Resposta de cultivares de soja de ciclo precoce à adubação nitrogenada e à inoculação com bradyrhizobium em semeadura do cedo na região sudoeste do Paraná

Abstract

A Nitrogen fertilization in soybean still causes much debate in the scientific community about it’s real need, efficiency and economic viability. The objective of this study was to evaluate the agronomic performance of soybean under different levels of nitrogen applied in the sowing row and the realization or not of the practice of inoculation. The tests were carried out in PatoBranco - PR, in the 2014/2015 season. Being two separated trials, with NS 4823 RR and P95Y21 RR cultivars, submitted to four nitrogen fertilization rates (0, 20, 40 and 60 kg ha-1 N) and the application or not of the inoculants based on Bradyrhizobium japonicum, applied in a dose of 1x1010 UFC kg-1 of seed. The variables evaluated were stand and plant height at thirty days after sowing, in R1, R5 and harvesting; number and dry mass of nodules per plant; average dry mass per node; height of first pod insertion, number of pods per plant, number of seeds per pod; thousand grain weight; productivity; protein content in grains; and N extraction by plants. The trials were evaluated in a split plot model. Data were submitted to analysis of variance by the F test, regression and comparison of means by Tukey at 5% probability. The N rates factor was significant for 18 of the 20 variables studied in both cultivars, while the inoculant factor was significant only for grain yield cultivar P95Y21 RR. The application of nitrogen in soybean sowing row contributed to the increase of soybean productivity. High N doses at sowing row reduced the plant stand, as well as the dry mass of nodules, due to lower average weight per node, and to a lower level by the number of nodes by plant. There was a positive relationship between increasing doses of N at sowing line, grain yield and N extraction by soybean up to a dose of 40 kg ha-1 of the nutrient, reducing after that dose, especially for P95Y21 RR cultivar. Low amounts of N at sowing line, along with the use of inoculants based on Bradyrhizobium japonicum, contributed to the productivity of the cultivar NS 4823 RR. To cultivate P95Y21 RR, this positive effect was up to the maximum dose of 60 kg ha-1. Soybean cultivars respond differently to nitrogen fertilization in the row and inoculation with Bradyrhizobium japonicum.