大気由来の窒素負荷が森林生態系の窒素循環および林床からのN_2O放出に与える影響

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(NO_3^–) 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(NH_4^+ and NO_3^–) flux in throughfall. The amount of nitrogen in the litterfall and the inorganic nitrogen(NH_4^+ and NO_3^–) flux in A_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 N_2O emissions from the forest floor and inorganic nitrogen(NH_4^+ and NO_3^–) 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 NH_4^+ and NO_3^– 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 NO3^–. 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 A_0 layer percolation in the summer accompanied rainfall events just after a prolonged dry period. At the mountainous sites in Oku-nikko(NM and NY) and in Norikura(SC and SD), the total NH_4^+ and NO_3^– flux in A_0 layer percolation were the same or less than those in throughfall. In coniferous forest at site SC, inorganic nitrogen that corresponding 15 times that in throughfall infiltrated the A_0 layer as NH_4^+, an exceptional value. At sites NM and NY, the NO3^– flux in A_0 percolation was very high during the snowmelt period. These results suggest that at temperate forest sites with high nitrogen deposition the inorganic nitrogen(NH_4^+ and NO_3^–) fluxes through the A_0 layer are high and are controlled by nitrogen mineralization during the decomposition of organic matter. In contrast, at mountainous sites with low nitrogen deposition, the inorganic nitrogen fluxes into the soil layer were very low. Thus, nitrogen immobilizations may outweigh nitrogen mineralization because of the poor nitrogen environment. In addition, relatively high NO3- flux pulses that appeared abruptly were detected after a prolonged dry period in summer or during the snowmelt period. Under these extreme conditions, microorganisms would be damaged and the increase in nitrogen mineralization was likely induced by the death or regeneration of these. A 2-yr nitrogen manipulation experiment was carried out in temperate forests(IK and IY). Nitrogen(NH_4NO_3) application equal to twice the amount in throughfall induced increases in inorganic nitrogen(NH_4^+ and NO_3^–) fluxes only in the A_0 layer, but no change was observed in the mineral soil layer. Nitrogen removal also resulted in no significant effect on NH_4^+ and NO_3^– fluxes in the mineral soil layer. These findings suggest that nitrogen cycling in temperate forest ecosystems is more strongly governed by the internal nitrogen cycling, which consists of decomposition and assimilation of organic matter, than by short-term changes in nitrogen deposition. Next, we focused on litterfall nitrogen as an index of internal nitrogen cycling. The amount of nitrogen in the litterfall and the C/N ratios of them were compared among sites. The characteristics of litterfall differed greatly between coniferous and deciduous trees. The C/N ratios in deciduous litterfall were large compared to those of coniferous litterfall, and they differed according to species. Among deciduous trees litterfall nitrogen also showed large variation across the site and was high at the sites with high nitrogen deposition. N_2O emissions from the forest floor was measured at the six sites as an index of nitrogen output from the ecosystems. High