Effect of temperature and time of storage of dry soil samples on a chemical index of N availability

Summary

The effect was studied of storage temperature on the index of available soil N wich uses U.V. absorbance of a 0.01M NaHCO₃ extract as an indicator. The U.V. absorbance was found to increase at a non-linear rate for four soils stored at temperatures of 50, 75, and 150°C. The change in extract absorbance due to extended soil storage at each of these temperatures was positively correlated to the percent organic matter, percent N, C/N value and concentration of humic substances in soils, but not to the extract absorbance prior to soil storage. These findings were not consistent with room temperature storage data which showed a linear increase in extract absorbance with soil storage time. The change in absorbance for the room temperature case was not related to any of the soil parameters mentioned above. Analysis of a soil stored at 105°C showed an increase in ninhydrin-detectable N, protein N and Kjeldahl N of the NaHCO₃ extract, while the apparent molecular weight distribution of extracted organic matter (as determined by gel filtration) showed only a slight change. As a comparison to the NaHCO₃ extract, a boiling CaCl₂ extract of the same soil was also analyzed; and the absorbance at 260 nm was found to increase in a curvilinear fashion with starage time at 75°C but to less of an extent than was noted with the NaHCO₃ extract. Nitrogen availability indexes based on the U.V. absorbance of these extracts, particularly those utilizing the NaHCO₃ extract, would be significantly affected by soil storage at elevated temperatures.