Contribution of dissolved organic nitrogen deposition to nitrogen saturation in a forested mountainous watershed in Tsukui, Central Japan

摘要

Nitrogen (N) budget was estimated with dissolved inorganic N (DIN) and dissolved organic N (DON) in a forested mountainous watershed in Tsukui, Kanagawa Prefecture, about 50 km west of Tokyo in Central Japan. The forest vegetation in the watershed was dominant by Konara oak (Quercus ser-rata) and Korean hornbeam (Carpinus tschonoskii), and Japanese cedar (Cryptomeria japonica). Nitrate (NO+3~-) concentration in the watershed streamwater was averagely high (98.0 ± 19 (± SD, n = 36) μmol L~(-1)) during 2001-2003. There was no seasonal and annual changes in the stream NO_3~- concentration even though the highest N uptake rate presumably occurred during the spring of plant growing season, a fact indicating that N availability was in excess of biotic demands. The DON deposition rates (DON input rates) in open area and forest area were estimated as one of the main N sources, accounting for about 32% of total dissolved N (TDN). It was estimated that a part of the DON input rate contributed to the excessive stream NO_3- output rate under the condition of the rapid mineralization and nitrification rates, which annual DON deposition rates were positively correlated with the stream NO_3~- output rates. The DON retention rate in the DON budget had a potential capacity, which contributed to the excessive stream NO_3~- output rate without other N contributions (e.g. forest floor N or soil N).