Microbial response to cooling

In temperate soil systems microbial biomass often increases during winter and decreases again in spring. This build up and release of microbial carbon could potentially lead to a build-up of stabilized soil carbon during winter times. The mechanism behind the increase in microbial carbon is not well understood. In this laboratory incubation study, we looked into microbial physiology as well as microbial glucose uptake and partitioning during cooling. Soils from a temperate forest and agricultural system were cooled down from field temperature of 11°C to 1°C. We added ¹³C-labelled glucose immediately and after an acclimation phase of 7 days and traced the ¹³C into microbial biomass, CO₂ respired from the soil and phospholipid fatty acids. In addition we determined microbial growth using ¹⁸O-incorporation into DNA.

First results show that while total respiration was strongly reduced when soils were cooled, glucose-derived respiration was as high in soils at 1°C as at 11°C. The same general pattern was found in soils during fast cooling and after an acclimation phase in agricultural and forest soils. We also saw an increased investment of glucose-derived carbon in unsaturated PLFAs. Since unsaturated fatty acids retain fluidity at lower temperatures compared to saturated fatty acids, this could be interpreted as precaution to reduced temperatures and potential freezing.

Our results show a distinct response of the soil microbial community to cooling. The maintained glucose-derived respiration and incorporation into PLFAs at low temperatures compared to field temperature might indicate a preferential use of labile C forms during cooling. Moreover, the ¹³C incorporation into PLFAs may signal the buildup of cooling resistant cell membranes. These findings will be discussed with results from the ¹³C label tracing into microbial biomass, extractable organic carbon and total soil carbon as well as data on microbial growth and carbon use efficiency.