Implications of seasonal variation in nitrate export from forested ecosystems: a review from the hydrological perspective of ecosystem dynamics

In recent decades, the seasonal variation in NO₃ ^? discharge from forest ecosystems has been used increasingly by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors underlying the seasonal patterns of stream NO₃ ^? concentration include seasonal variations in (1) ecosystem nutrient demand, (2) solute transport capability of the hydrological condition, and (3) in-stream nutrient usage and supply. In this article I show how case studies have helped elucidate the dominant controlling factors by comparing data from Japanese catchments with previously compiled data from studies in North America and Europe. Moreover, I explain the different influences that hydrological and biogeochemical controls exert in regions with rainy (Japan) and dry (northeastern United States and Europe) summers. The seasonal variation in hydrological conditions is the predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeastern United States and Europe. I emphasize the importance of comparing seasonal patterns among regions with differing climates to obtain more universal explanations of seasonal variations in NO₃ ^?-related biogeochemical and hydrological dynamics in a forest ecosystem. In addition, multi-scale investigations will be needed to provide insight into the relative contributions of hillslope biogeochemical effects and in-stream biological activities.