温带森林土壤CH4吸收和N2O排放对氮沉降和降水变化响应机制

There is very limited knowledge about the responses of methane consumption and nitrous oxide emission from forest soils to increasing atmospheric nitrogen deposition and climatic change. This is not extremely beneficial for our understanding of sinks and sources of greenhouse gases in forests of China. In this program, an undisturbed mature temperate Korean pine and broadleaf mixed forest and its secondary forest during sucession, which locates at the foot of Changbai mountains in northeastern China, can be selected to carry out in situ measurements of throughfall carbon and nitrogen inputs, and a series of field experiments concerning simulated nitrogen deposition at different levels and change in throughfall and the regulation of carbon input under forest floors. In the light of measurements of greenhouse gas fluxes, carbon and nitrogen inputs via throughfall and their leaching in the soil under these forest stands, laboratory simulation experiments and molecular biology studies, we shall try to quantatially assess the properties concerning methane consumption and nitrous oxide emission under temperate forest stands of northeastern China. It will be established some quantatial relationships of methane consumption and nitrous oxide emission from temperate forest stands to soil temperature and moisture, soil active carbon and nitrogen pools, and the amount of leached carbon and nitrogen from forest floors. It will be illustrated microbial population structure and microbial activities under forest floors related to soil methane consumption and nitrous oxide emission and their respective responses to changes in throughfall, nitrogen deposition and forest succession. Finally, we shall try to explain the mechanisms involving the effects of change in throughfall, increased atmospheric nitrogen deposition and forest types on the net fluxes of methane and nitrous oxide under temperate forests in northeastern China. The results obtained can improve one to understand the net fluxes of methane and nitrous oxide under temperate forests in China and their respective responses to increasing atmospheric nitrogen deposition and climatic change. Also, some important scientific methods can be provided to regulate the sinks and sources of greenhouse gases in northern forests in China under global change sceneries in the future.

目前森林土壤甲烷(CH4)和氧化亚氮(N2O)净通量对大气氮沉降增加和气候变化的响应研究十分缺乏，极不利于认识我国森林温室气体源汇特征及驱动机制。本项目拟在长白山成熟阔叶红松林及其演替次生林实施氮沉降模拟、降水变化和碳输入控制实验，通过林地温室气体净通量、碳氮湿沉降和淋溶通量观测、室内模拟实验和分子生物学研究，定量评估我国北方温带森林CH4吸收和N2O排放特征；建立林地CH4和N2O净通量与土壤温度、水分、土壤活性碳氮库和碳氮淋溶等因素之间的定量关系；阐明林地CH4吸收和N2O排放功能微生物丰度和群落组成及活性以及对降水和氮沉降改变及森林演替的响应；探讨氮沉降和降水改变及森林类型对温带森林土壤CH4和N2O净通量的影响机制。研究结果利于正确认识我国温带森林CH4和N2O净通量特征以及对未来氮沉降增加和气候变化的响应，为寻求合理措施来调节全球变化背景下森林温室气体源汇过程提供重要的科学基础。

当前我国仍缺乏长期研究典型区域森林土壤甲烷和氧化亚氮净通量潜力以及对大气氮沉降量增加和气候变化的响应。本项目基于长白山典型森林长期氮沉降量增加、森林穿透雨改变以及林地凋落物去除野外控制实验，围绕我国阔叶红松混交成熟林和次生白桦林土壤甲烷和氧化亚氮净通量以及有关环境因素开展数据收集工作。通过两种林分野外控制实验、森林穿透雨以及土壤渗漏液中溶解性有机碳和不同形态氮湿沉降量和淋溶通量观测，揭示了上述温带森林土壤甲烷和氧化亚氮净通量的季节性和年际变化及其差异性；分析了森林凋落物、模拟氮沉降量增加和穿透雨改变对森林土壤甲烷和氧化亚氮净通量的影响及其作用机制。基于野外控制实验多年观测结果，显示阔叶红松混交成熟林和次生白桦林在生长季土壤氧化亚氮年排放量分别为0.40~0.95(平均值0.61)和0.04~0.08(均值0.06) kg N2O-N ha-1，其温度效应Q10值分别为0.9~4.3(均值2.2)和1.3~3.5(均值2.1)；发现上述两种林分在生长季土壤甲烷年吸收量分别为1.69~2.76(平均值2.25)和1.36~1.51(均值1.41) kg CH4-C ha-1。在剔除林地凋落物微区，两种林分生长季土壤氧化亚氮年排放量均值分别增加0.37和0.74 kg N2O-N ha-1，甲烷年吸收量均值分别减少0.50和0.53 kg CH4-C ha-1。增施氮各处理使阔叶红松混交成熟林和次生白桦林在生长季土壤氧化亚氮年排放量均值分别增加2.09和1.44 kg N2O-N ha-1，约占每年施氮量5.9%和3.6%；两种林分施氮样地甲烷年吸收量均值分别降低24%和28%。根据森林穿透雨增加/减少控制实验五年观测，指出阔叶红松混交成熟林生长季穿透雨增加25%的实验样地氧化亚氮年排放量和甲烷年吸收量变化范围分别为0.91~2.31(均值1.37) kg N2O-N ha-1和0.27~1.64(均值0.97) kg CH4-C ha-1，高于林地穿透雨无改变处理氧化亚氮年排放量(变化范围0.67~2.10，均值1.17)、并低于甲烷年吸收量(变化范围0.40~1.70，均值1.13)；森林穿透雨减少处理样地在生长季氧化亚氮和甲烷年净通量均值分别减少20%和增加28%。研究结果为认识我国温带森林土壤甲烷和氧化亚氮净通量对气候变化和未来大气氮沉降量增加的响应提供重要支撑。