Nutrient Legacies and Time Lags: Understanding Catchment Biogeochemical Responses in Anthropogenic Landscapes

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Excess nutrients associated with high levels of fertilizer application and concentrated livestock operations find their way into both ground and surface waters, leading to problems of hypoxia, aquatic toxicity and drinking water contamination. Reductions in fertilizer application or implementation of other best management practices (BMPs) have been carried out to improve water quality, but often they do not yield expected results. Such lack of response is attributed to buildups of legacy stores that can cause significant time lags between BMP implementation and water quality improvements. Understanding how the accumulation and depletion of anthropogenic legacies alter water quality is the focus of my research program. The research not only addresses a critical gap in global nutrient budgets, but also helps to create a practical framework for setting realistic, time-specific goals for water quality management. ***The Discovery research will build on my recent work, which provides new evidence of organic nitrogen (N) accumulation in agricultural soils and raises important questions regarding the role of N legacy in catchment-scale N dynamics: (1) What mechanisms control legacy N accumulation and depletion within agricultural landscapes? (2) How do such legacies contribute to time lags between changes in management practices and improvements in water quality? ***In the proposed work, a model will be developed to explore hypotheses regarding soil carbon-nitrogen dynamics and to clarify controls on N accumulation and depletion. The model will be validated using both existing datasets and project-specific fieldwork. In collaborative work at the Harrow Agricultural Research Station, depth-dependent accumulation of legacy N will be determined under a range of fertilizer treatments, and isotope analysis will be used for source detection of nitrate in drainage water. In addition, field studies will be carried out across an identified chronosequence to determine N concentration trajectories after agricultural land retirement. The vadose zone legacy model thus developed will be used in a spatially explicit framework to quantify time lags at a range of scales, from plot to field to catchment, for watersheds both within and outside Canada. This will lay the groundwork for a more global-scale analysis of the role of legacy-related time lags in nutrient loading to coastal waters. *** Research funded through the Discovery will increase fundamental understanding of N legacies in anthropogenic landscapes and their impacts on water quality. The project will leverage existing university strengths in water and ecohydrology and contribute positively to Canada's economy by training of top HQP in the water and environmental sector. Research results will contribute significantly to the field of watershed-scale nutrient transport, thus allowing for more effective management of Canadian Water Resources.***********