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The influence of nitrogen (N) fertilizers, especially anhydrous ammonia, on soil quality has been questioned frequently by proponents of organic farming and low input sustainable agriculture. A 10-yr experiment was conducted on an Orthic Dark Brown Chernozemic loam, at Scott, Saskatchewan, to examine the influence of urea and anhydrous ammonia, at rates of N up to 180 kg h −1 , on yields of cereals and oilseeds. In the 10th yr, we sampled soil from the 0- to 7.5- and 7.5- to 15-cm depths of each treatment 3 d before and 6 and 26 d after fertilization to assess the impact of applied N on microbial populations and soil biochemical properties. The long-term residual effects of N fertilization on soil properties were evident prior to the 10th annual N application. The short-term effects were most pronounced 6 d after the 10th N application. Generally, effects were greater in the 7.5- to 15-cm depth, where N was placed. The soil, which was already acidic (pH = 5.2 in 0.01 M CaCl 2 ), decreased in pH in proportion to N rate and more so for anhydrous ammonia than urea. Generally, fungal and bacterial populations (plate counts) were positively related to N rate and were greater in soil treated with anhydrous ammonia than in urea-treated soil. In contrast, the actinomycete population was inversely related to N rate and was less for anhydrous ammonia than for urea. Nitrifier counts were increased by low rates of N (added substrate) but were similar to the check at high N rates (high acidity). There were no significant effects of N treatment on denitrifiers or yeasts. In contrast to the plate count results, microbial biomass decreased with increasing rates of N and was lower for anhydrous ammonia than for urea. However, the authenticity of this response is questionable because the fumigation-incubation method of biomass determination is compromised under acid conditions. Carbon mineralization was unaffected but N mineralization and nitrification tended to decrease at the 180 kg N ha −1 rate of anhydrous ammonia. Significant nitrification occurred at pH < 5.0 suggesting possible adaptation of nitrifiers in this acid soil. Wet aggregate stability (WAS) was unaffected by N treatments. We concluded that, if producers in the Dark Brown soil zone apply fertilizers at rates less than 90 kg N h −1 , deterioration of soil quality should be minimal. Key words: Anhydrous ammonia, microbial biomass, nitrification, mineralization, soil microflora, urea