Abstract Burial of organic matter in deep peat deposits has already been experimentally demostrated to slow down or even inhibit anaerobic decomposition due to lack of diffusive transport and end-product accumulation. However, so far little is known about potential biogeochemical or thermodynamic indicators for the observed inhibition of further decomposition. For example, theoretical energy yields for methanogenesis, hydrogen partial pressures, stable isotope fractionation factors between CO2 and CH4, and electrochemical properties of dissolved organic matter have been proposed as thermodyamic indicators for such inhibition. To test the applicability and explanatory power of these indicators to identify conditions inhibiting organic matter decomposition, we incubated homogenized ombrotrophic peat for 300 days at 20 °C under diffusive flux conditions as control, and compared the observed effects to a treatment with vertical advective transport by water circulation and to a treatment in which both the unsaturated and water-saturated zone of the peat profile were kept anoxic. Results of energy yields of acetoclastic and hydrogenotrophic methanogenesis were compared to hydrogen partial pressures, to 13C isotope fractionation factors and to redox properties of dissolved organic matter as obtained from mediated electrochemical oxidation and reduction. While CO2 and CH4 production slowed substantially in the deep peat profile, a concomitant decrease of Gibs free energy yields available to hydrogenotrophic and acetoclastic methanogensis and hydrogen and acetate concentrations over time supported a thermodynamic constraint on methanogenesis. Although, energy yields for the hydrogenotrophic pathway were close to or below the theoretical energy minimum levels already after 15 days. Transiently elevated H2 concentrations, not related to actual methanogenesis rates were observed for about 150–225 days. Thereafter, hydrogen concentrations diminished to levels below thresholds to thermodynamically support ongoing methanogenesis. Thus even on incubation timescales of 150–225 days, steady-state hydrogen concentrations as would be expected from thermodynamic considerations did not adjust on the bulk scale of observation. Gibbs free energy estimates for methanogenesis based on hydrogen partial pressures were consequently biased and did not reach the minimum required threshold despite ovious net CH4 production. Ratios between electron accepting (EAC) and donating (EDC) capacity of dissolved organic matter, however, turned out to provide suitable indicators of predominant redox conditions along gradients, stabilizing at low values upon onset of methanogenesis. Thus, our study demonstrated that the thermodynamically driven slow down of decomposition in deep peat deposits, preventing the peat to decompose further, cannot be easily identified based on a single indicator. However, constant and high concentrations of decomposition end-products, indicating zero net turnover and low energy yields, and constantly low EAC/EDC ratios, indicating no further availability of terminal electron acceptors, seem to be characteristic of the onset of conditions inhibiting further significant decomposition of peat.

中文翻译：

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评价泥炭产甲烷条件和热力学约束的生物地球化学指标

摘要 由于缺乏扩散运输和终产物积累，深部泥炭沉积物中的有机质埋藏已经被实验证明可以减缓甚至抑制厌氧分解。然而，到目前为止，对于观察到的进一步分解抑制的潜在生物地球化学或热力学指标知之甚少。例如，甲烷生成的理论能量产率、氢分压、CO2 和 CH4 之间的稳定同位素分馏因子以及溶解有机物的电化学特性已被提议作为这种抑制的热力学指标。为了测试这些指标的适用性和解释力，以确定抑制有机物分解的条件，我们在扩散通量条件下在 20 °C 下将均质化的同养泥炭培养 300 天作为对照，并将观察到的效果与通过水循环进行垂直平流传输的处理以及其中不饱和和水饱和区的处理进行比较泥炭剖面均保持缺氧。将醋酸碎屑和氢营养产甲烷的能量产率结果与氢分压、13C 同位素分馏因子和溶解有机物的氧化还原特性进行比较，如通过介导的电化学氧化和还原获得。虽然在深层泥炭剖面中 CO2 和 CH4 的产生显着放缓，随着时间的推移，可用于氢营养和乙酰碎屑产甲烷作用的 Gibs 自由能产量以及氢和乙酸盐浓度随着时间的推移而减少，支持了对产甲烷作用的热力学约束。尽管如此，氢营养途径的能量产量在 15 天后已经接近或低于理论能量最低水平。在大约 150-225 天内观察到与实际产甲烷速率无关的 H2 浓度暂时升高。此后，氢气浓度降低到阈值以下的水平，以在热力学上支持正在进行的产甲烷作用。因此，即使在 150-225 天的孵化时间尺度上，从热力学考虑所预期的稳态氢浓度也不会在大量观察范围内进行调整。因此，基于氢分压的甲烷生成的吉布斯自由能估计存在偏差，尽管存在明显的 CH4 净产量，但并未达到所需的最低阈值。然而，溶解有机物的电子接受 (EAC) 和捐赠 (EDC) 能力之间的比率证明提供了沿着梯度的主要氧化还原条件的合适指标，在产甲烷开始时稳定在低值。因此，我们的研究表明，热力学驱动的深层泥炭沉积物分解减缓，阻止泥炭进一步分解，无法根据单一指标轻松识别。然而，分解终产物的浓度恒定且高，表明零净周转率和低能量产率，以及持续低的 EAC/EDC 比率，