averaged N_2O emission rates were detected at sites with high nitrogen deposition or high nitrogen internal cycling via litterfall. Moreover, N_2O emission rates showed a seasonal pattern similar to that of inorganic nitrogen(NH_4^+ and NO_3^–) fluxes in A_0 layer percolation. These results suggest that N_2O emission will occur at sites where there is a surplus of inorganic nitrogen, over both the short and long terms, for N_2O is generated with mineralized nitrogen. To confirm whether the positive correlation between nitrogen deposition and N_2O emission noted at the study sites was applicable to other sites, N_2O emission was measured at about 60 points across Japan. The concentration of NO_3^– in both soil water and stream water had a significant positive correlation with the estimated nitrogen deposition. There was also a significant positive correlation(but with a low coefficient) between N_2O emission rates and estimated nitrogen deposition. Our findings indicated that the nitrogen deposition level near urban areas in Japan, where nitrogen saturation has been suspected, is approximately the same as that in nitrogen-saturated forests in Europe. The chronic nitrogen deposition has increased the amount of nitrogen cycling within the plant–soil system. Our findings also suggest that the effect of increased nitrogen deposition in forest ecosystems depends on their capacity for nitrogen maintenance. The N_2O emission rate, which is one nitrogen output from an ecosystem, showed a positive correlation with total nitrogen deposition and internal cycling of nitrogen, as well as NO3^– leaching from soils. N_2O shows clear seasonal patterns and is an effective index of short-term changes in the supply and demand of inorganic nitrogen.世界で人口増加や経済発展が進む中、食糧増産のための化学肥料の投入や増加する化石燃料の利用にともなう燃焼時のNOx放出などにより、大気圏および生物圏において反応性窒素は急激に増加しており、陸上生態系の窒素循環に及ぼす影響が懸念されている。本研究では、増加する大気由来の窒素負荷が森林の窒素動態に与える影響を明らかにするために、窒素負荷量の異なる関東周辺の6地点で窒素循環量を比較した。 林内降水中のNO_3^–およびNH_4^+を生態系への窒素のインプットとし、土壌からのNO_3^–およびNH_4^+の流出量を生態系からのアウトプットとして比較した。2から29kg N ha^ yr^の窒素負荷量レンジに対し、流出量は、1kg Nha^ yr^以下から13kg N ha^ yr^の範囲であった。窒素負荷量が小さい地点では、流出量も小さい傾向にあったが、窒素負荷量が10-15kg N ha^ yr^ の範囲では、窒素流出量は大きくばらついた。このことから窒素アウトプット量は窒素インプット量だけでは説明できないことが明らかになった。 窒素負荷量が多い茨城の2地点では、浸透水が有機物層 (A_0層) を通過する際、無機態窒素量は林内降水の約2.5倍に増加した。一方、窒素負荷量が少ない奥日光や乗鞍の山岳地点 (乗鞍の針葉樹林を除く) では、A_0層浸透水の無機態窒素量は、林内降水と同程度かやや下回った。すなわち、リターフォールなど有機物の分解過程で生じる無機態窒素量は地点間で変動が大きいことが示された。 更に、2年間にわたって実施した窒素添加・除去実験の結果、窒素添加により、A_0層浸透水のNO_3^–およびNH_4^+フラックスは増加したが、鉱質土壌層の土壌水では有意な差は認められなかった。一方、除去実験では、A_0層浸透水のNO_3^–およびNH_4^+フラックスが減少する場合もみられたが、鉱質土壌層での変化は認められなかった。すなわち、窒素負荷量の多い茨城の調査地では、外部から負荷される窒素量の短期的な変化よりも、微生物による窒素の有機化や無機化など内部循環にともなう無機態窒素の動態の寄与が大きいことが示された。 林床からの亜酸化窒素 (N_2O) 放出量を比較した結果、窒素負荷量やリターフォール窒素量が多い地点で多く、窒素流出量と同様の傾向があった。国内の59か所で実施した短期一斉観測では、土壌および渓流水のNO_3^–濃度は、N_2O放出量と共に、各地点の推定窒素負荷量と有意な正の相関を示した。 リターフォール窒素量については、落葉広葉樹林と針葉樹林で窒素負荷量との関係が異なることが知られているが、本調査でも、針葉樹林に比べ落葉広葉樹林でリターフォール窒素量の変動幅が大きかった。一方、林外降水窒素量に対する林内降水窒素量の比は、針葉樹林で落葉広葉樹林に比べて大きかった。このことから、同じ地点でも、植生タイプの違いにより、林内降水窒素量とリターフォール窒素量のバランスが異なることが示唆された。長期にわたる窒素負荷は、土壌-植物系の内部循環窒素量の変化をともなう場合もあることから、林床への窒素インプットとして、窒素負荷量 (林内降水窒素量) と内部循環窒素量 (リターフォール窒素量) の両方を勘案することが妥当と考えられた。生態系からの窒素アウトプット経路の一つであるN_2O放出は、土壌からのNO_3^–流出と同様に、林内降水窒素量とリターフォール窒素量との合計と高い正の相関を示した。本研究により、森林生態系からの窒素流出は、降水にともなう大気由来の窒素負荷量と系内の循環窒素量とによって支配されていることが明らかになった