Increasing atmospheric nitrogen (N) deposition can significantly change carbon (C) cycling rates and budget in the terrestrial ecosystem, and is generally considered to be an important pathway of missing of the sink. However, the contribution of atmospheric N deposition to C sequestration in the terrestrial ecosystem is controversial. It is, therefore, essential to accurately evaluate the effects of rate and type of N deposition on amount, composition and stability of soil organic carbon (SOC). Stable C isotope natural abundance (or δ¹³C) contains various information concerning C cycling processes, and hence makes it feasible to track SOC in its transfer, transformation and accumulation processes. The technique of Stable ¹³C natural abundance provides a way to characterize the dynamics of SOC with different turnover times. It is hypothesized in this study that N deposition increases biomass of plants and¹³C-depleted plant debris that leads to expansion of the fraction of coarse particle-sized SOC. Meanwhile, increased N deposition promotes activities of soil microbes, and increases emission of ¹³C-depleted CO2, thus leading to decrease in SOC content and accumulation of δ¹³C. Consequently, this study is mainly aimed at determination of the effects of rate and type of N deposition on contents of SOC and its various particle-size fractions in the soil and quantification of relative contributions of the changes in these fractions to the change in total under the condition of N accumulation. Through the above-described researches, it is expected a better in-depth knowledge could be obtained regarding mechanisms of the effects of N deposition on deposit and stability of SOC. For that end a controlled multi-form, low-rate N addition field experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station in 2007. The experiment was designed to have three types of N fertilizers, NH₄Cl, (NH₄)₄SO₄ and KNO₄, and four N application rates: 0, 10, 20 and 40 kg hm^-2 a^-1, and three replicates for each treatment. Soil samples, 10 in each treatment lot,were collected randomly with augers at 10cm intervals to a depth of 30 cm. A total of 1080 samples were collected in 2011 for determination of content and δ¹³C value of dissolved organic carbon, as well as contents of the three fractions of SOC: macro particle-sized organic carbon (MacroPOC, >250 μm), micro particle-sized organic carbon (MicroPOC, 53~250 μm) and mineral bonded organic carbon (MAOC, <53 μm). It was found that low N input significantly increased the contents of soil MacroPOC and MAOC, while high N input acted reversely; and N input, no matter high or low, decreased the content of MicroPOC. Besides, the effect of nitrate fertilizer was more significant than that of ammonium fertilizer. In short, For the entire soil profile, low N input increased the SOC storage in the 0～30cm soil layer by 4.5%, while medium or high N input decreased it by 5.4% or 8.8%, respectively. In the treatments of low N input, the increment of C consisted mainly of MicroPOC, while in the treatments of high N input, the decrement did mainly of MicroPOC, too. N application for 5 years in a row significantly promoted decomposition of the POC fraction of SOC, thus leading to increase in the proportion of stable fractions of SOC. The above-described findings indicate that N input may affect, increase or decrease, SOC storage in the alpine meadow, with the critical rate being set at 20 kg hm^-2 a^-1 while taking into account the ambient N deposition rate of 10 kg hm^-2 a^-1. Therefore, as long as the exogenous N input level is kept lower than this critical value, it is safe and beneficial to SOM accumulation in the alpine meadow on the Qinghai-Tibetan Plateau in the current period of time. These findings may serve as a theoretical basis for C and N management of alpine meadows in the future. Further study is still needed to explore microbial mechanisms of N input affecting SOC in quality and quantity.