Nitrogen budget of a forest-urban-agricultural Canada-USA transboundary watershed

Quantification of nitrogen (N) fluxes at the watershed scale can inform efforts to understand and sustainably manage the drivers of nitrogen release to the environment and their impact on air and water quality. We constructed a comprehensive N budget for the transboundary Nooksack River Watershed (NRW; BC, Canada and WA, USA) using locally-derived data, national statistics and standard parameters. We quantified N fluxes in the three main sectors: urban, fishery, and agriculture.

Results showed that feed imports for dairy (mainly in the US) and poultry (mainly in Canada) accounted for 29 and 28% of the total N input to the watershed, respectively, and the differences are driven by contrasting economic factors in the two countries. Nitrogen imports as synthetic fertilizer (2200 tonnes N) was the next largest source, representing 21% of the total N input. Food import for humans and pets accounted for 9.4 % of total inputs, slightly lower than atmospheric deposition, which contributed 10%. Returned salmon represented only 0.056% of total N input but is important for the natural part of the ecosystem.

N export was 80% of total N input. Ammonia emission into the atmosphere was the largest estimated output of N from the watershed (32% of total export). Export of animal products was the second largest flux of N out of the watershed (31%): milk and cattle in the USA and poultry products in Canada. We estimated crop N use efficiency in US-NFT dairy system as 74% for applied manure and fertilizer N. Riverine export of N was estimated 28% of total N export.

As the dominant activity, agriculture was the primary source of N released to the environment, despite widespread adoption of conservation practices such as cover cropping. There are possibilities for improvements in all sectors such as reducing food waste, which exceeds N fluxes in sewage and septic. The N budget results allow us to link activities across sectors and national boundaries help create environmentally and economically viable and effective solutions.