氮素富集条件下丛枝菌根真菌对青藏高原高寒草甸土壤有机碳周转的调控机理

Arbuscular mycorrhizal (AM) fungi are well acknowledged to play important roles in regulating the turnover of soil organic carbon (SOC) by directly contribute to SOC sequestration and indirectly impact on SOC decomposition via influencing decomposer communities. However, the mechanisms of how AM fungi regulate the decomposition and sequestration of SOC in the alpine meadow on the Qinghai-Tibetan Plateau under nitrogen (N) enrichment remains unclear. This gap of knowledge has greatly constrained the prediction of direction and extent of changes in SOC of this region under increasing atmospheric N deposition. The present project aims to unveil the mechanisms responsible for the regulation of AM fungi on SOC dynamics based on a long-term research platform with multi-form and low-level N addition located in Haibei Alpine Meadow Ecosystem Research Station, Chinese Academy of Sciences. Firstly, traditional mycorrhizal biological and molecular techniques will be used to analyze the effect of N rate and form on the abundance and composition of AM fungal community. Secondly, soil aggregate will be fractioned into different size-fraction and the content of glomalin related soil protein (GRSP) will be quantified in each fraction to evaluate the contribution of GRSP to aggregate stability and SOC sequestration. Finally, in situ 13C-CO2 pulse labeling experiment in combination with high-throughput pyrosequencing and analyses of enzyme activity will be employed to investigate the variations in community composition and activity of 13C-utilizing microbes located in mycorrhizosphere under N enrichment. Taken together, the present project will improve mechanistic understanding of the regulation of soil microbes on SOC decomposition and sequestration in alpine ecosystem, which will help to improve current C and N cycling coupled biogeochemical model for terrestrial ecosystem. In addition, results from the present project will provide scientific basis for the sustainable management of alpine meadow ecosystem under future global change scenarios.

丛枝菌根（AM）真菌能够直接或通过影响菌根际微生物群落碳分解活性间接影响土壤有机碳（SOC）周转。然而，现有研究尚未系统阐明氮素富集条件下AM真菌对青藏高原高寒草甸SOC的微生物学调控机制，这是导致大气氮沉降增加下该区域SOC演变方向不明确的核心问题之一。本项目拟依托长期增氮控制试验，利用菌根生物学和分子生物学方法，阐明施氮类型和剂量对AM真菌丰度和群落组成的影响；结合团聚体分组与球囊霉素相关土壤蛋白（GRSP）含量的测定，量化氮素富集下GRSP积累对高寒草甸SOC含量的直接贡献；构建原位13C-CO2脉冲标记试验，结合DNA-SIP、高通量测序及土壤酶活性测定，剖析不同施氮处理菌根际13C利用活性微生物群落组成及其SOC分解活性差异。研究结果有助于丰富调控高寒草甸SOC周转的微生物学机理，完善陆地生态系统碳-氮循环耦合模型，并可为高寒草甸生态系统应对全球变化以及可持续管理提供科学依据。

高寒草甸土壤有机碳（SOC）的分解和固持过程能否在大气氮沉降持续增加的现状下保持平衡是目前尚未明晰的关键问题，而解决这个问题的核心是明确氮素富集条件下调控高寒草甸土壤有机碳周转和稳定的微生物学机制。丛枝菌根（AM）真菌能够直接或通过影响菌根际微生物群落碳分解活性间接影响土壤有机碳周转，然而现有研究尚未系统阐明氮素富集条件下AM真菌对高寒草甸土壤有机碳分解和固持的调控机制。本项目以高寒草甸菌根际系统为研究对象，依托长期增氮控制试验，利用菌根生物学和分子生物学方法，研究了施氮剂量和类型对AM真菌多样性和群落组成的影响、球囊霉素累积及其与土壤团聚体和有机碳组分的耦联关系、菌根际碳利用活性微生物组成与功能及其对土壤有机碳周转和稳定性的影响三个核心内容，从而揭示氮素富集条件AM真菌对高寒草甸土壤有机碳周转的调控机制。研究结果表明：（1）施氮剂量而非氮素类型显著影响了高寒草甸AM真菌丰度、多样性及群落组成，氮素富集加剧了球囊霉科AM真菌在群落中的优势，而无梗囊霉科AM真菌的相对丰度则在高氮条件下显著受到抑制；（2）氮素富集条件下球囊霉素相关土壤蛋白（GRSP）含量显著降低，并与土壤团聚体稳定性及物理稳定性有机碳组分含量呈显著正相关关系，表明了GRSP对土壤有机碳稳定性的积极作用是AM真菌调控高寒草甸土壤有机碳固持的重要机制；（3）土壤氮素富集显著影响了菌根际微生物的群落组成，而这些群落组成上的变化与具有不同周转和稳定性特征的土壤有机碳组分的变化存在紧密关联。此外，氮素富集条件下土壤酶生态化学计量比值（酶C:N）的变化也与土壤有机碳组分周转动态有显著相关性，揭示出菌根际土壤微生物需求与底物之间的生态化学计量偏差是驱动氮素富集条件下土壤有机碳周转动态的重要机制。以上研究结果为深入认识高寒草甸土壤有机碳周转和稳定的微生物驱动机制提供了科学依据。