GROWTH OF NITROGEN-FIXING BACTERIA IN THREE DIFFERENT ENVIRONMENTS

Microorganisms in the soil perform essential functions in maintaining the environment and biodiversity, decomposition of organic matter and biogeochemical cycles. In the biogeochemical cycles, one of the most important is the nitrogen cycle, where the fixing-bacteria play a fundamental role, associated with the roots, capture and fix the atmospheric nitrogen and convert it into forms that can be used by the plants. Among them, the bacteria of the genus Azotobacter sp. and Azomonas sp., are indispensable for the fixation of this element. The objective of this work was to quantify nitrogen-fixing bacteria (Azotobacter sp and Azomonas sp.) present in the soil of three different environmental. The experiment was conducted at the Laboratory of Forest Pathology and Soil Microbiology of the College of Agronomic Sciences - CAS, of the São Paulo State University - UNESP in Botucatu, SP. Soils were collected from three different environments at 20 cm depth, from a seven years old eucalyptus forest area, an area of pasture (more than 10 years) and an agricultural area with corn, where exist crop rotation management, located in CAS/UNESP. Five grams of each treatment was added in saline solution and diluted until concentration of 105. In the Petri dishes, was added 1 ml of solution of each concentration, followed by the addition of LGI medium culture, selective for bacteria of the genus Azotobacter sp. and Azomonas sp., with three replicates for each concentration and treatment. The Petri dishes were maintained in a B.O.D chamber with photoperiod of 12 hours at 28°C±2 during 48 hours. After this period the number of bacterial colony forming units (CFU) was quantified. The CFU averages for each soil were compared by the Tukey test at 1% probability. The soils collected from the forest area and the agricultural area with crop rotation showed a greater number of CFUs with 28.42x104 and 32.01x104 respectively, without significant differences to each other, followed by pasture soil with 5.24x104 CFU, differing statistically from the other two environments. There was lower incidence of Azotobacter sp., representing 0.06% of CFU of the total quantified of the three treatments. We conclude that the soils of the forest area and of the agricultural area has higher incidence of nitrogen-fixing bacteria, because these soils has high amount of plant material and consequently a greater microbial activity.