Report on the Agricultural Experiment Stations, 1952 Page: 50
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50 REPORT ON EXPERIMENT STATIONS, 1952
Oat straw and alfalfa hay, decomposed under different conditions
in the presence of added inorganic N in studies at the Iowa station,
gave no loss of N by denitrification under acid conditions, but resulted
in a loss under alkaline conditions. This effect was found to be due to
nitrite toxicity rather than to acidity alone. Because fungi are more
tolerant to both nitrites and acidity than are bacteria, they predominated
under the acid conditions to the extent that there was not sufficient
bacterial activity to cause denitrification. Buffering decomposing
plant material to an acid reaction eliminated such N losses as
occurred under alkaline conditions.
Research at the New York (Cornell) station, using nitrate labeled
with N15 isotope, showed that nitrate can be lost from soils through
denitrification under fully aerobic conditions, even in the absence of
added energy material, such as crop residues. The nitrate N that disappeared,
under either aerobic or anaerobic conditions, was assimilated
by micro-organisms or was lost from the soil as N2 gas. The
higher the amount of oxygen present, the less was the loss of N by
denitrification. Regardless of the concentration of oxygen present,
only a small quantity of the added nitrate was reduced to the ammonium
form. This research also established that ammonium N is not
converted to N2 gas directly by micro-organisms. The results indicate
that added nitrate N may be lost to an appreciable extent on poorly
drained soils, and that ammonium N is therefore a more efficient
source of N under conditions of moderate to poor soil aeration.
The Iowa station found that the concentration of oxygen required
to insure the biological process of nitrification at optimum speed was
about the same as that in ordinary air. Some nitrification occurred
at oxygen concentrations slightly less than 0.4 percent. Approximately
half as much nitrate was produced when oxygen was maintained
at 2.1 percent as at 20 percent, although at 11 percent the rate
of nitrification was almost as great as at 20 percent. The nitrification
rate also varied with the amount of soil moisture present, being optimum
when the soil pores were well filled with water providing there
was ample space for air. In addition, the rate of nitrification was
found to be lower in soils of pH 6.5 to 7.0 than in those above pH 7.1,
and soils low in available phosphorus or potassium had lower nitrification
rates than those high in these elements. A more rapid method
was developed for determining the rate of nitrification of soils. Results
by this method correlated well with crop response to applications
Experiments carried on at the Illinois station showed that, contrary
to former beliefs, additions of large amounts of carbonaceous materials
did not increase the number of Azotobacter (N-fixing bacteria)
even at a relatively high soil pH. Additions of N fertilizers had little
effect on the number of Azotobacter in the topsoil, but greatly increased
the number in the subsoil. Azotobacter grew well in all soils
when sucrose was added as the source of energy, but only in certain
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United States. Office of Experiment Stations. Report on the Agricultural Experiment Stations, 1952, book, January 1953; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc5990/m1/52/: accessed March 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.