MEASUREMENT OF MICROBIAL BIOMASS BY FUMIGATION-EXTRACTION IN SOIL STORED FROZEN

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