Stable isotopic evidence for the excess leaching of unprocessed atmospheric nitrate from forested catchments under high nitrogen saturation
Owing to the elevated loading of nitrogen through atmospheric deposition, some forested ecosystems become nitrogen saturated, from which elevated levels of nitrate are exported. The average concentration of stream nitrate eluted from upstream and downstream of the Kasuya Research forested catchments...
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Published in | Biogeosciences Vol. 20; no. 3; pp. 753 - 766 |
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Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
Katlenburg-Lindau
Copernicus GmbH
16.02.2023
Copernicus Publications |
Subjects | |
Online Access | Get full text |
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Summary: | Owing to the elevated loading of nitrogen through atmospheric deposition,
some forested ecosystems become nitrogen saturated, from which elevated
levels of nitrate are exported. The average concentration of stream nitrate
eluted from upstream and downstream of the Kasuya Research forested
catchments (FK1 and FK2 catchments) in Japan were more than 90 µM,
implying that these forested catchments were under nitrogen saturation. To
verify that these forested catchments were under the nitrogen saturation, we
determined the export flux of unprocessed atmospheric nitrate relative to
the entire deposition flux (Matm/Datm ratio) in these catchments;
because the Matm/Datm ratio has recently been proposed as a
reliable index to evaluate nitrogen saturation in forested catchments.
Specifically, we determined the temporal variation in the concentrations and
stable isotopic compositions, including Δ17O, of stream nitrate
in the FK catchments for more than 2 years. In addition, for comparison, the
same parameters were also monitored in the Shiiba Research forested
catchment (MY catchment) in Japan during the same period, where the average
stream nitrate concentration was low, less than 10 µM. While showing
the average nitrate concentrations of 109.5, 90.9, and 7.3 µM in FK1,
FK2, and MY, respectively, the catchments showed average Δ17O
values of +2.6 ‰, +1.5 ‰, and +0.6 ‰ in FK1, FK2, and
MY, respectively. Thus, the average concentration of unprocessed atmospheric
nitrate ([NO3-atm]) was estimated to be 10.8, 5.1, and 0.2 µM in FK1, FK2, and MY, respectively, and the Matm/Datm ratio was estimated to be 14.1 %, 6.6 %, and 1.3 % in FK1, FK2, and MY,
respectively. The estimated Matm/Datm ratio in FK1 (14.1 %)
was the highest ever reported from temperate forested catchments monitored
for more than 1 year. Thus, we concluded that nitrogen saturation was
responsible for the enrichment of stream nitrate in the FK catchments,
together with the elevated NO3-atm leaching from the
catchments. While the stream nitrate concentration ([NO3-]) can be
affected by the amount of precipitation, the Matm/Datm ratio is
independent of the amount of precipitation; thus, the Matm/Datm
ratio can be used as a robust index for evaluating nitrogen saturation in
forested catchments. |
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ISSN: | 1726-4189 1726-4170 1726-4189 |
DOI: | 10.5194/bg-20-753-2023 |