dehydrogenease activity, soil respiration.