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Second Lotic Intersite Nitrogen eXperiment (LINX-2) |
Researchers: Ashley Helton, Geoffrey Poole, Judy Meyer, Chris Bennett,
Clay Arango, Linda Ashkenas, Cliff Dahm, Walter Dodds, Stan Gregory,
Nancy Grimm, Robert Hall, Steve Hamilton, Sherri Johnson, William
McDowell, Patrick Mulholland, Bruce Peterson, Jennifer Tank, Maury
Valett, and Jack Webster.
Patterns of nitrogen delivery to and uptake within stream networks are
important determinants of the fate and transport of nitrogen in the
landscape. Eco-metrics, Inc. is collaborating with universities
across the United States as part of the "LINX-II" experiment; a five-year study to investigate
nitrate dynamics that determine the fate of nitrate delivered to
streams. Nitrate dynamics in headwater streams located in eight
different biomes (located in Coastal Massachusetts, Western North
Carolina, Southern Michigan, Central Kansas, Western Wyoming, Western
Oregon, Arizona, and Puerto Rico) and across different land uses are
being measured by adding minute quantities of 15N (a stable isotope of nitrogen) to streams and tracking the fate of the isotopes within the stream ecosystem.
Researchers at Eco-metrics, Inc. are developing a stream
network-scale model of nitrate delivery to streams and biotic
processing therein. The model will expand the results of
reach-scale field experiments to predict nitrogen dynamics in stream
networks and will facilitate inter-site comparisons across the eight
study biomes. The model will be applied to one 4th-6th order
drainage network in each biome. This project allows us to
identify variation in drivers of both nitrate processing and loading
across our study streams, a crucial step in understanding the fate of
nitrogen delivered to stream networks.
The model simulates expected patterns of nitrate concentration within
stream networks based on nitrate delivery to streams (“loading”),
in-stream uptake, network structure, and stream channel
morphology. We will implement the model for each network under
two different scenarios in order to establish whether or not in-stream
nitrate uptake is an important determinant of nitrate concentration on
the stream-network scale. First, the model will assume that
in-stream net removal of nitrate is negligible (i.e., uptake = 0), so
that nitrate accumulates in streams with little processing.
Second, the model will assume that nitrate uptake is a function of
nitrate concentration, based on biome-specific uptake data derived from
field measurements. By comparing model result from simulations
that incorporate uptake with result from simulations that do not, we
will determine how important nitrate removal is within each stream
network. In biomes where removal is important, we will also
examine where uptake is occurring in the network to determine the
proportion of removal occurring in streams of different sizes, since
recent research suggests that headwater streams are important sites for
nitrate processing and denitrification. Finally, we will perform
a cross site comparison of network scale nitrate removal.
Although other researchers have found strong correlations in large
rivers between land cover type in a drainage basin and annual nutrient
loading rates to the river, our initial results suggest that these
correlations are much weaker at finer spatial and temporal scales,
likely due to variation in the processes governing both loading and
uptake across seasonal climate cycles. This suggests that
successful mechanistic approaches to predicting the behavior of nitrate
in stream networks based on reach-scale measurements will consider
seasonal variation in patterns of loading and biotic activity.
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Research 
