暖温带与中温带过渡区典型森林非生长季土壤CO2通量及其对冻融的响应机制

Forest soil CO2 flux in non-growing season play an important role in regional carbon balance. Snowpack and litter on the surface soil which can be an effective thermal insulation layer in non-growing season, and freezing and thawing alternation process which is an important period of non-growing season in seasonal frozen area. Their contributions to soil CO2 emission cannot be ignored; however, the response mechanism of soil CO2 flux to these impact factors are still unknown. In this study, the temperate econtone which is very sensitive to global climate change was chosen as the samples belt, and the theory of soil and forest ecology were used, combined with field research and control experiment were applied, with two typical forests in this area as the research object. We study the dynamics of soil CO2 flux and its components for the two typical forests in temperate econtone in non-growing season and how they are relevance to the differences of non-growing season characteristics. Quantification of contribution of soil CO2 flux and its components for different surface covers in non-growing season, and reveals the inherent mechanism of their effects. Based on the natural soil freeze - thaw process set the simulation freeze - thawing pattern conditions for revealing the response characters of natural freeze - thaw pattern on soil CO2 efflux. It has great significance for precise evaluation of contribution of soil carbon sequestration to the ecosystem carbon budget and responses of forecast ecosystem carbon cycle to global climate change.

森林非生长季土壤CO2通量是区域碳收支非常重要的组成部分。作为非生长季内土壤有效保温层的长期雪被以及季节性冻土区非生长季内经历的冻融交替过程对土壤CO2排放的影响不容忽视，然而有关它们对土壤CO2通量的响应机制研究还很欠缺。本研究选择在对全球气候变化极为敏感的暖温带与中温带过渡区域建立观测样带，以研究区内两种典型森林为研究对象，应用土壤学和森林生态学等理论，结合野外原位研究和控制实验模拟，研究过渡区两种典型森林非生长季土壤CO2通量及其组分动态变化规律与非生长季特征因子相互作用机制；量化地表覆被物对非生长季土壤CO2通量及其组分影响的贡献并揭示其影响的内在机理；基于自然土壤冻融过程设置冻融格局模拟条件，研究自然状态下冻融格局对土壤CO2排放的响应规律。本研究对准确评估土壤碳固存对生态系统碳收支的贡献和预测生态系统碳循环对全球变化的响应具有极其重要的意义。

大量研究证实了低温下部分土壤微生物仍具有活性因此非生长季内能够维持较高的有机碳分解速率，其土壤CO2释放量的份额能达到全年总量的17-30%。冻融交替作用是季节性冻土区非生长季土壤CO2排放的一个主导影响因素，它对非生长季土壤CO2的排放起着决定性的作用。本项目选择对全球气候变化极为敏感的暖温带与中温带生态过渡区，运用土壤学和森林生态学理论，综合野外原位观测和控制实验模拟研究典型森林非生长季土壤CO2通量及其对冻融的响应机制。结果显示，非生长季落叶松人工林平均土壤呼吸速率年际间的差异很小，最小值出现在1-2月份与当年最低气温的出现时间相吻合。非生长季内土壤呼吸速率及土壤碳排放的贡献率均为秋冬冻融期最大，冬春冻融期次之，冻结期最小，表明土壤在进入完全冻结阶段前经历的冻融交替激发释放CO2气体速率的能力比在进入完全融解阶段前经历的冻融交替更快。应用冻融交替模拟实验分析冻融格局对土壤呼吸速率的影响规律，得到弱度冻融交替（-8℃）时土壤呼吸速率变化较平缓，未出现明显的峰值；强度冻融交替（-16℃）时，土壤呼吸速率波动比弱度冻融交替剧烈，在第1和第3次冻融交替时出现了两个明显的峰值，随后逐渐趋于平缓。应用冻融交替模拟实验结合野外原位研究揭示单日和季节尺度上土壤呼吸速率变化特征，得到土壤呼吸速率日动态呈单峰型变化，模拟未发现固定峰值，野外则在14:00时达到峰值；土壤呼吸速率春季冻融时期土壤呼吸速率随着解冻进程而增大，模拟时强度冻融出现两次峰值，弱度冻融无峰值，而野外也未发现峰值。经过冻融交替后，土壤理化性质变化趋势为容重和毛管孔隙度变大，非毛管孔隙度和总孔隙度变小，有机碳含量降低，配对样本t检验结果显示冻融模拟实验的影响更显著；经过冻融交替后，土壤生物学特性的变化规律为，土壤真菌、细菌和放线菌数量都显著剧烈减少。2010-2014期间土壤年CO2释放量在521-763 g C m-2之间，非生长季土壤CO2释放量占17-22.2%。指数模型能够较好的模拟土壤呼吸速率随10cm深度土壤温度的变化规律。生长季Q10值在2.18-3.29之间，非生长季在2.45-5.47之间，表明非生长季土壤呼吸速率的温敏性强于生长季。本研究对于科学评估暖温带与中温带过渡区典型森林非生长季土壤CO2释放规律，揭示其内在影响机制以及预测生态系统碳循环对全球变化的响应具有相当重要的现实意义。