The experiment was carried out to evaluate the influence of N on the agronomic characteristics of soybean crop under no-tillage system over forage sorghum intercropped with U. ruziziensis and on straw degradation. Four treatments (nitrogen doses) were used: 0; 20; 40 and 60 kg of N ha-1. N were applied at V3-V4 stage, using ammonium nitrate as the N source, containing 30% N. The experimental design was a randomized block design with four replications. The evaluation of dry matter quantity of forage sorghum and U. ruziziensis straw was carried out before the sowing of forage sorghum and U. ruziziensis and after soybean harvest. The soybean crop parameters evaluated were, as follows: first pod insertion height and plant height, final stand ha-1 and grain yield ha-1. Evaluations were performed at the soybean crop harvest. Nitrogen fertilization did not influence the straw decomposition of U. ruziziensis and forage sorghum nor the first pod insertion height, plant height and final stand ha-1 of soybean crop. Nevertheless, fertilization showed a quadratic behavior in the 2015/16 crop in relation to grain yield, with a reduction in yield at doses greater than 21 kg of N ha-1, and a linear increase in the 2016/17 crop.

Keywords: Ammonium nitrate, Glycine max L., no-tillage.