西南喀斯特山区典型森林生态系统生物固氮速率及控制机理

Biological nitrogen fixation (BNF) is the key pathway of nitrogen input to natural terrestrial ecosystems. Knowledge of rates and controls of BNF is crucial for understanding the processes and function of terrestrial ecosystems and their responses to global changes. There is still huge uncertainty in terms of the rates and controls of BNF in terrestrial ecosystems. Karst ecosystems are a major part of terrestrial ecosystems, but so far no study has been conducted to investigate the rates and controls of BNF for karst ecosystems. In this study, typical karst forests which cover a succession gradient will be selected in a national natural reserve in southwest China. Different approaches will be used including field sampling, incubation, molecular techniques and laboratory analyses. Both symbiotic N2 fixation and asymbiotic N2 fixation will be measured. The project will firstly measure the spatial-temporal patterns of BNF rates and relates them to the influences of tree species and soil physiochemical properties in order to find out the major regulating factors. Laboratory incubation experiment will be used to identify whether asymbiotic N2 fixation from soil and litter layer is limited by phosphorus or molybdenum or both. Meanwhile, seedlings of two common legume species are used in a full factorial nitrogen, phosphorus and molybdenum addition experiment to evaluate whether the symbiotic N2 fixation is limited by either of the nutrients and what is the role of nitrogen in the responses of BNF to phosphorus or molybdenum addition. In order to link asymbtioc N2 fixation from soil and litter layer to free-living N2-fixer microbial community composition, nifH functional gene in soil and litter layer will be analyzed. Based on the above experiments, the project is aimed to reveal the rates and controls of BNF in the typical forests of southwest China. The obtained results will be useful for the understanding of nitrogen cycling in typical karst forests and for in turn help to guild the ecological restoration in this region.

生物固氮（BNF）是陆地自然生态系统外源氮输入的主要途径，对BNF速率及控制机理的认识对理解森林生态系统过程与功能及其对全球变化响应具有重要意义。然而目前对森林BNF速率及机理的认识尚很不足。项目以喀斯特山区典型森林为对象，采用野外实验、培养实验、分子生物学和室内分析多种手段，首先较系统测定森林共生与非共生固氮速率，并分析其时空变化特征；在此基础上，探讨树种类型与理化性质对BNF速率时空变化的影响，找出主控因子；继而通过养分添加实验明确森林共生与非共生固氮速率受钼和磷的限制情况；为了从微观尺度揭示非共生固氮速率变化机理，将通过对土壤/凋落物nifH基因的研究分析固氮微生物群落组成、丰度与多样性，并分析其与非共生固氮速率的相关性，确定最能反映固氮速率的微生物群落指标；最终阐明西南喀斯特山区典型森林BNF速率水平及控制机理。结果将有助于理解喀斯特森林氮循环和为生态恢复实践提供科技支撑。

生物固氮是多数自然生态系统外源氮输入最主要的途径，然而，目前对非共生固氮控制机理的认识非常欠缺，生态模型仍不能通过机理过程对生物固氮进行模拟与预测。本项目重点开展了以下几个方面的研究内容：喀斯特山区典型森林共生与非共生固氮速率及主控因素；树种类型对非共生固氮速率的影响及机理；通过养分添加实验确证典型森林共生与非共生固氮速率限制因子及土壤及凋落物固氮微生物群落及其与非共生固氮速率的相关性。重要发现包括：（1）桂西北喀斯特山区典型森林非共生固氮速率较低，约为0.01 ± 0.00 至 2.34 ± 0.13 kg N ha-1 yr-1，其中土壤固氮约占了85%，森林非共生固氮的最强解释因子因基质和森林而异，总体上包括凋落物含水率、全氮、全磷、交换性钙和土壤有机碳；（2）树种和岩性分别为解释森林内部和森林之间非共生固氮速率的空间变异提供了新机制；（3）地形可能调控非共生固氮速率对氮沉降的响应，但地形的调控效应可能因生态系统组分而异，因此，为了更好反映生物固氮的空间差异，生态模型需要考虑地形部位对大气氮沉降的差异化响应；（4）森林苔藓、凋落物和土壤固氮速率对温度和湿度的敏感性存在明显差异，因此，为了更好预测生物固氮对气候变化的响应，生态模型需要针对不同生态系统组分采用不同的温度或湿度敏感性；（5）相邻但优势树种不同的森林之间非共生固氮速率存在明显差异，因此，为了获取代表性数据，需要开展更多的研究；（6）磷和钼添加促进喀斯特山区常见固氮植物生物固氮速率，但促进作用因树种而异，意味着同一生态系统内部不同固氮树种之间生物固氮的限制性元素可能存在差异。