Evaluation of pesticides degradation by microorganisms in the soil of a new cropping system : Isolation and characterization of pesticide-degrading strains of soil microorganisms

Abstract

Among the process that determine the fate of pesticides in soils, microbial degradation is recognized as one of the most important. Biodegradation of pesticides seems to be achieved through the contribution of a few key species that exhibit degradation capabilities. However, in field conditions the real effect of these key species on the fate of pesticides is still poorly understood. We attempted to isolate and identify some of them capable of degrading metamitron, metazachlor, bentazon, s-metolachlor and chlortoluron after selective enrichmeent cultures in minimal medium. Their degradation capacities were evaluated in a minimal liquid medium with pesticide as the only source of carbon and nitrogen. The feasibility of assessing the degradation behaviour of degrading strains on undisturbed soil columns was performed. A special focus was made on the effect of autoclaving on soil properties that might control the fate of pesticides. In total, 85 pesticide-resistant cultures were isolated. 27 of them showed significant enzyme capacity with ABTS as substrate which may indicate their ability to degrade phenolic compounds. Three isolated strains (SM 12, SM 2 and SM 14) were suggested to be able to degrade metolachlor. Three other strains (MET 8, MET 17 and MET 12) were proposed to be able to degrade metamitron. One strain (MZ 3) was potentially able to degrade metazachlor. The soil column study revealed for the first time that autoclaving may influence the soil hydraulic conductivity by affecting the soil structure. This can lead to inaccurate assessments of pesticide fate in experiments conducted under abiotic soil columns and inoculated with degrading strains.