研究生知识库

在全球变化的大背景下，地球陆地生态系统碳循环的研究一直是近些年国内外研究的热点之一。占地球陆地面积约12%的喀斯特地区是全球陆地生态系统的重要组成部分，具有岩石碳库大、碳循环速度快、对全球变化响应和反馈敏感等特征，理应受到更多的关注。不可否认，这些年大家对岩溶作用的碳汇效应、过程机制开展大量的理论、实测和模拟研究，也获得了突破性的进展。但是，对于喀斯特土壤碳循环方面的研究相对薄弱，亟待加强。之所以突出喀斯特土壤碳循环研究的重要性，原因有二：一是，喀斯特土壤作为生态系统的关键组成部分，不仅其本身碳的蓄积和排放对于整个生态系统碳平衡具有重要意义，而且更是岩溶碳汇效应的核心驱动因子；二是，喀斯特具有双层结构，导致地上-地下相互连通、土壤厚度小且连续性差、对气候变化的响应敏感等，喀斯特土壤的非地带性区域特征非常鲜明。在气候变化的背景下，喀斯特土壤呼吸对于温度、降水变化的响应研究是自然而然的命题。根据喀斯特土壤的基本特征，按照降水变化导致土壤干湿交替发生变化--土壤干湿交替变化导致土壤环境及微生物发生变化---土壤呼吸发生变化这一逻辑规律和机理图像，我们选择“喀斯特土壤呼吸对干湿交替的响应规律及其微生物学机制研究”作为学位论文的题目。本学位论文首先获取贵州省84个气象站点近60年的逐日数据来揭示全球变化背景下贵州降水的变化性及其空间分异，其次依托定位连续观测来揭示土壤含水量对降水变化的响应规律，再者依托定位连续观测来揭示土壤呼吸对土壤含水量变化的响应，最后通过控制性试验来解析土壤呼吸对土壤含水量变化响应的微生物学机制，获得了如下几个方面的结果和认识。贵州省1961-2017年平均降水量为1209.47mm，年降水量总体呈现减少趋势，减少速率为10.78mm/10a；贵州省降水格局发生了变化，降水期的特征更为明显、更为集中，有雨日降水量和最大日降水量均呈现出增加趋势，连续多天无降水的概率增加。受季风影响及周期性制约，贵州省降水的时间、空间分异明显：按东西划分，西部地区年降水量和变异性均高于东部地区，按南北划分，南部地区年降水量明显高于北部地区；全省年降水量具有近30年的周期变化性，连续无降水的周期性更为明显，即1961-1972和2004-2017年为相对低概率期、1973-2003年为相对高概率期，可能意味着周期性是本区域乃至更大区域气候变化的主要制约因素。在4个水文年的监测期内，土壤相对含水量低于40%（干旱严重）的天数占全年的8.4%、土壤相对含水量为40-60%（呈现旱象）的天数占全年的31.6%，显示全年有40%的时间土壤水分处于不利于生物生长的状况；喀斯特土壤旱象、干旱严重的状况不仅出现于旱季，相对含水量低于40%、40-60%的状况在旱季出现的比重分别为5.4%、20.1%，在雨季出现的比重分别为3.0%、15.3%。喀斯特土壤水分对降水的响应呈现出不同的态势：土壤水分对降水没有响应，对于小于小雨、中雨、大雨均有无响应的情形出现，无效比例分别为49.3%、9.7%、13.8%，其原因各有不同；土壤水分的丰、缺对不同雨强的降水的响应方式和过程均有所不同，体现出一定的复杂性。在连续无降雨的状况下喀斯特土壤水分衰减呈现一定的规律性，无论雨季还是旱季土壤相对含水量均快速地降至60%，呈现旱象；但是，土壤相对含水量从60%降至40%所用时长呈现出明显的季节差异性，雨季的时长明显小于旱季，可能意味着雨季温度驱动的蒸发作用明显强于旱季。土壤呼吸的长期定位观测发现，喀斯特土壤呼吸在降雨前后存在着明显的差异性，我们认为降水对喀斯特地区的土壤呼吸有着更大的激发效应，忽视这种激发效应可能会低估喀斯特生态系统通过土壤呼吸向大气排放CO2的通量；多重干湿交替室内模拟实验干旱土壤复水后，土壤呼吸明显增加，观察到较为明显的Birch效应，复湿后的24小时土壤呼吸产生的CO2分别占每个周期总释放量的24.43%、24.53%和22.27%。对实验过程中CO2释放量以及土壤水分进行拟合，发现二者呈极显著相关关系，土壤含水量对土壤呼吸的解释度为79.5%。结合多种环境因子对土壤呼吸进行RDA分析，土壤含水量在三个干湿交替循环过程中对土壤呼吸的权重最大，影响显著。室内模拟干湿交替实验结果得知，频繁的干湿交替作用造成微生物量及微生物活性降低，干旱土壤复水后矿化作用显著增强，并且复水事件刺激了这些微生物的大量繁殖。干湿交替对微生物量的作用表现在每次干旱后期微生物量均较低，在干旱条件下有部分微生物死亡，胞内有机质释放，在土壤重新湿润时被其他微生物分解利用，为Birch效应提供物质基础。 通过MiSeq Illumina平台对土壤样品细菌、真菌和古菌的高通量测序获取了大量序列，测序结果显示本研究区土壤细菌分属32个门58个纲826个属；土壤真菌分属42个门52个纲876个属。对干湿交替处理土壤进行微生物群落结构分析，经过三个循环的干湿交替处理，细菌和真菌群落结构均发生改变，丰富度和多样性降低，古菌群落对干湿交替变化不敏感。

In the context of global climate change, the study of the carbon cycle of terrestrial ecosystems has been one of the hotspots of research at home and abroad in recent years. The karst area, which accounts for about 12% of the earth's land area, is an important part of the global terrestrial ecosystem. It has the characteristics of large rock carbon pool, fast carbon cycle, sensitivity to global changes and feedback, and should receive more attention. It is undeniable that in recent years, a lot of theoretical, measured and simulation studies have been carried out on the carbon sink effect and process mechanism of karst, and breakthrough progress has also been made. However, research on the carbon cycle of karst soils is relatively weak and needs to be strengthened. The importance of karst soil carbon cycle research is highlighted for two reasons. First of all, karst soil is a key component of ecosystems, not only is its carbon accumulation and emissions important for the entire ecosystem carbon balance, but it is also the core driving factors of karst carbon sink effect. Secondly, the karst has a two-layer structure, resulting in above-ground and underground connectivity, small soil thickness, poor continuity, sensitivity to climate change, etc., and the non-zonal regional