17:15 〜 18:30
[MIS11-P07] Nitrogen budget over two decades in a headwater catchment of a natural cool-temperate forest
キーワード：窒素動態、硝酸態窒素、粗大有機物、渓流水質、ナラ枯れ
Recently, forest ecosystem has faced various disturbances such as global warming, increasing nitrogen deposition, intensive grazing by herbivorous mammals, insect attacks, and so on. These disturbances will cause not only the loss of biodiversity, but also the alteration of nutrient cycling and ecosystem services. To elucidate the complicated effects of various disturbances under natural condition, a long-term assessment of forest dynamics and nutrient cycling is necessary. We examined biomass and primary production of trees, atmospheric deposition and nutrients, especially nitrogen export to stream over two decades in a 7-ha headwater catchment of a cool-temperate natural forest in Ashiu Research Forest, Field Science Education and Research Center, Kyoto University.
Above-ground net primary production was constantly increased from 1992 to 2004, but it decreased from 2004 to 2010 partly because of extensive dieback of Japanese oak Quercus crispula driven by ambrosia beetle during this period. After that, it increased again due to the rapid growth of Cryptomeria japonica. From 1990 to 2002, nitrogen deposition doubled and stream water nitrate concentration increased over 4 times. Since 2002, a clear temporal trend in nitrogen deposition was not found (6.3 to 11.3 kgN ha-1 yr-1), while stream nitrate concentration and inorganic nitrogen exports were increasing gradually from 2002 to 2008 and decreasing from 2009 to 2018. Under the condition that nitrogen deposition was relatively constant, an increase in stream nitrate concentration and inorganic nitrogen exports would be caused by the Japanese oak wilt dieback. This trend, however, continued for only a few years, mainly owing to the compensation of C. japonica growth for the dieback.
An annual export of coarse particulate organic matter (CPOM) and nitrogen (CPON) for two decades fluctuated largely year by year, ranging from 1.8 to 47.3 kg ha-1 yr-1 and from 0.02 to 0.36 kgN ha-1 yr-1, respectively, both of which were strongly regulated by the maximum daily flow rate in each year. The ratio of annual CPON export to inorganic nitrogen export was 1- 12 %. The results indicate that the inorganic nitrogen export is directly affected by the oak dieback, while CPON export highly depends on the storm intensity.
Above-ground net primary production was constantly increased from 1992 to 2004, but it decreased from 2004 to 2010 partly because of extensive dieback of Japanese oak Quercus crispula driven by ambrosia beetle during this period. After that, it increased again due to the rapid growth of Cryptomeria japonica. From 1990 to 2002, nitrogen deposition doubled and stream water nitrate concentration increased over 4 times. Since 2002, a clear temporal trend in nitrogen deposition was not found (6.3 to 11.3 kgN ha-1 yr-1), while stream nitrate concentration and inorganic nitrogen exports were increasing gradually from 2002 to 2008 and decreasing from 2009 to 2018. Under the condition that nitrogen deposition was relatively constant, an increase in stream nitrate concentration and inorganic nitrogen exports would be caused by the Japanese oak wilt dieback. This trend, however, continued for only a few years, mainly owing to the compensation of C. japonica growth for the dieback.
An annual export of coarse particulate organic matter (CPOM) and nitrogen (CPON) for two decades fluctuated largely year by year, ranging from 1.8 to 47.3 kg ha-1 yr-1 and from 0.02 to 0.36 kgN ha-1 yr-1, respectively, both of which were strongly regulated by the maximum daily flow rate in each year. The ratio of annual CPON export to inorganic nitrogen export was 1- 12 %. The results indicate that the inorganic nitrogen export is directly affected by the oak dieback, while CPON export highly depends on the storm intensity.