Seasonal and annual fluxes of atmospheric nitrogen deposition and riverine nitrogen export in two adjacent contrasting rivers in central Japan facing the Sea of Japan

Study regionKita and Minami River basins in Japan.Study focusThe coastal watershed in central Japan along the Sea of Japan has suffered large amounts of atmospheric nitrogen (N) deposition from northeastern Asia. However, the quantitative influences of atmospheric N deposition onto forested watersheds in the two basins and riverine N export into coasts remain unclear. To evaluate the current contribution of atmospheric N deposition, N deposition rates from the atmosphere to both basins, and N export rates from the rivers to the sea were quantified.New hydrological insight for the regionDeposition rates of bulk N in each basin exceeded 1000 mg m−2 year−1, more than 60% of which was supplied from winter to early spring by westerly winds. Annual deposition rates in the two basins did not differ, but annual export rates of inorganic N from the Kita River were significantly higher than those from the Minami River. These results suggest that symptoms of N saturation in the Kita River forested watershed are more serious. Furthermore, recent increasing trends of riverine N concentrations may have caused shifts in the limiting nutrient for coastal primary production from N to phosphorous. We suggest reductions in nitrate exports from forests as a strategy to improve nitrate pollution to both downstream waters and coastal ecosystems; however such efforts would involve intercontinental-scale actions in reducing N emissions.