湿热流域源区坡面径流及壤中流驱动的碳的生物地球化学循环

In the humid subtropical headwater catchments, those factors including a larger storage of soil organic carbon per unit area, a weaker intensity of water erosion and a lesser disturbance by human activities together had resulted in the significant difference between the headwater catchments and the lower reaches and estuary of a river in the biogeochemical processes of soil organic carbon carried by the flowing water. The applicant had committed to the study on the biogeochemical cycle of riverine carbon in the main-stem and estuaries in the Pearl River drainage basin. In this study, we would jointly use the methods of geomorphology, hydrology and biogeochemistry, and choose 4 slopes in the headwater catchments located in the two fixed observational stations in South China, to collect and analyze periodically the aquatic carbon contents and their isotopic compositions, and the aquatic carbonate system in the slope runoff and the subsurface flow of the soil, referring to the compositions of organic matter and minerals in the soil profiles, to investigate the biogeochemical changes of the eroded soil matter under a weaker water erosion environment of the headwater catchments, and to investigate the formation mechanism of recalcitrant carbon in chemistry, and the lateral transportation of CO2 in the subsurface flow. Those study results could provide the input information for further understanding of the evolution of terrestrial organic matter in rivers and in sedimentary basins, and would have an important scientific significance for elucidating the coupling relationship of geomorphic, hydrological and biogeochemical processes in the humid subtropical headwater catchments in global carbon cycle.

湿热地区流域源区单位面积的土壤有机碳储量较大、流水侵蚀强度较弱、人类活动的扰动较小，这些因素致使源区流水中土壤有机碳所经历的生物地球化学过程与下游河段及河口区差别明显。申请人曾长期致力于珠江流域干流与河口区碳的生物地球化学循环研究。在本项研究中，我们将地貌学、水文学与生物地球化学的研究方法交叉融合，在华南的两个定位观测站选取4处流域源区坡面作为调查对象，通过周期性采样分析源区坡面径流及壤中流中各种形态碳的含量和同位素组成及水体碳酸盐系统，参照源区土壤剖面中的有机质和矿物组成，探讨源区微弱流水侵蚀环境下产出的土壤物质在流水中经历的生物地球化学变化及化学稳态碳的形成机制，以及源区壤中流对CO2的侧向输运作用。研究结果可为进一步了解陆源有机质在河流及沉积盆地中的演化提供输入端数据，并对揭示湿热流域源区的地貌过程、水文过程与生物地球化学过程在全球碳循环中的耦合关系具有重要的科学意义。

为探讨源区弱侵蚀环境下土壤及溪流中的生物地球化学过程及碳的侧向输运作用，本项目在鼎湖山森林源区开展了4年的观测研究，得到如下结果。.鼎湖山源区溪流、土壤水和地下水呈弱酸性（pH均值5.33±1.23）。土壤水中K+、Ca2+、NO3-和SO42-含量显著高于地下水和溪流；而地下水的Na+、Mg2+、Cl-、DSi含量显著高于土壤水和地下水。硅酸盐风化和大气沉降是溪流阳离子的主要贡献者。溪流SO42 -含量与雨水相近，而NO3-含量远高于雨水。溪流中Ca2+、Na+、Mg2+、Cl-、SO42-、NH4+含量向下游渐增，而NO3-含量渐减。溪流DIC含量向下游渐增。地下水和溪流的DIC以游离CO2为主，这些CO2源于承压水的注入。溪流下游水体pH值升高，HCO3-成为DIC主体，游离CO2更多来自水体有机质降解。出露的地下水因减压和湍流效应，向大气扩散CO2，从而降低了水体的DIC含量。下游溪流中OC分解提高了水体DIC含量。夏秋季溪流和地下水的DIC响应径流的稀释效应，含量低于冬春季节。土壤水DOC含量显著高于溪流和地下水。夏秋季溪流和地下水DOC响应降水对土壤OC的冲刷效应，含量高于冬春季。旅游开发对溪流DOC含量的影响较小；源区溪流微生物对有机质的分解作用较弱。源区溪流DOC输出通量约为2.16 kg/km2•d，雨季高于旱季。地下水/溪流游离CO2的稳定碳同位素组成（δ13C）约为-24.05‰/-21.69‰，表明游离CO2主要来自土壤有机碳的降解。δ13CDIC (-28.01~-11.36‰)与水体TDS、HCO3-、pH值相关性显著，表明DIC受岩石风化和人类活动的叠加影响。H2SO4和HNO3在鼎湖山岩石风化中起主要作用，上游水体DIC主要来自土壤CO2，δ13CDIC值偏低，弱酸性径流极大影响了DIC的组分，抑制了土壤CO2对硅酸盐的风化；下游河段地层中含有碳酸盐，其风化对HCO3-贡献明显，δ13CDIC值增大。溪流CO2、N2O、CH4扩散过程受地形影响存在较大空间变异性，通量分别为335.6~6884.2 mmol m-2 d-1、-8.5~114.0 mmol m-2 d-1、-1.7~3.1 mmol m-2 d-1。.上述结果对揭示湿热流域源区的地貌过程、水文过程与生物地球化学过程在全球碳循环中的耦合关系具有重要参考价值。