Headwater streams drain >70 % of global land areas but are poorly monitored compared with large rivers. The small size and low water buffering capacity of headwater streams may result in a high sensitivity to local hydrological alterations and different carbon transport patterns from large rivers. Furthermore, alpine headwater streams on the “Asian water tower”, i.e., Qinghai–Tibetan Plateau, are heavily affected by thawing of frozen soils in spring as well as monsoonal precipitation in summer, which may present contrasting spatial–temporal variations in carbon transport compared to tropical and temperate streams and strongly influence the export of carbon locked in seasonally frozen soils. To illustrate the unique hydro-biogeochemistry of riverine carbon in Qinghai–Tibetan headwater streams, here we carry out a benchmark investigation on the riverine carbon transport in the Shaliu River (a small alpine river integrating headwater streams) based on annual flux monitoring, sampling at a high spatial resolution in two different seasons and hydrological event monitoring. We show that riverine carbon fluxes in the Shaliu River were dominated by dissolved inorganic carbon, peaking in the summer due to high discharge brought by the monsoon. Combining seasonal sampling along the river and monitoring of soil–river carbon transfer during spring thaw, we also show that both dissolved and particulate forms of riverine carbon increased downstream in the pre-monsoon season due to increasing contribution of organic matter derived from thawed soils along the river. By comparison, riverine carbon fluctuated in the summer, likely associated with sporadic inputs of organic matter supplied by local precipitation events during the monsoon season. Furthermore, using lignin phenol analysis for both riverine organic matter and soils in the basin, we show that the higher acid-to-aldehyde ($\mathrm{Ad}/\mathrm{Al}$) ratios of riverine lignin in the monsoon season reflect a larger contribution of topsoil likely via increased surface runoff compared with the pre-monsoon season when soil leachate lignin $\mathrm{Ad}/\mathrm{Al}$ ratios were closer to those in the subsoil than topsoil solutions. Overall, these findings highlight the unique patterns and strong links of carbon transport in alpine headwater catchments with local hydrological events. Given the projected climate warming on the Qinghai–Tibetan Plateau, thawing of frozen soils and alterations of precipitation regimes may significantly influence the alpine headwater carbon transport, with critical effects on the biogeochemical cycles of the downstream rivers. The alpine headwater catchments may also be utilized as sentinels for climate-induced changes in the hydrological pathways and/or biogeochemistry of the small basin.

中文翻译：

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流域碳的时空变化受高山流域局部水文事件的强烈影响

死水径流> 70 占全球陆地面积的百分比，但与大河流相比，监测不力。上游水源的大小小和水缓冲能力低，可能导致对当地水文变化和来自大河的不同碳传输模式的高度敏感性。此外，“亚洲水塔”（即青藏高原）上的高山源头水流受春季冻土融化和夏季季风性降水的严重影响，与碳运量的时空变化相比，存在差异。进入热带和温带河流，并强烈影响季节性冻结土壤中碳的出口。为了说明青藏高原源头河流中河流碳的独特水生生物地球化学，在此，我们根据年度通量监测，两个不同季节的高空间分辨率采样以及水文事件监测，对沙流河（一条小水源汇入高山的小河流）中的河流碳迁移进行基准研究。我们发现，沙流河中的河流碳通量以溶解的无机碳为主，由于季风带来的高排放，在夏季达到峰值。结合沿河的季节性采样和春季融化过程中土壤-河流碳转移的监测，我们还表明，由于季风融化土壤中有机质的贡献增加，溶解和颗粒形式的河流碳在季风前期的下游都增加了。河流。相比之下，河流碳在夏季波动较大，可能与季风季节当地降水事件提供的零星有机物输入有关。此外，使用木质素酚分析盆地中的河流有机质和土壤，我们发现较高的酸-醛（$\mathrm{广告}/\mathrm{铝}$）季风季节中河流木质素的比率反映出与季风前季节土壤沥滤液木质素相比，可能通过增加地表径流量增加了表土的贡献 $\mathrm{广告}/\mathrm{铝}$比率比表土解决方案更接近地下土壤中的比率。总体而言，这些发现凸显了高山源水集水区碳迁移与当地水文事件的独特模式和紧密联系。考虑到预计的青藏高原气候变暖，冻融土壤的融化和降水方式的变化可能会显着影响高山源水的碳迁移，从而对下游河流的生物地球化学循环产生重要影响。高山源水集水区也可以用作小流域水文路径和/或生物地球化学中气候引起的变化的前哨。