Characteristics of Dissolved Organic Matter in Alpine Mountain Soils and Its Effect on Riverine Dissolved Organic Matter Export
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摘要: 冻土区土壤中存储有大量有机碳.目前对高寒山区多年冻土区和季节冻土区土壤有机质特征及其差异研究较少，对土壤中溶解性有机质（dissolved organic matter，DOM）特征及其对河水中DOM输出的影响认识尚不明确.为了解高寒山区土壤中溶解性有机质的分布规律、成分特征及对水体中DOM输出特征的控制作用，本研究采集青藏高原东北部葫芦沟小流域中多年冻土区和季节冻土区不同深度（< 1 m）土壤样品，对土壤中总有机碳（soil organic carbon，SOC）和溶解性有机碳（dissolved organic carbon，DOC）含量、DOM的光谱特征、DOC的生物可降解性（biodegradable dissolved organic matter，BDOC）进行分析，并将其与不同水体中DOM特征的季节性变化进行对比.研究发现：多年冻土区与季节冻土区土壤在DOC的生物可降解性及微生物活动方面存在明显差异；多年冻土区土壤中SOC含量较高，但DOC含量较低，DOM的腐殖化程度和芳香性低于季节冻土区；季节冻土区的土壤中BDOC占比高于多年冻土.研究表明：高寒山区土壤水文特性对土壤有机质含量和特征的显著影响，其中土壤含水率是重要影响因素；多年冻土区浅层土壤DOM对河水DOC浓度和成分变化起决定性作用；相比之下，季节冻土区土壤对河水DOC浓度和成分变化直接影响较小，水文条件影响着水体中DOM的输出特征.本研究成果对高寒山区冻土退化条件下的碳循环研究具有指导意义.Abstract: The soil layers in permafrost regions store a large amount of organic carbon. However, the understanding of the influence of dissolved organic matter (DOM) in permafrost and seasonally frozen ground on the DOM characteristics of riverine output is still unclear due to limited existing studies on the characteristics of soil organic matter in permafrost and seasonally frozen ground and their differences in alpine catchments. To understand the distributions, and controlling mechanisms of dissolved organic matter in the soil of alpine catchments on aquatic DOM, this study collected soil samples of permafrost and seasonally frozen ground in the Hulugou catchment in the northeastern part of the Tibetan plateau and analyzed the soil organic carbon (SOC), dissolved organic carbon (DOC) content, spectral characteristics of DOM, and its biodegradable dissolved organic carbon (BDOC). Then, DOM characteristics in soils were compared with those from different water bodies at different seasons. The study reveals significant differences between permafrost and seasonally frozen soils in terms of dissolved organic carbon (DOC) biodegradability and microbial activity. Permafrost soils have higher soil organic carbon (SOC) but lower DOC, and their dissolved organic matter (DOM) is less humified and aromatic compared to seasonally frozen soils, where biodegradable DOC (BDOC) proportions are higher. The findings indicate that soil hydrological traits in cold mountain areas significantly influence soil organic matter, highlighting soil moisture as a critical factor. In permafrost areas, shallow soil DOM crucially affects river water DOC concentrations and composition. In contrast, seasonal permafrost zone soils have less direct influence on changes in stream DOC concentration and composition, with hydrological conditions shaping DOM's output features in water bodies. This research is crucial for understanding carbon cycling under permafrost degradation in cold mountain regions, condensed into a comprehensive summary within the specified word limit.
