Water availability controls microbial temperature responses in frozen soil CO₂ production

Abstract

Soil processes in high-latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO 2 production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO 2 production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO 2 temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO 2 production at low temperatures is crucial for a thorough understanding of low-temperature soil processes and for accurate predictions of C-balances in northern terrestrial ecosystems.

Both catabolic and anabolic heterotrophic microbial activity proceed in frozen soils

“…Microbial processes and production of greenhouse gases continue during the winter months in soils of high-latitude regions (2)(3)(4)(5)(6), and a significant fraction of the carbon fixed during the growing season can be lost during the following winter (7,8). Hence, knowledge about winter microbial processes is essential for understanding carbon mineralization in these ecosystems and its coupling to climate change.…”

“…When soil freezes, the liquid water content is reduced (10), and the solutes in the liquid water phase must be concentrated for the unfrozen water to reach the required water potential for a specific subzero temperature (11). That soil microbial catabolic processes occur in frozen soils is known through the detection of biogenic CO 2 production (2,3,12), but knowledge about anabolic processes and the partitioning of the carbon flow between catabolic and anabolic processes in frozen soil is less complete.…”

Seasonal and interannual dynamics of soil microbial biomass and available nitrogen in an alpine meadow in the eastern part of Qinghai–Tibet Plateau, China

“…Previous studies indicated that substantial mi-568 B. Xu et al: Seasonal and interannual dynamics of soil microbial biomass and available nitrogen crobial activity exists in frozen soils during cold seasons even at temperatures lower than −5 • C (Brooks et al, 1996;Edwards et al, 2006;Panikov et al, 2006;Jefferies et al, 2010). Although microbial activity is limited by cold temperatures and substrate transport (Deming, 2002;Lipson et al, 2002;Oquist et al, 2009), its cumulative effects on organic matter decomposition in soil during long cold seasons significantly influence annual N pools in Arctic and alpine ecosystems (Lipson et al, 1999;Schmidt and Lipson, 2004;Schmidt et al, 2007;Buckeridge and Grogan, 2008). Thus, by understanding microbial activities in winter, we can broaden our current knowledge regarding nutrient supplies for plants and microbes during the subsequent growing season.…”

Effects of soil organic matter composition on unfrozen water content and heterotrophic CO2 production of frozen soils

“…The samples came from different pine (Pinus sylvestris, L.) and spruce (Picea abies, L.) dominated sites and were sampled at 1-3 depths (n = 9) and 1-2 depths (n = 5), respectively, see Oquist et al (2009) for more detailed information on sampling sites and soil characteristics. We investigated potential effects of air-drying versus freeze-drying, and steel ball milling versus agate ball milling of the samples on NMR results before more detailed experiments were performed, and detected no differences between the treatments.…”