The influence of nitrogen fertilization on microbial activity and yield of oat crop in a soil of Chimangual (Andisol) of Narino department of Colombia was mesasured. The experiment was designed using a factorial design arranged in a CBD. The three N sources consisted of: potassium nitrate, 13%N, ammonium sulfate 21%N and colacteos 27%N (a local nitrogen fertilizer). Also, three doses (25,75 y 150 kg N ha-1) and three application times were used: at sowing time, total at 45 days an fractional, 50% at sowing and 50% at 45 days after sowing. The higher respiration values at week 2 and 7 were observed after the higher dose of N (75 and 150 kg N ha-1) and fractional application were done. Potassium nitrate, colacteos and ammonium sulfate caused the higher increments in soil respiration and yield of green forage these fertilizer were applied at sowing time fractional and at 45 days respectively. Respiration positively correlated with yields of the established crop.