【作者】 王雯；

【导师】 廖允成；

【作者基本信息】 西北农林科技大学 ， 作物栽培学与耕作学， 2013， 博士

【摘要】 研究黄土高原旱作麦田生态系统CO₂通量时空变化特征及其环境响应机制，对于探明旱区农田生态系统的碳循环过程，构建气候变化对农业生产影响的综合评估模型具有重要意义。本文基于涡度相关技术对黄土高原旱作麦田生态系统2008～2009和2009～2010冬小麦生育年的CO₂通量进行长期连续观测，主要结论如下：1. CO₂通量数据质量分析非稳态测试和湍流积分统计性检验的结果表明观测数据的质量比较可靠。功率谱分析和协谱分析的结果显示长武站的涡度相关系统对湍流信号在高频段的响应能力可以满足本研究的观测要求。能量平衡闭合分析的结果显示能量平衡闭合程度较好，2008～2009和2009～2010冬小麦生育年和生长季的能量平衡闭合率（斜率）分别为0.83、0.80和0.93、0.91。通量贡献区分析结果显示观测数据主要来源于研究所关注的区域（目标区）。在冬小麦生长盛期，白天约有90%的涡度相关数据来源于目标区，而夜间约有77%的涡度相关数据来源于目标区。数据质量评价的结果表明，在研究期内涡度相关系统运行状况良好，观测数据的质量比较可靠。2. CO₂通量的日、季节和年变化特征（1）CO₂通量的日变化特征。从返青期到灌浆期CO₂通量的日变化基本上呈“U”型曲线。最大CO₂净吸收峰值出现在2008～2009的抽穗期(-0.65±0.02mg CO₂m^-2s^-1)和2009～2010的拔节期(-0.51±0.02mg CO₂m^-2s^-1)。最大CO₂释放峰值同样出现在2008～2009的抽穗期(0.18±0.01mg CO₂m^-2s^-1)和2009～2010的拔节期(0.16±0.01mgCO₂m^-2s^-1)。（2）CO₂通量的季节变化特征。冬小麦生长季的净生态系统CO₂交换量（NEE）曲线呈现初期变幅小，中期迅速下降，后期逐步上升的变化趋势。最大日均NEE（绝对值）出现在2008～2009的抽穗期(-5.26±0.10g C m^-2d^-1)和2009～2010的拔节期(-5.68±0.11g C m^-2d^-1)，最大日均生态系统总初级生产力（GPP）也出现在这一时期，最大值日均生态系统呼吸通量(R_eco)则出现在生长季中后期。总体上，2008～2009和2009～2010冬小麦生长季的NEE总量分别为^-263.2±13.8和^-218.9±11.5g C m^-2d^-1，这表明在冬小麦生长季该生态系统具有较强的碳汇潜力，但是当考虑收获后冬小麦籽粒中的含碳量后，该麦田生态系统由强碳汇转变为弱碳汇(-65.4～^-36.2g C m^-2)。（3）CO₂通量的年变化。2008～2009和2009～2010生育年的NEE总值分别为-71.6±5.7和-65.3±5.3g C m^-2s^-1。夏季休闲期对年碳收支的影响不可忽视，该时期麦田生态系统表现为一个强碳源。两个夏季休闲期的累积R_eco削减了约25%～30%的生长季固碳量。考虑到影响年碳平衡的内部和外部因素，本文建议采取两方面措施来提高黄土高原旱作农田生态系统的固碳能力。3. CO₂通量的环境响应机制（1）CO₂通量对气象因子的响应。在冬小麦生长中期（返青期至灌浆期），光合有效辐射（PAR）和NEE的回归系数（R²）的范围为0.72～0.89。而且，在不同的气温（Ta）和饱和水汽压差（VPD）等级下，白天NEE的变化趋势存在差异。当15＜Ta＜25℃时，白天CO₂净吸收速率随着Ta上升进入稳定增长阶段，而当Ta超过30℃时，CO₂净吸收速率明显下降；当0＜VPD＜1kPa时，白天CO₂净吸收速率随着VPD的增大而增加，而当VPD超过2kPa时，CO₂净吸收速率开始下降。（2）CO₂通量对土壤因子的响应。一方面，除越冬期、灌浆期和成熟期外，其他生育期和夏季休闲期麦田生态系统呼吸对土壤温度（T_s）的变化反应较为敏感。另一方面，在冬小麦生长盛期（拔节期至灌浆期），在不同土壤水分条件下，白天NEE和PAR、夜间R_eco和T_s的相关关系存在差异。当土壤含水量（SWC）在0.15和0.21m³m^-3之间时，NEE-PAR、R_eco-T_s的相关性最高。4. CO₂通量对降水的响应机制（1）在冬小麦生长季麦田生态系统CO₂通量对有效降水（日降水量＞5mm）反应敏感。从返青期到灌浆期，在有效降水后1～3天，日最大30min R_eco较雨前增加了约70%～630%。这种“脉冲式”响应的原因可以归结为：物理替代效应、有机质矿化机制、微生物对干旱胁迫的反应机制。然而，在夏季休闲期，强降水事件（日降水量＞40mm）和连续降水事件（降水天数＞4天）使雨后1～2天的日R_eco值明显低于雨前水平。（2）雨前土壤含水量的影响。在冬小麦生长中后期，雨前土壤含水量（SWC）解释了约40%～42%的R_eco变化，而降水量解释了约25%～27%的R_eco变化。而在夏季休闲期，前者解释了约50%～63%的R_eco变化，而后者与R_eco基本无相关性。因此，对于旱作麦田生态系统而言，雨前土壤含水量对雨后R_eco的变化的影响力要高于降水量。综上所述，黄土高原旱作麦田生态系统具有较强的固碳能力，CO₂通量表现出明显的日、季节和年变化特征。CO₂通量受到气象因子、土壤因子以及降水事件等环境因子的综合影响表现出复杂的响应机制。本文的研究结论为探明黄土高原旱作农田生态系统碳循环过程，构建碳减排技术体系提供理论参考和技术支持。更多还原

【Abstract】 Study on the variation of CO₂fluxes and its mechanism of environmental response playsignificant roles in understanding the process of the carbon cycling, and constructing anevaluation model for the influence of the climate change on the agricultural production in therain-fed agro-ecosystem. Based on the eddy covariance （EC） technique, a long-termcontinuous observation was conducted in the rain-fed winter wheat ecosystem of ChineseLoess Plateau during2008^-2009and2009^-2010crop years. The main results are as follows:1. Data quality analysis of CO₂fluxesThe results of non-stationary nonstationarity test and integral turbulence characteristicsshowed that the quality of observation data were reliable. The results of power spectrum andcospectrum analyses indicated