Comparative evaluation on nitrogen saturation of forest catchments in Japan and northeastern United States

To analyze the differences in the status and processes of nitrogen saturation in Japan and northeastern United States, we examined the hydrobiogeochemistry of nitrogen of forested watersheds in these regions. Two distinct differences were found between watersheds in Japan compared with those in US. 1) In Japanese watersheds, marked decreases of NO3- concentration in surface waters during the summer growing season were not found and NO3- concentrations sometimes increased especially in the summer at nitrogen saturated sites. This contrast with watersheds in US where decreases in NO3- concentration during the summer are commonly observed except those watersheds in advanced stages of nitrogen saturation. These differences in NO3- concentration relationships can be attributed to climatic differences, with Japan having high precipitation and high discharge during the summer, while in many regions of North America lowest discharges are found in the summer. The climatic regime in Japan leads to high rates of mineralization and the rapid transport of NO3- to streams in summer. 2) Japanese watersheds, even those with high NO3- concentrations in surface waters, show little evidence of acidification. This is in contrast to sites in US where increased NO3- concentrations, especially during episodic events, result in surface water acidification.