Soil bacterial growth after a freezing/thawing event

Bacterial growth after freezing/thawing was studied in two soils with a history of annual freezing/thawing events. Soil samples were frozen for 1 week at -3 degrees C or 18 degrees C, thawed at +4 degrees C, and respiration and bacterial growth (estimated using leucine incorporation) were compared with reference soils kept at +4 degrees C. There were no major differences between soils. A respiration pulse, peaking within 9 h, was found, but after 30-100 h respiration had decreased to that in the reference. Freezing at -18 degrees C resulted in 2.2-2.5 times higher cumulative respiration than the reference, while at -3 degrees C 1.6-1.8 times higher respiration was found. Bacterial growth rates immediately after thawing were 43-44% of the reference in the -3 degrees C and 23-26% in the -18 degrees C treatment. Growth rates then increased linearly, recovering after 36 h and around 50 h in the -3 degrees C and -18 degrees C freezing, respectively. Growth rates then increased further in the -18 degrees C, but remained lower or similar to the reference in the -3 degrees C treatment. The microbial response to freezing/thawing thus appeared similar to mild drying/rewetting (type 1 response sensu Meisner et al. (2015)). (C) 2016 Elsevier Ltd. All rights reserved.