Where Have All the Nutrients Gone? Long-Term Decoupling of Inputs and Outputs in the Willamette River Watershed, Oregon, United States

Better documentation and understanding of long-term temporal dynamics of nitrogen (N) and phosphorus (P) in watersheds is necessary to support effective water quality management, in part because studies have identified time lags between terrestrial nutrient balances and water quality. We present annual time series data from 1969 to 2012 for terrestrial N and P sources and monthly data from 1972 to 2013 for river N and P for the Willamette River Basin, Oregon, United States. Inputs to the watershed increased by factors of 3 for N and 1.2 for P. Synthetic fertilizer inputs increased in total and relative importance over time, while sewage inputs decreased. For N, increased fertilizer application was not matched by a proportionate increase in crop harvest; N use efficiency decreased from 69% to 38%. P use efficiency increased from 52% to 67%. As nutrient inputs to terrestrial systems increased, river concentrations and loads of total N, total P, and dissolved inorganic P decreased, and annual nutrient loads were strongly related to discharge. The N:P ratio of both sewage and fertilizer doubled over time but there was no similar trend in riverine export; river N:P concentrations declined dramatically during storms. River nutrient export over time was related to hydrology and waste discharge, with relatively little influence of watershed balances, suggesting that accumulation within soils or groundwater over time is mediating watershed export. Simply managing yearly nutrient balances is unlikely to improve water quality; rather, many factors must be considered, including soil and groundwater storage capacity, and gaseous loss pathways.

Impact/Purpose

Watershed nitrogen and phosphorus budgets are increasingly being used by decision-makers to assess and manage nutrient impacts on water quality. Many of these budgets are constructed for one year, independently examining the inputs and fates over that year. But emerging research is determining that nutrients can be retained within watersheds, for example stored in soils or groundwater, and accumulate within the system in ways that may affect surface water and groundwater quality. EPA scientist Jana Compton, along with current and former National Research Council post-docs and faculty from Washington State University examined a 40+ year record of N and P inputs and hydrologic export in the Willamette River Basin of Oregon. They found that there was indeed a disconnect between annual inputs and annual export, with a net loss of N from the watershed in the 1970s and 1980s, now moving into an accumulation of N and P within the basin. This eventual net accumulation may mean that current N inputs will accumulate within the watershed in soils and groundwater, and the watershed may just be reaching a tipping point where additional N and P inputs may begin to increase stream N and P export or affect water quality. If this is the case, continued improvements in wastewater treatment, and effective agricultural nutrient management to reduce nutrient surpluses on the landscape will be essential to avoid passing this point as it may be difficult to improve water quality quickly after this point has been passed.

Citation

Metson, G., J. Lin, J. Harrison, AND J. Compton. Where Have All the Nutrients Gone? Long-Term Decoupling of Inputs and Outputs in the Willamette River Watershed, Oregon, United States. American Geophysical Union, Washington, DC, 125(10):e2020JG005792., (2020). [DOI: 10.1029/2020JG005792]

DOI: Where Have All the Nutrients Gone? Long-Term Decoupling of Inputs and Outputs in the Willamette River Watershed, Oregon, United States