基于干旱和半干旱天然草地中长期氮添加背景下土壤氮有效性和土壤酸化对碳循环调节机制研究不足的现状，本项目以干旱和半干旱天然草地长期(15年)多梯度氮素添加实验为平台，结合酸添加(土壤酸化)和硝酸钾添加(土壤氮有效性)补充实验，利用根系隔离+刈割法区分土壤呼吸的两个基本组分(有机质分解呼吸和根系呼吸)，从氮有效性增加和土壤酸化角度研究土壤呼吸对长期氮添加的响应机制：(1) 探测土壤呼吸及其组分(有机质分解呼吸和根系呼吸)对长期氮素添加、酸添加、硝酸钾添加的响应规律以及响应的异同；(2) 比较植物群落、微生物群落和酶活性、土壤有机质组分、土壤氮循环、土壤阳离子等对长期氮素添加、酸添加、硝酸钾添加的响应阈值和敏感性；(3) 联系植物、微生物和土壤等因子分析土壤呼吸及其组分对长期氮素添加的响应规律，探测土壤氮有效性和酸化对土壤呼吸调节的相对重要性，深化全球变化背景下草原生态系统C-N耦合机制的理解。

Nitrogen enrichment could alter the below-ground SOC cycling in grassland ecosystem via direct effect of nitrogen availability and indirect effect of soil acidification on biotic communities and SOC availability. However, the direct evidence of how soil acidification and nitrogen availability affect belowground carbon cycling via plant and soil microbial communities is less conducted. Here, using trenching and root excision methods, we separated soil respiration into three components (SOM-derived respiration and root respiration) in long-term nitrogen, and KNO3 addition experiments. Our objective in this proposal was to evaluate the plausibility of different mechanisms whereby the effects of soil acidification and nitrogen availability on the responses of soil respiration components to long-term nitrogen addition via effects on above-ground and below-ground communities, soil cations, N cycling, and their linkages to the below-ground carbon cycling. Specifically, we attempt to address three questions: (1) the responses of soil respiration components to nitrogen addition and acid addition gradient; (2) the responses of plant community, soil microbial community and enzyme activity, soil nitrogen cycling, soil cations, and SOC stability to nitrogen addition and acid addition gradient; and (3) the plausibility of different mechanisms whereby the effects of soil acidification and nitrogen availability on the responses of soil respiration components to long-term nitrogen addition.