Abstract
Abstract. 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 17O anomalies (Δ17O) 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 17O anomalies with Δ17O values ranging from +20.8‰ to +34.5‰ (n = 32) with +26.2‰ being the annual average. The maximum Δ17O value of +34.5‰, obtained for precipitation on the 23rd to 24th of February 2007, was an extraordinarily large value among values for all samples of precipitation in Rishiri. Most nitrate in the sample might have been produced via NO3 radical in a highly polluted air mass that had been supplied from megacities on the eastern coast of the Asian continent. On the other hand, nitrate in groundwater had small Δ17O values ranging from +0.9‰ to 3.2‰ (n = 19), which corresponds to an mixing ratio of atmospheric nitrate to total nitrate of (7.4±2.6)%. Comparing the inflow and outflow of atmospheric nitrate in groundwater within the island, we estimated that the direct drainage accounts for (8.8±4.6)% 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.
Highlights
Anthropogenic activities have increased emissions of fixed nitrogen from land to the atmosphere
Water chemistry has been routinely measured through many programs on a regional to national scale, our understanding of the mechanisms that regulate the transport of atmospheric deposition to streams in forested ecosystems is still limited because the fate of atmospheric nitrate deposited onto a forest is complicated; the fate is determined through an interplay of several processes such as (1) dilution through nitrification, (2) uptake
Nitrate in groundwater of Rishiri had the 17O values ranging from +0.9‰ to 3.2‰ (n = 19), which corresponds to an mixing ratio of atmospheric nitrate to total nitrate of (7.4±2.6)%
Summary
Anthropogenic activities have increased emissions of fixed nitrogen from land to the atmosphere. 17O is stable in mass-dependent isotope fractionation processes and so we can use 17O as a conserved tracer of NO−3 (atm) and trace NO−3 (atm) irrespective of partial removal through denitrification and/or uptake reaction subsequent to deposition. We determined 17O values of nitrate in spring, lake, and stream water on the island of Rishiri (Japan) as those representing groundwater charged by natural forested watersheds on the surface, where atmospheric fixed-N input has been low; less than 5 kg ha−1 a−1. Using the 17O tracer, we quantified the fraction of NO−3 (atm) within the total nitrate output in groundwater to gain insight into the processes controlling the fate and transport of NO−3 (atm) deposited on the forested watershed with small anthropogenic fixed-nitrogen input. The quantified NO−3 (atm) output will be useful in future studies to evaluate the amounts of NO−3 (atm) eluted from the region with different NO−3 (atm) inputs, including the amount eluted by the same forested watershed in the case of probable elevated NO−3 (atm) input in the future
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