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TECHNICAL REPORTS

Surface Water Quality

Nitrate-Nitrogen, Land Use/Land Cover, and Soil Drainage Associations at Multiple Spatial Scales

^a Dep. of Forest Engineering, Resources and Management, Oregon State Univ., Corvallis, OR 97331
^b Current address: Ministry of Forests and Range - Coast Forest Region, 2100 Labieux Rd., Nanaimo, BC V9T 6E9 Canada
^c Dep. of Forest Engineering, Resources and Management, Oregon State Univ., Corvallis, OR 97331
^d Current address: Virginia Water Resources Research Center, 210 Cheatham Hall, Virginia Tech, Blacksburg, VA 24061
^e USDA-ARS National Forage Seed Production Research Center, 3450 SW Campus Way, Corvallis, OR 97331
^f Western Ecology Division, USEPA National Health and Environmental Effects Research Laboratory, 200 WE 35th St., Corvallis, OR 97333
^g USDA-ARS Office of National Programs, 5601 Sunnyside Ave., Beltsville, MD 20705

^* Corresponding author (stephen.schoenholtz{at}vt.edu).

Received for publication February 23, 2008. Managing non–point-source pollution of water requires knowledge of land use/land cover (LULC) influences at altering watershed scales. To gain improved understanding of relationships among LULC, soil drainage, and dissolved nitrate-N dynamics within the Calapooia River Basin in western Oregon, we selected 44 watersheds ranging in size between 3 and 33 km² for monthly synoptic sampling of surface water quality between October 2003 and September 2004. Seasonal associations were examined between dissolved nitrate-N and proportion of woody vegetation or poorly drained soils at differing scales (10, 20, 30, 60, 90, 150, 300 m, and entire watershed), which we defined as influence zones (IZs), surrounding stream networks. Correlations between nitrate-N and proportion woody vegetation or poorly drained soil at each IZ were analyzed for differences using the Hotelling-Williams test. We observed negative correlations (r = –0.81 to –0.94) between nitrate-N and proportion of woody vegetation during winter and spring. Poorly drained soils had positive correlations (r = 0.63–0.87) with nitrate-N. Altering the scale of analysis significantly changed correlations between nitrate-N and woody vegetation, with IZs <150 m being stronger than the watershed scale during winter. However, absolute differences in correlation values were small, indicating minimal ecological consequence for significant differences among scales. In contrast, nitrate-N correlations with poorly drained soil were stronger at the watershed scale than the 10- through 90-m IZs during winter and spring, and absolute differences were sufficient to suggest that scale is ecologically important when determining associations between dissolved nitrate-N and poorly drained soils.

Abbreviations: DEM, digital elevation model • DN, dissolved nitrogen • HW, Hotelling-Williams • LULC, land use/land cover • IZ influence zone