Increasing atmospheric nitrogen (N) deposition caused by human activities significantly changes carbon cycles and carbon budgets in terrestrial ecosystems. Compared with plant carbon pools, soil pools are more complex in their components and they respond in a variety of ways to N addition. Thus, contrasting conclusions have been reached as to the consequences of N addition for carbon storage in N-limited forest and grassland ecosystems including promotion, no change and inhibition. Alpine meadows are a N-limited grassland ecosystem on the Qinghai-Tibetan Plateau, where plants and soil microorganisms have adapted to the environment of low available N. N addition might be expected to affect inputs and outputs of soil organic carbon (SOC) via changing returns of plant residues and soil CO₂ release. However, a related study of this ecosystem has not so far been carried out. To assess the effects of atmospheric N deposition on SOC dynamics and the stability of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, a multi-form, low-level N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station in 2007. Three N fertilizers, NH₄Cl, (NH₄)₂SO₄, and KNO₃, were added at four rates: control (0 kg N·hm^-2·a^-1), low N (10 kg N·hm^-2·a^-1), medium N (20 kg N·hm^-2·a^-1), and high N (40 kg N·hm^-2·a^-1). Each N treatment had three replicates. Each plot had an area of 9 m² (3 m × 3 m) and a 2 m isolation band was established between adjacent plots. During the 2010 growing season, soil samples were collected to 30cm depth at 10cm intervals in mid-May, July and September. The contents of three size SOC fractions, coarse particulate organic carbon (CPOC, >250μm), fine particulate organic carbon (Fine POC, 53-250μm) and mineral associated organic carbon (MOC, <53μm) as well as POC/MOC ratios were measured to examine the dynamics, shifts and stability of SOC caused by N addition. Soil POC in the alpine meadow mainly accumulated in the top 10cm and accounted for more than 64% of the total SOC content, reflecting the lability and poor stability of the soil organic matter. Three-year N addition significantly changed the contents of soil CPOC, FPOC and MOC, and there were significant differences between various N levels, rather than N forms. Both soil POC and MOC responded in contrasting ways to N addition in the early, end and peak of the growing season, suggesting that temporal variability in the dynamics of SOC components responded to N addition. N addition tended to increase soil CPOC and FPOC contents in the peak of the growing season, while significantly reducing them in the early and end of the growing season. However, soil MOC content responded insensitively to N addition. N addition also significantly lowered the topsoil POC/MOC ratio in the early growing season, suggesting an increase in the stability of SOC. These results suggest that increasing atmospheric nitrogen deposition in the future may cause significant short-term changes in soil organic carbon composition and stability in the alpine meadow due to its lability.