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The growing population of Oregon’s Willamette River Basin places an increasing
demand on the basin’s surface waters. Watershed-scale research addressing spatial trends
of dissolved nitrogen (DN) and its relationship with landuse and soil N dynamics, such as
N mineralization, is sparse in the Willamette Basin. I measured DN along 124 km of the
Calapooia River, a tributary to the Willamette River, and in 44 non-nested sub-basins to
the Calapooia River for three years. Relationships between land-use and DN
concentrations were explored using correlation and regression analysis. Additionally, I
measured net N mineralization at monthly intervals for one year in surface soils (0-15cm)
of the riparian zone at 32 locations along the length of the Calapooia River.
Results show that there was consistently more DN in lower sub-basin surface
waters dominated by poorly drained soils and agricultural production when compared to
upper sub-basins that are dominated by well-drained soils and timber production.
Nitrate-N was >10 mg L-1 for eight lower-basins and total N was >10 mg L-1 for nine
lower basins during at least one sample period. Dissolved organic N (DON) represented
a greater proportion of DN in the upper basin, but had lower concentrations relative to the
lower basin. Seasonal nitrate-N concentrations had strong positive correlations to the
percent of a sub-basin that was managed for agriculture (%AG) in all seasons except
summer, whereas seasonal DON concentrations had strong positive correlations to %AG
in all seasons. This study indicated that DN concentrations and components varied
widely among seasons and years relative to precipitation amount and timing, soil
drainage, and land management. Efforts to reduce or regulate DN in the Calapooia Basin
or similar basins in the Willamette Basin must address this large temporal variability and
should include consideration of soil drainage because of its influence on hydrological
connections between terrestrial and aquatic systems.
Net N mineralization in riparian soils had seasonal trends with relatively low
mean net mineralization rates in the fall and winter and relatively high mean rates in the
spring and summer when conditions for microbial activity and decomposition were
enhanced. Annual net N mineralization was positively correlated with total N and labile
N in surface soils and with basal area of hardwoods within the riparian zone. Annual net
N mineralization per unit area was lower in riparian soils along the upper reaches of the
basin compared to the lower reaches. This difference was primarily caused by higher
amounts of coarse fragments in soils along the upper reaches. This demonstrates that
there is an inherent likelihood of more N mineralization in riparian soils of the lower
Calapooia Basin because of a lack of coarse fragments when compared to the upper
basin. Net N mineralization per kg of soil-size fraction may be a more appropriate
measure when exploring relationships with riparian conditions on watershed scales. |
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