Impact of land use patterns and agricultural practices on water quality in the Calapooia River Basin of western Oregon

Agricultural practices, including tillage, fertilization, and residue management, can affect surface runoff, soil erosion, and nutrient cycling. These processes, in turn, may adversely affect (1) quality of aquatic resources as habitat for amphibians, fish, and invertebrates, (2) costs of treating surface and ground water to meet drinking water standards, and (3) large-scale biogeochemistry. This study characterized the surface water sources of nitrogen (N) (total, nitrate [NO3 −], ammonium [NH4 +], and dissolved organic N) and sediment active within 40 subbasins of the Calapooia River Basin in western Oregon in monthly samples over three cropping years. The subbasins included both independent and nested drainages, with wide ranges in tree cover, agricultural practices, slopes, and soils. Sediment and N form concentrations were tested against weather and agricultural practice variables. Subbasin land use ranged from 96% forest to 100% agriculture. Average slopes varied from 1.3% to 18.9%, and surface water quality ranged from 0.5 to 43 mg L–1 (ppm) total N maxima and 29 to 249 mg L–1 suspended sediment maxima. Total N during the winter was positively related to percentage landcover of seven common agricultural crops (nongrass seed summer annuals, established seed crops of perennial ryegrass [Lolium perenne L.], tall fescue [Schedonorus phoenix {Scop.} Holub], orchardgrass [Dactylis glomerata L.], clover [Trifolium spp.], and newly planted stands of perennial ryegrass and clover) and negatively related to cover by trees and one seed crop, Italian (annual) ryegrass (Lolium multiflorum). Results for NO3 − and total N were highly similar. Sediment concentrations were most strongly related to rainfall totals during periods of 4 and 14 days prior to sampling, with smaller effects of soil disturbance. Fourier analysis of total N over time identified four prominent groups of subbasins: those with (1) low, (2) medium, and (3) high impacts of N (up to 2, 8, and 21 mg L–1, respectively) and a strong cyclical signal peaking in December and (4) those with very high impact of N (up to 43 mg L–1) and a weak time series signal. Preponderance of N in streams draining agriculturally dominated subbasins was in the form of the NO3 − ion, implying mineralization of N that had been incorporated within plant tissue following its initial application in the spring as urea-based fertilizer. Since mineralization is driven by seasonal rainfall and temperature patterns, changes in agronomic practices designed to reduce prompt runoff of fertilizer are unlikely to achieve to more than ~24% reduction in N export to streams.