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

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.