Soil microbial and biochemical properties after ten years of fertilization with urea and anhydrous ammonia

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 Chemozemic loam, at Scott, Saskatchewan, to examine the influence of urea and anhydrous ammonia, at rates of N up to 180 kg ha(-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 CaCl2), 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 ha(-1), deterioration of soil quality should be minimal.