Nitrate variability in three small Oregon Coast Range streams

摘要

Potential increases of nitrate-nitrogen in stream water following timber harvesting remain a water quality concern. The Alsea Watershed Study (1959-1973) was a paired watershed study in the Oregon Coast Range testing in-stream physical, chemical, andbiological effects of two timber harvesting techniques (clear-cut without restriction and patch-cut with riparian buffers) compared with an unharvested control. Nitrate-nitrogen export increased three-fold in the two years following the clear-cut. Concentrations in this catchment returned to pre-treatment levels between the third and sixth year following harvest. No significant increase in nitrate-nitrogen export was observed in the patch-cut catchment. The control catchment has consistently had highernitrate-nitrogen concentrations than the clear-cut catchment. The Alsea Watershed Study Revisited, a project initiated in 2005 to investigate effects of contemporary forest practices on water resources, has created an opportunity to assess current nitrogen concentrations and export. The original gauging stations were reactivated and a monitoring network for monthly synoptic sampling of water quality was established. Results from monitoring in 2006 indicate that the clear-cut catchment has increased nitrate-nitrogen export by 14 kg/ha/yr over the AWS pre-harvest mean (4 kg/ha/yr) and 3 kg/ha/yr over the post-harvest mean (15 kg/ha/yr). The control catchment exports more nitrate-nitrogen than the clear-cut catchment (30 kg/ha/yr and 18 kg/ha/yr, respectively, in 2006). Export in the patch-cut catchment was similar to levels measured during the original Alsea Watershed Study (31 kg/ha/yr in 2006). Spatial variability of nitrate-nitrogen concentrations was evident longitudinally within the mainstem reachof the clear-cut catchment, whereas variations were more pronounced amongst tributaries for the patch-cut and control catchments. This temporal and spatial variability of nitrate-nitrogen should be taken into account when assessing water quality in headwater streams of the Oregon Coast Range.