Report on the Agricultural Experiment Stations, 1952 Page: 49
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SOIL SCIENCE 49
fur was oxidized to the sulfate form at about the same rate as was
elemental sulfur. Orzan decomposed in the soil about one-third
as rapidly as added dextrose, but 50 percent more rapidly than pine
Utilization of Nitrogen by Soil Micro-organisms
Studies at the Iowa station (coop. USDA) using isotopic nitrogen
(N15) have further established the fact that ammonium N is preferred
over nitrate N by soil micro-organisms. When both forms of N were
present, the ammonium form was used almost exclusively. These
findings help to explain why immediate recovery by crops from
applications of ammonium N is nearly always low in comparison with
recovery from a like amount of nitrate N. Soil microbes utilize the
ammonium N, thus leaving less N for the growing crop. Nitrogen tied
up in the bodies of micro-organisms is not lost from the soil, however,
but becomes available upon their decay and then may be utilized by
the growing crop or tied up again in newly synthesized cell materials.
When the nitrate form is applied as fertilizer, soil micro-organisms
use less of it, and thus more is available to the immediate crop and
recovery is higher.
The preference of bacteria for ammonium N was also demonstrated
in experiments at the Oklahoma station. After bacteria had become
adapted to growing on nitrate N by prolonged culture on a nitrate
medium and then were transferred to a similar medium containing
the stable isotope N15, it was observed that the nitrate (NO8) in the
medium disappeared within 24 to 36 hours. When ammonium nitrate
was added to these cultures, the NO8 continued to disappear at a rapid
rate. The amount of ammonium N also decreased, but at a much
slower rate. Increases in total cellular N were closely correlated with
disappearance of the ammonium N from the culture.
Aeration of the cultures did not appreciably reduce the rate at
which nitrate disappeared from the medium. The fact that N from
the nitrate showed up only to a slight extent in the bodies of the
bacteria is further evidence that the nitrate form of N is lost by
denitrification under well-aerated conditions, and that bacteria prefer
ammonium to nitrate N.
Loss of Nitrogen by Volatilization
Investigations on the loss of N from flooded soils by the Louisiana
station have shown that the addition of organic residues increased
slightly the amount of N lost by volatilization (denitrification).
Flooding, as in rice culture, caused a loss of 78 parts per million or 8
percent of the N present. Where clover residue was added prior to
flooding, the loss was 12 percent, and where rice straw was added, it
was 10 percent. It was apparent that the nitrate-reducing capacity of
the soil was increased by both flooding and organic matter. No loss
of N was evident when the soil was held at optimum moisture. The
numbers of aerobic N-fixing bacteria were increased by the organic
matter but were reduced by flooding, whereas the anaerobic N-fixers
increased in numbers with both flooding and the addition of organic
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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/m1/51/: accessed January 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.