古尔班通古特沙漠生物结皮在地-气界面CO2交换中的作用
文献类型：学位论文
| 作者 | 吴林 |
| 学位类别 | 博士 |
| 答辩日期 | 2015 |
| 授予单位 | 中国科学院大学 |
| 授予地点 | 北京 |
| 导师 | 张元明 |
| 关键词 | 干旱地区 生物结皮 光合固碳 土壤呼吸 碳交换 降雨 |
| 学位专业 | 理学博士 |
| 中文摘要 | 生物结皮是由蓝藻、绿藻、地衣和藓类等为主的生物体所组成的有机复合体，其覆盖约占干旱区地表活体覆盖面积的40%以上，在一些受干扰较少的极端环境群落中生物结皮甚至达到了70%的活体盖度。作为荒漠系统的主要构建者和生态系统工程师，生物结皮中地衣的光能利用率为0.5～2%，与维管束植物处于同一量级，而在条件适宜时它们的光合速率在两个量级之间变化，即在0.1～11.5 μmol CO2 m-2s-1之间，而后者与同一生物气候区旱生灌木的光合速率相当。虽然生物土壤结皮的瞬时光合速率较维管束植物低，但如果考虑到生物土壤结皮在干旱、半干旱地区盖度的面积，生物土壤结皮的光合固碳是这些地区的一个潜在碳汇。另外，生物结皮作为干旱区主要的地表覆被类型，其独特的生物学和物理学特征不但影响土壤水文过程，还影响土壤结构，从而间接影响土壤碳的释放过程。因此，了解生物土壤结皮及其发育土壤的碳过程是全面解析荒漠地区碳循环不可或缺的重要环节。本研究以古尔班通古特沙漠为研究对象，通过野外调查摸清研究区生物结皮的空间分布情况，然后通过野外原位和模拟实验观测生物结皮土壤碳通量特征变化特征，同时结合室内控制实验分析影响生物结皮光合固碳的水热因子。目的是为了：（1）从全新的角度理解生物结皮对荒漠地区碳循环的影响；（2）阐明其影响过程和程度；（3）最后为荒漠地区管理提供决策意见。主要结果包括以下几个方面：生物结皮的分布和发育特征本文通过数码照相法估算了整个沙漠地区生结皮盖度为41.34%，其中地衣-藻结皮盖度占到了90%以上。尽管与维管植物生物量相比，生物结皮层的碳储量可以忽略不计，但是其广泛的分布面积及其对土壤碳通量的影响是不可小视的。另外，通过分析生物结皮分布与环境因子关系，本研究发现随着年降雨量的增加，生物结皮的盖度从25% 显著增加到94%，并且，苔藓结皮在结皮总盖度中的比例也从0增加到18%。说明，随着降雨量的增加，光合能力相对高的苔藓结皮的优势度逐渐增加；与此同时，随着降雨量的增加，土壤粘粉粒含量、养分含量和有机碳储量也显著提高。以上结果说明，降水是主导古尔班通古特沙漠生物结皮分布和土壤碳储量的主要环境因素。生物结皮对土壤有机碳的影响生物结皮除了自身能够进行光合固碳外，还能够捕捉沉降物，从而改变土壤有机碳含量。本研究通过调查不同演替序列结皮发育土壤有机碳的分布情况，发现随着结皮发育程度的增加，土壤有机碳含量显著增加，且土壤有机碳主要集中在表土层。另外，结皮去除后，土壤有机碳相比完整结皮样地下降得更快，间接说明结皮去除后土壤呼吸释放碳增加。而随着后期生物结皮的重新定植（叶绿素含量逐渐增加），6个月后土壤有机碳含量又开始增加。因此，通过对比土壤有机碳在时空尺度上变化，可以清晰地认识到生物结皮是荒漠地区土壤有机碳的主要贡献者之一。生物结皮对土壤水分、温度的影响在该地区，生物结皮减小降雨入渗，使得土壤水分浅层化，结皮层有储存水分的作用。同时，结皮的存在又会增加土壤温度，促进土壤水分蒸发。而土壤水分、温度变化直接控制土壤呼吸过程，因此，生物结皮会通过调控土壤水热状况变化间接影响土壤碳通量过程。生物结皮对土壤呼吸的影响生物结皮除了光合作用还进行呼吸作用，直接参与土壤呼吸过程。另外生物结皮通过改变土壤水热状态间接影响土壤呼吸。当结皮去除后，土壤呼吸发生明显改变。干旱季节，结皮去除后，土壤碳释放量增加；湿润降雨季节，结皮去除后，土壤呼吸减弱。干旱季节，结皮处于休眠状态，结皮本身不参与土壤呼吸，但是结皮层能够对下层土壤呼吸释放的CO2产生一个物理阻隔作用, 其阻隔量约占土壤呼吸的5%-27%。湿润季节，降雨主要刺激结皮层呼吸，有结皮覆盖土壤呼吸显著大于去结皮土壤。一次降水过程（从降水到结皮干燥无活性为止）生物结皮层呼吸约占整个土壤呼吸的65%-75%；降雨强度越大，结皮层呼吸所占比重越大。生物结皮的光合固碳生物结皮中的植物体都是变水植物，因此生物结皮的光合固碳过程必然跟水分有关。该沙漠地区降雨和融雪水是生物结皮光合固碳的主要水分来源。通过室内模拟研究发现，生物结皮净光合作用所需的最适水分为60%（质量含水量），温度为15-20oC。另外考虑到温度越高，水分蒸发越快，因此，水热耦合过程是生物结皮光合固碳的最关键因素。本研究发现相对较低的温度（5-15oC）和较高的水分条件（40%-80%）有利于生物结皮光合固碳，而这一适宜的环境条件主要出现在该沙漠地区的早春融雪期和早春降雨后，因此，春季势必是生物结皮光合固碳高峰期。野外观测结果证实了这一假设，全年地衣-藻结皮的光合固碳总量约为21.77 ± 2.14 gCm-2，苔藓结皮为26.68 ± 4.09 gCm-2，其中春季固碳量为11.77-14.07 gCm-2，占到全年总固碳量的47%-49%。生物结皮对地-气界面CO2交换的影响生物结皮是地表碳通量的主要参与者。生物结皮土壤碳通量在时间尺度上的变化主要受土壤水分和温度影响，其中，土壤含水量能够解释其71-74%的变异。土壤类型（结皮和去结皮土壤）同样显著影响土壤碳通量（P<0.01），但是影响过程依赖降雨。在无降雨条件下，生物结皮处于休眠状态，结皮的存在能够减缓土壤碳的释放。降雨驱动结皮直接参与土壤碳交换过程，小量降雨后（2 mm），结皮层光合不及自身呼吸，导致生物结皮土壤碳通量表现为碳释放。当降雨强度等于或者超过5 mm后，结皮总光合大于结皮呼吸和下层土壤呼吸之和，生物结皮土壤碳通量表现碳吸收。并且，降雨强度越大，碳吸收能力越强。结皮层的光合固碳和自身呼吸调控着土壤碳通量的流向和流速。整体上，生物结皮全年净固碳量为4.21-6.27 gCm-2，可以抵消土壤总释放碳的11%～13%。最后，考虑到生物结皮广泛的分布面积以及结皮层的碳交换主要受降雨驱动，而其碳交换过程深刻影响土壤碳循环。在全球气候变化背景下，荒漠地区降雨将发生很大改变，理解生物结皮碳交换过程对土壤碳通量的影响有助于我们清晰地认识荒漠生态系统碳平衡。 |
| 英文摘要 | Biological soil crusts (BSCs) are composed of various components of cyanobacteria, algae, fungi, lichens and mosses and other organisms that may constitute as much as 40% of the living cover in dry land ecosystems, even reach to 70% in some extreme environment regions where interference is less. BSCs are the main builder of desert ecosystem and ecological system engineer, the light efficiency ratio of biocrust lichen is 0.5-2%, which is in the same order with vascular plant, its photosynthetic rate ranges from 0.1 to 11.5 μmol m-2 s-1. Although instantaneous photosynthetic rates of BSCs are less than those of vascular plants, coverage of BSCs in semiarid and arid areas is huge, thus making them potential contributors of soil organic carbon (SOC) in arid and semiarid regions. In addition, as the main types of ground cover in arid areas, unique biological and physical characteristic of BSCs significantly influence