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[MIS14-P03] Determination on the Triple Oxygen Isotopic Composition of Nitrous Oxide Emitted from Soils
Keywords:Nitrous Oxide, Triple Oxygen Isotopic Composition, Nitrification, Denitrification, Soil
Nitrous oxide (N2O) is a long-lived trace gas playing important roles in the atmosphere, such as stratospheric ozone depletion and global warming. The production of N2O is closely related to multiple microbial processes in terrestrial ecosystems, in which N2O emissions from both agricultural and natural soils represent 50 - 70% of total emission. The stable isotopic compositions of N2O (δ15N and δ18O) have been widely used as tracers to identify the production pathways of N2O to establish strategies reducing N2O emission from anthropogenic sources traditionally. However, possible progress of the microbial isotope fractionation processes in the heterogenous soil ecosystems, often made it difficult to identify the production pathways of N2O. Based on the stability during the microbial processes, we proposed to use the Δ17O of N2O as an additional, more robust tracer to clarify the potential microbial production pathways of N2O emitted from soil environments.
N2O produced via nitrification pathway has oxygen atom originates from atmospheric oxygen molecules (O2: Δ17O = –0.4 ‰) while the oxygen atom of N2O produced via denitrification pathway originates from nitrate (NO3-: Δ17O > 0 ‰) or water (H2O: Δ17O = 0 ‰). Therefore, it is expected that the N2O production pathways could be elucidated from the Δ17O value. In this study, we quantified the Δ17O of N2O emitted from soils, the forest areas in Gifu and Aichi prefectures and the tea plantations in Mie prefecture, to determine the contribution (mixing ratio) of the nitrification and denitrification. In the forest area of Gifu prefecture, we collected soil N2O accumulated in snow. In Aichi and Mie prefectures, N2O emitted from soils were collected using an open-flow chamber. In addition, in order to confirm the effect of atmospheric NO3-(Δ17O = 26.3 ± 3 ‰) on the N2O production in soil ecosystems during the raining events, we also performed chamber experiments both during and after the natural raining events.
The Δ17O values of N2O emitted from the forested soils, both in Gifu and Aichi prefecture, were -0.4 ‰, implying that oxygen atom in N2O originates from atmospheric O2 via microbial nitrification. On the other hand, the Δ17O value of N2O emitted from tea plantation field was -0.2‰, implying that the oxygen atom in N2O derives from O2, H2O and soil NO3-(Δ17O = +0.5 ‰) via both nitrification and denitrification. The Δ17O value of N2O emitted from forested soils in campus (Aichi prefecture) during the natural raining days reached up to +5 ‰, implying that all the oxygen atoms in N2O were derived from NO3-, and was probably derived directly from part of the atmospheric NO3-via denitrification in the surface soils. We concluded that the main microbial processes to produce N2O within the soil ecosystems have been changed from nitrification to denitrification during the raining events.
N2O produced via nitrification pathway has oxygen atom originates from atmospheric oxygen molecules (O2: Δ17O = –0.4 ‰) while the oxygen atom of N2O produced via denitrification pathway originates from nitrate (NO3-: Δ17O > 0 ‰) or water (H2O: Δ17O = 0 ‰). Therefore, it is expected that the N2O production pathways could be elucidated from the Δ17O value. In this study, we quantified the Δ17O of N2O emitted from soils, the forest areas in Gifu and Aichi prefectures and the tea plantations in Mie prefecture, to determine the contribution (mixing ratio) of the nitrification and denitrification. In the forest area of Gifu prefecture, we collected soil N2O accumulated in snow. In Aichi and Mie prefectures, N2O emitted from soils were collected using an open-flow chamber. In addition, in order to confirm the effect of atmospheric NO3-(Δ17O = 26.3 ± 3 ‰) on the N2O production in soil ecosystems during the raining events, we also performed chamber experiments both during and after the natural raining events.
The Δ17O values of N2O emitted from the forested soils, both in Gifu and Aichi prefecture, were -0.4 ‰, implying that oxygen atom in N2O originates from atmospheric O2 via microbial nitrification. On the other hand, the Δ17O value of N2O emitted from tea plantation field was -0.2‰, implying that the oxygen atom in N2O derives from O2, H2O and soil NO3-(Δ17O = +0.5 ‰) via both nitrification and denitrification. The Δ17O value of N2O emitted from forested soils in campus (Aichi prefecture) during the natural raining days reached up to +5 ‰, implying that all the oxygen atoms in N2O were derived from NO3-, and was probably derived directly from part of the atmospheric NO3-via denitrification in the surface soils. We concluded that the main microbial processes to produce N2O within the soil ecosystems have been changed from nitrification to denitrification during the raining events.