黄土高原聚湫近200年沉积物DNA组成及反演生态环境和人地关系

A series of ecological restoration engineering have been carried out on the Chinese Loess Plateau (CLP) during the past 60 years. How to evaluate the ecological restoration effect depends on the understanding of the historical processes and mechanism of the eco-environment change on the CLP to a large extent. Nevertheless, little is known about the historical eco-environment change and its coupling relationships with human activities on the CLP due to the lack of the idea geological carriers and methods for extracting the information of eco-environment and human activities. The sediment sequences within landslide-dammed reservoirs and check dams，which is vertical stacked by deposition couplets induced by individual storms, are one of the idea objects to reconstruct recent eco-environment change and human/environment interactions on the CLP. In this project, the Huabaowan and Laochengzheng landslide-dammed reservoirs in Jingbian and Heshui counties, respectively, which lies in the hilly-gully area and plateau-gully area on the central CLP, respectively, will be selected as research objects. The discrete samples will be collected by equal intervals along the silt surfaces and slopes in the reservoirs catchments. The floristic DNA database, the vegetations types (region and local family, genus and species) by pollen and sedimentary DNA, and modern ecological information database will be built based on the surface sediments in the catchments. Further, the eco-environment change and native natural vegetation during the past 200 years will be reconstructed by sedimentary DNA of the deposition couplets within the landslide-dammed reservoirs. By comparing with a previous reconstruction of contemporary soil erosion and storms frequencies, the coupling relationships among soil erosion, eco-environment, climate and major human activities will be understood during the past 200 years in these two catchments. Moreover, the driven mechanism and regional differences of soil erosion and eco-environment change in the representative landform areas on the CLP will be uncovered. Finally, the theoretical foundation will be provided for evaluating ecological restoration effect and for generating strategies of social economic sustainable development on the CLP.

黄土高原近60年开展了一系列大规模的生态环境修复工程，如何评价其修复效果有赖于历史时期生态环境演变过程和驱动机制的认识。然而，由于缺乏提取生态环境与人类活动信息的有效载体和方法，黄土高原历史时期生态环境演变及其与人类活动的耦合关系所知甚少。以沉积旋回为特征的黄土高原聚湫沉积序列是高分辨率重建小流域历史时期生态环境和人地关系的良好载体。本项目拟选取黄土高原中部丘陵沟壑区靖边县和高塬沟壑区合水县两个聚湫为研究对象，在建立两个流域表土样品DNA基因库、植物种类和生态信息资料库的基础上，基于聚湫沉积旋回的DNA组成重建近200年生态环境演变、自然植被类型；结合同时期土壤侵蚀和暴雨历史的反演，明确两流域近200年土壤侵蚀、生态环境与气候和重大人类活动的耦合关系，揭示黄土高原典型地貌区土壤侵蚀和生态环境演变的驱动机制及区域差异，进而为评价黄土高原生态修复效果和制定社会经济可持续发展策略提供理论依据。

作为世界上典型的生态环境脆弱区和人类活动主要区域，黄土高原的生态演变、传统农业社会发展、人地关系和区域平衡均关系着该区域及整个中国的社会-生态系统稳定和高质量发展。但由于低分辨率的黄土沉积和文献记录的主观性，对生态环境恶化程度和人类活动强度均达历史峰值的黄土高原近200年人地关系演变的认识较为有限，限制了“以史为鉴”地合理评价当代生态治理效果和制定未来管理方向。本项目选取黄土高原特有的聚湫流域沉积序列作为研究载体，以具有确切形成年代的陕西省靖边县花豹湾聚湫和甘肃省合水县老城镇聚湫为研究对象，构建了年际冻融层、137Cs比活度分布、现代降水记录和历史洪涝灾害记录相结合的聚湫沉积序列年代框架的确立方法。两个聚湫流域近200年的土壤侵蚀过程和侵蚀性降雨阈值被厘清，结果表明靖边聚湫流域1855-2014年的年际产沙模数介于0.09×104 t/km2/a和23.40×104 t/km2/a间，合水1811-1996年间的年际产沙模数范围为0.20×104 - 20.61×104 t/km2/a，总体表现为黄土高原在19世纪初到20世纪中叶经历了极为严重的土壤侵蚀，自1960年后有所缓解；两个聚湫流域发生泥沙侵蚀和淤积的降水量阈值分别为30mm和40mm。ENSO事件驱动的东亚季风降水变化是驱动黄土高原土壤侵蚀年际变化的主要影响因子，且在La Nina年份和El Nino次年更易发生特大暴雨和土壤侵蚀。此外，人类活动也在一定程度上影响了各流域的侵蚀强度，但不同类型的人类活动方式导致的地表侵蚀过程有所差异。最后，在国内范围率先构建了流域范围沉积物DNA提取和鉴定方法，全面鉴别了聚湫流域表土DNA的浓度和空间分布。黄土高原近200年人地关系演变的创新性认识为该区域未来的洪涝灾害防治、水土治理、生态修复和土地利用等方面提供了历史依据，也为黄河流域生态保护和社会经济高质量发展奠定了科学基础。