凝结水在生物结皮碳库维持中的作用及其机理

Biological soil crusts (biocrusts) are a soil surface community made up of cyanobacteria, lichens, and moss; are globally widespread; and are an important part of the soil community in many desert ecosystems. They have long been paid much more attention due to the potential foreground in desertification control of arid and semi arid regions. And recently they are also be widely reconsidered as an important and missing carbon sink of terrestrial ecosystem. However, the carbon in desert biocrusts is not stable, even labile, water is regarded as the most important factor that leads to carbon loss from biocrusts. But non-rainfall water is far more important water than occasional precipitation in this area. Therefore this program would do the explorations on non-rainfall water and maintenance of bicrustal carbon. The main contents are the following sides. .The field experiments would all performed in Hopq Desert at the constructed mesocosms. The amount of non-rainfall water comes from air and soil during the period of this water occurrence would be firstly measured, the crucial environmental factors that obviously affect the sources of non-rainfall water would be found, thus some reasons on the different distributions of every types biocrusts would be recognized. Then the amount of this water, dark respiration, photosynthetic activity and net photosynthesis productivity during this period, would all be in situ simultaneously tested, the pattern and effeciency of CO2 exchange during this period would be revealed. Third, chlorophyll, the total organic carbon, biomass carbon and remnant carbon content at the stage of desiccation would be long-term measured, the amount and pattern of carbon pool non-rainfall water maintained would be obtained. By combination with climatic data, the change speed in amount and pattern of carbon pool and crucial influencing factors would also be obtained.

生物结皮作为新的碳汇被高度关注，但水分引起的数据不稳使估算和稳定性有很大出入。凝结水是最频繁影响结皮的水源，而这个方面长期以来一直被忽视。本任务拟以不同发育阶段生物结皮为研究对象，通过原位的微宇宙实验，研究凝结水发生期间各结皮所能获得的凝结水量、来源差异和影响这一过程的主要因素，并通过暗呼吸和净光合效率的昼夜连续检测，探究它们的分布位置与微环境条件间的内在机理，从而初步确定凝结水在生物结皮碳稳定中的贡献。

非降雨型水(NRW)是生物结皮用于固碳的最频繁水源，我们知道生物结皮对NRW积累有显著促进作用，但我们对其促进机制、结皮利用NRW的情况还知之甚少。我们在该课题的支持下，选择进化地位不同的4类(5种)结皮(2种蓝藻结皮AC，1种蓝藻地衣结皮LC1，1种绿藻地衣结皮LC2，1种藓结皮MC)，在中国内蒙库布齐沙漠的野外试验中，通过室内和原位的实验比较，从结皮属性影响NRW积累的机制、NRW积累期间结皮的光合活性和CO2交换量等方面进行了系统研究。结果发现，结皮属性均随演替明显在变化，而这些变化又反馈影响了气象因素对NRW积累的影响程度，最终使地表温度的影响随结皮发育逐渐减弱，地表下5cm地温和光强的影响逐渐增强；受气压、水汽压、地表温度、气温-露点温差的影响，蓝藻结皮中近50%的NRW来自结皮下土壤，绿藻地衣结皮和藓结皮中近80%来自大气；受自养生物活性的明显调节，蓝藻地衣结皮与流沙有相似的来源模式，但日NRWmax量主要由结皮厚度和光合自养生物生物量及其相关属性决定。此外，沙漠环境下影响结皮碳交换的主要因素有光强、气温、地温和NRW量，但对AC气温最重要，对LC2和MC气温和NRW量同等重要。.在NRW积累过程中，蓝藻结皮的光合活性恢复得最早(20:30起)，蓝藻地衣结皮和藓结皮较晚(0:30起)，蓝藻地衣结皮一直没能恢复；CO2交换以蓝藻结皮和藓结皮最早(多0:30起)，绿藻地衣结皮较晚(多2:30起)，但蓝藻结皮的交换效率低，绿藻地衣结皮和藓结皮的交换效率高。从各结皮维持碳平衡所需日NRWmax量来看，蓝藻结皮最低(ca.0.08mm)，绿藻地衣结皮较高(ca.0.17mm)，藓结皮最高(ca. 0.20 mm)。由此推测，利用蓝藻生物结皮的固沙技术边界应该在NRW丰富季节里0.08mm以上日NRWmax持续约2小时以上的地域。