Soil Microbial Mineralization of Cellulose in Frozen Soils

Abstract:

Soils of high-latitude ecosystems store a large fraction of the global soil carbon pool. In boreal forests, the mineralization of soil organic matter (SOM) during winter by soil heterotrophic activity can affect the ecosystems net carbon balance. Recent research has shown that microorganisms in the organic surface layer of boreal forest soil can mineralize and grow on simple, monomeric substrates under frozen conditions. However, any substantial impacts of microbial activity in frozen soils on long-term soil carbon balances depend on whether soil microorganisms can utilize the more complex, polymeric substrates in SOM. In order to evaluate the potential for soil microorganisms to metabolize carbon polymers at low temperatures, we incubated boreal forest soil samples amended with [¹³C]-cellulose and studied the microbial catabolic and anabolic utilization of the substrate under frozen and unfrozen conditions (-4 and +4°C). The [¹³C]-CO₂ production rate in the samples at +4°C were 0.524 mg CO₂ SOM ^-1 day^-1 while rates in the frozen samples (-4°C) were 0.008 mg CO₂ SOM ^-1 day^-1. Thus, freezing of the soil markedly reduced microbial utilization of the cellulose. However, newly synthetized [¹³C]-enriched cell membrane lipids, PLFAs, were detected in soil samples incubated both above and below freezing, confirming microbial growth also in the frozen soil matrix. The reduced metabolic rates induced by freezing indicate constraints on exoenzymatic activity, as well as substrate diffusion rates that we can attribute to reduced liquid water content of the frozen soil. We conclude that the microbial population in boreal forest soil has the capacity to metabolize, and grow, on polymeric substrates at temperatures below zero. This also involves maintaining exoenzymatic activity in frozen soils. This capacity manifests the importance of SOM mineralization during the winter season and its importance for the net carbon balance of soils of high-latitude ecosystems.