Microbial utilization of simple carbon substrates in boreal peat soils at low temperatures

Northern peatlands are considered to be a contemporary carbon (C) sink and currently store an estimated 25% of the world's soil C. In boreal peatlands, CO2 emissions during the long winters (which can last up to 6 months), can account for 16–80% of the growing season's net CO2 assimilation. Thus, low-temperature biogeochemical processes are crucial for the long-term net ecosystem C balance. However, our conceptual understanding of the controls on microbial metabolic activity driving C mineralization in peat at temperatures below freezing is limited. We characterized the microbial communities and incubated peat-soil samples amended with 13C-labeled glucose and followed the fate of the labeled material as it decomposed. We show that microorganisms in the acrotelm of frozen boreal peat soils utilize simple C substrates to sustain both catabolic and anabolic metabolism at temperatures down to c. –5 °C. In the frozen matrix, small temperature changes can have strong effects on process rates. Thus small differences in winter soil temperature can have very large implications for the winter C dynamics of northern peatlands. The observed microbial metabolic capacity in winter peat suggests that persistent degradation of soil organic matter (SOM) and concomitant synthesis of new microbial biomass during winter occurs in boreal peatlands at environmentally relevant timescalesand temperatures.