This study was conducted to evaluate the accumulation and displacement of N-NO 3- in the soil after pig slurry application in no-tillage maize in southern Brazil. The doses of 0, 40 and 80 m 3/ha pig slurry were applied annually, for three years, on the mulch of cover crop of black oats [ Avena nuda] and of winter spontaneous vegetation, preceding maize sowing. The N-NO 3- concentration was evaluated in different soil layers to a depth of 60 cm and on six dates, from the slurry application until maize tasseling. The amount of N-NO 3- increased quickly in the soil surface layer with the pig slurry application, evidencing the high nitrification rates of ammoniacal N in the slurry. N-NO 3- produced in the surface layers moved down quickly in the soil profile. At a dose of 80 m 3/ha slurry the amounts of N-NO 3- in the 30-60 cm soil layer on the 30th day of the first year, 29th day in the second and 36th day in the third year were higher than the average of the treatments without slurry in 9, 21 and 32 kg N-NO 3-/ha, respectively. In the first two years the amount of soil N-NO 3- in the surface layer did not differ with slurry application on mulch of oats or spontaneous vegetation, indicating the low potential of grass mulch in promoting microbial N immobilization. The high rate of nitrification of ammoniacal N in the slurry and the fast displacement of N-NO 3- in the soil profile when maize N demand was still small indicate a greater susceptibility of N-NO 3- losses by leaching with slurry application, especially at a dose of 80 m 3/ha, where the average amount of total applied N in the three years was 244 kg/ha per year.
