Key Components and Contrasts in the Nitrogen Budget Across a U.S.‐Canadian Transboundary Watershed

Abstract

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, USA) using locally derived data, national statistics, and standard parameters. Feed imports for dairy (mainly in the United States) 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 United States and poultry products in Canada. Riverine export of N was estimated at 28% of total N export. 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.

Long‐Term Disconnect Between Nutrient Inputs and Riverine Exports in a Semi‐Arid, Agricultural Watershed: Yakima River Basin 1945–2012

“…Perhaps a portion of manure produced by dairies is exported from the watershed or, in the case of N, losses from NH 3 volatilization could be larger than 20%. A recent N budget in the Nooksack River Basin found that 32% of total N losses were due to volatilization; three quarters of which were attributed to dairy manure alone (Lin et al., 2020). The growing dairy industry within the lower Yakima valley has led to major efforts to manage nutrients, such as the passage of the Dairy Nutrient Management Act, 1998 (Washington State) (RCW 90.64).…”

“…Similar to Lin et al. (2020), we assumed these volatilization losses happened during CAFO housing and the remainder was applied to fields. We did not account for manure exports from the basin but rather assumed all CAFO waste was either applied within watershed boundaries or lost via volatilization.…”

“…We assumed 20% of annual manure N inputs were lost via NH 3 volatilization (Figure 2) based on USDA National Resources Conservation Sources (NCRS) estimates for western Washington (USDA, 1998). Similar to Lin et al (2020), we assumed these volatilization losses happened during CAFO housing and the remainder was applied to fields. We did not account for manure exports from the basin but rather assumed all CAFO waste was either applied within watershed boundaries or lost via volatilization.…”

“…Long-term watershed dynamics of N and P in agricultural regions with mediterranean to semi-arid climates (characteristic of the western U.S.) are not well-understood. There have been localized budget efforts for mediterranean basins in the Pacific Northwest (PNW) and Central Valley in California, which focused on seasonal N dynamics in the early to mid-2000s (Compton et al, 2019;Johnson et al, 2013;Lin et al, 2019Lin et al, , 2020Sobota et al, 2009). Only one of these recent studies evaluated a coupled, multidecadal N and P input-output budget; this was a study of the Mediterranean-like Willamette river basin, located in western Oregon (Metson et al, 2020).…”