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Biogeosciences An interactive open-access journal of the European Geosciences Union
Biogeosciences, 13, 3441-3459, 2016
https://doi.org/10.5194/bg-13-3441-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
15 Jun 2016
Accurate and precise quantification of atmospheric nitrate in streams draining land of various uses by using triple oxygen isotopes as tracers
Urumu Tsunogai1, Takanori Miyauchi1, Takuya Ohyama1, Daisuke D. Komatsu1,a, Fumiko Nakagawa1, Yusuke Obata2,b, Keiichi Sato3, and Tsuyoshi Ohizumi3,c 1Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
2Faculty of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu 514-8507, Japan
3Asia Center for Air Pollution Research, 1182 Sowa Nishi-ku, Niigata 950-2144, Japan
anow at: School of Marine Science and Technology, Tokai University, 3-20-1 Orito, Shimizu, Shizuoka 424-8610, Japan
bnow at: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
cnow at: Niigata Prefectural Institute of Public Health and Environmental Sciences, 314-1, Sowa, Niigata, Niigata 950-2144, Japan
Abstract. Land use in a catchment area has significant impacts on nitrate eluted from the catchment, including atmospheric nitrate deposited onto the catchment area and remineralised nitrate produced within the catchment area. Although the stable isotopic compositions of nitrate eluted from a catchment can be a useful tracer to quantify the land use influences on the sources and behaviour of the nitrate, it is best to determine these for the remineralised portion of the nitrate separately from the unprocessed atmospheric nitrate to obtain a more accurate and precise quantification of the land use influences. In this study, we determined the spatial distribution and seasonal variation of stable isotopic compositions of nitrate for more than 30 streams within the same watershed, the Lake Biwa watershed in Japan, in order to use 17O excess (Δ17O) of nitrate as an additional tracer to quantify the mole fraction of atmospheric nitrate accurately and precisely. The stable isotopic compositions, including Δ17O of nitrate, in precipitation (wet deposition; n =  196) sampled at the Sado-seki monitoring station were also determined for 3 years. The deposited nitrate showed large 17O excesses similar to those already reported for midlatitudes: Δ17O values ranged from +18.6 to +32.4 ‰ with a 3-year average of +26.3 ‰. However, nitrate in each inflow stream showed small annual average Δ17O values ranging from +0.5 to +3.1 ‰, which corresponds to mole fractions of unprocessed atmospheric nitrate to total nitrate from (1.8 ± 0.3) to (11.8 ± 1.8) % respectively, with an average for all inflow streams of (5.1 ± 0.5) %. Although the annual average Δ17O values tended to be smaller in accordance with the increase in annual average stream nitrate concentration from 12.7 to 106.2 µmol L−1, the absolute concentrations of unprocessed atmospheric nitrate were almost stable at (2.3 ± 1.1) µmol L−1 irrespective of the changes in population density and land use in each catchment area. We conclude that changes in population density and land use between each catchment area had little impact on the concentration of atmospheric nitrate and that the total nitrate concentration originated primarily from additional contributions of remineralised nitrate. By using the average stable isotopic compositions of atmospheric nitrate, we excluded the contribution of atmospheric nitrate from the determined δ15N and δ18O values of total nitrate and estimated the δ15N and δ18O values of the remineralised portion of nitrate in each stream to clarify the sources. We found that the remineralised portion of the nitrate in the streams could be explained by mixing between a natural source with values of (+4.4 ± 1.8) and (−2.3 ± 0.9) ‰ for δ15N and δ18O respectively and an anthropogenic source with values of (+9.2 ± 1.3) and (−2.2 ± 1.1) ‰ for δ15N and δ18O respectively. In addition, both the uniform absolute concentration of atmospheric nitrate and the low and uniform δ18O values of the remineralised portion of nitrate in the streams imply that in-stream removal of nitrate through assimilation or denitrification had little impact on the concentrations and stable isotopic compositions of nitrate in the streams, except for a few streams in summer with catchments of urban/suburban land uses.

Citation: Tsunogai, U., Miyauchi, T., Ohyama, T., Komatsu, D. D., Nakagawa, F., Obata, Y., Sato, K., and Ohizumi, T.: Accurate and precise quantification of atmospheric nitrate in streams draining land of various uses by using triple oxygen isotopes as tracers, Biogeosciences, 13, 3441-3459, https://doi.org/10.5194/bg-13-3441-2016, 2016.
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Short summary
We quantified the influence of changes in land use and population density between each catchment area on the fate of atmospheric nitrate, by determining the areal distribution and seasonal variation in stable isotopic compositions of nitrate for more than 30 streams within a same watershed. We found that the absolute concentrations of atmospheric nitrate were almost the same between the streams irrespective to the changes in land use and population density.
We quantified the influence of changes in land use and population density between each catchment...
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