Fixação biológica de nitrogênio e eficiência de uso de nitrogênio em duas variedades de cana-de-açúcar inoculadas com bactérias diazotróficas

Abstract

One of the factors responsible for the low levels of sugarcane yield in Brazil is the inadequate supply of N. In this context, studies about BNF in this crop have scientific relevance. The objectives of the current research were: (i) to evaluate the interactive effect of N fertilization and diazotrophic bacteria in the accumulation of N and the yield in two commercial varieties, (ii) to evaluate non-fixing plants and the strategy of sampling in different parts of sugarcane plants to estimate BNF using the 15N natural abundance technique in sugarcane plant inoculated or not with diazotrophic bacteria. The experiment was carried out in vases conditions with sugarcane from pre-sprouted seedlings in pots containing 100 kg of soil from 0-40 cm layer of an Ultisol. In the first chapter, the tested effect of inoculation (with and without) and N (0 and 45 kg N ha1, ammonium sulphate with 2% excess of 15N atoms) in the accumulation and in the yield of two sugarcane varieties (RB867515 e RB92579) harvested at 90, 180, 270 and 360 days after transplanting (DAT). In the second chapter, the δ15 values were determined in roots, leaves, straw and in three stalk fractions (bottom, medium and upper) in the last sugarcane harvesting (360 DAT) in pots without N fertilization e the excess% of 15N atoms was determined in the same sugarcane parts and harvesting, although only in the pots fertilized with N.The addition of N-fertilizer sporadically inhibited or even caused an antagonistic effect of inoculation in some growth variables (stalk diameter, leaf dry matter, straw dry matter, N content and accumulation in leaf), but with no significant effect on stalk yield. The BNF contributed, on average, with 48% of the N accumulated in the variety RB92579 and 46% in the variety RB867515. There was no significant variation in the 15N natural abundance of available N in the soil estimated by different non-fixing plants. On the other hand, there was a significant variation in the values of δ 15N in different parts of sugarcane plant, indicating that the sampling strategy for BNF quantification by 15N natural abundance technique should consider the whole plant. However, when there was enrichment of the plant with 15N from the fertilizer to study the NUE, no significant variation in the values of excess 15N atoms in the different parts of the plantswas observed. This indicates that, in fertilizer 15N recovery studies, only specific parts of the plant can be sampled, representing a significant reduction in the time and cost of sampling and analysis. The NUE was 60% for both varieties, with no significant effect of inoculation. Studies with other varieties and in different soil and climate conditions are still needed to define scenarios with greater possibility of positive effect of inoculation of sugarcane with diazotrofic bacteria.