Vo. Biederbeck et al., SOIL MICROBIAL AND BIOCHEMICAL-PROPERTIES AFTER 10 YEARS OF FERTILIZATION WITH UREA AND ANHYDROUS AMMONIA, Canadian Journal of Soil Science, 76(1), 1996, pp. 7-14
The influence of nitrogen (N) fertilizers, especially anhydrous ammoni
a, on soil quality has been questioned frequently by proponents of org
anic farming and low input sustainable agriculture. A 10-yr experiment
was conducted on an Orthic Dark Brown Chemozemic loam, at Scott, Sask
atchewan, to examine the influence of urea and anhydrous ammonia, at r
ates of N up to 180 kg ha(-1), on yields of cereals and oilseeds. In t
he 10th yr, we sampled soil from the 0- to 7.5- and 7.5- to 15-cm dept
hs 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 bioch
emical properties. The long-term residual effects of N fertilization o
n soil properties were evident prior to the 10th annual N application.
The short-term effects were most pronounced 6 d after the 10th N appl
ication. Generally, effects were greater in the 7.5- to 15-cm depth, w
here N was placed. The soil, which was already acidic (pH = 5.2 in 0.0
1 M CaCl2), decreased in pH in proportion to N rate and more so for an
hydrous 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 cont
rast, the actinomycete population was inversely related to N rate and
was less for anhydrous ammonia than for urea. Nitrifier counts were in
creased by low rates of N (added substrate) but were similar to the ch
eck at high N rates (high acidity). There were no significant effects
of N treatment on denitrifiers or yeasts. In contrast to the plate cou
nt results, microbial biomass decreased with increasing rates of N and
was lower for anhydrous ammonia than for urea. However, the authentic
ity 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 nitrifi
cation tended to decrease at the 180 kg N ha(-1) rate of anhydrous amm
onia. Significant nitrification occurred at pH < 5.0 suggesting possib
le 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 k
g N ha(-1), deterioration of soil quality should be minimal.