Effect of nitrogen deposition on fluxes of green house gases in a forest ecosystem

已知氮沉降影响森林生态系统中温室气体的排放和吸收。氮沉降增加森林土壤氧化亚氮排放，抑制森林土壤甲烷排放。氮沉降对森林生长和CO_2固存有促进或抑制作用。然而，目前的研究大多集中在氮沉降高的氮饱和森林（DIN > 10 kgN ha^<-1> y^<-1>），氮沉降对低氮沉降森林（DIN < 10 kgN ha^<-1> y^<-1>）温室气体通量的影响尚需进一步研究。IPCC提出了“全球变暖潜势”（GWP）的概念，作为衡量生态系统生物地球化学过程对全球变暖影响的综合指标。以前的研究主要集中在这些温室气体中的一种，如二氧化碳、一氧化二氮和甲烷。很少有研究报告了这些气体或全球变暖潜能值对森林生态系统的总影响。本研究旨在揭示极低氮沉降对落叶松森林GWP的影响。我们在日本北部札幌市白羽山落叶松森林的五个地点进行了研究。采用密闭室法测定了土壤/积雪表面N_2O排放和CH_4吸收。通过测量树木高度和呼吸高度时的直径、异速生长方程和室内湿润土壤培养测量的微生物呼吸来估算co2的吸收。无雪期氮沉降范围为0.8 ~ 1.8 kgN ha^<-1> y^<-1>。CH_4年吸收量为5 ~ 7 kgC ha^<-1> y^<-1>，与氮沉降无相关性。但与文献数据比较表明，该研究地点的高CH_4吸收率是由于氮沉降非常低而抑制CH_4吸收。由于N_2O排放与土壤温度和土壤湿度没有相关性，本研究将氧化亚氮排放归为背景型排放。背景型N_2O排放被认为不受氮沉积的影响。N_2O年排放量范围为0.02 ~ 0.09 kgN ha^<-1> y^<-1>。无雪期累积N_2O排放量与有雪期氮沉降呈显著正相关。与文献中背景型N_2O排放数据的比较表明，即使是背景型N_2O排放也受到氮沉降的影响。净生态系统生产力（NEP）为0.2 ~ 2.1 tC ha^<-1> y^<-1>。无雪期NEP与氮沉降呈显著正相关。全球变暖潜能值的计算结果表明，森林植被对co2的吸收是研究区全球变暖潜能值的最大组成部分；N_2O排放和CH_4吸收对GWP的贡献可以忽略不计。综上所述，氮沉降对全球变暖有负作用，氮沉降促进树木生长的作用远大于氮沉降增加N_2O排放和减少CH_4吸收的作用。少

Nitrogen deposition has been known to affect emission and absorption of green house gases in forest ecosystems. Nitrogen deposition enhances nitrous oxide emission from forest soils and inhibits methane emission into forest soils. Nitrogen deposition might enhance or diminish forest growth and CO_2 sequestration. Most researches, however, focus on nitrogen-saturated forests receiving high deposition of nitrogen (DIN > 10 kgN ha^<-1> y^<-1>), and we need more information on the effect of nitrogen deposition on green house gases fluxes in forests receiving low nitrogen deposition (DIN < 10 kgN ha^<-1> y^<-1>). IPCC proposed the concept of 'Global Warming Potential' (GWP) as a comprehensive measure of effect of ecosystem biogeochemical processes on global warming. Previous studies mainly focused on one of these green house gases, e.g., carbon dioxide, nitrous oxide and methane. Very few studies reported total effect of these gases or GWP of forest ecosystems. The purpose of this study is … More to reveal the effect of very low nitrogen deposition on GWP in a larch forest. We studied at five sites in larch forest in Mt. Shirahata, Sapporo city, northern Japan. N_2O emission and CH_4 absorption at the surface of soil/snowpack were measured with closed chamber method. CO_2 sequestration was estimated with measurement of tree heights and diameters at breath height, allometric equation, and microbial respiration measured by incubation of moist soils in laboratory.Nitrogen deposition during no-snow season ranged 0.8 〜 1.8 kgN ha^<-1> y^<-1>. Annual CH_4 absorption ranged 5 〜 7 kgC ha^<-1> y^<-1> and showed no correlation with nitrogen deposition. But comparison of our result with literature data suggested that high CH_4 absorption in this study site was caused by very low nitrogen deposition that is known to inhibit CH_4 absorption. Nitrous oxide emission in this study was classified as background type emission because N_2O emission showed no correlation with soil temperature and soil moisture. Background type N_2O emission has been believed not to affected by nitrogen deposition. Annual N_2O emission ranged 0.02 〜 0.09 kgN ha^<-1> y^<-1>. Accumulated N_2O emission during no-snow season showed positive correlation to nitrogen deposition during now-snow season. Comparison of our results with literature data on background type N_2O emission suggested that even background type N_2O emission is affected by nitrogen deposition. Net ecosystem productivity (NEP) ranged 0.2 〜 2.1 tC ha^<-1> y^<-1>. NEP showed positive correlation to nitrogen deposition during no-snow season. Calculation of GWP revealed that CO_2 sequestration by forest vegetation was largest component in GWP in this study site ; contribution of N_2O emission and CH_4 absorption to GWP was negligible. In summary, nitrogen deposition has a negative effect on global warming and tree growth enhancement by nitrogen deposition has much larger role than enhanced N_2O emission and reduced CH_4 absorption by nitrogen deposition. Less