西南喀斯特山区生态恢复早期阶段共生固氮及调控机制

Symbiotic N2 fixation (ANF) is the major pathway of external nitrogen input to unmanaged natural terrestrial ecosystems in tropics or subtropics, and hence greatly impact an ecosystem’s structure, processes and functions. As nitrogen (N) is the major limiting nutrient of plant growth in most terrestrial ecosystems, the rates of ANF have great implications in the restoration of degraded ecosystems. Nevertheless, knowledge is much poor so far in terms of the ANF rates, regulations and strategies. Consequently, a mechanistic representation of ANF has been one of the biggest challenges in earth system models, which are the primary tool used for predicting climate - C cycle feedbacks. Karst ecosystems are widely distributed across the global land surface, especially in the southwest China. However, karst ecosystems remain poorly investigated in terms of the biogeochemical processes. Karst ecosystems are unique in multiple ecological and chemical properties and characterized by abundant mineral nutrients (e.g., N, P, Mg, Ca), relatively high soil pH and SOC storage capacity, which results in unique biological traits in both plant and microbial communities. In the current study, ANF will be investigated in the early stages of ecological restoration, including grassland, grass-shrub mixed field, and shrubland in the karst areas, southwest China. The project will firstly measure the spatial-temporal patterns of ANF rates and relates them to the influences of soil physiochemical properties in order to find out the major regulating factors. The key limiting factors will be identified via the investigation based on nutrient gradient transect and nutrient manipulation experiment. In combination with investigations based on nutrient gradient transect, nitrogen addition experiment on grassland planted with nitrogen-fixing plants and pot experiment, the N2 fixation strategies for the dominant N2-fixing plants will be identified. In addition, the N2-fixing plants that suitable for enhancing ecological restoration of the degraded lands in the karst areas will be selected. Based on the above experiments, the project is aimed to reveal the rates and controls of ANF during the early stages of ecological restoration. The results will be useful for the understanding of nitrogen cycling in in the early successional stages and in turn for guiding the ecological restoration in the karst region, southwest China.

共生固氮是热带/亚热带自然生态系统外源氮输入的主要途径，对生态系统结构和功能具有重要影响，也对促进退化土地生态恢复具有重要意义。当前对生物固氮速率、调控机制及固氮策略认识非常欠缺，导致生态模型仍不能通过机理对生物固氮进行模拟或预测。项目以喀斯特山区生态恢复早期典型植被为对象，首先在原位条件下测定共生固氮速率，分析其时空变化特征，甄别共生固氮主控因素；在此基础上，结合原位与盆栽实验筛选适用于促进喀斯特生态恢复的高效固氮植物；继而通过养分梯度样带研究与养分添加盆栽实验明确典型固氮植物固氮速率的关键限制因子；随后结合养分梯度样带、引种固氮植物的草地氮添加实验及盆栽实验等多种手段确定喀斯特山区生态恢复早期阶段典型固氮植物固氮策略。最终阐明西南喀斯特山区生态恢复早期阶段典型固氮植物固氮速率及调控机制，为深入理解喀斯特生态系统氮循环提供理论基础及为喀斯特退化山地生态恢复及生态服务功能提升提供科技支撑。

共生固氮是热带/亚热带自然生态系统外源氮输入的主要途径，对生态系统结构、功能和过程具有重要影响，也对促进退化土地生态恢复具有重要意义。当前对生物固氮速率、控制机制及固氮策略认识仍非常欠缺，导致生态模型仍不能通过机理对生物固氮进行模拟或预测。本项目重点研究了生态恢复早期阶段的野葛群落共生与非共生固氮速率及主控因素；典型固氮植物固氮速率比较研究；典型固氮植物对不同形态氮添加及环境变化的响应。重要发现包括：（1）野葛群落生物固氮速率为7.12-61.93 kg N/ha/yr，共生和土壤非共生固氮对野葛群落生物固氮的贡献大小相当，二者均受土壤湿度显著影响，此外，共生固氮速率受土壤磷可利用水平影响，而非共生速率受土壤钒和铁可利用水平的影响；共生固氮速率受固氮微生物的影响较大，而土壤非共生固氮速率受土壤性质的影响较大。（2）树种和土壤类型影响共生和非共生固氮对不同形态氮添加的响应。（3）桤木的固氮速率在城区环境中最高，而木豆的固氮速率在郊区环境中更高。生物固氮对多重环境变化的响应不仅在共生和非共生形式间存在差异，也在不同固氮树种间存在差异。（4）盐度增加对共生与非共生固氮的影响存在差异；共生固氮速率受植物叶片性状、土壤理化性质与固氮微生物组成共同调控；土壤非共生固氮速率受土壤理化性质与固氮微生物群落组成的影响。（6）银合欢共生固氮速率随纬度增加显著上升，而根瘤中的固氮微生物群落香农多样性随纬度增加显著降低，年均温升高通过改变固氮微生物组成显著抑制共生固氮速率。（7）基于盆栽实验比较了喀斯特山区生态恢复早期常见的五种固氮植物（桤木、木豆、银合欢、山毛豆和紫穗槐）的固氮速率，山毛豆固氮速率最高，可作为生态恢复早期的优良树种。项目成果有助于深入理解喀斯特生态系统氮循环及为喀斯特退化山地生态恢复提供科技支撑。

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