Evaluation of rhizobial populations ability to degrade herbicides and denitrificate

VERCELLINO M; GOMEZ MA

Villa Carlos Paz, Córdoba

Congreso; VI CONGRESO ARGENTINO DE MICROBIOLOGÍA GENERAL; 2009

SAMIGE

Herbicides are widely used in agriculture worldwide and they can compromise water and soil quality. Besides, excessive use of nitrogen fertilizers can be harmful to ecosystems, resulting in a significant source of nitrate pollution in soil and water. Rhizobia have a wide variety of functions associated with the agricultural environment where they thrive. The objective of the present work was to evaluate the capabilities of rhizobial populations to degrade herbicides and the reduction of nitrates and/or denitrification. We evaluated 81 rhizobial strains belonging to 4 different genera. The ability of rhizobia to use herbicides 2,4-Dichlorophenoxyacetic acid (2,4-D), Glyphosate (GF) and Atrazine (AT) as a source of nutrients was assessed in mineral-based medium (MB). Liquid cultures were inoculated (10 ¥ìl strain-1) in the control (MB) and in the following treatments: 1) MB + 2,4-D; 2) MB + GF; 3) MB + AT, and incubated at 28¨¬C in a moist chamber for 36 days. Nitrate reduction was assessed by inoculating each strain in tubes containing modified Bergersen liquid medium and Durham tubes. Tubes were incubated in anaerobic conditions at 28¡ÆC for 14 days. The presence/absence of nitrogen gas bubbles was recorded and nitrate reduction was tested by GRIESS-ILOSVAY‟s Nitrite Reagent. In this study, herbicide AT was the most degraded by the rhizobial populations assessed. Most strains had normal development in the evaluation of degradation of 2,4-D and GF. Fifty-three strains reduced nitrate (no gas) and seven strains appeared to be potential denitrifiers (gas production). Until now, genes norC and nosZ were detected in one of the strains. Three Bradyrhizobium strains were both able to degrade AT and denitrificate, and we called them "double capacity strains¡±. Their ability to degrade AT was assessed weekly (during 42 days) determining the absorbance at 620 nm. Turbidity increased with time, demonstrating that strains can use AT as a source of nutrients. It is suggested that "double capacity strains" are important because they might be able to degrade the herbicide under anaerobic conditions. Knowledge of these functions in rhizobia broadens the expectations at the farm and environmental level as well as economic level. The possibility of using these N2 fixing bacteria as inoculants may be advantageous as it would allow appropriate fertilization management and also help in the biodegradation of pesticides in agricultural soils.