Abstract
Characteristics and research questions in nitrogen cycling in natural forest ecosystems of northern Hokkaido were discussed based on the case studies in Hokkaido University's experimental forests. Most atmospheric nitrogen deposition was net retained in forest ecosystem and the nitrate leaching from soil was mostly occurred during the snowmelt period. The rapid microbial immobilization of nitrogen in surface soil was important processes for the nitrogen retention. However, the removal of soil surface and vegetation for the forest practices caused the increase of nitrate leaching potential due to the decreases of the nutrient uptake and the microbial nitrogen immobilization. On the other hands, the increase of fine root biomass and the nitrogen uptake by dense understory vegetation, Sasa dwarf bamboo prevented the nitrate leaching to stream against the clear-cutting of trees in the basin. The riparian processes (denitrification and biological nitrogen uptake) were important for the stream chemistry in the basin with gentle topography in northern Hokkaido. These results address that development of the process model including the biological and hydro-topographical controls under the cold and snowy climate would be important for the further understandings of the ecosystem responses against the various environmental changes.
