Testing a framework of soil-stream interfaces that expand and contract to affect biogeochemistry in headwater catchments

教科书经常把流动的淡水溪流与土地和土壤之间的界面描绘成一条与溪流平行的狭长地带。该项目用一个新的框架挑战了标准范例，该框架将土壤-河流界面描述为一个动态扩展和收缩的体积，包括近河流区域和周期性饱和的高地地区。新的框架对水质政策和管理具有重要意义。当氮和磷等营养物质从高地浸出并在土壤-溪流界面遇到水饱和的情况时，由植物和土壤微生物介导的一系列反应可以在营养物质进入溪流之前过滤掉它们。在水被过量氮和磷污染的地方，比如这项研究将要进行的切萨皮克湾盆地，各州和市政府经常通过资助沿河岸种植数百英里的森林来履行执行《清洁水法》的法律义务。这些近溪流森林（通常被称为河岸缓冲带）有望过滤营养物质并改善水质。然而，如果像本项目所断言的那样，土壤-溪流界面和相关的生态过滤扩展到高地土壤，那么仅仅关注河岸的森林种植可能不足以改善水质和管理国家的淡水资源。在测试新的土地/河流概念模型的同时，该项目还将吸引大学预科生、本科生和研究生，包括那些来自代表性不足群体的学生参与研究。这个项目利用了页岩山天文台的长期研究和模型开发历史，页岩山天文台是宾夕法尼亚州中部的一个小集水区。按照传统的定义，页岩山的任何部分都不是在土壤-溪流的界面上，但每年春天，谷底的土壤都是饱和的，一股短暂的溪流会以不同的时间流动到夏天。页岩山有明显的沟壑——汇聚的水流路径切割成平坦的斜坡——也周期性地饱和。新的实地测量将量化这些谷底和沼泽土壤何时通过厌氧营养循环与集水口相连接，这是土壤-溪流界面的特征。土壤湿度、土壤气体、土壤孔隙水化学和地下水化学将在整个流域的关键地点取样。在整个流域尺度上，溶解气体和离子的地表径流输出和涡动相关塔将测量水、能量和碳通量。现场数据将与一个新的空间分布、耦合、水文-反应性运输-生态系统生物地球化学模型进行同化，该模型可以模拟膨胀-收缩土壤-溪流界面的瞬态饱和生物地球化学。综合现场测量和模型结果将验证以下假设：强调土壤-河流界面的时间（假设1）和空间（假设2）变化，动态土壤-河流界面对生态系统碳和氮库和通量的影响（假设3），以及用新的耦合模型模拟动态土壤-河流界面的能力（假设4）。该项目还将扩大参与范围，并为大学预科生、本科生和研究生提供研究培训。该项目由BIO/DEB的生态系统科学集群和GEO/EAR的水文科学项目共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Textbooks often depict the interface between a flowing freshwater stream and the land and soil over and through which it flows as a narrow band running parallel to the stream. This project challenges the standard paradigm with a new framework that depicts the soil-stream interface as a dynamic expanding and contracting volume encompassing near-stream zones and periodically saturated upland areas. The new framework has important implications for water quality policy and management. As nutrients like nitrogen and phosphorus leach from uplands and encounter water-saturated conditions at the soil-stream interface, a suite of reactions mediated by plants and soil microorganisms can filter out nutrients before they enter the stream. In locations where water is polluted by excess nitrogen and phosphorus, such as the Chesapeake Bay Basin where this research will take place, states and municipalities often meet their legal obligations to Clean Water Act enforcement, in part, by funding the planting of hundreds of miles of forest along streambanks. These near-stream forests (often called riparian buffers) are expected to filter nutrients and improve water quality. Yet, if the soil-stream interface and associated ecological filtering expand into upland soils as this project asserts, then forest plantings focused solely on streambanks may be an inadequate approach to improving water quality and managing the Nation's freshwater resources. While testing a new land/stream conceptual model the project will also engage pre-college, undergraduate and graduate students, including those from underrepresented groups, in the research.This project leverages a long history of research and model development at the Shale Hills Observatory, a small catchment in central Pennsylvania. No part of Shale Hills is at the soil-stream interface by the conventional definition, but each spring, soils in the valley floor are water-saturated and an ephemeral stream flows for a variable duration into summer. Shale Hills has distinct swales - convergent flow paths that cut into otherwise planar slopes – that are also periodically saturated. New field measurements will quantify when these valley floor and swale soils are hydrologically connected to the catchment outlet with anaerobic nutrient cycling that is characteristic of soil-stream interfaces. Soil moisture, soil gases, soil pore water chemistry, and groundwater chemistry will be sampled at key locations throughout the catchment. At the whole-catchment scale, surface runoff export of dissolved gases and ions and an eddy covariance tower will measure water, energy, and carbon fluxes. Field data will be assimilated with a new spatially distributed, coupled, hydrology-reactive transport-ecosystem biogeochemistry model that can simulate the transient saturated biogeochemistry of an expanding-contracting soil-stream interface. Integrated field measurements and model results will test hypotheses that emphasize temporal (hypothesis 1) and spatial (hypothesis 2) variation in the soil-stream interface, the impacts of a dynamic soil-stream interface on ecosystem carbon and nitrogen pools and fluxes (hypothesis 3), and the ability to simulate dynamic soil-stream interfaces with the new coupled model (hypothesis 4). The project will also broaden participation and include research training for pre-college, undergraduate and graduate students.This project is co-funded by the Ecosystem Science Cluster in BIO/DEB and the Hydrologic Sciences program in GEO/EAR.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.