With climate change, streams and rivers are at increased risk of droughts and flow intermittency. The full implications of these conditions for fluvial carbon (C) processing and stream-atmosphere CO₂ emissions are not well understood. We performed a controlled drought experiment in outdoor hyporheic flumes. We simulated small rain events that increase sediment moisture content, but do not cause streamflow in order to investigate how these events affect streambed dissolved organic C dynamics, biofilm respiration and enzyme activity, and bacterial community composition. Flumes were subject to a non-flow phase of one month with small rain events with varying frequency (weekly, 3 × weekly, and no rain). Sediment was sampled at the surface and from the hyporheic zone at the end of the non-flow phase. We quantified microbial respiration of the dry sediments and sediment DOC leaching after simulated flow resumption. We found that, at the surface, more frequent rain events significantly increased microbial respiration from 12.6 ± 0.25 µg CO₂ g⁻¹ DW h⁻¹ to 26.5 ± 11.3 µg CO₂ g⁻¹ DW h⁻¹ between the control and 3 × weekly rain events. The average amount of DOC leached from surface sediments during flow resumption was reduced by 0.813 ± 0.62 mg L⁻¹ with more frequent rain events. More frequent rain events also resulted in the leaching of fresher DOM with increased tryptophan fluorescence and a higher BIX. This, along with higher glucosidase activity in the biofilms, indicates higher OC processing during the drought period with more frequent rain events. Small rain events also enhanced Shannon diversity of microbial communities, with a stronger presence of ‘terrestrial-like’ bacterial clades. We propose that rain events during drought, even those of small size, are highly relevant for fluvial organic C processing during the dry phase. Future research should explicitly consider small rain events when investigating C fluxes in intermittent streams to fully understand the C processing in these systems with climate change. We conclude that small rain events impact DOM dynamics during reflow and likely impact the cascading C processing in the downstream river network.

随着气候变化，溪流和河流发生干旱和流量间歇性的风险增加。这些条件对河流碳 (C) 处理和流大气 CO _{2的全部影响}排放量还不是很清楚。我们在室外低流水槽中进行了受控干旱实验。我们模拟了增加沉积物水分含量但不引起水流的小雨事件，以研究这些事件如何影响河床溶解有机碳动力学、生物膜呼吸和酶活性以及细菌群落组成。水槽经历了一个月的非流动阶段，小雨事件的频率不同（每周，3 × 每周，无雨）。在非流动阶段结束时，在地表和低流带区对沉积物进行采样。我们量化了模拟流动恢复后干燥沉积物和沉积物 DOC 浸出的微生物呼吸。我们发现，在地表，更频繁的降雨事件显着增加了 12.6 ± 0.25 µg CO₂ g ^-1 DW h ^-1到 26.5 ± 11.3 µg CO ₂ g ^-1 DW h ^-1在对照和 3 × 每周降雨事件之间。流量恢复期间从表层沉积物中浸出的 DOC 平均减少了 0.813 ± 0.62 mg L ^-1随着更频繁的降雨事件。更频繁的降雨事件也导致更新鲜的 DOM 浸出，色氨酸荧光增加和 BIX 更高。这与生物膜中更高的葡糖苷酶活性一起表明干旱期间的 OC 处理更高，降雨事件更频繁。小雨事件也增强了微生物群落的香农多样性，“类地”细菌进化枝的存在更强。我们提出干旱期间的降雨事件，即使是小规模的降雨事件，也与干旱阶段的河流有机碳处理高度相关。未来的研究应在调查间歇流中的 C 通量时明确考虑小雨事件，以充分了解这些系统中的 C 处理与气候变化。