Heterogeneous Atmospheric Nitrogen Deposition Effects Upon the Nitrate Concentration of Stream Waters in a Forested Mountain Area

Abstract

Nitrogen compounds generated by anthropogenic combustion deposits in forest watersheds and induce nitrogen saturation of the area. Because excess nitrogen is derived from atmospheric deposition, this action is expected to uniformly affect a wide area of forest soils. Geographically, heterogeneous nitrate concentration of stream water within a small area has been attributed to the tree type, geological setting and tree cut. In this article, we hypothesized that the effect of the atmospheric nitrogen deposition in the forest watershed may vary within a small area, and that such variation is induced by the degree of air mass containing a high concentration of nitrogen deposition of combustion origin. We measured major ion concentrations, including nitrate, nitrite oxygen and nitrogen stable isotope of nitrate sampled at 24 water streams in the Chichibu region, which is 50-100 km from the Tokyo metropolitan area. The nitrate concentration showed a wide range (25.6-237 μmol L −1 ) within 300 km 2 , which was explained sufficiently by the air mass advection path and its contact with the mountain's surface. The nitrate concentration showed a significant positive correlation with chloride (r=0.73; p<0.001). As chloride originates outside of the Chichibu region, the positive correlation between two ions showed that the nitrate concentration of the stream water was affected by the nitrogen compound from the Tokyo Metropolitan area as a form of atmospheric deposition. Between the nitrate concentration and the stable isotope ratio of oxygen of nitrate, there was a positive correlation until nitrate concentration of 100 μmol L −1 . When the nitrate is over 100 μmol L −1 , δ 18 O shows a stable value of ca. 5.7‰. This indicates that the nitrification proceeds when the nitrate concentration was low to middle, but the reaction slowed when the nitrate concentration became high. Oxygen stable isotope of nitrate along with a set of nitrate concentrations can be used as a good indicator of nitrogen saturation.

Varying sensitivity of mountainous streamwater base-flow $${\bf{N}}{{\bf{O}}}_{{\bf{3}}}^{-}$$concentrations to N deposition in the northern suburbs of Tokyo

“…In the present study, about 25% of samples were greater than 1 mg-N L −1 as , which was a larger distribution ratio than that in Japan-wide monitoring 25 . Such high N concentration samples in the study region are comparable with values in European forest 17 , 18 and the same Kanto-region forest catchment reported to have N-saturated forests 25 – 27 . However, contrary to the European forests, pH in 99% of samples were >6.5, suggesting streams without acidification despite relatively high acid deposition loadings.…”

Nitrate is an important nitrogen source for Arctic tundra plants

“…S2 ), including one tropical and four subtropical sites in southwestern China; nine temperate sites in central, southern, and western Japan; and four Arctic tundra sites in northern Alaska. Among them, Tsukuba Forest Experimental Watershed (TKB) and Tama-Kyuryo Field Museum upper slope (TMU) and lower slope (TML) ( SI Appendix , Table S1 ) are characterized by high soil NO 3 − or N saturation ( 49 , 64 , 65 ), while the Arctic sites TFS, Sagavanirktok River Valley (SAG), and IMT ( SI Appendix , Table S1 ) are characterized by unmeasurable nitrification rates and negligible soil NO 3 − and, thus, are assumed to be typically low-NO 3 − ecosystems ( SI Appendix , Fig. S2 ).…”

Drivers of atmospheric nitrate processing and export in forested catchments

“…As the growing season coincides with the rainy season in Japan, greater nitrate export observed during this season likely reflects the combined influence of increased microbial nitrate production and high water drainage [ Mitchell et al ., ; Mitchell , ]. In a study of forested catchments near Tokyo, Tabayashi and Koba [] reported higher stream nitrate concentrations and δ 18 O‐ values in areas receiving elevated N deposition inputs. However, maximum δ 18 O‐ values only reached +6‰, showing that nitrification sources dominated stream nitrate even in catchments with high stream nitrate concentrations.…”