EFFECT OF NITROGEN DEPOSITION ON STREAM WATER NITRATE AND NITROUS GAS EMISSION IN LARCH FOREST IN THE NORTHERN JAPAN

摘要

In this report, we show that the spatial variation in nitrogen deposition enables to detect the effect of anthropogenic N deposition on biogeochemical processes in forest ecosystems. We measured nitrogen deposition (throughfall fluxes), stream water chemistry and trace gases emission from forest floors over five adjacent small catchments in which soil types (Hapludants) and vegetation composition (fifty to sixty years old larch plantation) were fairly identical. Thirty-two throughfall collectors were set up in the five catchments (six to eight collectors in each catchment) and throughfall samples were collected after a rain event, while stream water samples were collected once or twice a month. The monitoring was carried out during a period of six months (2002 June to 2002 November). In July 2002 fluxes of trace gases (N_2O, NO, CH_4) were measured beside throughfall collectors. Throughfall DIN (dissolved inorganic nitrogen) fluxes were highly variable: the highest N input, 1.32 kg-N·ha~(-1) 6 months~(-1), was 66 times higher than the lowest input, 0.02 kg·N·ha~(-1) 6 months~(-1). The DIN inputs showed a sgnificant correlation (p < 0.05) with the stream nitrate concentrations (r = 0.88) and with N_2O emission from forest floors (r = 0.40), suggesting that the N deposition less than 10 kg-N·ha~(-1)·yr~(-1) is enough to affect NO_3 export and N_2O emission from forest ecosystems.