In grassland ecosystems, nitrogen (n)-induced changes in plant community often affect grassland soil organic carbon (SOC) decomposition and accumulation. However, there is no consensus on the biological and chemical mechanisms that underlie the response of SOC decomposition to n-addition. In this study, we did a litter addition experiment to study the decomposition of SOC with added litter of six dominant plant species collected from n alone or n combined with phosphorus (n+P) fertilization treatments in a 7-yr field experiment. The experiment showed that there were no consistent responses of plant chemical properties to n alone or n+P addition. Fertilization altered plants species’ dominance in community, in particular the grass Elymus nutans. Litter addition significantly increased C decomposition, with the highest decomposition rates of SOC with added litter of all six plant species in n+P treatment. In general, both plant litter properties of organic C, total n (Tn), lignin concentrations, and soil properties of SOC and Tn, but not soil microbial biomass C, were the main factors determining cumulative decomposition of SOC with added litter. Litter properties were more important than abiotic soil properties in controlling C decomposition. This study indicated that fertilization with n alone or n+P can increase decomposition of SOC with added litter and lower SOC accumulation through changes in plant species’ dominance and chemical properties of litter. In terms of maintaining high SOC and increasing soil C, n fertilization, whether in n alone or n+P, should not be used in these alpine meadows.