17:15 〜 19:15
[AHW27-P02] Response of stream nitrate (NO3-) and dissolved organic nitrogen (DON) to decreases in atmospheric nitrogen (N) deposition in the forested watersheds, western Japan
キーワード：渓流水質、窒素沈着、大気汚染、硝酸塩、溶存有機態窒素
Background and Research gaps
Atmospheric N deposition is an important N resource for forest ecosystems. However, when atmospheric N deposition exceeds the biological demands of the ecosystem, the forest may become “N-saturated” with large amounts of NO3- leaching into streams. These increased leachates can degrade stream water quality and lead to environmental issues, such as acidification and eutrophication.
Recently, atmospheric N deposition has been declining in Japan. However, there is still a gap in the understanding of the response of stream NO3- to the decreased atmospheric N deposition in forested watershed. Additionally, DON plays a key role in the forest N cycle, serving as a nutrient source for plants and microorganisms while also contributing to soil N mineralization. Decreasing atmospheric acid deposition may alter the solubility and mobility of soil organic matter, leading to increased stream DON, which has been linked to freshwater “browning”. However, there are no studies have assessed the response of stream DON to decreases in atmospheric N deposition in Japan.
Methods and Materials
From January to November 2023, stream pH, total nitrogen (TN), dissolved total nitrogen (DTN), and major ionic components (NO3-, SO42-, Cl-, NH4+, Na+, K+, Mg2+, Ca2+) were measured 6 times in the forested watersheds in the upstream areas of the four rivers in the Tatara Watershed in western Japan. The bicarbonate (HCO3−) concentration was calculated from the anion and cation concentration balance using the equation
[HCO3−] (µmol L−1) = ([Na+] + [K+] + [NH4+] + 2 [Ca2+] + 2 [Mg2+]) – ([Cl−] + [NO3−] + 2 [SO42−]).
DON was calculated by subtracting dissolved inorganic nitrogen (NO3- and NH4+) from DTN
[DON] (µmol L-1) = [DTN] – ([NO3−] + [NH4+]).
The measured water chemistry data were compared with water chemistry data measured at the same locations in 2008-2009.
Results and Discussion
After a 14 years of decrease in atmospheric N deposition, the stream NO3- concentrations were significantly lower in 2023 (40.1 ± 3.7 µmol L-1) than in 2008-2009 (55.8 ± 4.7 µmol L-1), reflecting the decreases in atmospheric N deposition. In contrast, the stream DON concentrations did not differ significantly between 2023 (10.4 ± 1.2 µmol L-1) and 2008-2009 (11.3 ± 1.5 µmol L-1). Stream pH showed a slight increase from 7.45 in 2008-2009 to 7.61 in 2023, and stream ionic strength did not change significantly (1026.1 ± 90.1 µmol L-1 in 2008-2009 vs. 1077.9 ± 122.2 µmol L-1 in 2023). These results suggest that there may have been minimal changes in solubility and mobility of soil organic matter, resulting in unchanged stream DON concentrations.
Atmospheric N deposition is an important N resource for forest ecosystems. However, when atmospheric N deposition exceeds the biological demands of the ecosystem, the forest may become “N-saturated” with large amounts of NO3- leaching into streams. These increased leachates can degrade stream water quality and lead to environmental issues, such as acidification and eutrophication.
Recently, atmospheric N deposition has been declining in Japan. However, there is still a gap in the understanding of the response of stream NO3- to the decreased atmospheric N deposition in forested watershed. Additionally, DON plays a key role in the forest N cycle, serving as a nutrient source for plants and microorganisms while also contributing to soil N mineralization. Decreasing atmospheric acid deposition may alter the solubility and mobility of soil organic matter, leading to increased stream DON, which has been linked to freshwater “browning”. However, there are no studies have assessed the response of stream DON to decreases in atmospheric N deposition in Japan.
Methods and Materials
From January to November 2023, stream pH, total nitrogen (TN), dissolved total nitrogen (DTN), and major ionic components (NO3-, SO42-, Cl-, NH4+, Na+, K+, Mg2+, Ca2+) were measured 6 times in the forested watersheds in the upstream areas of the four rivers in the Tatara Watershed in western Japan. The bicarbonate (HCO3−) concentration was calculated from the anion and cation concentration balance using the equation
[HCO3−] (µmol L−1) = ([Na+] + [K+] + [NH4+] + 2 [Ca2+] + 2 [Mg2+]) – ([Cl−] + [NO3−] + 2 [SO42−]).
DON was calculated by subtracting dissolved inorganic nitrogen (NO3- and NH4+) from DTN
[DON] (µmol L-1) = [DTN] – ([NO3−] + [NH4+]).
The measured water chemistry data were compared with water chemistry data measured at the same locations in 2008-2009.
Results and Discussion
After a 14 years of decrease in atmospheric N deposition, the stream NO3- concentrations were significantly lower in 2023 (40.1 ± 3.7 µmol L-1) than in 2008-2009 (55.8 ± 4.7 µmol L-1), reflecting the decreases in atmospheric N deposition. In contrast, the stream DON concentrations did not differ significantly between 2023 (10.4 ± 1.2 µmol L-1) and 2008-2009 (11.3 ± 1.5 µmol L-1). Stream pH showed a slight increase from 7.45 in 2008-2009 to 7.61 in 2023, and stream ionic strength did not change significantly (1026.1 ± 90.1 µmol L-1 in 2008-2009 vs. 1077.9 ± 122.2 µmol L-1 in 2023). These results suggest that there may have been minimal changes in solubility and mobility of soil organic matter, resulting in unchanged stream DON concentrations.