Nitrogen and phosphorus dynamics at plant soil ecosystems of Ozegahara mire under impacts of floodings

第4次尾瀬総合学術調査の一環として，尾瀬ヶ原泥炭土壌の物理化学的性状と植生土壌生態系の窒素・リン動態の調査が尾瀬湿原39地点で行われた（2017年7月～2018年8月）．泥炭土壌は水飽和状態に近く，弱酸性で，河川流路に近いヤチヤナギが高密度に分布するバンクホロー複合体の湿原凹地では，土壌の可給態リン量が多く，NO3-量，窒素代謝活性と酸化還元電位，および可給態リン量が水位変化と密に関連していた．下田代の湿原凹地のヤチヤナギ窒素固定活性の調査から，ヤチヤナギの窒素固定量と樹高の間に有意な相関関係が見出された．土壌の窒素・リン量とヤチヤナギの分布パターンとの関係の検討により，河川洪水氾濫水により湿原に運ばれたリンの湿原凹地への沈積が，ヤチヤナギ増殖活発化を招いたと推論された．葉生産から出発し，落葉の分解無機化，土壌中の微生物活動による窒素固定と脱窒，ヤチヤナギの根粒微生物による窒素固定を経て，植物体再生産に至る窒素循環の量的検討により，尾瀬ヶ原の植物土壌システムの窒素収支は，降水に伴う窒素供給を含めると，ほぼ植物葉の生産を賄うことが示された．この窒素収支の検討から，洪水に伴う外部からの窒素負荷増加は，植物生産増加をまねく可能性が示された．今後，河川洪水に伴う窒素・リンの尾瀬ヶ原湿原への供給と流出を含めた尾瀬ヶ原の土壌・植生システムの窒素，リン動態のさらなる詳細な量的調査が必要とされる．As a part of the 4th Scientific Research the Oze mire, nitrogen and phosphorus dynamics of the soil-plants systems were studied at a number of selected survey sites of Ozegahara (July 2017-August 2019). Wet hollow mire and fen dich under the impacts of flooding were characterized by abundant occurrence of Myrica gale var. tomentosa in association with plentiful available phosphorous in the soils. Transect survey through many mire sites at the bank-hollow complex indicated significant correlations among NO3- concentration, denitrification activity, redox-potential, and water level in the soil. We also found a significant correlation between nitrogen fixation activity and stem height of the Myrica at Shimotashiro mire. Abundant occurrence of phosphorus in the soils at mire hollows closing to the rivers was regarded to be caused by flood associated external supply of phosphorus. Analysis of nitrogen transfer through plants and soil ecosystems at 12 survey sites indicated almost balanced nitrogen cycles through soil-plant ecosystem started by leaf production followed by mineralization of fallen leaves, microbial nitrogen fixation and denitrification at soil system, nitrogen fixation at root nodules of the Myrica and nitrogen returning to leaf production at respective ecosystem under external input of nitrogen by falling rain. Further detailed studies are needed on the effects of external supply of nitrogen by flooding on production of Myrica gale and other plants as wall as nitrogen cycle through plants soil ecosystem at Ozegahara