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Measurement of Microbial Biomass by Fumigation-Extraction in Soil Stored Frozen

Department of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada N1G 2W1

* Corresponding author.

ABSTRACT

This research examined the effect of freezing on soil microbial C and N measured by CHCl₃ fumigation-extraction (CFE). A soil was stored frozen (–15°C) and sampled after 1, 7, and 170 d. Compared with a nonfrozen soil, freezing had no effect on microbial C or N, except after 7 d when C increased by 24% and N increased by 34%. The soil was also subjected to freeze-thaw cycles (18/6 h). Microbial C and N were unaffected by one cycle, increased with two, and decreased with three cycles. In a second experiment, soil was amended with ¹⁴C-glucose and incubated for 1, 7, or 35 d (25°C), then stored frozen for either 1, 7, 21, or 35 d. Soil incubated for 1 d showed a decline in microbial ¹⁴C (measured by CFE) after freezing for 7 to 35 d. Soil incubated for 7 and 35 d contained a ¹⁴C microbial population that was unaffected by freezing. At first glance, these observations indicated that the CFE assay was not greatly affected by freezing soils; however, closer examination indicated that microbial mortality had been masked by a freezing-induced improvement in CFE efficiency. Increased efficiency may have resulted from improved aggregate dispersal following freezing. Homogenization of soil samples improved extraction of microbial C by 40%. In conclusion, we don't recommend that soils be frozen prior to CFE. Also, the physical disruption of soil during CFE must be severe to remove any interference of aggregate stability on the results of this assay.

Received for publication June 3, 1992.

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