Tracing the fate of atmospheric nitrate deposited onto the ecosystem using the triple oxygen isotopes as tracers

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The stable isotopic compositions of nitrate in precipitation (wet deposition) and groundwater (spring, lake, and stream water) were determined for the island of Rishiri, Japan, so as to use the ^<17>O anomalies (Δ^<17>O) to trace the fate of atmospheric nitrate that had deposited onto the island ecosystem, which is a representative background forest ecosystem for eastern Asia. The deposited nitrate had large ^<17>O anomalies with Δ^<17>O values with +26.2 ‰ being the annual average. On the other hand, nitrate in groundwater had small Δ^<17>O values ranging from +0.9‰ to 3.2‰, which corresponds to an average mixing ratio of atmospheric nitrate to total nitrate of 7%. Thus, we estimated that the direct drainage accounts for 10.5 ± 5.2% of atmospheric nitrate that has deposited on the island and that the residual portion has undergone biological processing before being exported from the forest ecosystem. The stable isotopic compositions of nitrate were determined in the oligotrophic water column of Lake Mashu, Japan, as well. These data were then used to quantify the geochemical dynamics of nitrate in the lake, by using the deposition rate of the atmospheric nitrate onto the entire catchment area of the lake.