人工湿地中植物多样性与生态系统功能关系对废水碳氮比的响应及机理

Increasing plant species diversity in constructed wetlands improves ecosystem functioning, such as enhancing nitrogen (N) removal rate and reducing emissions of greenhouse gases. However, the huge difference in carbon (C) and N concentration of wastewater makes these effects uncertain. The effects of C/N ratio on the relationship between plant species diversity and ecosystem functioning are also new scientific questions in other ecosystems. Based on the previous projects that focused on the effects of plant diversity on N removal and greenhouse gas emission in constructed wetlands, this project tries to analyze the difference and stability of the effects of plant diversity (species richness, species identity, and species combination) on N removal, greenhouse gas emission, productivity, and substrate C sequestration under different C/N ratios. We plan to set the input conditions of wastewater with high N concentration but different C/N ratios, and use mass balance method and the technology of stable C and N isotope to analyze the mechanism through C and N processes in the substrate. We then analyze the effects of C/N ratio on the relationship between diversity and net ecosystem services. We also try to trade-off between N removal (the target service of constructed wetland) and the other services, thus to find the specific species and combination that have high efficient, economical and stable ability for treating wastewater with different characteristics of C and N. This study will deepen the understanding of biodiversity-ecosystem functioning, and provide theoretical and technical support for enhancing the efficiency of constructed wetlands for treating various sources of wastewater.

在人工湿地中提高植物多样性可以提高氮去除率、减低温室气体排放等。但在废水碳氮比例的巨大差异下这些多样性效应存在不确定性。碳氮比对生物多样性-生态功能关系的影响在其他生态系统中也是新的科学问题。本项目拟在以往几个关于植物多样性对人工湿地氮去除和温室气体排放等功能影响项目基础上，设置高氮含量下不同碳氮比的废水输入条件，综合物质平衡法和稳定同位素技术，研究植物多样性（丰富度、种特性、种组合）对生态系统氮去除、温室气体排放、生产力、基质碳固持等功能的效应在不同废水碳氮比下的差异和稳定性，通过基质碳氮过程分析其机理；分析碳氮比对多样性-净生态系统服务关系的影响及机理；针对氮去除这项人工湿地的目标服务，与其他服务进行权衡分析，找出处理不同碳氮含量废水的高效、经济、稳定的工具植物种类和植物组合。本研究将深化生物多样性-生态系统功能的研究，并为强化人工湿地处理多种废水的效能提供理论和技术支持。

以往研究已发现在人工湿地（CWs）中提高植物多样性（丰富度、种特性、种组合）可以提高氮（N）去除率、减少温室气体（GHG）排放等，但在废水碳氮比例（C/N）的巨大差异下这些多样性效应存在不确定性。碳氮比对生物多样性-生态功能关系的影响在其他生态系统中也是新的科学问题。本项目设置了两类调节C/N比废水的输入条件。其中一个是高N含量下不同C浓度的C/N比梯度，另一个是相同C浓度下不同氮浓度的碳氮比梯度。综合物质平衡法和稳定同位素技术，研究植物多样性对生态系统氮去除、温室气体排放等功能的影响。提高C/N比从1到5和提高植物多样性对GHG排放的影响的实验表明：（1）当C/N比为1时，增加物种丰富度使N2O和CH4的温室气体排放量减少，整合二者的全球增温潜势（GWP）降低70%；（2）CN比1下有芦竹的人工湿地中GWP减少72%；（3）将C/N比从1提高到5，单种人工湿地的GWP减少98%；（4）在CN比1下配置芦竹×紫露草×吉祥草混种人工湿地的GWP减少92%，单位氮去除的GWP与C/N比为5时单种均值无差异。我们的结论是，在砂培CWs处理C/N比为1污水时，提高植物多样性对GHG的减排作用与提高C/N比至5相当。提高C/N比从5到10和植物多样性对模拟漂浮人工湿地氮去除和GWP的影响实验表明：（1）C/N比为5时，随着物种丰富度的增加，出水TN浓度降低65%；（2）当C/N比从5增加到10时，单种人工湿地出水总氮（TN）浓度降低36%；（3）当C/N比为5时，物种丰富度增加时，非二氧化碳排放的GWP与生物能替代化石能缓解的GWP相结合的GWPnet减少120%；（4）在C/N比为5的条件下，4物种混种具高N去除和低GWPnet；（5）在C/N比为10的条件下，水芹×少花象耳草×鸢尾三物种混种具高N去除和低GWPnet。本研究得出的结论是，在合适C/N比条件下，在人工湿地中配置高植物物种丰富度和特定植物群落组合是一种更有效、更清洁的方法。本研究将深化生物多样性-生态系统功能的研究，并为强化人工湿地处理多种废水的效能提供理论和技术支持。