Diazotrophic plant growth-promotion bacteria and sugarcane straw levels: Effects on sugarcane productivity and on soil enzymatic and microbiological activities

Due to the rising demand for bioenergy, a certain amount of sugarcane straw is often removed for ethanol production, but this practice may compromise health soil. In addition to the amount of straw, growing concern with the environment has made the use of diazotrophic plant-growth promotion bacteria (DPGPB) an alternative to the use of nitrogen fertilizers. A research was designed with the aim of evaluating the effect of different amounts of straw kept on the soil on soil microbial and enzymatic parameters and on sugarcane yield during the first ratoon. The field experiment was carried out in randomized blocks, using nitrogen fertilizer (NF) or DPGPB (BI), both with four straw levels. The parameters evaluated were: β-glucosidase (GA), urease (UR) and acid phosphatase (APA) activities, fluorescein diacetate activity, basal soil respiration, microbial biomass N, total C, total N and labile carbon. Responses were not obtained for various parameters with respect to the straw levels which was attributed to the short experimental period. In general, the evaluated parameters were always higher in the BI treatment, except for APA, which was lower in this treatment. This fact could be attributed to the greater capacity of the inoculated plants to make phosphorus available to the plants, which could reduce APA. The absence of a response for sugarcane yield between the NF and BI treatments demonstrates the beneficial effect of the bacteria in the inoculant in suppressing plants with adequate nitrogen contents. Greater yields were obtained for the L64 and L100 straw levels with no significant difference. This demonstrates the adequateness of removing part of the straw to produce second generation ethanol.