Banca de DEFESA: CAROLINA ALMADA GOMES DE OLIVEIRA

Biological Nitrogen Fixation as a sustainable strategy in the production of energy biomass: Elephant grass and sugarcane. 

bioenergy, renewable energy, ¹⁵N, nitrogen, C₄ grasses, sustainability.

The increasing demand for energy is directly related to the growth of the human population. The environmental impacts resulting from the burning of fossil fuels for energy generation reinforce the need to seek renewable and sustainable energy sources. Elephant grass and sugarcane are two C4 photosynthetic grasses that possess characteristics suitable for energy biomass production associated with biological nitrogen fixation (BNF) due to their high capacity to accumulate dry mass and photosynthetic efficiency. This thesis was structured into two chapters, and the hypotheses raised in the study were BNF is the main external source of nitrogen in the cultivation of ten elephant grass genotypes (Chapter 1) and three sugarcane varieties (Chapter 2) cultivated in an Argisol soil with low N availability. The general objectives of the study were to evaluate the effect of BNF as the main external source of nitrogen (N) providing high dry mass productivity and nitrogen accumulation in elephant grass and sugarcane crops. To this end, two different experiments were installed in the experimental area of Embrapa Agrobiology located in the municipality of Seropédica – RJ, where the contribution of BNF to biomass productivity was evaluated using the natural abundance method of 15N. In Chapter 1, ten elephant grass genotypes were evaluated for three consecutive years, without the application of nitrogen fertilizer, in a concrete tank with dimensions of 120m2 and 0.5m deep filled with soil labeled with 15N. The experimental design was a randomized block design with four replications, where each genotype corresponded to a plot, totaling forty plots in the experiment. In Chapter 2, three commercial varieties of sugarcane were evaluated over five consecutive years without the application of nitrogen fertilizer in a concrete tank with characteristics similar to those of experiment 1. The experimental design was a randomized block design with four replicates, with each variety corresponding to one plot, totaling twelve plots. Harvests in experiment 1 occurred every six months of the cycle, and in experiment 2, they occurred annually. During the harvests, all above-ground plant material grown in the tanks was cut, weighed in the field, sampled, and taken to a drying oven until the dry biomass stabilized. In each experiment, the following variables were evaluated: total fresh weight of the above-ground part, total dry weight, total accumulated N, 15N abundance, contribution of BNF, and N derived from BNF. In Chapter 1, it was observed that five elephant grass genotypes (P13G13, BRS Capiaçu, P11G7, P21G15, and P27G19) showed the best average dry matter yields, around 30 Mg ha-1 year-1, with a statistical difference between them during the cultivation years. The average contribution of BNF was 35%, which represented an average accumulation of 35 kg ha-1 year-1 of N derived from BNF. In Chapter 2, no statistical difference was also observed between the treatments. The sugarcane varieties RB867515, RB 92579, and SP81-3250 showed high dry matter productivity and N accumulation, and an average BNF contribution of 33%, which represented an average accumulation of 30 kg ha-1 of N derived from BNF. The results obtained demonstrate that, even in low-fertility soils and without nitrogen fertilization, elephant grass and sugarcane showed high biomass productivity sustained by BNF. It is concluded that the P13G13 genotype and the RB867515, RB92579, and SP81-3250 varieties can be recommended for energy use based on biological nitrogen fixation (BNF). These results highlight the essential role of BNF as an important external source of N for these crops and reinforce its strategic importance in the production of energy biomass.