『Abstract
We evaluated nitrogen (N) export for various catchments in the
San Pedro River watershed of South-central Chile (39゜20' to 40゜12'S)
during the dry season (February to March). We measured concentrations
and export of the various N species at 16 points from the Andean
headwaters to the lowland portion of the watershed: eight main
nested points along the main watershed and eight secondary points
on tributaries. We expected that, given a downstream increase
in pastureland and decrease in native pristine forest cover, inorganic
forms of N (DIN) would increase downstream, while conversely,
dissolved organic nitrogen (DON) would decrease compared with
concentrations in the forested headwaters. Nitrogen concentrations
did not show statistically significant differences among the nested
catchments. However, there were statistically significant differences
in N concentrations associated with land cover among the tributaries.
The results suggest that in the presence of base flow, natural
landscape properties (barren land, lakes and rivers), explained
most of the spatial variation in the N exports, while anthropogenic
disturbance was not detectable. There was a negative relationship
between DIN export and the coverage of lakes and rivers, suggesting
that lakes might be acting as N traps. On the other hand, DIN,
DON and total N exports were positively associated to barren land.
Total nitrogen export during this 60-day dry season was less than
20 kg km-2 and the annual export was not larger than
100 kg km-2. This study documents the as yet pristine
conditions of rivers in southern Chile.
Keywords: Catchment; Land cover; Nitrogen export; DIN/DON ratio』
Introduction
Study area
Methods
Sampling design
Water chemistry
Streamflow
Nitrogen export
Data analysis
Results
Land cover
Hydrology
Nitrogen concentration
Nitrogen export
Discussion
Magnitude of N exports and concentrations
Trends in N concentration and exports
Conclusions
Acknowledgments
References