Temperature sensitivity of dark CO 2 fixation in temperate forest soils

[摘要] Globally, soil temperature to 1 m depth is predicted to be up to 4  ∘ C warmer by the end of this century, with pronounced effectsexpected in temperate forest regions. Increased soil temperatures willpotentially increase the release of carbon dioxide ( CO 2 ) from temperate forest soils,resulting in important positive feedback on climate change. Dark CO 2 fixation by microbes can recycle some of the released soil CO 2 , and CO 2 fixation rates are reported to increase under higher temperatures.However, research on the influence of temperature on dark CO 2 fixationrates, particularly in comparison to the temperature sensitivity ofrespiration in soils of temperate forest regions, is missing. To determinethe temperature sensitivity ( Q 10 ) of dark CO 2 fixation andrespiration rates, we investigated soil profiles to 1 m depth from beech(deciduous) and spruce (coniferous) forest plots of the Hummelshain forest,Germany. We used 13 C - CO 2 labelling and incubations of soils at 4and 14  ∘ C to determine CO 2 fixation and net soil respirationrates and derived the Q 10 values for both processes with depth. Theaverage Q 10 for dark CO 2 fixation rates normalized to soil dryweight was 2.07 for beech and spruce profiles, and this was lower than themeasured average Q 10 of net soil respiration rates with ∼2.98 . Assuming these Q 10 values, we extrapolated that net soilrespiration might increase 1.16 times more than CO 2 fixation under aprojected 4  ∘ C warming. In the beech soil, a proportionallylarger fraction of the label CO 2 was fixed into soil organic carbonthan into microbial biomass compared to the spruce soil. This suggestsa primarily higher rate of microbial residue formation (i.e. turnover asnecromass or release of extracellular products). Despite a similar abundanceof the total bacterial community in the beech and spruce soils, the beechsoil also had a lower abundance of autotrophs, implying a higher proportionof heterotrophs when compared to the spruce soil; hence this might partly explainthe higher rate of microbial residue formation in the beech soil.Furthermore, higher temperatures in general lead to higher microbialresidues formed in both soils. Our findings suggest that in temperate forestsoils, CO 2 fixation might be less responsive to future warming than netsoil respiration and could likely recycle less CO 2 respired fromtemperate forest soils in the future than it does now.

[学科分类] 大气科学