A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams |
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Authors: | Daniel J Sobota Sherri L Johnson Stan V Gregory Linda R Ashkenas |
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Institution: | (1) Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331, USA;(2) US Forest Service, Pacific NW Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331, USA |
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Abstract: | The influence of land use on potential fates of nitrate (NO3
−) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here,
we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use
setting influences fates of NO3
− in streams. We used 24 h releases of a stable isotope tracer (15NO3-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO3
− uptake processes. NO3
− uptake lengths varied two orders of magnitude (24–4247 m), with uptake rates (6.5–158.1 mg NO3-N m−2 day−1) and uptake velocities (0.1–2.3 mm min−1) falling within the ranges measured in other LINX II regions. Denitrification removed 0–7% of added tracer from our streams.
In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO3
−, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban
streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured
in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer
was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short
N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range
of NO3
− uptake processes in human-altered streams. |
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