Dissolved organic matter (DOM) in freshwaters is recognised as a significant and active component of the global carbon budget. DOM exported from terrestrial ecosystems may be compositionally and functionally altered by chemical and biological reactions as it is transported downstream. The processes affecting DOM in headwater streams remain uncertain but are potentially highly variable depending on DOM composition and nutrient availability as a function of soils, land-use, and human pressures. To investigate variability of DOM reactivity we took water samples from two contrasting headwater catchments, considered functional ‘end-members’ for DOM export: one rich in aromatic DOM and low in inorganic nutrients (peatland), and another (nearby) catchment characterized by less aromatic DOM and high nutrient loadings (agricultural grassland). Under controlled laboratory conditions, we evaluated the effects of light, presence/absence of aquatic biota and nutrient enrichment on short-term changes in DOM quantity and quality in these samples. For the peat stream, exposure to sunlight (with UV) resulted in net abiotic DOM removal, whereas in the agricultural stream it led to net biological DOM production. Nutrient addition accelerated DOM production in both streams. We conclude that in-stream changes in DOM quantity and quality represent the net effect of multiple consumption and production processes whose relative importance is strongly influenced by source-dependent DOM composition and environmental factors such as inorganic nutrient content and sunlight exposure. Our findings suggest that headwater streams may be more active processors of carbon and nutrients than presumed hitherto.

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源头水中溶解有机物的动态：源头流与 DOM 和营养成分对比的比较

淡水中的溶解有机物 (DOM) 被认为是全球碳预算的重要且活跃的组成部分。从陆地生态系统输出的 DOM 在向下游运输时可能会因化学和生物反应而在成分和功能上发生改变。影响源头水流中 DOM 的过程仍然不确定，但根据 DOM 组成和养分可用性（作为土壤、土地利用和人类压力的函数）可能存在很大差异。为了研究 DOM 反应性的变异性，我们从两个对比鲜明的源头流域采集水样，这些流域被认为是 DOM 输出的功能性“终端成员”：一个富含芳香族 DOM 且无机养分含量低（泥炭地），另一个（附近）流域的特征是芳香 DOM 和高营养负荷（农业草地）。在受控实验室条件下，我们评估了光照、水生生物群的存在/不存在和营养物质富集对这些样品中 DOM 数量和质量的短期变化的影响。对于泥炭流，暴露在阳光下（带有紫外线）导致净非生物 DOM 去除，而在农业流中，它导致净生物 DOM 产生。营养添加加速了两个流中的 DOM 生产。我们得出的结论是，流中 DOM 数量和质量的变化代表了多种消费和生产过程的净效应，其相对重要性受到依赖源的 DOM 组成和环境因素（如无机营养成分和阳光照射）的强烈影响。我们的研究结果表明，源头溪流可能是比迄今为止假设的更活跃的碳和营养物质处理器。