Changes in the extractable ammonium‐ and nitrate‐nitrogen contents of soil samples during freezing and thawing

Abstract

Deep‐freezing is the method most commonly applied when soil samples are stored for inorganic nitrogen determination. Usually the samples are thawed before analysis to allow homogenization and weighing. The results of several studies on the subject are nevertheless inconsistent. 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, two laboratory experiments were performed to study the effect of deep‐freezing 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. In the first experiment, where the soils were thawed, ground, or ground and thawed after deep‐freezing, there was no change in nitrate nitrogen content in any of the soils. However, the content of ammonium‐nitrogen was 40–65%, 9–192%, and 66–405% greater than in the fresh soil after the soil was thawed, ground, and ground and thawed, respectively. In the second experiment, where the changes in ammonium‐ and nitrate‐nitrogen contents in soil during freezing and thawing were investigated, there was no change in either form of nitrogen during freezing of the soil nor up to four 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. 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. This period should not be extended longer than is necessary to render the soil loose enough for homogenizing and weighing. The temperature of the soil should not rise above zero for more than the minimum time.