Seasonal Patterns of Nitrate Discharge from Forested Catchments: Information Derived from Japanese Case Studies

In recent decades, the seasonal variation of NO 3 − discharge from forest ecosystems has been increasingly used by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors controlling the seasonal patterns of stream NO 3 − concentrations include seasonal variations in (i) nutrient demands of the ecosystem, (ii) solute transport capability of the hydrological condition, and (iii) in‐stream nutrient usage and supply. In this article, we 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, and explain the different influences hydrological and biogeochemical controls exert in rainy summer regions ( Japanese catchments) and dry summer regions (sites in the northeast United States and Europe). The seasonal variation of hydrological conditions is a predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeast United States and Europe. We emphasize that it is important to compare seasonal patterns among different climate regions to obtain more universal explanations of the seasonal variation in stream NO 3 − concentration. In addition, multi‐scale investigations will be needed to provide insight into the relative contribution of hillslope biogeochemical effects and the influences of in‐stream biological activities.