characteristics of karst soil are very distinct.In the context of climate change, the study of the response of karst soil respiration to temperature and precipitation changes is a natural proposition. According to the basic characteristics of karst soil, the change of soil drying and rewetting changes caused by precipitation-the change of soil environment and microbes caused by soil drying and rewetting changes-the logical law and mechanism image of soil respiration changes. We choose "response of soil respiration to repeated drying-rewetting cycles and microbiological mechanism in karst"as the subject of a dissertation. This dissertation firstly obtained the daily data of 84 meteorological stations in Guizhou Province for nearly 60 years to reveal the variability and spatial differentiation of precipitation in Guizhou under the background of global change. Secondly, relying on continuous observation to reveal the response of soil water content to precipitation changes. Furthermore, relying on continuous observation to reveal the response of soil respiration to changes in soil water content, and finally through the control experiment to analyze the microbiological mechanism of soil respiration response to soil water content changes, the following results and understanding.The average precipitation in Guizhou Province from 1961 to 2017 was 1209.47mm, and the annual precipitation showed a decreasing trend with a decreasing rate of 10.78mm/10a. The precipitation pattern in Guizhou Province changed, and the characteristics of the precipitation period were more obvious and concentrated. The precipitation on the rainy day and the maximum daily precipitation showed an increasing trend, and the probability of no precipitation for several consecutive days increased.Influenced by the monsoon and cyclical constraints, the precipitation and spatial differentiation of precipitation in Guizhou Province are obvious: according to the east and west, the annual precipitation and variability in the western region are higher than that in the eastern region. According to the north and south, the annual precipitation in the southern region is significantly higher than that in the north. The annual precipitation of the province has a cyclical variability of nearly 30 years, and the periodicity of continuous no precipitation is more obvious, that is, the relatively low probability period of the year from 1961 to 1972 and the year from 2004 to 2017, and the relatively high probability period of the year from 1973 to 2003. It may mean that cyclicality is a major constraint to climate change in the region and beyond.During the monitoring period of the four hydrological years, the number of days with relative soil water content below 40% (serious drought) accounted for 8.4% of the year, and the relative soil water content was 40-60% (presenting drought). 31.6%, showing that 40% of the year's soil moisture is not conducive to biological growth; karst soil drought, severe drought conditions not only occur in the dry season, relative water content is less than 40%, 40-60% in the dry season The proportions appearing are 5.4% and 20.1%, respectively, and the proportions in the rainy season are 3.0% and 15.3%, respectively.The response of karst soil moisture to precipitation showed different trends: soil moisture did not respond to precipitation, and there was no