soil structure and soil hydrological processes, then indirectly affect the release process of soil carbon. Therefore, understanding the carbon exchange of BSCs and biologically crusted soil is an important part of soundly revealing carbon cycle in desert ecosystem. In the current study, we chose the Gurbantunggut Desert as a study site to survey the spatial distribution of BSCs in the desert, to examine the way in which CO2 flux rates of biologically crusted soil respond to changes in soil moisture and temperature and how removal of BSCs impacts on soil CO2 flux, and measure carbon fixation of BSCs using field observation and laboratory simulation methods. We specifically aimed to: (1) obtain new data and insights into the effect of BSCs on carbon flux and SOC storage; (2) determinate the extent to which BSCs enhance carbon sequestration and minimize carbon loss; (3) provide valuable data in optimizing land management practices to protect desert lands. The main results as follows: Distribution and development characteristic of BSCs: In this study, we tried to get the distribution information of BSCs and calculated the coverage of each component using maximum likelihood supervised classification. Results indicated that the coverage of BSCs reached to 41.34% in the whole desert, and the coverage of lichen/cyanobacteria crusts accounted for more than 90%. Despite their low biomass relative to other primary producer, crust organisms have a disproportionally large influence on biogeochemistry in arid regions. In addition, we found that BSCs coverage significantly increased from 25% to 94% with the increase of annual precipitation, and moss crust coverage increased from 0% to 18%. Similarly, the content of clay and soil organic carbon (SOC) increased with the increase of rainfall. These results indicated that precipitation was the key factor driving the distribution and development of BSCs and SOC content. The influence of BSCs on soil organic content: BSCs not only fix carbon during photosynthesis, but also be able to catch carbon from deposition. The spatial distribution of SOC in different succession of biologically crusted soil was investigated. Results showed that SOC significantly increased with the increase of BSCs development, and the occurrence of BSCs could significantly enhance the accumulation of SOC in the surface 0-5 cm soil layer compared with areas devoid of crust cover. Moreover, we found that once BSCs had been removed, in the absence of autotrophs there was no primary production and SOC gradually decreased. However, SOC began to slowly increase 6 months later, accompanied by a continuous increase in chlorophyll a and this appeared to be as a result of re-colonization of the bare soils. Thus, the dynamic change in SOC driven by BSCs and the significant difference in SOC between biologically crusted soils and bare soils clearly demonstrated the effect of BSCs on SOC. The influence of BSCs on soil moisture and temperature: In the desert, BSCs reduce rainfall infiltration, and make soil moisture shallow stratification. Also, BSCs can increase soil temperature and promote soil moisture evaporation. Soil moisture and temperature control soil respiration, thus BSCs can indirectly affect soil respiration by regulating soil water content and temperature. The influence of BSCs on soil respiration: BSCs not only directly involved in the process of soil respiration, but indirectly affect soil respiration by changing soil moisture and temperature. Results indicated that soil respiration significantly changed when BSCs were removed. Soil