Abstract. To clarify the biological processing of nitrate within temperate forested
catchments using unprocessed atmospheric nitrate exported from each catchment
as a tracer, we continuously monitored stream nitrate concentrations and
stable isotopic compositions, including 17O excess
(Δ17O), in three forested catchments in Japan (KJ, IJ1, and
IJ2) for more than 2 years. The catchments showed varying flux-weighted
average nitrate concentrations of 58.4, 24.4, and
17.1 µmol L−1 in KJ, IJ1, and IJ2, respectively, which
correspond to varying export fluxes of nitrate: 76.4, 50.1, and
35.1 mmol m−2 in KJ, IJ1, and IJ2, respectively. In addition to stream
nitrate, nitrate concentrations and stable isotopic compositions in soil
water were determined for comparison in the most nitrate-enriched catchment
(site KJ). While the 17O excess of nitrate in soil water showed
significant seasonal variation, ranging from +0.1 ‰ to
+5.7 ‰ in KJ, stream nitrate showed small variation, from
+0.8 ‰ to +2.0 ‰ in KJ, +0.7 ‰ to
+2.8 ‰ in IJ1, and +0.4 ‰ to +2.2 ‰ in IJ2.
We conclude that the major source of stream nitrate in each forested
catchment is groundwater nitrate. Additionally, the significant seasonal
variation found in soil nitrate is buffered by the groundwater nitrate. The
estimated annual export flux of unprocessed atmospheric nitrate accounted for
9.4 %±2.6 %, 6.5 %±1.8 %, and 2.6 %±0.6 % of the annual deposition flux of atmospheric nitrate in KJ, IJ1,
and IJ2, respectively. The export flux of unprocessed atmospheric nitrate
relative to the deposition flux showed a clear normal correlation with the
flux-weighted average concentration of stream nitrate, indicating that
reductions in the biological assimilation rates of nitrate in forested soils,
rather than increased nitrification rates, are likely responsible for the
elevated stream nitrate concentration, probably as a result of nitrogen
saturation. The export flux of unprocessed atmospheric nitrate relative to
the deposition flux in each forest ecosystem is applicable as an index for
nitrogen saturation.