Nutrição nitrogenada de cana-de-açúcar com fertilizantes ou bactérias diazotróficas

Resumo

Nitrogen is required in large quantities for biomass production. Around 23% of the fertilizer N in Brazil is used in sugarcane. Besides being the most expensive plant nutrient, N fertilizers are an important component of the environmental budget of biofuel production. It is estimated that synthesis of N fertilizer accounts for about 25% of all the fossil energy spent in field operations for ethanol production from sugarcane in Brazil. Emission of nitrous oxide, a potent green house gas associated with fertilizer use, also adds to the environmental costs of ethanol. There are evidences that biological nitrogen fixation (BNF) is responsible for supplying part of the N required by sugarcane plants because several diazotrophic microorganisms have been isolated in that crop. Besides, the amounts of N fertilizer applied to sugarcane in many cases do not replenish the N removed from the fields with the harvest or lost as part of management practices. However, old sugarcane fields generally do not show signs of soil degradation. The actual contribution of BNF to sugarcane under field conditions is controversial. Some authors have expressed their view that BNF is of little relevance for sugarcane N nutrition. However, it is generally recognized that BNF presents great potential especially in Brazil where many studies have shown promising results. BNF is affected by plant variety, bacteria species, and plant-bacteria interactions. An inoculant produced with five strains of N2-fixing diazotrophs was recently developed by Embrapa but it has not been extensively tested under field conditions. This project has the objective of studying the contribution of BNF to sugarcane production compared with the use of synthetic N fertilizer under different soils, environments, and sugarcane cultivars, evaluating the emission of N2O from sugarcane fields fertilized with N, and testing an inoculant produced with endophytic bacteria. At the same time new N2 fixing organisms are being searched for that are adapted to the sugarcane growing conditions of the State of São Paulo. The project will be complemented with studies on genetic traits of sugarcane associated with the capacity of N2 fixation, which may help to obtain new varieties that can make better use of BNF. (AU)