Microbial respiration, biomass, and endo- exo-enzyme activities measurements from Arctic soil microcosms incubated with cellulose polymers, Alaska, 2021

Faster decomposition and respiration of soil carbon with global warming is a concern for the Arctic, which contains one of the largest stocks of soil carbon (C) and is warming three times faster than the global average, especially in winter. Microbes remain active in frozen soils, but little is known about controls on decomposition and respiration below 0 Celsius (˚C). Determining the controls on microbial respiration in frozen soils is needed to predict winter warming effects on soil carbon dioxide (CO2) emissions. To identify these controls, we incubated Arctic soil microcosms between -10 and 10 °C with cellulose polymers of different lengths and measured microbial respiration, biomass, and endo- exo-enzyme activities. With declining temperatures below 10 °C, the breakdown of long cellulose chains was increasingly inhibited by slowing rates of enzyme hydrolysis, and eventually stopped. Changes in relative activities of enzymes cleaving cellulose chain interiors (endo-cellulases) with declining temperatures indicate abrupt thresholds at 10 °C, between -2 and -6 °C, and at -10 °C. Due to rapid winter warming, soil temperatures are now above these thresholds for longer periods than in the past. Incorporating these mechanistic temperature thresholds for decomposition into models may improve predictions of increasing Arctic winter soil CO2 emissions.