Soils substantially contribute to the terrestrial fluxes of CO₂ to the atmosphere. Dark CO₂ fixation, the microbial process by which pore space CO₂ is reduced to organic matter, may recycle and trap some of the CO₂ respired in soils before it can escape to the atmosphere. To evaluate its potential significance for global temperate forest soil carbon stocks, we quantified dark CO₂ fixation rates in a temperate beech forest soil down to 1 m depth over a range of 2–20% (v:v) headspace CO₂ concentrations, by tracing incorporation of a¹³C–CO₂ label into microbial biomass carbon and soil organic matter. We found that fixation rates under a concentration of 2% CO₂ decreased with depth from 0.86 to 0.06 μg C normalized to g(dw) soil⁻¹ d⁻¹. However, when dark CO₂ fixation rates were normalized to soil microbial biomass carbon, no significant differences between depths were observed. Higher CO₂ concentrations increased fixation rates, with a linear 2-fold increase between 2% and 10% CO₂. Molecular analysis revealed the dominance of heterotrophs, along with the presence of autotrophs mainly employing the Calvin Benson Bassham (CBB) pathway followed by the reductive citric acid (rTCA) pathway. Although community composition varied with depth, the relative fraction of autotrophs determined by qPCR of RuBisCO (cbbL IA, cbbL IC) and ATP-citrate lyase (aclA) genes remained stable at approximately 0.5% of the total community. Dark CO₂ fixed carbon accounted for up to 1.1% of microbial biomass carbon and up to 0.035% of soil organic carbon after 28 days. We estimated a fixation flux of 25 ± 7.2 g C m⁻² yr⁻¹ to 1 m depth for the Hainich forest soil under field conditions. Without this process, Hainich forest soil CO₂ emissions would be 5.6% higher, recycling a fraction of carbon large enough to potentially affect carbon isotope signatures in SOC. If this is held for all temperate forest soils globally, the annual rate of dark CO₂ fixation would be 0.26 ± 0.07 Pg C yr⁻¹ to a depth of 1 m, without considering contributions from other biomes. In conclusion, microbial biomass carbon and CO₂ concentration appear to be the main drivers of dark CO₂ fixation in temperate forest soils, and dark CO₂ fixation may maintain Hainich forest soil carbon stocks by moderating a significant fraction of soil CO₂ emissions annually.

土壤实质上是导致CO ₂进入大气的陆地通量。深色的CO ₂固定（通过微生物将孔隙空间中的CO ₂还原为有机物的过程）可能会循环并截留一些在土壤中呼吸的CO ₂，然后才能逃逸到大气中。为了评估其对全球温带森林土壤碳储量的潜在重要性，我们通过以下方法对深达1 m的温带山毛榉森林土壤中2-20％（v：v）顶空CO ₂浓度范围内的深色CO ₂固定率进行了量化，方法是跟踪纳入¹³ C–CO ₂标记为微生物生物量碳和土壤有机质。我们发现，在2％CO ₂浓度下，固定化率随深度（从0.86到0.06μgC相对于g（dw）土壤^-1 d ^-1标准化而降低）。然而，当将深色CO ₂固定率标准化为土壤微生物生物量碳时，在深度之间未观察到显着差异。较高的CO ₂浓度会提高固定率，在2％和10％之间的CO ₂线性增加2倍。分子分析揭示了异养生物的优势，以及自养生物的存在，主要采用Calvin Benson Bassham（CBB）途径，其次是还原性柠檬酸（rTCA）途径。尽管群落组成随深度而变化，但通过RuBisCO（cbbL IA，cbbL IC）和ATP-柠檬酸裂解酶（aclA）基因的qPCR测定的自养生物的相对分数保持稳定，约为总群落的0.5％。28天后，深色的CO ₂固定碳占微生物生物量碳的1.1％，占土壤有机碳的0.035％。我们估计定影通量为25±7.2 g C m ^-2 yr ^-1田间条件下，海尼希森林土壤的最大深度为1 m。如果没有此过程，海尼希（Hainich）森林土壤的CO ₂排放量将增加5.6％，回收的一部分碳足够大，有可能影响SOC中的碳同位素特征。如果全球范围内所有温带森林土壤都保持这种状态，则不考虑其他生物群落的贡献，暗CO _2的年固定率将为0.26±0.07 Pg C yr ^-1到1 m的深度。总之，微生物生物量碳和CO ₂浓度似乎是温带森林土壤中深色CO ₂固定和深色CO ₂的主要驱动因素。固定可以通过减少每年土壤CO ₂排放量的很大一部分来维持海尼希森林的土壤碳储量。