respiration increased when BSCs were removed in dry season and decreased in wet season. BSCs don’t participate in soil respiration when organisms are disccated and inactive, but have a barrier effect on subsoil respiration and reduced 5% to 27% of soil respiration. In wet season, precipitation primarily drives crust-derived respiration, which significantly influences the magnitude of soil CO2 efflux. During a rainfall event (from wet to dry), crust’s respiration contributed 65% to 75% to the whole soil respiration (crust respiration and subsoil respiration), and the ratio of crust respiration increased with increase of rainfall intensity. Photosynthetic carbon sequestration of BSCs: Biocrust microphytes are poikilohydric organisms in which the water content of cells is in equilibrium with the surrounding environment. Rainfall and snowmelt water is the main moisture source of BSCs photosynthesis in the desert. Laboratory experiment showed that maximum rates of gross and net carbon fixation were typically reached at 60% relative mass water content. Optimal temperatures for photosynthesis occured between 10 and 20oC. And, higher temperature reduced wet duration, the effective photosynthetic time decreased. Interaction of moisture and temperature should be taken into consideration for estimating total carbon sequestration of BSCs. Field observation and laboratory simulation experiment demonstrated that relative lower temperature (5-15oC) and higher water content (40%-80%) resulted in lager carbon sequestration, and suitable environment conditions occur in spring in the desert. The annual carbon fixation was 21.77 ± 2.14 gCm-2 for lichen-cyanobacteria dominated crusts and 26.68 ± 4.09 gCm-2 for moss dominated crusts, and about 47%-49% of annual carbon fixation was got in spring. The influence of BSCs on soil-atmosphere CO2 exchange: Temporal variation of NCE varied with soil moisture and temperature. Soil moisture alone could explain 71-74% of variation in NCE. Soil type (moss crusted soil or bareland) also had a significant effect on NCE (P < 0.01), but this was dependent on soil moisture which was directly linked to precipitation pulse. Without precipitation, organisms in BSCs were disccated and inactive, BSCs reduced soil carbon efflux. After a 2 mm precipitation pulse, the crust gross photosynthetic rate (GPc) was lower than the crust respiration rate (Rc), resulting in C efflux. When precipitation intensity was equal to or greater than 5 mm, GPc fully offset total respiration, resulting in an increase in C uptake. C gain was positively correlated with intensity of precipitation pulse. Regardless of different precipitation intensities, Rc was significantly higher than that of subsoil respiration. Thus, the regulation of atmospheric-soil C balance by BSCs depends on the intensity of precipitation. Overall, net carbon fixation of BSCs was 4.21-6.27 gCm-2, which can offset 11%～13% carbon release from soil respiration. Lastly, given the large areas covered by BSCs in arid and semiarid lands, the unequivocal consideration of crust-derived CO2 exchange in future empirical and modeling studies may greatly contribute to the understanding of the global C cycle and enable better prediction of the effects of global environmental change on soil C balance. |
| 学科主题 | 植物学 |
| 语种 | 中文 |
| 源URL | [http://ir.xjlas.org/handle/365004/14936]  |
| 专题 | 新疆生态与地理研究所_研究系统_荒漠环境研究室 |
| 作者单位 | 中科院新疆生态与地理研究所 |
| 推荐引用方式 GB/T 7714 | 吴林. 古尔班通古特沙漠生物结皮在地-气界面CO2交换中的作用[D]. 北京. 中国科学院大学. 2015. |
入库方式： OAI收割
来源：新疆生态与地理研究所
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