研究生知识库

碳循环一直以来都是全球变化研究的重点、热点，也是难点。碳循环在地表环境演化中起着重要的作用，理解碳循环对预测未来气候至关重要。“遗失碳汇”这一问题制约了对现代碳循环的理解。碳酸盐在碳循环中扮演着重要作用，不仅碳酸盐是地表圈层最大的碳储库，而且碳酸盐快速的风化过程中吸收大量大气二氧化碳，形成巨大的碳汇通量，使得碳循环更加活跃。碳酸盐风化能形成长久碳汇的关键在于不稳定的溶解无机碳通过光合作用转化为有机碳进入沉积物埋藏下来。然而对由碳酸盐风化形成的有机碳汇通量有多大尚缺乏足够的了解。内陆水域中水库的碳汇通量最大，具有很强的内源碳汇潜力。因此，研究水库内源碳汇对碳酸盐风化碳汇的研究具有启示意义。本文选取位于贵州喀斯特区的平寨水库、普定水库和红枫湖为研究对象，研究了水化学和沉降颗粒物的性质，采用碳同位素和C/N比值对有机质的来源进行了判别，并估算了水库内源碳汇通量，得出以下几点认识：（1）水库水化学存在显著的时空变化，分层期间光合使得表层水体具有高的pH、溶解氧浓度和δ13CDIC值，低的电导率和DIC浓度。基于DIC浓度和δ13CDIC的联合示踪表明在表层DIC残余比例低于0.7（对应低的溶解二氧化碳浓度）时，藻类对HCO3-的利用比例加大；然而δ13CDIC同δ13CPOC的联合示踪表明，即使在低溶解二氧化碳浓度条件下，溶解二氧化碳依旧是微藻的主要碳源，对HCO3-的利用不占主导。（2）分层期间藻类强烈光合作用产生了大量的POC，加强了DIC向有机碳的转化。δ13CPOC指示水体POC主要为内源，但内源POC具有相对外源POC更低的沉降速率。（3）水库颗粒物的沉降通量存在显著的时空差异，受水动力条件的控制。有机质的C/N比值和δ13COC表明沉降颗粒物有机质的内外源比例即受控于水动力条件也受控于光合作用强度。自普定水库、平寨水库和红枫湖，随水动力减弱，内源比例升高；自春季至夏季，光合作用增强，内源有机碳比例增高，而自夏季至冬季，光合作用减弱，内源比例减弱。通过C/N比值法对内外源有机碳的定量辨识表明，普定水库、平寨水库和红枫湖的内源有机碳比例分别为23%、36%和56%，内源有机碳沉降通量分别为96 t C/km2·yr、46 t C/km2·yr和48 t C/km2·yr，水库总内源有机碳沉降通量分别为1975 t C/yr、693 t C/yr和1041 t C/yr。自平寨水库、普定水库至红枫湖，随富营养程度的增加，内源有碳汇能力先增加后减少，说明一定程度的富营养化可能有助于提高内源碳汇能力。

Carbon cycle has always been one of the focuses in Global Change research, and it's also a quite tough part. Carbon cycle plays a crucial role in Earth's surface environmental evolution, thus understanding global carbon cycle is important for predicting future climate change. “Missing carbon sink” has regulated our understanding on modern global carbon cycle. Carbonate plays a curial role in carbon cycle, is the largest carbon reservoir in the Earth's surface layer and carbon cycle is intensified by carbonate weathering during which process a significant amount of atmospheric carbon dioxide is absorbed and carbon sink flux is produced. The key in producing carbon sink by carbonate weathering is conversation of unsteady dissolved inorganic carbon to organic carbon, which is then buried into sediment. However, we still lack enough understanding on the amount of organic carbon sink flux accompanied by carbonate weathering. Reservoirs may have the largest organic flux, thus a great autochthonous organic carbon sink potential. Hence, understanding carbon cycle in reservoirs is of great importance for carbonate weathering carbon sink study.Pingzhai Reservoir, Puding Reservoir and Hongfeng Lake, which are located in karst region in Guizhou Province, were investigated with properties of hydrochemistry and sinking particles were detailly examined. Carbon isotopes and C/N ratios were used to distinguish organic carbon sources and autochthonous organic carbon sink flux was estimated. The following conclusions are made:Spatiotemporal variations of hydrochemistry existed in the reservoirs. Photosynthesis caused higher pH, DO and δ13CDIC values and lower EC and DIC concentration in epilimnion. HCO3- utilization by algae was enhanced when residual fraction of DIC below 0.7 which corresponds to low CO2(aq) condition based on DIC and δ13CDIC tracing. But HCO3- pathway had never dominated carbon fixation by algae and CO2(aq) is the main inorganic source even under low CO2(aq) condition as revealed by δ13CDIC and δ13CPOC tracing.Intensified photosynthesis resulted in high POC production and enhanced DIC conversion to organic carbon. POC in water column was mainly autochthonous and autochthonous POC sinks slower than allochthonous POC as indicated by δ13CPOC values.Hydrodynamic condition has controlled the spatiotemporal variations of particles' sinking pattern. C/N ratios and δ13COC indicated that autochthonous organic portion in sinking particles was governed by both hydrodynamic condition and photosynthesis intensity. The portion of autochthonous OC increased from Puding Reservoir (23%) to Pingzhai Reservoir (36%) and Hongfeng Lake(56%), with weakened hydrodynamic condition. The portion of autochthonous OC increased from spring to summer induced by intensified photosynthesis and decreased from summer to winter. Reservoir surface area normalized autochthonous organic carbon sink fluxes were 96 t C/km2·yr,46 t C/km2·yr and 48 t C/km2·yr, the total autochthonous organic carbon sink fluxes were 1975 t C/yr, 693 t C/yr and 1041 t C/yr for Puding Reservoir, Pingzhai Reservoir and Hongfeng Lake, respectively. Autochthonous organic carbon sink is enhanced by eutrophication, increasing from Pingzhai Reservoir to Puding Reservoir and decreasing following further increased eutrophication state from Puding Reservoir to Hongfeng lake, which indicates that eutrophication may increase autochthonous organic carbon sink within a certain degree.