There are large uncertainties on the responses of community carbon and nitrogen to grazing in alpine grasslands, which limits our ability to accurately predict the responses of the vegetation growth, and carbon and nitrogen storage in alpine grasslands to grazing. A field fencing experiment has been conducted in an alpine meadow located at three elevations: 4300, 4500 and 4700 m, in the Damxung county of the Tibet since July, 2008. The carbon and nitrogen contents and the ratio of carbon to nitrogen of the community aboveground and belowground parts in the alpine meadow at the three elevations were measured in August, 2011, September, 2011 and August, 2012. Our results showed that fencing significantly decreased the carbon content of community aboveground parts by 2.5% at elevation 4700 m, but significantly increased the nitrogen content of community belowground parts by 9.8% at elevation 4500 m and significantly increased the carbon content of community belowground parts by 4.2% at elevation 4700 m, compared to grazing. Response of plant community carbon and nitrogen to grazing differed with different sampling dates. Therefore, the effects of grazing on community carbon content, nitrogen content and the ratio of carbon to nitrogen varied with elevations and sampling dates.%关于高寒草地植被碳氮含量如何响应放牧活动还存在着很大的不确定性，这限制了准确预测高寒草地植被生长及其碳氮贮存量对人类放牧活动的响应。基于2008年7月布设在藏北高原3个海拔高度（4300、4500和4700 m）上的围栏样地，通过对比分析围栏内外2011年8月份、2011年9月份和2012年8月份的植物群落地上和地下部分的碳含量、氮含量及碳氮比，探讨了藏北高原高寒草甸植物群落碳含量、氮含量和碳氮比对放牧的响应。与自由放牧条件下相比，围栏显著降低了海拔4700 m处2.5%的植物群落地上部分碳含量，显著增加了海拔4500 m处9.8%的植物群落地下部分氮含量，显著增加了海拔4700 m处4.2%的植物群落地下部分碳含量。不同采样日期的植物群落碳氮含量对放牧的响应存在差异。因此，放牧活动对高寒草甸植物群落碳含量、氮含量和碳氮比的影响随着海拔高度和采样日期而发生变化。
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