The variation of the levels of nitrogen within the microbial biomass of a volcanic soil (Medial, thermic, Typic Distrandept), was measured in three management systems: a) permanent grassland under grazing, with and without N fertilization, b) rotation ofwheat after sugarbeet or corn, and c) a rotation ofwheat-oats with two sources of N fertilization, urea and sodium nitrate. The microbial biomass was determined by the "fumigation-incubation" method. Within the grassland system there was not significant difference (P > 0.05) between the Ntreatments. In the rotation sugar beet-wheat, the level of N-biomass was 18% greater than that measured in the corn-wheat rotation (P < 0.05). Within the rotation of wheat-oat, nitrate increased N-biomass 40% over the control without N fertilization (P < 0.05), while the urea treatment increased the N-biomass only 6% (P > 0.05). Among all of the management systems, the N-biomass in the grassland (mean =44.5 μg g-1) more than double what was measured in the crop rotations (mean = 19.5 μg g-1).