中文摘要：

由于人类活动和气候变化的共同作用，大气氮（N）沉降日益加剧，使得陆地生态系统中的可利用性N显著增加，生态系统更易受其他元素如磷fP）的限制。然而，目前关于N、P养分添加对草原生态系统不同组织水平的影响研究较少，相关机制尚不清楚。该文以内蒙古典型羊草（Leymus chinens＆）草原为研究对象，通过连续两年（2011-2012年）的N和P养分添加实验，研究建群种羊草的生理生态性状、种群生物量和群落初级生产力对N、P添加的响应及其适应机制。结果表明：羊草草原不同组织水平对N、P添加的响应不同。群落水平上，地上净初级生产力在不同降水年份均受N和P元素的共同限制，N、P共同添加显著提高了地上净初级生产力；物种水平上，N、P添加对羊草种群生物量和密度，以及相对生物量均没有显著影响，表明羊草能够维持种群的相对稳定；个体水平上，在正常降水年份（2011年），羊草生长主要受N素限制，而在湿润年份（2012年），降水增加使得羊草生长没有受到明显的养分限制。羊草通过增加比叶面积、叶片大小和叶片N含量，提高整体光合能力，以促进个体生长。总之，内蒙古典型草原群落净初级生产力受N、P元素共同限制，作为建群种的羊草，其对N、P添加的响应因组织水平而异，也受年际间降水变化的影响。

英文摘要：

Aims The accelerated atmospheric nitrogen （N） deposition due to human activity and climate change greatly increases the availability of reactive N in terrestrial ecosystems, leading to limitations of other nutrient elements such as phosphorus （P）. The effects of N and P additions on grassland ecosystems across different organizational levels, however, have rarely been studied; particularly the underpinning mechanisms remain unclear. Our objec- tive is to examine the effects of N and P additions on aboveground net primary productivity and functional traits of Leymus chinensis, which is a dominant species in the typical steppe of Inner Mongolia, and to study nutrient limitations at different organizational levels in a L. chinensis steppe induced by N and P additions. Methods We conducted a field manipulation experiment with additions of N （10.5 g N＇m-La-1） and P （32 g P2Os＇m-Z＇a-1） in a L. chinensis steppe ecosystem in Inner Mongolia in 2011 and 2012. The abovegr0und primary productivity, population biomass and density, whole-plant traits （e.g., individual biomass and stem-leaf biomass ratio）, leaf physiological （e.g., photosynthetic rate, water use efficiency, and leaf N content） and morphological traits （e.g., leaf area and specific leaf area） ofL. chinensis were investigated. Important findings Our results showed that N and P additions had different effects on the aboveground biomass at different organizational levels. The aboveground net primary productivity was greatly enhanced by the com- bined N and P additions in 2011 （normal precipitation） and 2012 （above average precipitation）, indicating the co-limitation of N and P at the community level. At the species level, N and P additions had no significant effects on population biomass, density, and relative biomass of L. chinensis in both years, indicating that this species could maintain population stability. The individual biomass of L. chinensis was increased by N addition in 2011; whereas it was not affected by eithe