| 摘要 | 黄河三角洲滨海湿地是陆海交互作用而形成的独特生态系统, 由于其较高的碳固定能力和较低的碳降解能力而成为缓解全球变暖有效的蓝色碳汇。但在全球气候变化背景下, 湿地能否继续保持碳汇地位存在着极大的不确定性。本研究通过涡度相关和微气象观测技术, 对黄河三角洲芦苇湿地净生态系统CO2交换(NEE)以及环境、生物因子进行了观测, 以探讨(1) 黄河三角洲芦苇湿地净生态系统CO2交换(NEE)的变化规律; (2) 环境因子和生物因子对黄河三角洲芦苇湿地NEE的影响; (3) 阴晴天对黄河三角洲芦苇湿地NEE的影响; (4) 黄河三角洲芦苇湿地NEE组分区分及碳收支评估。
结果表明: (1) 2013年湿地生态系统各月NEE日均值呈明显“U”型曲线, 表现为白天NEE为负值(净CO2吸收), 夜间为正值(净CO2排放)。但生长季与非生长季间NEE日振幅具有明显差异。在1-4月和11-12月的非生长季, NEE波动性比较小, 规律不明显。在5-10月的生长季, NEE日变化规律明显, 整体呈现明显的“U”形。湿地于6月达到NEE日均极值(-10.0 μmolCO2m-2s-1), 而夜间生态系统排放最大值出现在7月(3.6 μmolCO2m-2s-1)。
(2) NEE与环境因子中光合有效辐射(PAR)、气温(T)、土壤温度(Tsoil)、土壤湿度(SWC)以及降雨量(PPT)的季节变化均呈生长季5-10月较高, 非生长季1-4月和11-12月较低分布。PAR月平均最高值出现在6月(500.8 μmolm-2s-1), 6月以后由于降水增多, PAR离散趋势明显, 且呈现下降趋势。全年日均气温为12.1 oC, 生长季日平均气温为21.9 oC, 其中8月份月平均气温最高(27.7 oC), 12月份最低(-1.3 oC)。全年Tsoil日均值为12.6 oC, 生长季日均值为22.0 oC, 与气温相比, 波动较小。2013年生长季总降水量为434mm, 占全年降水的近68%, 其中8月降水占到全年的60%。地上生物量与叶面积指数在植被生长初期开始增长, 分别在8月底(635.5 gm-2)和7月底(0.6)达到最大值。NEE具有明显的季节变化特征, 呈“U”型曲线, 生长季表现为碳汇, 波动比较大; 非生长季表现为碳源, 波动比较小。在7月30日达到NEE日吸收极值(-7.0 gCm-2d-1) , 在6月22日达到NEE日排放极值(2.3 gCm-2d-1)。
(3) 生长季白天各月生态系统NEE和光合有效辐射(PAR)之间均呈直角双曲线关系, 通过Michaelis-Menten 模型对生长季各月的参数进行了估计, 其中α与Reco,daytime均于8月达到最大值, 分别为0.08 μmolCO2μmol-1PAR和10.8 μmolCO2m-2s-1, Amax于7月达到最大值为25.4 μmolCO2m-2s-1。夜间生态系统呼吸(Reco,nighttime)受气温和土壤含水量的共同影响。当SWC小于40%时, Q10最大(2.8), 随着SWC增加, Q10逐渐减小; 在整个生长季中, 湿地Q10为2.5, 年尺度上, 湿地Q10为3.6。
(4) 在自然条件下选择12对相邻阴晴天数据, 在生物要素(生物量、叶面积指数)、土壤水分以及养分特征保持不变的前提下, 揭示阴晴天变化对湿地生态系统NEE光响应和温度响应的影响。12对阴晴天生态系统NEE的日平均动态均呈“U”型曲线, 但阴天NEE的变幅较小。晴天条件下湿地生态系统NEE日均值显著高于阴天 (P < 0.01)。阴晴天湿地生态系统NEE与PAR之间均呈直角双曲线关系, 但晴天条件下, 最大光合速率Amax显著大于阴天(P < 0.01), 同时白天生态系统呼吸Reco,daytime也显著大于阴天(P < 0.01)。阴晴天条件下, Reco,daytime与气温均存在显著的指数关系。晴天湿地生态系统呼吸的温度敏感系数Q10 (5.5)远大于阴天(1.9)。阴晴天昼间PAR差值以及气温差值对NEE差值的协同影响达到63%。
(5) 2013年, 黄河三角洲芦苇湿地生态系统表现为CO2的汇, 其中生态系统呼吸(Reco)为757.0 gCm-2, 生态系统总初级生产力(GPP)为1003.7 gCm-2, 生态系统总净固碳量为247.2 gCm-2, 而生长季Reco、GPP与NEE分别占到全年的91%、89%和 95%。 |
| 其他摘要 | Coastal wetlands, the interfaces between terrestrial and ocean ecosystems, can be as an efficient sink due to their higher primary productivities and lower carbon decomposition rates. However, the global climate change can effect their carbon sinks. Using the eddy covariance technique, we measured CO2 flux between the ecosystem and atmosphere, environmental and biological factors over a reed wetland in the Yellow River Delta. Our objectives were (1) to characterize diurnal and seasonal variations of the net ecosystem CO2 exchange (NEE) of the ecosystem; (2) to identify the main abiotic and biotic drivers of NEE at different timescales; (3) to illustrate the effect of sunny and cloudy days on NEE (4) quantify the carbon sink ability of the coastal wetland .