response to less than light rain, moderate rain and heavy rain. The invalid ratios were 49.3%, 9.7%, and 13.8%, respectively and for that the reasons are different. The abundance and deficiency of soil moisture are different in response to different precipitations and processes, reflecting a certain complexity.The soil moisture attenuation of karst showed a certain regularity in the absence of rainfall. The relative water content of the soil in the rainy or dry season decreased rapidly to 60%, showing drought condition. However, the relative water content of the soil decreased from 60% to 40%. The duration of the season shows obvious seasonal differences. The duration of the rainy season is significantly lower than that of the dry season, which may mean that the evaporation driven by the temperature in the rainy season is significantly stronger than that in the dry season.Long-term localization observation of soil respiration shows that there is a significant difference in karst soil respiration before and after rainfall. We believe that precipitation has a greater stimulating effect on soil respiration in karst areas. Neglecting this stimulating effect may underestimate the karst ecosystem through soil. The flux of breathing CO2 to the atmosphere;Multiple dry and wet alternate indoor simulation experiments after rehydration of reclaimed soil, soil respiration increased significantly, and Birch effect was observed obviously. The CO2 produced by soil respiration after 24 hours of rehydration accounted for 24.43% ,24.53% and 22.27% of the total release per cycle respectively. The CO2 emission and soil moisture were fitted during the experiment, and it was found that there was a significant correlation between them. The interpretation of soil water content to soil respiration was 79.5%. The RDA analysis of soil respiration was carried out by combining various environmental factors. The soil water content had the greatest weight to soil respiration during the three dry-wet alternate cycles, and the effect was significant.The results of indoor simulated dry-wet experiments showed that the frequent dry and wet alternating effects caused the microbial biomass and microbial activity to decrease, and the mineralization of the dry soil was significantly enhanced after rehydration, and the rehydration event stimulated the proliferation of these microorganisms. The effect of dry-wet alternation on microbial biomass was observed in the microbial population at the end of each drought period. Under drought conditions, some microbes died, intracellular organic matter was released, and other microbes were decomposed and utilized when the soil was rewetting, which providing substances for the Birch effect.A large number of sequences were obtained by high-throughput sequencing of soil samples of bacteria, fungi and archaea through the MiSeq Illumina platform. The sequencing results showed that the soil bacteria in this study area belonged to 32 phylum and 58 classes and 826 genera; the soil fungi belonged to 42 phylum 52 classes and 876 genera. The microbial community structure was analyzed in the dry-wet alternately treated soil. After three cycles of dry-wet treatment, the bacterial and fungal community structure changed, the richness and diversity decreased, and the archaea community was not sensitive to the alternating wet and dry changes.