Stable isotopic evidence for the excess leaching of unprocessed atmospheric nitrate from forested catchments under high nitrogen saturation

[摘要] Owing to the elevated loading of nitrogen through atmospheric deposition,some forested ecosystems become nitrogen saturated, from which elevatedlevels of nitrate are exported. The average concentration of stream nitrateeluted from upstream and downstream of the Kasuya Research forestedcatchments (FK1 and FK2 catchments) in Japan were more than 90  µ M,implying that these forested catchments were under nitrogen saturation. Toverify that these forested catchments were under the nitrogen saturation, wedetermined the export flux of unprocessed atmospheric nitrate relative tothe entire deposition flux ( M atm / D atm ratio) in these catchments;because the M atm / D atm ratio has recently been proposed as areliable index to evaluate nitrogen saturation in forested catchments.Specifically, we determined the temporal variation in the concentrations andstable isotopic compositions, including Δ 17 O, of stream nitratein the FK catchments for more than 2 years. In addition, for comparison, thesame parameters were also monitored in the Shiiba Research forestedcatchment (MY catchment) in Japan during the same period, where the averagestream nitrate concentration was low, less than 10  µ M. While showingthe average nitrate concentrations of 109.5, 90.9, and 7.3  µ M in FK1,FK2, and MY, respectively, the catchments showed average Δ 17 Ovalues of + 2.6 ‰, + 1.5 ‰, and + 0.6 ‰ in FK1, FK2, andMY, respectively. Thus, the average concentration of unprocessed atmosphericnitrate ([NO 3 - atm ]) was estimated to be 10.8, 5.1, and 0.2  µ M in FK1, FK2, and MY, respectively, and the M atm / D atm ratio was estimated to be 14.1 %, 6.6 %, and 1.3 % in FK1, FK2, and MY,respectively. The estimated M atm / D atm ratio in FK1 (14.1 %)was the highest ever reported from temperate forested catchments monitoredfor more than 1 year. Thus, we concluded that nitrogen saturation wasresponsible for the enrichment of stream nitrate in the FK catchments,together with the elevated NO 3 - atm leaching from thecatchments. While the stream nitrate concentration ([NO 3 - ]) can beaffected by the amount of precipitation, the M atm / D atm ratio isindependent of the amount of precipitation; thus, the M atm / D atm ratio can be used as a robust index for evaluating nitrogen saturation inforested catchments.

[学科分类] 大气科学