Key components and contrasts in the nitrogen budget across a US-Canadian transboundary watershed & links to Puget Sound

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 (British Columbia, Canada and Washington, 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 contributing 21% of inputs. Food imports for humans and pets together accounted for 9% of total inputs, lower than atmospheric deposition (10%). N imported by returning salmon representing marine derived nutrients accounted for <0.06 % of total N input. 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 at 28% of total N export. While the climate and physiography are similar between the US and Canada in the NRW, the different sides of the border provide contrasts in N management and use efficiency: Crop NUE was higher on the US side of the watershed, but both the farm-gate and commercial whole-farm NUEs were higher in Canada-NRW. These differences were driven by the types of animals raised, manure management regulations and reporting, and farm economics. As might be expected, different policy frameworks had a large impact on key components of nutrient management in different portions of the watershed. The commonly used crop nitrogen use efficiency (NUE) metric alone did not provide sufficient information on farming activities but in combination with other criteria such as farm-gate NUE may better represent management efficiency. Agriculture was the primary driver of N inputs to the environment as a result of its regional importance; the N budget information can inform management to minimize N losses. The N budget provides key information for stakeholders across sectors and borders to create environmentally and economically viable and effective solutions.

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. This presentation will be given to a local interest group on nutrient management, and is based on a paper that was recently published in the Journal of Geophysical Research Biogeosciences.

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 &amp; links to Puget Sound. Pacific and Mountain West Nutrient Cycling, Soil Health and Food Safety Virtual Conference, Corvallis, OR, October 27 - 28, 2020.