The effects of land use on stream nitrate dynamics

The effects of land use and land use change on stream nitrate are poorly understood. While case studies have been presented, most process work has been done in areas with one land use (minimally disturbed or agricultural) and areas with substantial atmospheric deposition. In this paper we present results from three neighboring headwater catchments in western Oregon with similar (low) atmospheric deposition, size, and geology but with different, spatially consistent land use expressions: forest, agriculture, and residential. The climate in western Oregon has a distinct pattern of a three-month rainless period in the summer, a wetting up with many storms in the fall and winter, and a decrease of storms in the spring. We investigate how human activity alters the export of nitrate, whether the input of nitrate changes throughout the year which may affect storm response (i.e., depletion of soil water nitrate, addition of fertilizer, etc.), and how the changing contribution of source waters throughout the year affects streamflow concentrations. Our results showed marked differences in export rates between the three catchments. The forested catchment showed minimal export for three monitored storms (fall, winter, spring) through the seasonal wetting up of the catchments, and the residential catchment showed high export for all three storms. While the agricultural catchment displayed elevated export in the fall (similar to the residential catchment), exports decreased progressively throughout the rainy period (following late summer manure and green bean application). Overall, our results of storm event nitrate concentrations suggest that varying nitrate inputs have a large affect on nitrate dynamics. While within-storm nitrate concentration response patterns in the residential catchment were the same as the patterns in the reference forested catchment (a “concentration” pattern throughout the year), a “dilution” pattern was observed in the fall and winter and a “concentration” pattern was observed in the spring in the agricultural catchment.