Effect of nitrogen deposition on nitrogen cycling in forested ecosystems and N2O emission from the forest floor

Human-induced increases in reactive nitrogen contribute to detrimental changes in nitrogen cycling in terrestrial ecosystems. The effects are expanded from near the source to the surrounding area by atmospheric transportation or solution infiltration in soils. Forests in Japan provide various benefits to people such as supplying organic material to agricultural fields and maintaining the functioning of aquatic systems. Perhaps the most important function, providing a safe and steady water supply, seems to have been weakened in recent years. The release of nitrate ions (NO3sup(-)) into stream water has begun to be reported in suburban forests in Japan where nitrogen deposition is high due to atmospheric transport from polluted urban area. The purpose of this study was to clarify the nitrogen status of Japanese forests that have received chronic nitrogen deposition. We compared nitrogen cycling in six forest ecosystems with different levels of nitrogen deposition. As nitrogen input to the forest ecosystems, we measured inorganic nitrogen (NH4sup(+) and NO3sup(-)) flux in throughfall. The amount of nitrogen in the litterfall and the inorganic nitrogen (NH4sup(+) and NO3sup(-)) flux in Asub(0) layer percolation and soil water were measured to investigate the internal nitrogen cycling in the plant-soil system. As nitrogen outputs from the forest ecosystems, we measured N2O emissions from the forest floor and inorganic nitrogen (NH4sup(+) and NO3sup(-)) leaching from soils. Ibaraki sites (IK and IY), where nitrogen depositions in rainfall ranged from 15 to 19kg N/ha/yr were sites with high nitrogen deposition. Two Oku-nikko sites (NM and NY) located on the ridge and lower slope of Mt Maeshirane (2373 m) had 7 and 19kg N/ha/yr of nitrogen deposition, respectively. The other mountainous sites (SC and SD) are located hillside of Mt. Norikura (3026 m) and have very low nitrogen deposition of 4kg N/ha/yr. Among ecosystems with different temperature, precipitation and nitrogen deposition rate, various seasonal patterns of ion fluxes were noted. In temperate sites (IK and IY), the NH4sup(+) and NO3sup(-) fluxes increased to 2.5 times those of throughfall by passing through the organism layer. From 80 to 90% of inorganic nitrogen flux was accounted for by NO3sup(-). The seasonal change in that flux is high in summer and low in winter, which seems to correspond roughly with changes in the amount of precipitation. Cases of extremely high nitrogen fluxes in Asub(0) layer percolation in the summer accompanied rainfall events just after a prolonged dry period.