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图 1 葫芦沟在黑河上游的位置(a)和葫芦沟流域的地形地貌和采集点分布(b)
Fig. 1. The position of the Hulugou catchment within the Heihe River basin (a) and the extent of the Hulugou catchment, soil sampling points, and vegetation types in the Hulugou catchment (b)
图 2 葫芦沟流域内7个采样点土壤的容重、重量含水量、SOC含量、DOC含量随深度的变化
Fig. 2. Changes in soil bulk density (BD), soil water content (SWC), SOC and DOC with depth at seven sampling points within the Hulugou catchment watershed
图 3 葫芦沟流域35个土壤样品浸提液中DOM的6个主要荧光组分
Fig. 3. Six major fluorescent components of DOM in the leachate of 35 soil samples from the Hulugou catchment
图 4 葫芦沟流域内7个采样点上土壤浸提液DOM中6个主要荧光组分的占比(a、b、c代表不同样点间的显著性差异，具有相同的字母符号代表组间无显著性差异，显著性水平p < 0.05)
Fig. 4. The proportions of the six main fluorescence components of DOM in the soil leachate from seven sampling points within the Hulugou catchment
图 5 葫芦沟流域内7个采样点上土壤浸提液中DOM光谱特征参数随深度的变化
Fig. 5. Changes in the spectral characteristic parameters of DOM in the soil leachate with depth at seven sampling points within the Hulugou catchment
图 6 葫芦沟流域7个采样点上土壤浸提液中BDOC占比随深度的变化(a)以及BDOC占比与C2组分(b)、C6组分(c)变化量的线性拟合
Fig. 6. Changes in the proportion of BDOC in the soil leachate with depth at seven sampling points in the Hulugou catchment watershed (a), linear fitting of BDOC with changes in components C2 (b) and C6 (c)
图 7 流域内不同水体中DOC浓度和DOM的各特征光谱参数在不同冻融时期的变化特征
Fig. 7. Variation characteristics of DOC concentration and DOM optical indices in different water bodies within the watershed during different freeze-thaw periods
图 8 环境因子与DOM成分特征BDOC的冗余分析(RDA) (a)和BDOC与环境因子、DOM成分特征的相关性热度图及Mantel检验(b)
Fig. 8. Redundancy analysis (RDA) of environmental factors and DOM component characteristics BDOC (a) and correlation heat map and Mantel test of BDOC correlation with environmental factors and DOM component characteristics (b)
表 1 土壤采样点位信息
Table 1. Information on soil sampling locations
采样点编号 海拔高度（m） 采样深度(cm) 冻土类型 S01 2 990 80 季节冻土 S02 3 140 80 S03 3 340 80 多年冻土 S04 3 465 100 S05 3 540 100 S06 3 655 40 S07 3 950 60 
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表 2 光谱特征参数的计算公式和指示意义
Table 2. Basic information of spectral characteristic parameters
光谱参数 计算公式 参数意义 SUVA254 $ \frac{\mathrm{a}\mathrm{b}{\mathrm{s}}_{254\times 100}}{{C}_{\mathrm{D}\mathrm{O}\mathrm{C}}} $ 表征DOM中芳香性化合物含量，与DOM芳香化程度呈正相关关系. SR S275-295/S350-400 反映有机质来源与类型，包括分子量大小、光漂白活性. HIX λEx=254 nm
ΣEm435-480/ΣEm300-445表征DOM的腐殖化程度，值越高表示DOM腐殖化程度越高. FI λEx=370 nm
Em470 nm/Em520 nm表征芳香族有机质和非芳香族有机质对DOM荧光强度的相对贡献. BIX λEx=310 nm
Em380 nm/Em430 nm反映DOM的生物内源和外源输入的相对贡献程度. 注：abs254代表紫外可见光谱在波长为254 nm时的值；S275-295代表紫外可见光谱在波长为275~295 nm波长范围的光谱斜率系数；Ex代表激发波长；Em代表发射波长.
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表 3 葫芦沟流域35个土壤样品浸提液中DOM的6个主要荧光组分的特征
Table 3. Characteristics of the six major fluorescent components of DOM in the leachate of 35 soil samples of Hulugou catchment
组分 激发波长最大值(nm) 发射波长最大值(nm) 对应有机质特征 可能来源 C1 250(300) 420 来自陆地水生环境的微生物衍生的腐殖质样组分 陆生植物和土壤有机质；微生物过程 C2 < 250(300) 425 陆地来源腐殖质 陆生植物和土壤有机质 C3 220 550 陆地来源的腐殖质样组分或芳香族共轭大分子物质 陆生植物和土壤有机质 C4 265(370) 465 来自陆地水生环境腐殖质样组分 陆地植物和土壤有机质 C5 265(450) 495 陆地来源腐殖质；较大分子量和较强的芳香性 陆生植物和土壤有机质 C6 275 345 类蛋白质 色氨酸样；微生物来源
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