氮磷同位素自然丰度解析海螺沟冰川退缩迹地植被原生演替的营养驱动机制

Elucidating the driving factors of successional dynamics in terrestrial ecosystems is an important issue in ecology. The simultaneous availability of successional stages provides necessary conditions to use a “space-for-time substitution” approach on plant and soil biota succession. Thus, the Hailuogou Glacier Chronosequence, located on the south-eastern fringe of the Tibetan Plateau, provides an excellent place to study the relationship between vegetation succession and soil development. Nitrogen and phosphorus use strategy is critical for plants to exert their ecological functions under different nutrient conditions. Nature abundance in isotopic signature of N and P in plant-soil systems provide pivotal information for better understanding the shifts in plant N (P) acquisition in situ and ecosystem N (P) cycling. Based on the gradients of soil N availability, we will measure the concentration and stable isotope of bulk N and dissolved N species, and tissue nitrate in plants; Then we will apply stable isotope fractionations and mass-balance calculations to diagnose plant nitrate use strategies, N preferences, N allocation and use efficiency. Furthermore, through comprehensive evaluating the isotopic composition of oxygen in phosphate (δ18O-Pi), we can explore the importance of different biological processes on P cycling and P use efficiency in different successional plants. Lastly, controlled N and P addition experiment is performed to determine the strength of N, P and N:P stoichiometric homeostasis for the ecosystem structure, functioning and stability. Our research has implications for integrated studies on biogeochemical impacts of vegetation changes and nutrient status development. Further, our results will contribute to improved predictions of the direction and intensity of primary succession, and also to improved management practices related to nutrient limitation during long-term soil development.

冰川退缩迹地为研究植被演替规律提供了天然实验室，氮磷同位素自然丰度是理解植物氮磷原位利用策略的关键。本项目借助新的同位素方法(磷酸盐和硝酸盐氮氧同位素)，以海螺沟冰川退缩迹地的植被-土壤系统为研究对象，结合野外原位监测和控制实验，研究不同演替阶段下土壤营养状态和植物氮磷利用策略，以期揭示植被演替与土壤发育的耦合关系和协同效应：整合各演替阶段土壤中氮磷含量、化学组成和同位素特征，揭示土壤氮磷可利用性、循环过程与植被演替间的相互关系；应用同位素分馏和质量平衡原理，判别主要氮源及贡献，定量评价植物对不同氮源的相对偏好；总结不同演替阶段下植物氮磷利用策略的可塑性和稳定性的权衡关系，将表型可塑性、化学计量自稳态与生态系统结构、功能和稳定性相联系。本研究能为认识高山植物种间相互作用、群落的构建机制以及环境变化下植被组成和演替动态提供科学依据，实践中也为退化山地植被恢复与管理提供技术指导。

依托本项目，我们深入研究了冰川退缩迹地原生演替养分限制、氮贡献与偏好、功能性状经济型谱以及土壤微食物网群落构建等内容，取得以下主要结果：.一、揭示了群落演替中限制养分由氮到磷的转变过程.通过群落权重氮磷生态化学计量，发现随演替的进行叶片N:P从8.2增加至20.1，限制养分由早期的氮向后期的磷转变。类似的，胞外酶化学计量和阈值元素比率分析，表明微生物早期受碳和氮限制，而后期磷限制更严重。氮磷添加实验也表明，土壤氮磷有效性以及由此调控的种间关系是冰川退缩原生演替的重要驱动因素。.二、定量了各阶段优势物种不同氮形态的贡献与偏好.氮形态偏好上，演替早期物种硝态氮贡献最大，逐渐过渡到铵态氮，晚期云冷杉与外生菌根共生，能有效利用有机氮。氮形态的偏好与各阶段优势氮形态相吻合，表明充分利用优势氮资源的物种更容易占据优势生态位。.三、阐明了优势物种叶片功能性状经济型谱演化规律.演替早期固氮植物由于奢侈的资源利用策略，导致低的光合氮和磷利用效率；中期氮限制，阔叶树种更有效的氮分配，获取型资源策略导致“以快制胜”。后期磷限制，针叶树种增加代谢磷和核酸磷的分配，最高的叶片寿命/偿还时间，即保守型策略，从而“稳中求胜”。.四、明确了土壤微食物网群落构建机制与主控因素.在土壤微食物网的群落构建中，早期和晚期分别受土壤因素和生物因素的调控；且协同进化，微生物由细菌为主演替到后期的真菌为主，能量通道也由快速的细菌通道，到中速的植物通道，最后到慢速的真菌能量通道，均处于“快速投资－收益”到“慢速投资-收益”的演替中，植物适者生存。.在国内外学术期刊上发表论文6篇，包括5篇一区SCI论文，第一标注的Plant Soil、Soil Biology and Biochemistry和Geoderma，以及第二标注的两篇Tree Physiology，培养博士生和硕士生各一名。本研究深化了对植被演替-土壤发育-土壤微生物的耦合关系和协同效应的认知，实践中也为退化山地植被恢复与管理提供技术指导。