Seasonal and annual watershed nitrogen export within the Willamette River Basin

Background/Question/Methods .

Nitrogen export from watersheds is influenced by hydrologic flows, land use/cover and the timing and spatial arrangement of N inputs and removal within the basin.  We examined the relationship between N inputs and watershed N export for 25 monitoring stations between 1996 and 2006 within the Willamette River Basin, western Oregon USA.  We hypothesized that N export would be strongly correlated with N inputs, and that much of the N inputs come from agricultural activities in the valley.  We also expected that N export would be strongly seasonal, reflecting the Mediterranean climate of western Oregon. 

Results/Conclusions

We found a wide range of areal N export from the monitored WRB sub-basins, ranging from 1 to nearly 70 kg N ha^-1 yr^-1.  Watersheds with lower per unit area export reflected their origins in the predominantly forested Cascades, while the higher N export basins had a greater proportion of agricultural areas, particularly areas with high N-requirement cultivated crops.  Export varied greatly from year to year, responding to interannual changes in precipitation and runoff.  Export was strongly seasonal, with at least 50% and often 75% of the N export occurring during the fall and winter months.  Cascade-dominated streams tended to maintain flow and N export during the summer, compared with the basins dominated by Coast-Range and valley areas which have less snow and spring inputs to maintain summer flow.  Agricultural N inputs of synthetic and manure fertilizer were well correlated with N export from the sub-basins.  Across the WRB, N export appears to be more strongly related to fertilizer application rates, as opposed to agricultural areas, indicating the importance of specific crops and crop practices as opposed to considering all agricultural lands the same.  This reinforces the need for good tracking of N inputs to inform water quality monitoring and management.  Annual N export was strongly driven by precipitation and runoff, suggesting that changes in hydrology will have important effects on N export downstream and to coastal areas in the future.