11:00 AM - 11:15 AM
[MIS06-08] Accurate quantification of atmospheric nitrate in stream water eluted from a small forested watershed using triple oxygen isotopic composition
Keywords:nitrogen saturation, triple oxygen isotopic composition, forest ecosystem
The samples of rainfall, soil solution, and stream water in the study site were collected about once a month since December, 2012 to December,2014. Nitrate concentrations were measured by ion chromatography. The stable isotopic compositions including the triple oxygen isotopic compositions of dissolved nitrate were determined using Continuous-Flow Isotope Ratio Mass Spectrometry (CF-IRMS) system in Nagoya University (Tsunogai et al., 2010).
Although the triple oxygen isotopic compositions of nitrate in soil solutions showed significant seasonal variation from +0‰ to +6‰, these in stream water samples showed small variation. On the other hand, the concentration-weighted mean of the soil solutions (+0.8‰) was consistent with that of stream water samples (+1.3‰). We conducted that the major source of the stream water was groundwater in the forest, in which seasonal variation in soil solutions had been smoothed.
The estimated annual export flux of atmospheric nitrate occupied 9.2±4.4% of the annual deposition flux of atmospheric nitrate in the study site. Previous study showed that the ratio (i.e. direct elution ratio of atmospheric nitrate within total nitrate) increased in proportion to the forest declined (Durka et al., 1994; Tsunogai et al., 2014; Rose et al., 2015). In particular, the ratio obtained in Kajikawa site coincided with those eluted from the nitrogen saturated forest in West Virginia under the Stage 3. We concluded that the ratio likely reflected the turnover time of nitrate with in the forest soil and thus we can apply the ratio in each forest ecosystem. As a result, we conducted that the triple oxygen isotopic composition can be applicable as the indicator of nitrogen saturation.
Stoddard(1994) proposed that the disappearance of seasonality in the nitrate concentrations in stream water as the indication of nitrogen saturation in forest ecosystems. Mitchell et al. (1997), however, reported that the seasonality in stream nitrate concentrations was already lost in those eluted from normal forest in Japan, so that the seasonality was not a reliable index of nitrogen saturation in Japanese forests. Present study implied that the direct elution ratio of atmospheric estimated from the triple oxygen isotopic composition can be an alternative index of nitrogen saturation.