【期刊】四川大学生态水文团队：北半球冰冻圈河流有机碳的时空变化规律

导读 | Introduction

气候变化背景下寒区显著变暖导致冰冻圈加速退化。冰川和冻土是冰冻圈的重要有机碳（OC）库，也受到不同程度的扰动，其中的大量有机碳被输送到河流中。河流不仅是陆地和海洋之间物质输移的通道，也是生物地球化学反应器。冰冻圈水文过程和生物地球化学的同时变化，使得河流网络成为碳输出和排放的关键途径之一。河流输送的有机碳包括溶解有机碳（DOC）和颗粒有机碳（POC），它们是水生生态系统代谢的主要能量和物质来源，在全球碳循环、温室气体排放和河流生态系统健康中发挥着重要作用。北半球冰冻圈内不同地区的河流OC动态已得到广泛研究，然而，目前缺乏对北半球冰冻圈河流OC时空动态和调控因素的全面评估。因此，我们从已发表文献中汇编了北半球冰冻圈河流OC特征数据集（包括浓度，放射性碳（14C）同位素和芳香性指数（SUVA254）），旨在提升当前对河流OC特征时空规律的认识。

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. Streams and rivers serve as the channels connecting the land and ocean and also as biogeochemical reactors, which transport, process, and store large amounts of terrigenous carbon. Simultaneous changes in the cryosphere hydrology and biogeochemistry have made fluvial networks one of the crucial pathways for the carbon export. Dissolved and particulate OC (DOC and POC, respectively) are the two main forms of OC exported by rivers. They are the main energy and substance source for aquatic metabolism, which is important for river ecosystem health and ecological sustainability. In recent years, numerous studies investigated and analyzed patterns and drivers of OC characteristics in streams and rivers across the northern cryosphere. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC. Here, we compiled an extensive dataset of riverine OC characteristics from 138 published literature, including concentration, radiocarbon (14C), and specific ultraviolet absorbance at 254 nm (SUVA254) of DOC. This study aims to provide a systematic overview of the current understanding of the spatiotemporal variations of riverine OC characteristics.

一、 河流有机碳整体特征

 Overview of OC characteristics

图1显示北半球冰冻圈内河流OC在空间上表现出较大的变化范围。DOC浓度范围为0.0066 ~ 71.2 mg C L−1，平均浓度为6.34 mg C L−1，高于全球河流的平均浓度5.75 mg C L−1。POC浓度范围为0.0011 ~ 56.0 mg C L−1，平均浓度为2.61 mg C L−1，远低于DOC浓度。Δ14C-DOC范围为−615‰ ~ +143‰，平均值为−131‰；Δ14C-POC范围为−874‰ ~ +53‰，平均值为−419‰，河流输出的OC碳龄普遍表现出老化趋势。

OC characteristics exhibited high variations in northern cryosphere streams and rivers (Fig. 1). DOC concentrations varied across the surveyed sites, ranging from 0.0066 to 71.2 mg C L−1 with the mean concentration of 6.34 mg C L−1, which is higher than 5.75 mg C L−1 across global rivers. POC concentrations varied between 0.0011 and 56.0 mg C L−1 with the mean concentration of 2.61 mg C L−1, which are substantially lower than DOC concentration, indicating the dominance of DOC in the riverine OC pool. Δ14C-DOC signatures ranged from −615‰ to +143‰, corresponding to 14C age from ~7600 years BP to modern. Δ14C-POC signatures varied between −874‰ and +53‰, corresponding to 14C age from ~16600 years BP to modern. Compared to in global rivers (Δ14C-DOC = +22‰, Modern; Δ14C-POC = −204‰, ~1800 years BP), the average Δ14C-DOC and Δ14C-POC signatures were more depleted in northern cryosphere rivers (Δ14C-DOC = −131‰, ~1100 years BP; Δ14C-POC = −419‰, ~4300 years BP), indicating an aged carbon pool in northern cryosphere fluvial systems.

图1 北半球冰冻圈河流站点平均有机碳特征值的空间分布

Fig. 1 The spatial distribution of site-averaged (A) DOC concentration, (B) POC concentration, (C) Δ14C-DOC, and (D) Δ14C-POC data in the Northern Hemisphere cryosphere streams and rivers. Cryosphere abbreviations are C, continuous permafrost; D, discontinuous permafrost; S/I, sporadic/isolated permafrost; G, glacier.