Results show: (1) Diurnal patterns of NEE among different months were very similar in shape but varied substantially in amplitude. From May to October, average NEE for each month was negative (a CO2 sink) during the daytime and positive (a CO2 source) during the nighttime in wetland. While during growing season, the maximum of the averaged daily CO2 uptake and release rate in wetland ecosystem were -10.0 μmolCO2m-2s-1 and 3.6 μmolCO2m-2s-1.
(2) The values of NEE, photosynthetic active radiation (PAR), air temperation (T), soil temperature (Tsoil) and soil water content (SWC) were higher during the peak growing season (from July to September) and lower during the initial (from January to June) and late (from October to December) growing season. Average monthly PAR reached its maximum in June (500.8 μmolm-2s-1), and then decreased gradually. Annual mean temperature was 12.1 oC with minimum mean daily temperatures of -1.3 oC in December and maximum of 27.7 oC in August, respectively. Soil temperature at the 10 cm depth in the wetland was 12.6 oC, while it was 22.0 oC during the growing season. The total precipitation during the growing season was 434 mm with nearly 68% of the annual precipitation. The abovegroud biomass (AGB) and leaf area index (LAI) increased through the growing season until a maximum value was reached in August (635.5 gm-2) and July (0.61), respectively.
(3) During each month of the growing season, the response of daytime NEE to PAR can be expressed by a rectangular hyperbolic function. The seasonal variations of parameters (apparent quantum yield (α), maximum photosynthesis rate (Amax) and daytime ecosystem respiration (Reco, daytime)) could be represented as single peak curves and be described by quadratic models. The parameters of α and Reco, daytime reached maximum in August (0.08 μmolCO2μmol-1PAR, 10.8 μmolCO2m-2s-1), while Amax reached its maximum in July (25.4 μmolCO2m-2s-1).
SWC and Tsoil were the main factors controlling the nighttime CO2 flux. An exponential relationship existed between nighttime NEE (Reco,nighttime) and soil temperature at 10 cm depth under different SWC levels. The results showed that the temperature sensitivity of ecosystem respiration (Q10) evidently declined with increases in soil moisture. Q10 reached its maximum (2.8) when SWC was less than 40%. The temperature sensitivity of ecosystem respiration (Q10) were 3.6 in 2011, while it was 2.5 during the growing season.
(4) Diurnal pattern of NEE showed a distinct U-like course on both sunny and cloudy days, but they varied substantially in amplitude. During the daytime, NEE on sunny days was significantly higher (P < 0.01) than that on adjacent cloudy days, when data were averaged over the 12 paired sampling dates. The response of daytime NEE to PAR can be expressed by a rectangular hyperbolic function on both sunny and cloudy days. There was a significant reduction (P < 0.01) in Amax on cloudy daytime compared to the adjacent sunny daytime. Similarly, there was a statistically significant decrease (P < 0.01) in Reco,daytime on cloudy daytime as compared to that of adjacent sunny daytime. Though there were significant exponential relationships between Reco,daytime and air temperature on both sunny and cloudy days. In addition, Q10 on cloudy days (1.9) was significantly lower as compared to that of sunny days (5.5). Stepwise multiple regression analyses suggested that changes in PAR and T together explained 63% of the changes in NEE between sunny and cloudy days. By taking advantage of the natural shift of sunny and cloudy days without disturbance to the plant-soil system, the results indicated that cloud cover significantly inhibit the absorption capacity of CO2 in the wetland. Thus, it is necessary to take into account the influence of shift of sunny and cloudy days on NEE when predicting the ecosystem responses to future climate change in the wetland.
(5) The wetland was a carbon sink of 247.2 gCm-2 in 2013. Approximately 757.0 gCm-2was assimilated by GPP, and 1003.7 gCm-2 was released by Reco. During the growing season, Reco, GPP and NEE were nearly 91%, 89% and 95% of the annual values, respectively. |
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