Is Deep-Freezing a Safe Method for Storing Soil Samples for Inorganic Nitrogen Determination?

Abstract

Soil samples for inorganic nitrogen determination are usually stored deep-frozen to avoid microbial transformations of nitrogen and then thawed before analysis. In some studies, no changes have been observed in the ammonium or nitrate nitrogen content after freezing and thawing of samples, whereas in others considerable changes have been found in one or both.

In this work, three laboratory experiments were performed to study the effect of deep-freezing and thawing on the ammonium and nitrate nitrogen contents of a sandy soil, a clay soil, and a peat soil. A special mill for grinding the frozen samples to minimize these changes, was tested. One experiment was done to study whether the time of extraction of the samples could be extended to 20 hours.

Thawing of the samples increased the concentration of nitrate in the extracts in one experiment and that of ammonium in another experiment. Grinding increased the concentration of ammonium in both of the experiments. In the third experiment, there was no change in either form of nitrogen during freezing of the soil nor up to 4 hours of thawing. After that an increase occurred, especially in the content of nitrate nitrogen. This increase occurred few hours after a rise in temperature above zero degrees. In the fourth experiment, extending the time of extraction from 0.5 or 1 hour to 20 hours increased the concentration of ammonium in the extracts. Nitrate content was also increased slightly.

It is concluded that the divergence in the results of various studies on the subject are a consequence of mineralization of soil nitrogen during the thawing period. During this period, the temperature of the soil should not rise above zero for more than the minimum time. The period of extraction should not exceed two hours.