Jiajia Lin1, Jana Compton2, George Mueller-Warrant3, William Matthews4, Scott Leibowitz5
1 National Research Council, based at US Environmental Protection Agency, Western Ecology Division, 200 SW 35th St, Corvallis, OR 97333, [email protected]
2 US Environmental Protection Agency, Western Ecology Division, 200 SW 35th St, Corvallis, OR 97333, [email protected]
3 US Department of Agriculture, Agricultural Research Service, 3420 NW Orchard Ave, Corvallis, OR 97331, [email protected]
4 Oregon Department of Agriculture, Natural Resources Division, 635 Capitol St NE, Salem, OR 97301, [email protected]
5 US Environmental Protection Agency, Western Ecology Division, 200 SW 35th St, Corvallis, OR 97333, [email protected]
Reducing nitrogen (N) released into the environment through greater N use efficiencies (NUE) is a current challenge in watershed management. Examining N sources and sinks at local scales allows for better watershed-scale N use. We use data on land-use, CAFOs, N deposition, stream chemistry, and crop-level and county-level fertilizer use to assess the N inputs, exports and retention in the Calapooia River Watershed (CRW). The CRW is influenced by intensive agricultural activities, mostly in grass seed crops. Our results demonstrate that fertilizer is the dominant N input on agricultural land, with an average rate of 130 kg N/ha/yr on these subwatersheds. Deposition and alder fixation are the two main sources of N on forested land, with an average rate of <10 kg N/ha/yr. About 50-60% of the annual hydrologic N yield occurs during wet winter and reaches 40 kg N/ha per season. Summer TN yield is minimum, as low as <1 kg N/ha per season. At the CRW scale, annual stream export is 19% of the total N inputs. On average, about 41% of total N input is removed annually via crop harvest among the 58 subwatershed. The proportion of net N input that is “retained” in Calapooia is within the same range of estimates of northeastern watersheds. Our analysis also shows that runoff alone explains 62% of the variance in fractional N export in the U.S. watersheds.