全球变化，如大气CO2浓度升高、升温和降水模式的改变、以及氮非点源污染导致的氮增加等，对陆地生态系统的生态过程和功能产生重要影响。土壤微生物在陆地生态系统的生物地球化学过程发挥着重要作用。目前关于全球变化对土壤微生物群落和功能的影响还存在很多的未知和不确定性，主要是由于陆地生态系统的高度异质性和复杂性。探究土壤微生物群落和功能如何响应未来全球变化，是微生物生态领域的基本问题之一，对预测未来变化背景下生态系统的生态效应有着重要的意义。
本论文比较并量化了全球范围陆地生态系统土壤微生物群落、功能基因和酶活性对全球变化因子的响应；研究了森林和草原土壤微生物群落结构和酶活性对气候因子响应的异同及产生差异的原因；并通过模拟全球变化实验，阐明了全球变化敏感的草原生态系统土壤微生物群落结构和酶活性对全球变化因子的响应机制。本论文主要取得了以下研究结果：
（1）分析了全球范围的土壤微生物群落和功能对全球变化的响应，量化了全球变化因子对土壤微生物群落和功能的影响。CO2浓度升高增加了微生物群落的丰度（+40.5%）和多样性（+4.6%）；升温增加了细菌丰度（+2.1%）；降水减少和增多均导致真菌丰度的增加，分别增多6.8%和2.4%。升温和氮增加降低反硝化功能基因的丰度，且氮增加对碳循环相关基因的影响要大于对氮循环基因的影响。对于土壤胞外酶活性，氮增加对土壤胞外酶活性的影响最为显著；降水增加倾向于增加土壤胞外酶活性，而降水减少倾向于抑制土壤胞外酶活性。多种全球变化因子之间的交互作用，以加和作用为主，拮抗和协同作用较少。
（2）森林和草原生态系统土壤微生物群落和以酶活性为代表的生态系统功能的季节性变化和对气候因子响应存在差异性。不同生态系统中，酶活性均呈现出夏季高于春秋季的趋势。森林生态系统酶活性与降水显著正相关、与总氮显著负相关；草原中酶活性与碳的含量关系更为密切，主要是由于生态系统的限制性养分不同造成的。森林生态系统土壤微生物群落和功能与pH关系密切，对气候因子的响应不敏感，草原生态系统土壤微生物对气候因素的变化更敏感，说明森林生态系统的气候稳定性高于草原。在森林和草原生态系统中，细菌比真菌对气候因子的响应更敏感。
（3）气候变化敏感区的草原生态系统对全球变化因子（升温、大气CO2升高、降水减少、氮增加）响应的模拟实验表明，升温导致植物地上和地下生物量分别增加了108.07%和59.30%；氮增加显著增加植物地上地下植物量，增量分别为53.39%和78.78%。土壤微生物多样性、丰度以及主要物种组成受到升温的影响最大且显著，降水减少、氮增加和大气CO2升高的影响均不显著，且多因子间的交互作用不明显。升温显著降低土壤细菌和真菌的ACE和Shannon指数，增加OTU丰度，影响微生物分子生态网络。同时，升温降低土壤胞外酶活性，且升温条件下碳、氮和磷养分变化存在解耦现象，升温导致土壤中碳氮含量降低、而磷含量升高；升温可能导致养分限制从磷限制转变为碳和氮限制。全球变化直接或者通过改变土壤理化性质和养分含量间接影响微生物群落结构以及酶活性。但微生物群落和功能（酶活性）之间的关系基本不显著，酶活性主要受到环境和底物的影响。因此生态系统功能与微生物群落结构对全球变化的响应可能并不同步，全球变化下微生物群落和功能之间存在解耦现象。
本研究厘清了草原和森林生态系统土壤微生物群落和以酶活性为代表的生态系统功能对全球变化的响应机制，证明了将微生物群落和功能纳入生态功能过程评估的重要性和必要性，为预测全球变化对生态系统功能的影响提供重要指标和机制解释，以期为未来土壤管理和可持续发展提供数据支撑和管理依据。

The Earth is undergoing a host of global changes, such as a rising atmospheric CO2 concentration, warming, altered precipitation patterns, and aggravated non-point source pollution of nitrogen (N) due to the intensive use of chemical fertilizers in contemporary agriculture. These changes affect the process and functions of terrestrial ecosystems. Soil microbes, dominated by bacteria and fungi, play a crucial role in almost all of the biogeochemical processes in terrestrial ecosystems. There are still many uncertain impacts of global change on soil microbial communities and functions, mainly due to the high heterogeneity and complexity of terrestrial ecosystems. It is a crucial problem in the field of microbial ecology to explore how soil microbial community structure and function respond to global changes. It is of great significance to predict the ecological effect of the ecosystem under future change.
