高寒山区冻土水文过程及其对河流溶解性有机碳输出的作用机制

Dissolved organic carbon (DOC) fluxes in streams and rivers draining permafrost dominated areas contributes significantly to the global carbon cycle. Spatial and temporal patterns of DOC export in streams from mid-low latitude alpine areas and the effect of permafrost and hydrology on these patterns, which are expected to be different from those in boreal areas, has been poorly studied. The Hulugou catchment, an alpine catchment in headwater regions of the Heihe River, is selected as the studies area. Field survey, drilling and sediment investigation, and hydrological observation will be conducted in the catchment. In addition, water samples from stream, glacier meltwater, snowmelt, rainfall, soil, and aquifers will be collected, and then be analyzed for isotopic and hydrochemical composition, DOC species and optical properties of organic matte in laboratory. Spatial and temporal variation of DOC export in streams will be explored using the chemical and hydrometric data. Two and three-component hydrograph separations will be employed to identify the spatial and temporal sources of streamflow using the data of stable water isotopes and dissolved silica. PHREEQC hydrogeochemical model and end-member mixing diagram will be employed to trace flow paths in the catchments based on the hydrochemical data. The main emphasis of this research is on the role of permafrost hydrology in the origin, transport, geochemical and biogeochemical reaction of DOC within catchment and its export to the steams. Ultimately, the response of DOC exports to permafrost degradation will be predicted based on the above research. Result from this research is expected to improve our understanding of carbon cycle in alpine catchments, and consequently provide the theoretical basis for predicting responses of carbon cycle to future climate change.

多年冻土区河流中溶解性有机碳（DOC）的输出对全球碳循环有着重要贡献。中、低纬度高寒山区的河流DOC输出及控制其时空特征的流域水文过程可能有别于泛北极地区，但却很少有人研究。其研究的关键和难点在于冻土影响下的水文过程的精确刻画。本申请项目拟以黑河上游祁连山葫芦沟小流域为研究区，基于河流中DOC的浓度、组分和光学特征的动态观测，揭示高寒冻土区河流DOC输出的时空规律；联合应用同位素和水化学示踪、水文观测、水文地质钻探、水文地球化学模拟等方法，定量分割河道径流的水分来源，识别冻土影响下的水流路径，探析不同水流路径上DOC的迁移转化规律，进而揭示冻土水文过程对河流DOC输出的作用机制，预测多年冻土退化对河流DOC输出的影响，为高寒冻土区碳循环及其对气候变化的响应研究提供依据，为流域水文−生物地球化学耦合模型的构建奠定基础。

为揭示高寒山区河流DOC输出规律，探索冻土水文过程对其作用机制，以祁连山多年冻土–季节性冻土复合型流域——葫芦沟为研究区，应用水化学示踪、水文观测、水文地球化学模拟、室内实验等方法，开展了冻土水文过程、DOM来源、水文过程对DOM迁移转化及输出的作用机制、季节性冻土流域水文过程对DOC输出的作用机制等研究，形成如下认识：（1）地下水在径流调节中发挥重要作用：多年冻土层上水对山区产流有主要贡献，而山前孔隙地下水不仅维持着冷季时的基流，也是暖季时河道径流的最高贡献者；冻土分布及动态控制着地下水-地表水的转化，进而影响地下水的补径排特征。（2）土壤有机质含量高于青藏高原平均值，DOC生物可降解性与土壤含水率呈正相关，DOM以植物来源的腐殖质和类腐殖质组分为主，不易被微生物降解。（3）流域出口DOM输出特征与径流量高度相关，高流量期间陆源DOM比例高，DOC浓度低但通量大，低流量期间微生物来源DOM比例大，DOC浓度高但通量低。（4）冻融过程控制着多年冻土区的水流路径和DOM特征：冻土层上水在消融初期被限制在有机层内，导致河水中DOM芳香性强、蛋白质类组分少，夏季时活动层变厚使流动路径加深，DOM芳香性变弱，腐殖化程度增高；山前平原区“河水-地下水-河水”转化过程控制着季节性冻土区DOM输出特征，将来自多年冻土区的大量陆源DOM转化为类蛋白类组分，DOM降解率提升。（5）在以红泥沟为代表的高寒山区季节性冻土流域，DOC输出特征也与径流量高度相关，低流量期浓度高但通量低，高流量期浓度低但通量高；在高流量期间，控制水文过程进而影响DOC输出的机制并非冻土特征及动态，而是特殊的水文地质和地貌条件：薄层细粒残坡积物导水能力差，使降雨汇流路径以坡面流为主，稀释效应显著；广泛发育的冻融扰动地貌则使有机质层暴露，为DOC提供了来源。在春季消融期，冻土特征则是主控因素：过剩冰融水是径流的重要水源；冻土将汇流路径限制在浅部有机层；土壤冻结排盐使溶质向地表聚积，为DOC提供了额外来源。