The objective of this work was to identify the herbicides applied on sugarcane that do not affect the growth nor the process of biological nitrogen fixation (BNF) of the diazotrophic bacterium Azospirillum brasilense. Commercial doses of eighteen herbicides (paraquat, ametryn, amicarbazone, diuron, metribuzin [hexazinone + diuron] [hexazinone + clomazone] clomazone, isoxaflutole, sulfentrazone, oxyfluorfen, imazapic, imazapyr, [trifloxysulfuron-sodium + ametryn], S-metolachlor, glyphosate, MSMA and 2,4-D) were tested regarding their impact on the bacterial growth in liquid DIG medium. Thus, the carrying capacity of the culture medium, duration of lag phase and generation time of A. brasilense were determined by calculating the optical density data obtained at regular intervals during the incubation of the cultures for 55 h. The impact of herbicides on nitrogenase activity of A. brasilense was evaluated in semi-solid N-free NFB me dium by acetylene reduction assay (ARA). The effects of herbicides on the growth variables and ARA were compared with the control by the Dunnett test. Paraquat, oxyfluorfen, [trifloxysulfuron -sodium + ametryn] and glyphosate reduced the carrying capacity of DIG medium with respect to A. brasilense growth. This effect was associated with increases in both the length of the lag phase and generation time for [trifloxysulfuron-sodium + ametryn], and with the increase in gene ration time for glyphosate. MSMA, paraquat and amicarbazone reduce the BNF of A. brasilense; however, this reduction is more severe in the presence of paraquat. The other herbicides do not affect the growth or the BNF of A. brasilense.
Saccharum spp; biological nitrogen fixation; pesticides