that the response ability of EC system to high frequency signalmet the requirements of the study. The analysis of the energy balance closure showed that thedegree of energy balance closure was good, and the rate of energy balance closure （slope）during the2008^-2009and2009^-2010crop years and growing seasons was0.83,0.80,0.93,and0.91, respectively. The flux footprint analysis showed that the observation data mainlycame from the area of study interest （target area）, during peak growing seasons of winterwheat, there were about90%EC data came from the target area in the daytime, whereas therewere about77%of EC data came from the target area in the nighttime. The evaluation of dataquality suggested that the EC system was in good working conditions and the quality ofobservation data was reliable during the study period.2. Diurnal, seasonal and annual variations of CO₂fluxes（1） Diurnal variation of CO₂fluxes. Diurnal variation of CO₂fluxes displayed aU-shaped curve from returning green stage to grain filling stage. The maximum uptake peaksof CO₂occurred in heading stage of2008^-2009(-0.65±0.02mg CO₂m^-2s^-1) and jointingstage of2009^-2010(-0.51±0.02mg CO₂m^-2s^-1), respectively. The maximum emission peaksof CO₂also occurred in the heading stage of2008^-2009(0.18±0.01mg CO₂m^-2s^-1) andjointing stage of2009^-2010(0.16±0.01mg CO₂m^-2s^-1), respectively.（2） Seasonal variation of CO₂fluxes. The curve of net ecosystem CO₂exchange （NEE）displayed the trend of changed slightly in the early period, and then decreased rapidly in the middle period, finally increased gradually in the late period. The maximum daily mean NEE（absolute value） occurred in heading stage of2008^-2009(-5.68±0.11g C m^-2d^-1) and jointingstage of2009^-2010(-5.26±0.10g C m^-2d^-1), respectively, the maximum daily mean grossprimary productivity （GPP） also occurred in the periods, however, the maximum daily meanecosystem respiration (R_eco) occurred in the middle and late growing seasons. Overall, thetotal NEE for2008^-2009and2009^-2010growing seasons were^-263.2±13.8and^-218.9±11.5g C m^-2d^-1, respectively, indicating that the winter wheat ecosystem was a strong carbon sink.However, after considering the carbon in the grain after harvest, the ecosystem turned into aweak carbon sink (-65.4～^-36.2g C m^-2).（3） Annual variation of CO₂fluxes. The annual NEE for2008^-2009and2009^-2010cropyears were-71.6±5.7and-65.3±5.3g C m^-2, respectively. Summer fallow periods played animportant role in annual carbon budget, the winter wheat ecosystem acted as a strong carbonsource in the period. The accumulated R_ecoduring summer fallow periods cut the amount ofcarbon sequestered during growing seasons by25%^-30%. Considering the internal andexternal factors that influenced the annual carbon balance, the paper put forward twomeasures to improve the carbon sequestration ability in the rain-fed agro-ecosystem ofChinese Loess Plateau.3. Environmental response mechanism of CO₂fluxes（1） Response of CO₂fluxes to meteorological factors. The regression coefficientsbetween NEE and photosynthetically active radiation （PAR） was0.72to0.89in