Changes in bacterial communities by post-emergent herbicides in an Andisol fertilized with urea as revealed by DGGE

Large amounts of urea and various herbicides are applied to Chilean Andisols by farmers; however, how different herbicide types affect bacterial communities in urea-fertilized soils remains unknown. In this study, the effect of post-emergent herbicides (MCPA, flumetsulam, fluroxypyr, triclopyr, clopyralid and picloram) on total bacterial (TB) and ammonia-oxidizing bacteria (AOB) communities was investigated in a urea-fertilized soil microcosm using denaturing gradient gel electrophoresis at days 1 and 15 of incubation. The residual concentrations of herbicides in microcosm soils were measured by high-performance liquid chromatography. Differences in bacterial communities were analyzed and visualized with non-metric multidimensional scaling. All of the tested herbicides decreased in microcosm soils, reaching 46-98% of dissipation at 15 days. Changes in the TB and AOB communities were variable, depending on the urea and herbicide dosage, herbicide type and sampling time. Under the recommended dose of urea (570 mg kg−1 soil), soils that were treated with MCPA and triclopyr showed significant changes in TB and AOB community structures compared to control soils at both sampling times. When a 2-fold-higher recommended dose of urea (1140 mg kg−1 soil) was applied, the TB communities were mainly affected by flumetsulam and picloram at both sampling times. Changes in the AOB communities were mainly observed in soils that were treated with MCPA and fluroxypyr under both urea doses. This study determined which herbicides (and doses) significantly affect the bacterial communities in urea-fertilized soils. This evidence can be very useful when management practices are being designed, revised and/or established in Andisols.