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Doctoral thesis, 2010

Biogeochemical processes in frozen soils

Harrysson Drotz Stina


High latitude ecosystems are important for global carbon (C) balances and are among the most sensitive ecosystems to climate change. Microbial CO2 production in soil is known to proceed at temperatures < 0ºC in these regions and winter CO2 emissions can significantly affect annual C balances. However, the low-temperature processes involved were poorly understood. In frozen soils, the microbial activity must be confined to small pools of liquid water present in the bulk soil. Therefore, in laboratory incubation and NMR experiments I have investigated: the distribution of unfrozen water in frozen boreal forest soils; its implications for the mineralization of soil organic matter (SOM) and microbial substrate utilization under frozen conditions; factors controlling these phenomena and their temperature responses. The results show that the osmotic potential of unfrozen water contributes 20-69% of the total water potential in frozen soil, in contrast to unfrozen soil where its contribution is generally negligible. They also show that recalcitrant SOM components, such as aromatics and alkyl C, have positive effects on unfrozen water contents, and thus on CO2 production in frozen soils. Further, temperature responses of CO2 production in frozen soils are controlled by unfrozen water, and estimated biochemical Q10-values were consistent with thermodynamic theory after factoring out the effects of water availability. In addition, both catabolic and anabolic processes can proceed at -4ºC and no clear differences in C allocation patterns of metabolised substrates were observed across the -4°C to +9°C temperature range. However, at < 0ºC the soil microbes required longer times to adapt and utilise substrates maximally than at > 0ºC. At -4ºC, this adaptation was associated with increased cell membrane fluidity, and resulted in significant increases in CO2 production. The findings contribute to the general understanding of low temperature microbial processes; indicating that the hierarchy of controlling factors changes as soil freezes, but microbial metabolism is similar in frozen and unfrozen soil. The results have important implications for the conceptualization of processes related to soil C dynamics.


soil; frost; soil organic matter; carbon; carbon dioxide; soil water; soil temperature; catabolism; anabolism; carbon cycle; mineralization

Published in

Acta Universitatis Agriculturae Sueciae
2010, number: 2010:32
ISBN: 978-91-576-7509-5
Publisher: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences

Authors' information

Harrysson Drotz, Stina
Swedish University of Agricultural Sciences, Department of Forest Ecology and Management

UKÄ Subject classification

Soil Science
Forest Science

URI (permanent link to this page)