Comment on bg-2022-90

Abstract

Globally, soil temperature to 1 m depth is predicted to be up to 4 °C warmer by the end of this century, with pronounced effects expected in temperate forest regions. Increased soil temperatures will potentially increase the release of CO2 from temperate forest soils, resulting in important positive feedback on climate change. Dark CO2 fixation by microbes can recycle some of the released soil CO2, and CO2 fixation rates are reported to increase under higher temperatures. However, research on the influence of temperature on dark CO2 fixation rates, particularly in comparison to the temperature sensitivity of respiration in soils of temperate forest regions is missing. To determine the temperature sensitivity (Q10) of dark CO2 fixation and respiration rates, we investigated soil profiles to 1 m depth from beech (deciduous) and spruce (coniferous) forest plots of the Hummelshain forest, Germany. We used 13C-CO2 labelling and incubations of soils at 4 and 14 °C to determine CO2 fixation and net soil respiration rates and derived the Q10 values for both processes with depth. The average Q10 for dark CO2 fixation rates normalized to soil dry weight was 2.07 for beech and spruce profiles, and this was lower than the measured average Q10 of net soil respiration rates with ~2.98. Assuming these Q10 values, we extrapolated that net soil respiration might increase 1.16 times more than CO2 fixation under a projected 4 °C warming. In the beech soil, a proportionally larger fraction of the label CO2 was fixed into soil organic carbon than into microbial biomass compared to the spruce soil. This suggests primarily higher turnover (i.e. growth and death) of microbial cells, and we speculate that this is related to lower clay content in the beech soil. Despite a similar abundance of the total bacterial community in the beech and spruce soils, the beech soil also had a lower abundance of autotrophs, implying a higher proportion of heterotrophs when compared to the spruce soil, hence might partly explain the higher biomass turnover. Furthermore, higher temperatures in general lead to higher microbial biomass turnover in both soils. Our findings suggest that in temperate forest soils, CO2 fixation might be less responsive to future warming than net soil respiration and that variations in site-specific parameters might affect microbial biomass turnover and resultantly, dark CO2 fixation rates and its temperature sensitivity in temperate forest soils.