二、河流有机碳特征的季节模式

Seasonal patterns of OC characteristics

河流DOC和POC浓度、Δ14C-DOC和SUVA254呈现出显著的季节变化（图2）。春季河流输出的DOC浓度更高，碳龄更年轻且芳香性更高。夏季与秋季河流输出老化的DOC，浓度也最低。与DOC不同，POC在夏季浓度更高但碳龄同样更老，而在冬季浓度最低，碳龄也更年轻。在不同的季节，DOC年龄均比POC年龄更年轻，河流中DOC和POC的来源不同。

Riverine DOC and POC concentrations, as well as the Δ14C-DOC and SUVA254 values, exhibited significant seasonal variations (Fig. 2). The median DOC concentration, Δ14C-DOC signature, and SUVA254, were highest in spring and significantly higher than those in summer/autumn and winter, implying that riverine DOC was more concentrated, younger, and aromatic during the spring season. Meanwhile, the median DOC concentration was lowest and carbon age was oldest in summer/autumn. Different from DOC, the median POC concentration in summer was lower, but carbon age was still older, whereas it showed lower concentration younger carbon age in winter. DOC ages were generally much younger than POC ages, which may indicate different sources of DOC and POC in the rivers.

图2 河流有机碳特征的季节变化。

（A）DOC和POC浓度，（B）Δ14C-DOC和Δ14C-POC和（C）SUVA254。

Fig. 2 Boxplots showing (A) DOC and POC concentration, (B) Δ14C-DOC and Δ14C-POC, and (C) SUVA254 values for different seasons.

三、河流有机碳特征的空间变化

Spatial variability of OC characteristics

在冰冻圈内，与冰川主导的流域相比，在冻土主导的流域内DOC浓度更高、DOC和POC碳龄更年轻，但POC浓度更低。在不同生态区，北方森林流域中DOC浓度最高、碳龄也最年轻。此外，在冻土覆盖度更高、地下冰含量更高或以较厚覆盖层为特征的低地冻土流域，DOC浓度更高且芳香性更高。

Figure 3 shows spatial variations of OC closely linked to different land covers and landforms. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Across different ecoregions, DOC exhibited the highest concentration and youngest age in boreal forest watersheds, whereas it is the lowest concentration and oldest age in temperate forest regions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic.

图3 河流DOC和POC特征的空间变化。

（A）冰川和冻土，（B）生态区，（C）冻土类型，（D）地下冰含量和（E）地貌。

Fig. 3 Boxplots comparison of OC characteristics variables in rivers draining watersheds

(A) dominated by glacier or permafrost, (B) in different ecoregions, (C) in the different permafrost zone, (D) in different ground ice content, and (E) in different landform categories. Ecoregion abbreviations are BF, boreal forests; MGS, montane grasslands & shrublands; TF, temperate forests; TUN, tundra. Permafrost zone abbreviations are C, continuous permafrost; D, discontinuous permafrost; S/I, sporadic/isolated permafrost. Landform abbreviations are r, described as mountains, highlands ridges, and plateaus characterized by thin overburden cover and exposed bedrock; f, described as lowlands, highlands, and intra- and intermontane depressions characterized by thick overburden cover.

四、河流有机碳特征的环境影响因素 Environmental controls on OC characteristics

OC特征的变化受气候、水文、土壤有机碳含量和总悬浮物浓度的影响。气温和土壤有机碳含量是河流DOC的关键驱动因素，而河流POC主要受总悬浮物浓度调控。

OC characteristics are affected by climate, hydrology, soil organic carbon content and total suspended sediment concentration. Specifically, temperature and SOC content show strong and significant relationships with DOC characteristics (including concentration, aromaticity, and Δ14C), indicating that they were key drivers of riverine DOC, whereas riverine POC are primarily regulated by TSS.

