Nitrification and denitrification are the processes that give rise to the emissions of nitrous oxide (N2O) from the soil to the atmosphere, both processes being dependent on the availability of oxygen, but in opposite redox conditions. These reactions are affected by soil characteristics such as aeration, temperature, humidity, soil reaction, fertilizers, available organic matter, among others. The objective of this work was to evaluate the N2O emissions in a Brachiaria humidicola pasture, distributed in a toposequence in Seropédica, RJ. The area has variation of soil class and drainage influenced by topography. The experiment was conducted in the Milk Cattle Sector of the Federal Rural University of Rio de Janeiro (UFRRJ). The evaluated treatments were pasture planted under a Red-Yellow Argisol and pasture planted under the Gleissolo soil. The experimental design was in randomized blocks, with four replications and two treatments. The N2O emissions were evaluated using static chambers. Physical fractionation, aggregate weighted mean diameter (AMD) and water saturated pore space (EPSA) were also determined to understand the mechanisms related to N2O emission. The highest N2O fluxes were observed on the third and eighth day of sampling in the Red-Yellow Argisol (41 μg N-N2O m-² h-1). The accumulated emission of N2O during the monitoring period in the Red-Yellow Argisol (43.3 g N-N2O ha-1 day-1) was statistically significant in relation to Gleissolo (2.4 g N-N2O ha-1 day-1). The Gleissolo showed lower soil C input, measured by the free light fraction mass (FLL), and lower AMD. It concluded that, the Red-Yellow Argisol has an eighteen times greater potential to emit N2O than Gleissolo. Soil water content, FLL and percentage of stable aggregates in the Red-Yellow Argisol are factors that may be regulating the N loss pathways by nitrification.