『Abstract
Knowledge of import, export, and transport of nitrogen (N) in
headwater catchments is essential for understanding ecosystem
function and water quality in mountain ecosystems, especially
as these ecosystems experience increased anthropogenic N deposition.
In this study, we link spatially explicit soil and stream, data
at the landscape scale to investigate import, export and transport
of N in a 0.89 km2 site at the alpine-subalpine ecotone
in the Front Range of the Rocky Mountains, Colorado, U.S.A. For
two of the major N inputs to our site, N deposition in the snowpack
and N fixation, a complementary relationship was found across
the study site, with greater abundance of N-fixing plants in areas
with less snow and substantial snow inputs in areas with low N
fixer abundance. During the initial phases of snowmelt, mixing
model end members for oxygen isotopes in nitrate (NO3-)
indicated that a substantial quantity of NO3-
is transported downhill into the forested subalpine without being
assimilated by soil microbes. After this initial pulse, much less
NO3- entered the stream and most
but not all of it was microbial in origin. Rising δ15N
in stream NO3- indicated greater
influence of fractionating processes such as denitrification later
in the season. NO3- from both
atmospheric and microbial sources was not exported from our site
because it was consumed within the first several hundred meters
of the stream; ultimately, N exports were in the form of dissolved
organic nitrogen (DON) and particulate N (PN). The results of
this study suggest that the highest elevation dry alpine meadows
rely more heavily on N fixation as an N source and experience
less of the effects of anthropogenic N deposition than mid and
lower elevation areas that have more snow. Our data also suggest
that mid-elevation krummholz, moist meadows, and talus slopes
are exporting N as NO3- shortly
after the onset of snowmelt, but that this NO3-
is rapidly consumed as the stream flows through the subalpine
forest. This consumption by assimilation and/or denitrification
currently provides a buffer against increased inorganic N availability
downstream.
Keywords: δ17O; δ18O; Δ17O; Kriging;
Landscape continuum; LTER; Niwot Ridge; Dissolved organic nitrogen
(DON)』
Introduction
Methods
Study site
N inputs
N transport and exports
Results
N import from fixation and snow
N transport and export
Discussion
N import
N transport and export
Conclusion
Acknowledgments
References