农牧交错带半干旱草地生态系统碳通量的水-氮耦合效应及其生物学机制

The biological mechanism of carbon flux is the key to understanding of the carbon-nitrogen-water coupling cycle and analyzing the uncertainties of the interactive effects of water and nitrogen. It is an urgent problem to be solved how the plant and soil microbial communities will change under the interaction of water and nitrogen and what the relationship between these changes and the dynamics of carbon flux is. Therefore, relying on the water and nitrogen interaction experimental platform established in the semiarid grassland of the agro-pastoral ecotone in the north of China, this project intends to quantify the effects of water and nitrogen on the carbon dioxide and methane fluxes, and to analyze the relationship between carbon fluxes and plant community characteristics in the semiarid grassland ecosystem. Meanwhile, the effects of soil microorganisms on carbon fluxes will be explored using the GeoChip technique. The purposes of this study are to elucidate the coupling effects of water and nitrogen on carbon fluxes, and to reveal the biological mechanism of carbon flux in the semiarid grassland ecosystem. The results can provide in-situ data to accurately assess the changes of carbon source or sink function under the precipitation variations and atmospheric nitrogen deposition in the future, and a theoretical basis for the scientific management of the semiarid grassland in the agro-pastoral ecotone in northern China.

碳通量的生物学机制是深入理解碳-氮-水耦合循环，分析水、氮交互影响不确定性的关键。在水、氮交互影响下，植物、土壤微生物群落有何变化，这些变化与碳通量的变化有何关系，是亟待解决的问题。本项目依托国家重点研发计划在北方农牧交错带半干旱草地已建立的水、氮交互控制实验平台，量化水、氮交互影响下半干旱草地生态系统的二氧化碳和甲烷通量；分析植物群落特征变化与碳通量动态的关系；并利用微生物功能基因芯片（GeoChip）技术，探索土壤微生物对碳通量的影响机制。旨在阐明碳通量过程的水-氮耦合效应，揭示半干旱草地生态系统碳通量的生物学机制，为准确评估未来降水格局变化和大气氮沉降下半干旱草地生态系统碳源/汇功能的变化提供数据支撑，为北方农牧交错带半干旱草地的科学管理提供理论依据。

明确草地生态系统碳通量过程与机制，是提高草地碳汇能力、制定草地管理措施的关键。为阐明未来降水变化与氮沉降增加对草地生态系统碳通量的影响，本项目以农牧交错带半干旱草地为研究对象，对增水50%、减水50%、氮添加及其交互对二氧化碳（CO2）与甲烷（CH4）通量的影响进行研究。主要结论如下：单独的氮添加或增减降水对农牧交错带半干旱草地生态系统CO2通量[生态系统总生产力（GEP）、生态系统呼吸（ER）和生态系统净CO2交换（NEE）]无显著影响；但当降水减少时，氮添加会抑制NEE，而降水增加时，氮添加促进NEE，表明农牧交错带半干旱草地生态系统NEE受到氮和水的共同调控；而GEP 和ER 受土壤中阳离子（K+、Ca2+、Na+、Mg2+）交换量的调控；农牧交错带半干旱草地生态系统在生长季是弱的CH4汇，其平均CH4吸收速率为5.64 μg·m-2·h-1；氮添加和降水变化未影响产甲烷功能基因（mcrA）丰度和甲烷氧化功能基因（pmoA）丰度，因此氮添加、降水变化及两者交互并未对CH4通量产生显著影响；植物生物量、多样性、根冠比等是影响半干旱草地生态系统CH4通量的重要因素，进一步分析发现，相比于非根际土壤，植物根际土壤具有低pH和高可利用养分含量的特点，表明植物因素变化介导的根际作用变化对甲烷氧化菌的影响，很可能是植物群落特征影响CH4通量的途径。以上结果阐明了水、氮交互对两种含碳温室气体通量的影响，并为深入探究碳通量的生物学机制提供了理论依据与突破口。