Citation:

Lin, J., J. Compton, C. Clark, S. Bittman, D. Schwede, P. Homann, P. Kiffney, D. Hooper, G. Bahr, AND J. Baron. Key components and contrasts in the nitrogen budget across a US-Canadian transboundary watershed. Journal of Geophysical Research: Biogeosciences. American Geophysical Union, Washington, DC, 125(9):e2019JG005577, (2020). https://doi.org/10.1029/2019JG005577

Impact/Purpose:

The Nooksack Basin in Whatcom County, WA falls within a “very high” nitrate priority region according to the Washington Dept. of Ecology. Groundwater nitrate levels exceed the EPA recommended maximum contaminant level of 10 mg N/L in much of the region including the Nooksack-Sumas Aquifer, a drinking water source with connection to surface water. Contributing sources to groundwater vary from crop and animal agriculture, septic systems, and natural inputs, but levels are not well characterized for surface waters. The goals of the overall Nooksack-Fraser Transboundary Nitrogen (NFT-N) project are to 1) create a N inventory, 2) bring together stakeholders to review this inventory, and 3) work with stakeholders to develop a menu of strategies to protect local food production, the economy, and natural resources. Developing a common set of biophysical data and viewing them from multiple social and economic perspectives can help everyone to understand the problems and to identify preferred potential solutions to N-related issues. Our intent is to provide scientifically sound, objective information that can be used by local stakeholders to identify common regional goals and practical, comprehensive, and sustainable solutions where everyone, including people and their livelihoods, and the environment, benefits. This information will be incorporated into several existing stakeholder efforts to reduce stream, groundwater and coastal nitrogen-related pollution.

Description:

Watershed nitrogen (N) budgets provide insights into drivers and solutions for groundwater and surface water N contamination. We constructed a comprehensive N budget for the transboundary Nooksack River Watershed (BC, Canada and WA, US) using locally-derived data, national statistics and standard parameters. Feed imports for dairy (mainly in the US) and poultry (mainly in Canada) accounted for 30 and 29% of the total N input to the watershed, respectively. Synthetic fertilizer was the next largest source in the watershed contributing 21% of inputs. Food imports for humans and pets together accounted for 9% of total inputs, slightly lower than atmospheric deposition (10%). Returning salmon represented <0.06% of total N input but was an important ecological flux by importing marine-derived nutrients. Quantified N export was 80% of total N input, driven by ammonia emission (32% of exports). Animal product export was the second largest output of N (31%) as milk and cattle in the US and poultry products in Canada. Riverine export of N was estimated 28% of total N export. The commonly used crop nitrogen use efficiency (crop NUE) alone did not provide sufficient information on farming activities and should be combined with other criteria such as farm-gate NUE to understand management efficiency. Agriculture was the primary driver of N inputs to the environment despite widespread adoption of conservation practices, illustrating a need to optimize management to minimize hydrologic and volatilization losses. The N budget provides key information for stakeholders across sectors and borders to create environmentally and economically viable and effective solutions.

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