Different erosion processes deliver large amounts of terrestrial soil organic carbon (SOC) to rivers. Mounting evidence indicates that a significant fraction of this SOC, which displays a wide range of ages, is rapidly decomposed after entering the river system. The mechanisms explaining this rapid decomposition of previously stable SOC still remain unclear. In this study, we investigated the relative importance of two mechanisms that possibly control SOC decomposition rates in aquatic systems: (i) in the river water SOC is exposed to the aquatic microbial community which is able to metabolize SOC much more quickly than the soil microbial community and (ii) SOC decomposition in rivers is facilitated due to the hydrodynamic disturbance of suspended sediment particles. We performed different series of short-term (168 h) incubations quantifying the rates of SOC decomposition in an aquatic system under controlled conditions. Organic carbon decomposition was measured continuously through monitoring dissolved O₂ (DO) concentration using a fiber-optic sensor (FireStingO₂, PyroScience). Under both shaking and standing conditions, we found a significant difference in decomposition rate between SOC with aquatic microbial organisms added (SOC + AMO) and without aquatic microbial organisms (SOC − AMO). The presence of an aquatic microbial community enhanced the SOC decomposition process by 70 %–128 % depending on the soil type and shaking–standing conditions. While some recent studies suggested that aquatic respiration rates may have been substantially underestimated by performing measurement under stationary conditions, our results indicate that the effect of hydrodynamic disturbance is relatively minor, under the temperature conditions, for the soil type, and for the suspended matter concentration range used in our experiments. We propose a simple conceptual model explaining these contrasting results.

中文翻译：

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河流系统中土壤有机碳的快速分解：水生微生物群落和水动力干扰的影响

不同的侵蚀过程将大量陆地土壤有机碳（SOC）输送到河流。越来越多的证据表明，进入河流系统后，这种SOC的很大一部分显示了很宽的年龄范围，并迅速分解。解释这种先前稳定的SOC迅速分解的机制仍然不清楚。在这项研究中，我们研究了可能控制水生系统中SOC分解速率的两种机制的相对重要性：（i）在河水中，SOC暴露于水生微生物群落中，该微生物能够比土壤微生物更快地代谢SOC。 （ii）由于悬浮泥沙颗粒的水动力扰动，促进了河流中的SOC分解。我们进行了不同系列的短期（168 h）孵化，量化了在受控条件下水生系统中SOC分解的速率。通过监测溶解的O来连续测量有机碳的分解使用光纤传感器（FireStingO ₂，PyroScience）进行₂（DO）浓度测量。在这两种摇晃，站立的条件下，我们发现在SOC之间分解率显著差异添加了水生微生物（SOC  +  AMO）和无水生微生物（SOC  - AMO）。水生微生物群落的存在使SOC分解过程提高了70％–128％，具体取决于土壤类型和震动条件。尽管最近的一些研究表明，在固定条件下进行测量可能会大大低估了水生呼吸速率，但我们的结果表明，在温度条件下，对于土壤类型和悬浮物浓度，水动力扰动的影响相对较小。我们的实验中使用的范围。我们提出了一个简单的概念模型来解释这些对比结果。