This study compared and quantified the responses of soil microbial communities, functional genes and enzyme activities to global change factors in terrestrial ecosystems; investigated the different responses of soil microbial community structure and enzyme activity to climate factors in different forest and grassland ecosystems, as well as the drivers of the differences; carried out a global change experiment about soil microbial response mechanism to multiple global change factors (warming, elevated CO2 concentration, decreased precipitation and N addition) in a grassland ecosystem, which is reported sensitive to global change. The main results of this thesis are as follows:
A global database on the responses of soil microbial communities, functional gene abundances and enzyme activities to global change factors was established, and the effects of global change factors on soil microbial community and function were quantified. Elevated CO2 concentration increased microbial community richness (+40.5%) and diversity (+4.6%); warming increased bacterial richness (+2.1%); decreased precipitation and elevated precipitation increased fungal richness by 6.8% and 2.4%, respectively. Warming and N addition decreased denitrifying gene abundance, and N addition affected C-related functional gene abundance more than N-related functional genes. For soil extracellular enzymes, N addition had the most significant effect on soil extracellular enzyme activities. Elevated precipitation tended to increase soil extracellular enzyme activity, while decreased precipitation tended to inhibit soil extracellular enzyme activity. The interaction among multiple global change factors is mainly additive, with less antagonistic and synergetic.
Seasonal changes in soil microbial community and ecosystem function (represented by enzyme activities), and their responses to climate factors differed among forest and grassland ecosystems. Soil enzyme activity showed a pattern of higher in summer than in spring and autumn in almost all the studied ecosystems. However, soil enzyme activity in forest ecosystems was positively correlated to precipitation and negatively correlated to total nitrogen; while in grassland ecosystems, soil enzyme activity was more closely correlated with carbon content. This is mainly due to differences in the nutrient constraints of forest and grassland ecosystems, where forests may be nitrogen-limitated and grasslands may be carbon deficient. In addition, soil microbial community and function in forest ecosystems are closely related to soil pH and was relatively stable to climate factors. Soil microbial community was sensitive to climate factors, indicating higher stability than that of grassland. Moreover, bacteria is more sensitive than fungi to climate factors.
A simulation experiment of grassland ecosystem in climate change sensitive area was conducted on the effects of multiple climate change factors (warming, elevated atmospheric CO2, decreased precipitation, N addition). The results showed warming increased aboveground and belowground plant biomass by 108.07% and 59.30%, respectively, and N addition increased aboveground and blowground plant biomass by 53.39% and 78.78%, respectively. Soil microbial diversity, richness and main species composition were most affected by warming, while the effects of decreased precipitation, N addition and elevated CO2 were not significant, and the interactions between the multiple factors were not significant. Warming significantly decreased soil bacterial and fungal Shannon and ACE indexes but increased OTU richness, as well as affected microbial molecular ecological network. In addition, warming tended to decrease soil extracellular enzyme activities. The decoupling of total nutrients C, N and P was observed. Warming decreased soil C and N content, while increased P content. Therefore, warming may result in a shift of nutrient limitation, from P limitation to C or N limitation. Global change factors directly or indirectly affect soil physical and chemical properties and nutrient contents, microbial community structure and enzyme activities. However, the relationship between microbial community and function (enzyme activity) was not significant, and enzyme activities were mainly influenced by environmental factors and substrates. Therefore, the responses of ecosystem function and microbial community structure to global change may not be synchronous, and there is decoupling between microbial community and function under global change.
The thesis is expected to provide a database and mechanism for predicting the impacts of climate change on terrestrial ecosystem functions and ecological effects, as well as to provide a basis for soil management and sustainable development for future global change.