Comparison of nitrate export patterns in forested catchments in Japan and the Northeastern United States: A meta‐analysis

Despite the numerous field studies on watersheds in temperate forests, little attention has been paid to differences in the seasonality of NO₃‐ exports due to different seasonal patterns of precipitation. Factors that cause seasonal variation in NO₃‐ exports include the biological conditions involved in the production of NO₃‐ and the hydrological conditions of NO₃‐ transport and storage. Previous studies described in this paper reveal that differences in the seasonal patterns of precipitation result in differences in the NO₃‐ flux between sites in Japan and the Northeastern United States. In Japanese forests affected by the Asian monsoon and are subject to heavy rainfall and high‐flow conditions during the summer, the NO₃‐ concentrations in stream water often increase during the summer growing season. This is a major difference from the typical decrease in NO₃‐ concentrations that occur in summer in the Northeastern United States, which is not affected by monsoon conditions. In addition, the orogenically active geology differs in Japanese catchments from those in the United States and is composed of younger bedrock and steep topography in the former. It has been shown that the buffering capacity of stream water is higher in Japan than in the United States, resulting in that the degree of acidification of stream water due to the outflow of NO₃‐ is also low in Japan. These comparisons with the United States serve not only to indicate the hydrological and biogeochemical uniqueness involved in the export of NO₃‐ in Japanese forest catchments but also to provide a more universal understanding of the mechanisms underlying the differences. To elucidate these mechanisms clearly, a comprehensive database is required for both climatic zones that include the seasonality of nitrogen dynamics within the catchments and descriptions of the hydrological processes. Additionally, a process‐based model is needed, based on the results of a meta‐analysis using the database.