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 °C or −18 °C, thawed at +4 °C, and respiration and bacterial growth (estimated using leucine incorporation) were compared with reference soils kept at +4 °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 °C resulted in 2.2-2.5 times higher cumulative respiration than the reference, while at −3 °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 °C and 23-26% in the −18 °C treatment. Growth rates then increased linearly, recovering after 36 h and around 50 h in the −3 °C and −18 °C freezing, respectively. Growth rates then increased further in the −18 °C, but remained lower or similar to the reference in the −3 °C treatment. The microbial response to freezing/thawing thus appeared similar to mild drying/rewetting (type 1 response sensu Meisner et al. (2015)).