亚热带丘陵区典型农业小流域水系温室气体产生、扩散与排放机理研究

River system of small catchment in subtropical agricultural hilly region becomes an inconvenient greenhouse gases (GHG, methane and nitrous oxide) source by the influence of anthropogenic activities. Therefore, there are many kinds of stream/river system in the small watershed, the environmental element is also very complex, together with the lack of field experiments, this leading to huge uncertainties of GHG emissions from this kind of ecosystem. Obviously, this limited the scientificity and applicability of assessment methodology of GHG emissions by the Intergovernmental Panel on Climate Change (IPCC). Consequently, we proposed this project to focus on the classical agricultural small catchment in subtropical China based on the combination of lab incubation and long-term field monitor. The objectives is firstly to discover the mechanism of GHG production and the spatiotemporal variation of GHG diffusion from waterbody of small watershed, reveal the key process involved into the transportation, and address clearly the vital environmental factors that influence the whole procedure by multivariant nonlinear regression analysis. Then modification of default emissions factor of GHG by IPCC will be conducted, and then comprehensive evaluation of GHG emissions factor and total amount from river system of small catchment was undertaken by our experimental data and historical research. Conclusionly, our study can not only provide theoretical foundation and policy advices for the response of both carbon and nitrogen cycle and GHG emissions to the environmental factor in the small catchment scale, but also can shed light on the methodology of global budget of GHG from wetland ecosystem.

受人类活动影响，农业小流域水系碳氮养分循环特征带有明显的"人类"印记，成为不可忽视的温室气体排放源。小流域水系类型众多，环境要素复杂，加之试验观测缺失，导致其温室气体排放具有较大不确定性，这制约了政府间气候变化专门委员会(IPCC)水系温室气体排放系数及总量评估方法学的科学性和适用性。为此，本项目以典型亚热带农业小流域水系为研究对象，结合室内培养实验与野外长期观测，探析水系温室气体产生机理；多途径观测水体温室气体扩散，阐明水系温室气体时空变异规律，揭示其关键传输过程，明确其主控环境因子；进而修正IPCC指南默认值，整合相关研究成果和本项目实测数据，综合评估小流域水系温室气体排放系数及扩散总量。本项目通过对典型农业小流域水系温室气体产生、扩散及总量评估方法的综合研究，为该系统温室气体排放和碳氮循环对环境变化的响应提供理论依据和区域减排政策建议，也可为全球湿地温室气体排放估算提供方法学支撑

为探讨小流域水系温室气体产生、扩散和排放机理，项目选择我国典型亚热带农业丘陵小流域——湖南金井小流域水系为研究对象，基于野外原位观测和室内培养实验，应用扩散模型法和室内土样培养法，利用多元统计手段，经过连续4年的高精度观测，开展了以下4方面的研究：1）典型农业流域温室气体传输时空特征；2）环境要素时空变异及对水体温室气体扩散传输的影响；3）小流域土壤温室气体产生培养实验研究；4）河流氧化亚氮间接排放研究。发现，小流域温室气体时间序列传输特征主要受温度、降水、流量等因素的影响，空间分布特征则明显受人类活动的影响，表现在受人类活动影响最少的山涧溪流（W1河段）温室气体溶存浓度最低，甲烷和氧化亚氮值分别为35.75 µg C m -2 h -1 和14.95µg N m -2 h -1，另外3级河段受人类活动影响较大，温室气体传 输通量显著高于 W1 河段，但两种温室气体传输通量都表现为从W2河段起随级别增大而降低的趋势。河岸土壤铵态氮、硝态氮和DOC浓度均显著高于河水中相应的浓度，而河水中硝态氮浓度高于铵态氮浓度。随河流级别增加（W1到W4），以及从流域源头到出口，河水中铵态氮、硝态氮和DOC浓度相对增加，而河岸土壤中的各变量浓度呈下降趋势。受外源碳氮及支流河水输入及水文气象条件的影响，农业小流域水体养分及污染物逐级增加，而坡土的养分逐级分解、流失，呈现明显的梯级关系。多元决策回归树和因子重要性分析等统计分析表明，溶解氧饱和度是影响河水甲烷和氧化亚氮传输的首要环境要素。随河流级别的增加（流域源头到出口），稻田甲烷产生潜力逐渐下降而氧化潜力逐渐升高；河岸土壤反硝化速率降低和硝化速率均降低，而稻田土壤中反硝化速率增加，硝化速率则逐级降低。河流底泥δ13C、δ15N稳定同位素和C/N时空变化结果与此相一致。使用2种方法计算的EF5r均值仅为0.00004（0.00002~0.0005），均显著低于IPCC 2006默认值（0.0025），因此，目前的河流氧化亚氮排放间接排放系数被明显高估。本项目研究结果，为该系统温室气体排放和碳氮循环对环境变化的响应提供理论依据和区域减排政策建议，也可为全球湿地温室气体排放估算提供方法学支撑。