竹林扩张对凋落物分解的影响机制及其对环境变化的响应

Bamboo, mainly composed of woody species, is a kind of clonal plant. It has typical biological properties of invasive plants, including fast growth, high reproduction rate and adaptability. Therefore, bamboo expansion has been widely reported across the world. China has the most significant bamboo resources in the world, thus it is critical to understand the driving mechanism and ecological consequences of bamboo expansion, in order to propose reasonable and effective management strategies for bamboo forests. Plant litter decomposition is one of the cogs in the gear of ecosystem functioning. The decomposition process allows recycling of carbon and nutrients from dead organic matter, fuelling new primary production. Bamboo expansion is a process of changes in vegetation composition, microclimate and litter quality, while its effects on litter decomposition are still unclear. Whether the litter decomposition-driven nutrient cycling plays a facilitator of bamboo expansion is still unanswered. Due to the existence of physiological integration of bamboos, it is unknown if the effects of bamboo expansion on decomposition still follow the paradigm of general invasive species do to the decomposition. Will it increase the nutrient availability by accelerating decomposition in order to facilitate expansion, or decrease the nutrient availability by slowing down decomposition in order to inhibit the regeneration of the invaded community? Climate is one of the most important drivers of decomposition, but will climate warming alleviate the nutrient limitation to the invaded community caused by bamboo expansion then slow the expansion speed as a consequence? Additionally, the roles that different soil biota communities play in all abovementioned processes are unknown. We propose to choose nine locations across a national scale along a climatic gradient (15.6~24.5℃). In each location, sites with different bamboo density will be established in order to conduct both in situ and reciprocal decomposition experiment. Soil biota community will be manipulated by employing three mesh sizes litter bags or microcosms, enable us to distinguish three increasingly complete decomposer communities (small, medium-sized and complete). Dual-labelled (13C/15N) litter decomposition will be conducted to trace litter-derived C and N in soils. By investigating mass loss and nutrient release of all our decomposition experiments, we intend to answer above-developed questions.

竹类是木本为主的克隆植物，具有生长快、繁殖率高、适应性强等入侵植物生物学特性，因而竹林扩张被广泛报道。中国拥有世界最丰富的竹资源，认识竹林扩张的生态效应及驱动机制是制定竹林管理策略的重要依据。凋落物分解是养分循环的关键一环，影响生态系统生产力。竹林扩张伴随微环境及凋落物质量一系列变化，然而其对分解的影响机制尚不清楚。分解驱动的养分循环是否扮演竹林扩张的助推器有待解答；由于生理整合机制的存在，竹林扩张是否仍遵循入侵植物对分解影响的理论范式也未可知；气候是分解的重要驱动力，气候变化将缓解还是加速上述养分循环驱动的竹林扩张亟待解释。土壤生物在上述过程中又扮演怎样的角色？拟沿气候样带尺度（9个站年均温15.6~24.5℃）开展局地（竹林、竹阔界面、阔叶林）凋落物互置分解实验，结合土壤生物功能群和凋落物质量处理、13C/15N双标记凋落物跟踪碳、氮路径等手段观测分解速率及养分释放，回答上述命题。

凋落物分解是生态系统中碳和养分循环的基础，有关植物入侵对凋落物分解的影响通常通过叶片凋落物性状差异来预测，并且普遍认为外来入侵植物分解更快并籍以促进入侵地养分循环从而有利于其进一步入侵，即“养分促进假说”(Nutrient facilitation hypothesis)。然而，植物入侵还可能影响分解者的活性或改变分解者的功能多样性，从而改变凋落物分解，其机制尚不清楚。竹林扩张（尤其毛竹在中国是本地种，但因克隆生长造成入侵）是否遵循入侵植物对分解影响的理论范式尚未可知；气候变化将缓解还是加速上述竹林扩张驱动的养分循环亟待解释。本项目在全国7个地区建立竹林扩张梯度（竹林、竹阔界面、阔叶林）样地84个，重点就凋落物混合效应、土壤生物功能群多样性（包括微生物群落结构）、以及分解生境（即森林类型）阐明竹林扩张后对生态系统重要过程—凋落物分解的潜在影响极其调控机制，进一步探讨了上述过程对潜在环境变化的响应。项目完成了预期目标，阐明了毛竹扩张改变凋落物分解与土壤生物功能群多样性和土壤微生物群落组成关系的驱动机制，发现在考虑土壤生物功能群多样性后，竹林扩张对凋落物分解的影响或许不符合“养分促进假说”。此外，在受竹林扩张后，温湿度增加对凋落物分解的促进作用有所降低。首次揭示了土壤生物功能群与凋落物共同作用决定植被变化对土壤碳库影响的新机制，提出“植被变化通过土壤生物功能群与凋落物共同影响碳库”的新框架，改变了“植被组成变化主要通过凋落物数量和质量影响土壤碳库”的传统认识，丰富了土壤固碳机制科学认识，为发展土壤固碳稳定性机制提供创新思路。相关成果已发表论文5篇，其中包括Journal of Ecology（本项目为第一资助）等本领域国际知名杂志。