氮素富集条件下植物-土壤-微生物交互作用对高寒草甸土壤有机碳动态的调控机制

The alpine meadow on the Qinghai-Tibetan Plateau is a typical nitrogen-limiting terrestrial ecosystem. The increase in atmospheric nitrogen deposition tends to promote the aboveground biomass of vegetation, but the effects of nitrogen enrichment on belowground carbon processes and soil carbon sequestration are diverse and contradictory. Moreover, the knowledge on the interaction among plants, microbes, and soil organic matter (SOM) is lacking, and the microbial mechanisms responsible for its regulation to soil carbon sequestration are unclear. In this project, we will use the 13C-labeled CO2 and 13C-PLFA techniques to analyze the allocation pattern of new assimilated carbon through plant photosynthesis from aboveground vegetation components (leaf, shoot) to belowground components (e.g., roots, microbial biomass, and SOM, etc), as well as the effects of rhizosphere carbon deposition on active microbial groups. Also, building up two incubation experiments with various 13C-labelled substrates and nitrogen fertilizers (NH4+ and NO3-), we will examine the effects of substrate quality, nitrogen forms and their interaction on microbial carbon use efficiency and microbial biomass turnover, and assess the relative contributions of soil priming effects on SOC pools dynamics. In addition, using organic matter chemical (13C-NMR and Py-GC-MS) and microbial molecular ecology (rt-qPCR and clone sequencing) techniques, we will explore the coupling relationships between SOM chemical structure and functional microbial community abundance and composition under nitrogen enrichment. These research results will favor to clarify the mechanisms responsible for the regulation of plant-soil-microbial interaction to SOC dynamics in the alpine meadow, which will improve the build-up of carbon-nitrogen coupling cycle model in terrestrial ecosystems. Furthermore, these results will reduce the uncertainties of terrestrial carbon sink assessment in China, and provide scientific basis for the alpine meadow ecosystem management under global change.

大气氮沉降增加倾向于促进高寒草甸植被地上生物量，但是对地下碳过程和土壤碳截存的影响结果迥异，缺乏植物-微生物-土壤有机质交互作用的理解，对调控土壤碳截存的微生物学机理尚不清楚。本项目依托多形态增氮控制实验，利用原位13CO2标记和13C-PLFA技术分析植物同化碳的地下分配规律及其对活体微生物种群的影响；构建添加不同质量13C标记底物和18O-H2O的培养实验，探讨底物、氮肥类型对微生物碳代谢和周转的影响，量化激发效应对SOC动态的贡献；结合有机质化学（13C-NMR和Py-GC-MS）和微生物分子生态学（rt-qPCR和克隆测序）方法，研究有机质化学结构与功能微生物群落之间的耦合作用。研究结果有助于揭示植物-土壤-微生物交互作用对高寒草甸SOC动态的调控机制，完善陆地生态系统碳-氮耦合循环模型，有效降低中国陆地碳汇评估的不确定性，并可为高寒草甸生态系统应对全球变化提供科学依据。

不同氮素剂量和类型以及氮磷交互对青藏高原高寒草地土壤有机碳（SOC）截存的影响及其微生物学机制尚不清楚。本项目基于多类型、多剂量氮添加以及氮磷添加控制试验平台，构建13C标记底物的室内培养实验，利用SOM物理分组、13C-CP/MAS NMR、Pyrolysis-GC/MS和分子生物学等先进技术，研究不同氮素类型和剂量处理下SOC组成、来源、降解程度和化学稳定性的变化，量化氮磷富集下高寒草原生态系统SOC矿化及激发效应，探讨氮磷富集对介导土壤碳累积与释放的微生物群落结构及功能的影响。研究发现：（1）氮沉降增加对高寒草原表层SOC含量的影响取决于施氮类型和剂量。施氮显著改变高寒草原惰性SOM库的化学组成，增加其可降解性。施加铵态氮肥增加高寒草甸土壤重组中惰性化合物的比例，施加硝态氮肥增加轻组中活性化合物的比例。（2）施加高剂量的硝态氮肥导致高寒草原土壤NO3--N显著累积，加剧土壤酸化。施氮没有显著影响微生物数量和群落结构，与SOM中活性化合物的累积和消耗密切相关。施加铵态氮肥导致土壤微生物由细菌群落向真菌群落转变，减少溶解性有机碳（DOC）的产生；而施加硝态氮肥导致NO3--N显著累积，抑制了SOC的矿化，有利于SOC的截存。（3）活性碳添加显著促进高寒草原SOM矿化，产生正激发效应；氮素富集显著抑制高寒草原SOM矿化和激发效应，有利于土壤碳截存。高寒草甸土壤有机质矿化对活性碳输入的响应不敏感，在培养初期产生负激发效应。长期氮素富集促进高寒草甸SOM矿化和激发效应，不利于土壤碳截存。磷或氮磷共同输入对高寒草甸和高寒草原SOM矿化和PE的影响均不显著。（4）氮磷富集条件下高寒草原和高寒草甸SOM矿化和激发效应转变的微生物学机理迥异。活性碳输入引起高寒草原SOC矿化增加，产生正激发效应，由“共代谢”理论来解释。氮输入通过改变高寒草原土壤基本属性，影响土壤微生物群落组成和酶活性，进而抑制SOM矿化和激发效应，这可以由 “底物偏好利用”理论很好地解释。在长期氮素富集条件下，高寒草甸土壤过氧化物酶和子囊菌门相对丰度减少，但是活性碳输入增加了SOC矿化和激发效应。研究结果有助于提升高寒草地土壤养分利用效率，有效降低陆地“氮促碳汇”评估的不确定性，并可为高寒草地生态系统应对全球变化提供科学依据。

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