Fixação biológica de nitrogênio associada à cultura de cana de açúcar: eficiência e contribuição da inoculação com bactérias diazotróficas

Abstract

The association between the sugarcane plant and symbiotic N2 fixing bacteria can account for the production of up to half of the nitrogen (N) content of sugarcane crops, and N-fixing efficiency depends on plant genotype, soil and climatic conditions. To maximize the benefits from this plant/bacteria interaction, the present study tested the inoculation of sugarcane plants with a bacterial consortium of five diazotrophic species and evaluated N nutrition and stalk productivity in different sugarcane-growing regions of Brazil (Chapter 1). In addition, a long-term study determined soil carbon (C) and nitrogen (N) stocks of 10 sugarcane varieties cultivated without N-fertilization (Chapter 2). Chapter 1 describes an experiment that evaluated 12 commercial hybrid varieties of Saccharum sp, inoculated or not with the bacterial consortium, installed in seven sugar/ethanol mills, and another in the Experimental Field of Embrapa Agrobiologia. Two of the sugarcane varieties tested, RB867515 and RB72454, were also compared with a control treatment receiving 120 kg N ha-1. The experiment was conducted in a randomized block design with four replications and plots ranging from 22.4 to 90 m2. Chapter 2 describes a study on the natural potential of stalk production and N-fixing efficiency of sugarcane varieties RB72454, CB4789, CB453, SP70- 1143, SP79-2312, SP71-1406, SP71-6163, SP70-1284, Krakatau (Saccharum spontaneum) and Chunee (Saccharum barberi). The parameters evaluated in both chapters were fresh weight of stalks, straws and flag-leaves at harvest time, dry biomass accumulation, N content in the shoots, and contributions from biological N fixation (BNF). The 15N natural abundance technique was used to quantify N derived from BNF, using non-inoculated sugarcane and non-fixing plants grown under greenhouse conditions in soil taken from the experimental plots as reference. In variety RB867515, N-fertilization and bacterial inoculation increased stalk yield in 87.5% of the experiments. The bacterial inoculum increased crop yield by more than 10% in 80% of the experiments, but since it did not enhance BNF contributions in the genotypes tested, the benefits provided are likely a consequence of the growth-promoting effects of the inoculum. In the long-term study, without the use of N-fertilizer and with complete removal of post-harvest residues, the sugarcane varieties tested maintained their production potential in low-fertility soils, obtaining BNF contributions of approximately 50% of plant N. Moreover, C and N stocks were reduced in the long-term study, whereas N balance in the soil-plant system was positive for two (CB47-89 and SP70-1143) of the three varieties tested.