[MIS19-P06] Using triple nitrate isotopes to determine nitrate sources in the streamwater of tropical dry forest
Keywords:Oxygen isotope anomaly, Tropical dry forest, Nitrogen deposition
The study catchment was established in dry evergreen forest of the Sakaerat silvicultural research center, Nakohn Ratchasima Province in northeastern Thailand (35 ha). The altitude of the study catchment ranges 600 to 680 m. Annual mean precipitation was 1370 mm and annual mean temperature was 25.5 °C. Climate type was tropical savanna. The period between November and March is extremely dry, with monthly precipitation less than 50 mm. Using bulk sampler, we collected a precipitation in the morning of the day just after the rain event was observed. We mixed the daily precipitation into the monthly composite samples and stored it at freezer. Meanwhile, nitrate in soil water and stream water was collected by anion exchange resin method. The resins for soil water and stream water were put during ca. 6 month and 2 weeks, respectively. We extracted the nitrate from the recovered resins from the field after the period. We measured nitrate concentration and isotopic compositions of the nitrate for each water and extracts samples. δ15N and δ18O of nitrate were determined by the denitrifier method. And Δ17O was measured in Washington University after we transformed nitrate of the sampler to nitrous oxide by the denitrifier method.
As a result, in precipitation Δ17O -NO3- in precipitation was about 21‰ and did not largely fluctuate during wet season. In soil water Δ17O-NO3- was 1.3 and 1.4‰ at surface and sub soil, respectively, on the slope. These values decreased on riparian area. This tendency simply suggested that the contribution of atmospheric nitrate steeply declined in soil water in this study catchment. As for the seasonality, Δ17O-NO3- of soil water was higher in dry season compared to wet season. Finally, Δ17O-NO3- in the stream water ranged 6 to 12‰ during wet season. We also found the significantly relationship between δ15N and δ18O in the stream water, which suggested the strong contribution of the denitrifying processes within the study catchment.
We assumed that this technique was effective for distinguishing the atmospheric nitrate from microbe-oriented nitrate even in the tropical forest ecosystems. Meanwhile, in this study Δ17O-NO3- of precipitation (21‰) was relatively lower than the value in existing report at mid-latitude. In this presentation we are going to discuss the fraction of atmospheric nitrate to surface water nitrate by a quantitative approach.