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

Abstract

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. Plain Language Summary Too much nitrogen and phosphorus in fresh and coastal waters can cause problems for human, animal, and ecosystem health. For this reason, many entities have been working to reduce the amount of both nutrients entering waterways. In this study, we examine how sources of nitrogen and phosphorus on land have changed over 40+ years in the Willamette River Basin, Oregon, United States, and then compare these changes to water quality trends in the river. We found that sources of both nutrients increased over time, especially nitrogen fertilizer. Between 1969 and 2012, wastewater treatment was more effective at removing phosphorus than nitrogen. Changes in fertilizer use and sewage treatment have caused the ratio of nitrogen to phosphorus to increase over time. In contrast, the amount of nitrogen and phosphorus in the river has decreased, and there was no drastic change in the water nutrient ratio over time. The disconnect between nutrient sources on land and what is observed in the river over time suggests that a large part of the nitrogen and phosphorus used on land is being transferred to the soil, air, or groundwater. Simply managing yearly nutrient balances is unlikely to improve water quality; rather, many factors must be considered.

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

“…Incoming net nutrient inputs in the YRB appear somewhat delayed and smaller in magnitude when compared to basins such as the Mississippi or Chesapeake Bay at the whole watershed scale (Van Meter et al., 2017). Temporal trends in nutrient inputs are similar to those reported for the Willamette River Basin (Metson et al., 2020), another major tributary of the Columbia River.…”

“…The YRB illustrates how watersheds in semi-arid climates may have a decoupled relationship between nutrient inputs to land and surface water exports. Other recent budgeting efforts in watersheds with Mediterranean climates have revealed similar disconnects; with lower fractional exports and strong correlation between seasonality and nutrient losses (Compton et al, 2019;Lin et al, 2019;Sobota et al, 2009;Metson et al, 2020). In the YRB, nutrient retention in groundwater and N lost via denitrification can explain said disconnect, but the relative importance of these (and other) fates remains uncertain.…”

“…Agricultural balance was determined as the annual difference between total inputs (fertilizer, manure, and (for N) cultivated BNF, and N deposition) and outputs (crop harvest) on all pasture and cropland in the basin. We followed an approach similar to Metson et al (2020) where net agricultural balance (J Ag-net ) was defined as:…”

“…The fraction of N exported by the Yakima river was similar to export efficiencies reported for other watersheds in the western US, but the fraction of P exported was high relative to those same basins. For example, Metson et al (2020) estimated 53% of net P inputs to the Willamette River Basin were exported on average post-year 2000. This was higher than a majority of U.S. watersheds (Metson et al, 2017).…”

“…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). Findings suggest N and P retention dynamics in arid systems differ from wetter climates in the eastern U.S. (Compton et al, 2019;Howarth et al, 2006;Schaefer & Alber, 2007).…”