Long-term use of urea vs. anhydrous ammonia for N-fertilization of a Dark Brown loam: III. Soil microbial populations and activities
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The influence of fertilizers on soil quality, including soil microbial well-being, has been frequently questioned by proponents of organic farming and Low Input Sustainable Agriculture (LISA). A 10-yr experiment, conducted on a Dark Brown loam at Scott, in which the influence of urea and and anhydrous ammonia at rates up to 180 kg N ha-1 on yields of cereals and oilseeds was examined, provided the basis for this study. In the tenth year we took soil samples at 2 depths (0 to 7.5-, 7.5 to 15-cm) at 3 times (3 days before, 6 days, and 26 days after fertilizer application) . We assessed the impact of these treatments on populations of filamentous fungi, yeasts, bacteria, actinomycetes, nitrifiers, and denitrifiers. The fertilizer effects were most pronounced 6 days after N application but were also apparent just prior to N application, confirming a residual treatment effect. Generally, the effects were greater in the second depth where the fertilizer was placed. There were no obvious effects on yeasts or denitrifiers. Generally bacterial and fungal populations were directly related to N rates and were increased more by anhydrous ammonia than by urea. In contrast, actinomycete populations were inversely related to rate of N and the population was lower for anhydrous ammonia than for urea. Nitrifiers responded positively to N rate near the depth of N placement with the response peaking at the 90 kg ha-1 rate. The pHCaCl2 of the check soil, which was already low (5.2) was further decreased to 4.2 by 180 kg N ha-1, the decrease being greater for the anhydrous ammonia source. We believe positive responses in microbial populations are due to the nutritive value of N and negative responses related to soil acidification.
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