11:00 AM - 11:15 AM
[MIS11-08] Source identification of nitrate increased in forest streams during rainfall events
Keywords:nitrogen saturation, forest stream, Matm/Datm ratio, rainfall, unprocessed atmospheric nitrate
Forest ecosystems are deficient in nitrogen in general. Excess loading of nitrogen, however, often leads to “nitrogen saturation” (Aber et al., 1989) in forest ecosystems from which significant quantify of nitrogen is eluted as nitrate. Enrichment of nitrate often caused environmental problems such as degradation of water quality, eutrophication, N2O emission. Nakagawa et al. (2018) proposed to use the export flux of unprocessed atmospheric nitrate relative to the entire deposition flux (Matm/Datm ratio) in each forested catchment determined precisely using 17O-excess of eluted nitrate as a new index for the nitrogen saturation stage. The concentration of nitrate in forest stream, however, often shows significant increase during rainfall. On the other hand, little is known about the variation in the concentration of unprocessed atmospheric nitrate in forest streams in response to the nitrate increase during rainfall. Therefore, we must clarify the source of nitrate that increased during rainfall, and quantify the impact of the changes in the concentration of unprocessed atmospheric nitrate in forest streams during rainfall on estimating Matm/Datm ratio.
In this study, we determined the changes in the concentration of unprocessed atmospheric nitrate in response to rainfall at Kaji River forest catchment (KJ) in Niigata Prefecture and at Ijira forest catchment (IJ1) in Gifu Prefecture, where unprocessed atmospheric nitrate and Matm/Datm ratio in the forest stream eluted from each catchment had been determined and reported. In the Kaji River forest catchment (KJ), in addition to routine water samples taken once every month from May 2017 to March 2020 at the forest stream, special water sampling campaign was conducted every hour on August 12, 2019 (n=24) and on October 22, 2019 (n=23) when the flow rate of the forest stream increased due to rainfall. In the Ijira forest catchment (IJ1), stream water samples were taken twice every month from May 2017 to December 2019, and the changes in the concentration of unprocessed atmospheric nitrate in response to the increases in nitrate concentration in response to precipitation were determined precisely. The concentration of nitrate in each sample was quantified using ion chromatography. The stable isotopic compositions including the 17O-excess of dissolved nitrate were determined using Continuous-Flow Isotope Ratio Mass Spectrometry (CF-IRMS) system in Nagoya University (Tsunogai et al., 2010).
The nitrate concentrations in the stream water taken during the special water sampling campaign in the Kaji River catchment (KJ) increased in response to the increase in flow rate of stream water, from 30 µmol/L before the rainfall, to a maximum of 130 µmol/L. On the other hand, the Δ17O values of nitrate decreased in response to the increase in nitrate concentration, showing linear relationship between the Δ17O value and the reciprocal of nitrate concentration(p<0.001). The Δ17O value of the endmember obtained as the intercept of the linear regression line was +0.10 - +0.15‰, which was consistent with the Δ17O values of nitrate in soil water (+0.10 - +0.20‰; Nakagawa et al., 2018). We concluded that the increase in nitrate concentrations in forest stream was caused by leaching of soil nitrate enriched in the riparian area due to the raising of the stream due to rainfall. The average concentration of unprocessed atmospheric nitrate in the stream during the rainfall were 1.7±0.2 µmol/L in August and 1.8±0.3 µmol/L in October, which corresponded with those determined in the forest stream determined through routine monitoring (1.6±0.3 µmol/L in August and 1.8±0.3 µmol/L in October). We concluded that the observed increases in nitrate in response to rainfall were caused by leaching of the remineralized nitrate produced through nitrification in soil. As a result, the increase in nitrate concentration in stream had little impact on the elution of unprocessed atmospheric nitrate and thus Matm/Datm ratio.
In the Ijira Lake catchment (IJ1), similar enrichment of nitrate of nitrate (about 64.1 µmol/L, 2.5 times higher than the annual average), were observed in response to rainfall on August 12, 2018. In response to the increase, the concentrations of unprocessed atmospheric nitrate in stream water also increased to 3.4±0.4 µmol/L, from 1.5±0.2 µmol/L on July 31, 2018, and +1.2±0.1 µmol/L on August 31, 2018. We concluded that not only the increase in remineralized nitrate produced through nitrification in soil, but also the increase in unprocessed atmospheric nitrate was responsible for the increase in nitrate in the stream water during rainfall in IJ1.
In this study, we determined the changes in the concentration of unprocessed atmospheric nitrate in response to rainfall at Kaji River forest catchment (KJ) in Niigata Prefecture and at Ijira forest catchment (IJ1) in Gifu Prefecture, where unprocessed atmospheric nitrate and Matm/Datm ratio in the forest stream eluted from each catchment had been determined and reported. In the Kaji River forest catchment (KJ), in addition to routine water samples taken once every month from May 2017 to March 2020 at the forest stream, special water sampling campaign was conducted every hour on August 12, 2019 (n=24) and on October 22, 2019 (n=23) when the flow rate of the forest stream increased due to rainfall. In the Ijira forest catchment (IJ1), stream water samples were taken twice every month from May 2017 to December 2019, and the changes in the concentration of unprocessed atmospheric nitrate in response to the increases in nitrate concentration in response to precipitation were determined precisely. The concentration of nitrate in each sample was quantified using ion chromatography. The stable isotopic compositions including the 17O-excess of dissolved nitrate were determined using Continuous-Flow Isotope Ratio Mass Spectrometry (CF-IRMS) system in Nagoya University (Tsunogai et al., 2010).
The nitrate concentrations in the stream water taken during the special water sampling campaign in the Kaji River catchment (KJ) increased in response to the increase in flow rate of stream water, from 30 µmol/L before the rainfall, to a maximum of 130 µmol/L. On the other hand, the Δ17O values of nitrate decreased in response to the increase in nitrate concentration, showing linear relationship between the Δ17O value and the reciprocal of nitrate concentration(p<0.001). The Δ17O value of the endmember obtained as the intercept of the linear regression line was +0.10 - +0.15‰, which was consistent with the Δ17O values of nitrate in soil water (+0.10 - +0.20‰; Nakagawa et al., 2018). We concluded that the increase in nitrate concentrations in forest stream was caused by leaching of soil nitrate enriched in the riparian area due to the raising of the stream due to rainfall. The average concentration of unprocessed atmospheric nitrate in the stream during the rainfall were 1.7±0.2 µmol/L in August and 1.8±0.3 µmol/L in October, which corresponded with those determined in the forest stream determined through routine monitoring (1.6±0.3 µmol/L in August and 1.8±0.3 µmol/L in October). We concluded that the observed increases in nitrate in response to rainfall were caused by leaching of the remineralized nitrate produced through nitrification in soil. As a result, the increase in nitrate concentration in stream had little impact on the elution of unprocessed atmospheric nitrate and thus Matm/Datm ratio.
In the Ijira Lake catchment (IJ1), similar enrichment of nitrate of nitrate (about 64.1 µmol/L, 2.5 times higher than the annual average), were observed in response to rainfall on August 12, 2018. In response to the increase, the concentrations of unprocessed atmospheric nitrate in stream water also increased to 3.4±0.4 µmol/L, from 1.5±0.2 µmol/L on July 31, 2018, and +1.2±0.1 µmol/L on August 31, 2018. We concluded that not only the increase in remineralized nitrate produced through nitrification in soil, but also the increase in unprocessed atmospheric nitrate was responsible for the increase in nitrate in the stream water during rainfall in IJ1.