the middlegrowing seasons of winter wheat （returning green stage to grain filling stage）. Furthermore,there were different trends of daytime NEE at different classes of air temperature （Ta） andvapor pressure deficit （VPD）. The net CO₂uptake rate gradually increased with the increasingof the Taunder the condition of15＜Ta＜25℃, whereas it began to decrease when Ta＞30℃.The net CO₂uptake rate increased with the increasing of the VPD under the condition of0＜VPD＜1kPa, whereas it began to decrease when VPD＞2kPa.（2） Response of CO₂fluxes to soil factors. On the one hand, the ecosystem respirationwas sensitive to the changes in soil temperature （T_s） during winter wheat growing seasons（except for the overwintering stage, grain filling stage and ripening stage） and summer fallowperiod. On the other, during the peak growing seasons of winter wheat （jointing stage toheading stage）, there were different relationships of daytime NEE-PAR, and nighttime R_eco-T_sunder the different soil moisture conditions. On the range of soil water content from0.15to0.21m3m^-3, the highest correlations of NEE-PAR, and R_eco-T_swere observed.4. Response mechanism of CO₂fluxes to rainfall events（1） The ecosystem CO₂fluxes were sensitive to the effective rainfall events （daily precipitation＞5mm） during the growing seasons. From returning green stage to grain fillingstage, the maximum half-hourly R_eco1^-3 days after the effective rainfall events were70%-630%higher than that before the effective rainfall events. The reasons for the pulseresponse of R_ecowere as follows: the effect of physical alternative, the mechanism of organicmatter mineralization, the response mechanism of microorganism to drought stress. However,during the summer fallow periods, the daliy R_ecovalues1^-2 days after heavy rainfall event（daily precipitation＞40mm） and continuous rainfall event （rainfall days＞4d） were lowerthan that before these rainfall events.（2） The effects of the antecedent soil water content. During the middle and late growingperiods, the antecedent soil water content explained about40%-42%variation in R_eco,whereas precipitation explained about25%^-27%variation in R_eco. During the summer fallowperiods, the antecedent soil water content explained about50%-63%variation in R_eco,whereas there were almost no correlation between the precipitation and the R_eco. Thissuggested that the effect of antecedent soil water content on the variation of R_ecowas higherthan the effect of precipitation on the change of R_ecofor the rain-fed winter wheat ecosystem.In summary, the rain-fed winter wheat ecosystem had strong carbon sequestration ability.Meanwhile, there were obvious diurnal, seasonal, and annual variations in CO₂fluxes. CO₂fluxes were affected by the factors such as meteorological factors, soil factors, and rainfallevents, and displayed a complex response mechanism. The research results may providetechnical support and theoretical guidance for understanding the carbon cycling processes,and constructing the technical system of carbon emission reduction in the rain-fedagro-ecosystem of the Chinese Loess Plateau.更多还原