Nitrogen saturation of forested catchments in central Japan - Progress or recovery?

Figures & data

Figure 1. Locations and images of Lake Ijira catchment (IJR) and Kajikawa catchment (KJK) in central Japan (after Kamisako et al. 2008; Sase et al. 2019). Stream water samples were collected at the outlet of catchment areas (shown as the sampling points). In the case of Lake Ijira catchment (IJR), stream water samples have been collected at RW1 and RW2 since 2007 and 1988, respectively.

Table 1. Site characteristics of two study catchments, Lake Ijira catchment on the Pacific side and Kajikawa catchment on the Sea of Japan side

	Lake Ijira catchment (IJR)	Kajikawa catchment (KJK)
Catchment area (ha)	298	3.84
Annual precipitation (mm)	3,210^†	2,239 ^‡
Vegetation (tree species composition)	Japanese cypress (49%),broadleaf trees (29%),Japanese red pine (13%), andJapanese cedar (8%)	Japanese cedar (90%), andbroadleaf trees (10%)
Soil^§	Cambisols	Cambisols
Geology	Chert	Granodiorite
Stand age (as of 2018)	44–48 years old (Japanese cypress)	46 years old (Japanese cedar)
Forest management record (thinning of trees)	2002–2004, 2006–2007	2002

Note. †, the mean for the period from 2007 to 2019; ‡, the mean for the period from 2002 to 2019; §, classification according to the World Reference Base for Soil Resources (WRB).

Figure 2. Annual atmospheric fluxes of (1) SO₄–sulfur (S) and/or non-sea salt (nss) SO₄–S and (2) dissolved inorganic nitrogen (DIN) in (a) Lake Ijira catchment (IJR) and (b) Kajikawa catchment (KJK) (updated with the latest data after EANET 2020; Sase et al. 2021). Wet, fluxes by wet deposition at the Acid Deposition Monitoring Network in East Asia (EANET) site; RF, fluxes by rainfall outside canopy; TF+SF, fluxes by throughfall and stemflow.

Figure 3. Changes in (1) pH and pH 4.8 alkalinity and (2) concentrations of SO₄^2– and NO₃^– in stream water in (a) Lake Ijira catchment (IJR) and (b) Kajikawa catchment (KJK) (drawn with the latest data after Sase et al. 2019, 2021). For IJR, the historical data since 1988 at RW2 are plotted.

Figure 4. Relationships (a) between the atmospheric N inputs and NO₃^– concentrations in stream water and (b) between the atmospheric N inputs and N outputs from ecosystems in Lake Ijira catchment (IJR) and Kajikawa catchment (KJK) (EANET 2021). Three-year-moving averages are plotted for several periods to show chronological changes. Water year (WY) in IJR, from November to October in the next year, e.g., WY 2019 in IJR, from November 2018 to October 2019 (after Sase et al. 2019); WY in KJK, from June to May in the next year, e.g., WY 2018 in KJK, from June 2018 to May 2019 (after Sase et al. 2021).

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