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

Abstract

Excess inputs of reactive nitrogen (Nr) from the atmosphere will cause disturbances to forest ecosystems, including soil and stream water acidification, plant nutrient imbalances, alterations of species compositions (such as biodiversity losses), and nitrogen (N) leaching into stream water. Central Japan (or Chubu region) has experienced both domestic and transboundary air pollution since the 1950s. Emissions of nitrogen oxides in China peaked in 2011/2012, although the peak of ammonia emissions has not been confirmed. Changes in these emissions have been reflected gradually in atmospheric N deposition rates and the biogeochemical cycle in ecosystems in central Japan. We synthetically analyzed the current N saturation situation for forest ecosystems in central Japan based on our recent publications and the latest available dataset. The stream water in the Lake Ijira catchment (IJR) on the Pacific side in Gifu Prefecture became acidified and N-saturated in the mid-1990s but has begun to recover with a decrease in nitrate (NO 3 -) concentration since the mid-2000s. The progress and recovery of N saturation appeared to be related to the trends of atmospheric deposition, climatic anomalies (such as the cool and drought summers in 1993/1994), and forest management. The NO 3 concentrations in stream water within the forest catchment of Kajikawa (KJK) on the Sea of Japan side in Niigata Prefecture have continued to increase, although the catchment seems to be recovering from acidification owing to a decline in atmospheric sulfur (and probably N) deposition. Finally, the NO 3 concentration in stream water in KJK became higher than that in IJR, a direct inversion of the situation in the early 2000s. The recent analysis of 17 O excess in NO 3 estimated that the export of biologically unprocessed atmospheric NO 3 via stream water was larger in KJK than in IJR. The N cycle in soil-plant systems likely does not function as expected under excess atmospheric N input (still over 10 kg ha -1 year -1 ), especially in KJK. In addition to atmospheric N deposition, reduced N uptake rates as coniferous forests mature, and changing precipitation amounts/patterns appear to accelerate N leaching from forest catchments in central Japan.

Effects of climate and acidic deposition on inter-annual variations of stream water chemistry in forested watersheds in the Shimanto River Basin, southern Japan

“…Chiwa et al (2019) suggested that the NO 3 concentration in forest stream water has increased because deer browsing has reduced the understory vegetation. At the Sea of Japan side of the Kajikawa watershed, the stream NO 3 concentration has not decreased despite the lowering trend of nitrogen deposition (Sase et al, 2023). These results suggest that the recovery process from acidic deposition varies among forest ecosystems.…”

“…Fresh water chemistry has been generally recovering since the input of acidic deposition was reduced in the United States and European countries (Likens et al, 1996;Stoddard et al, 1999;Oulehle et al, 2017;Lucas et al, 2013) and long-term changes of stream water chemistry in watersheds have been reported in Japan (Chiwa, 2021;Sase et al, 2019;. The sulfate (SO 4 2-) concentration in Japanese watersheds has been decreasing since the 2000s (Sase et al, 2019; but the nitrate (NO 3 -) concentration is more variable, being significantly decreased in some forests but not in others (Chiwa et al, 2019;Sase et al, 2023). Chiwa et al (2019) suggested that the NO 3 concentration in forest stream water has increased because deer browsing has reduced the understory vegetation.…”

A Scientometric Analysis and Visualization of Forest Soil Contamination Research from Global Perspectives

“…Excessive inputs of reactive nitrogen (Nr) from the atmosphere can disrupt forest ecosystems in various ways. These disturbances include acidification of soil and stream water, imbalances in plant nutrients, changes in species compositions leading to biodiversity losses, and leaching of nitrogen (N) into stream water [82]. To understand how climate change will change the structure and function of forest ecosystems, Karnosky et al studied how O 3 affects the flow of carbon through ecosystems from the leaf level to the root and soil microorganisms under current and future atmospheric CO 2 conditions [83].…”

Stable isotopic evidence for the excess leaching of unprocessed atmospheric nitrate from forested catchments under high nitrogen saturation

“…Past studies reported that the maximum concentration of stream nitrate was 58.4 µM in the KJ forested catchment in Japan, with the maximum value of the M atm / D atm ratio as 9.4 % (Nakagawa et al, 2018;Sase et al, 2022). Whether the index of the M atm / D atm ratio can be applied to forested catchments, where the leaching of stream nitrate is much higher than the KJ forested catchment, remained unclarified.…”