Temperature sensitivity of dark CO2 fixation in temperate forest soils

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 release of CO2 from temperate forest soils, a potentially 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 will 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 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 will likely 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.

全球范围内，预计到本世纪末，土壤温度在1米深度处将上升至高达4摄氏度，且在温带森林区域将产生显著影响。土壤温度的升高有可能增加温带森林土壤中二氧化碳的释放，这可能是气候变化的一个重要正反馈机制。微生物对暗二氧化碳的固定能够循环部分释放的土壤二氧化碳，且已有研究表明，在较高温度下二氧化碳固定速率会增加。然而，关于温度对暗二氧化碳固定速率的影响研究尚显不足，尤其是在与温带森林土壤呼吸作用的温度敏感性进行对比时。为了确定暗二氧化碳固定和呼吸速率的温度敏感性（Q10值），我们调查了德国Hummelshain森林中栎树（落叶）和云杉（针叶）森林样地的1米深土壤剖面。我们利用13C-CO2标记和4°C及14°C条件下的土壤培养实验，确定了二氧化碳固定和净土壤呼吸速率，并据此推导出两种过程的Q10值随深度的变化。对于栎树和云杉土壤剖面，暗二氧化碳固定速率的Q10值（以土壤干重归一化）分别为2.07，低于净土壤呼吸速率的测量平均Q10值约为2.98。基于这些Q10值，我们外推，在预计的4°C升温条件下，净土壤呼吸将比二氧化碳固定增加1.16倍。在栎树土壤中，与云杉土壤相比，标记的二氧化碳以较大比例固定为土壤有机碳，而非微生物生物量，这表明微生物细胞的周转（即生长和死亡）速度更高，并且我们推测这与栎树土壤中较低的粘土含量有关。尽管栎树和云杉土壤中的总细菌群落丰度相似，但栎树土壤中自养生物的丰度较低，这表明与云杉土壤相比，异养生物的比例更高，这或许可以部分解释生物量周转率较高的原因。此外，两种土壤在较高温度下普遍导致微生物生物量周转率增加。我们的研究结果表明，在温带森林土壤中，二氧化碳固定可能对未来的升温反应不如净土壤呼吸敏感，并且特定地点的参数变化可能影响微生物生物量的周转，进而影响温带森林土壤中的暗二氧化碳固定速率及其温度敏感性。