图4河流OC特征与多年平均气温、多年平均降水、多年平均径流、土壤有机碳含量、总悬浮物浓度之间的关系

Fig. 4 OC characteristics versus MAT, MAP, Runoff, SOCC, and TSS and associated spearman rank correlation coefficients r with P-values. MAT, mean annual temperature (°C); MAP, mean annual precipitation (mm yr−1); runoff (mm yr−1); SOCC, soil organic carbon content (t hm−2); TSS, total suspended sediment (mg L−1).

五、河流单位流域面积内的OC通量

Riverine OC fluxes

本研究估算冰川和冻土河流中的OC通量分别为4.77和1.86 g C m−2 yr−1，冰川河流的单位流域面积OC通量超过多年冻土河流的2倍，显示了冰川河流较高的有机碳输出效率。其中，POC是冰川河流OC通量的主要贡献者，而DOC是多年冻土河流OC通量的主要贡献者。预计未来冰川河流单位流域面积将输出更多的OC通量。

We estimated OC fluxes of the glacier and permafrost-impacted rivers separately as 4.77 and 1.86 g C m−2 yr−1, respectively (Fig. 5). The OC fluxes of glacial rivers are more than twice that of permafrost-dominated rivers, with POC being the dominant contributor to OC fluxes in glacial rivers (DOC:POC ratio of 1:10) while DOC is the major contributor in permafrost rivers (DOC:POC ratio of 5:1). Assuming constant OC concentrations, conservative estimates of OC fluxes are made in conjunction with predictions of future runoff changes in different regions under various emission scenarios. We anticipate that due to the higher projected increase in glacier runoff compared to permafrost runoff, more specific OC fluxes will be exported from glacial rivers in the future.

总结 | Conclusions

本研究首次系统概述了北半球冰冻圈河流OC时空变化规律，并分析了其调控因子和通量特征。研究结果显示冰冻圈河流OC存在很大的时空异质性。冰冻圈的退化将对区域和全球碳循环产生深远影响，未来的研究应加强了解这些影响并预测其对气候变暖的反馈。

This study provides one of the first insights into spatiotemporal variations of riverine OC characteristics across the Northern Hemisphere cryosphere. Overall, this study highlights the complex and variable spatiotemporal patterns of riverine OC. We estimate the specific OC fluxes of glacier rivers are higher and increase faster than those of permafrost rivers in response to future warming. Given the important impact of cryosphere degradation on regional and global carbon cycling, further research is essential to comprehensively understand these impacts and predict their feedback on climate change.

扫二维码 | 查看原文

https://xiumi.us/studio/v5#/paper/for/457432585/cube/0

本文内容来自ELSEVIER旗舰期刊Sci Total Environ第906卷发表的论文：

Wang, Y., Wang, G., Sun, X., Li, J., & Song, C., 2024. Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere. Sci Total Environ 906, 167370

DOI: https://doi.org/10.1016/j.scitotenv.2023.167370

第一作者：王莹 硕士研究生

四川大学水利水电学院

现为四川大学水利水电学院硕士研究生。主要研究方向为寒区河流碳循环和生物地球化学过程。

通讯作者：宋春林 特聘副研究员

四川大学山区河流保护与治理全国重点实验室

四川大学山区河流保护与治理全国重点实验室特聘副研究员，硕士生导师，主要从事寒区水文学和生物地球化学领域的研究工作，研究方向包括河流碳循环、冻土水文学、寒区生态水文和流域生物地球化学，研究兴趣包括高寒山区陆源碳在河流等水体中的迁移转化过程、机制和影响以及冻土变化影响下的寒区流域水文过程和变化机制等。在PNAS、Water Resources Research、Environmental Research Letters、Journal of Hydrology、Science of The Total Environment、Agricultural and Forest Meteorology等刊物发表第一或通讯作者SCI论文10余篇，参编专著2部，获专利6项，主持国家及省部级科研项目多项。曾获中国科学院优秀博士学位论文奖。

近2年发表在Sci Total Environ上的论文：

1. Song et al., 2023. Riverine CO2 variations in permafrost catchments of the Yangtze River source region: Hot spots and hot moments. Sci Total Environ 863, 160948.

2. Sun et al., 2022. Contrasting water sources used by a coniferous forest in the high-altitude, southeastern Tibetan Plateau. Sci Total Environ 849, 157913.

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