Increasing exogenous nitrogen (N) input due to the elevated atmospheric N deposition strongly affects the carbon (C) cycle of terrestrial ecosystems. Nowadays, a large number of studies have demonstrated the positive effects of increasing N deposition on globally terrestrial vegetation C storage. While most of previous studies attributed it to the increased plant above-ground biomass due to the enhanced photosynthesis, recent studies have found that the effects of long-term N addition on below-ground biomass were also important for terrestrial C sinks. This study synthesized data from 181 available published papers across the Chinese major terrestrial ecosystems, and performed a meta-analysis to quantitatively evaluate the impact of the elevated N inputs on the biomass allocation of above- and below-ground in the Chinses terrestrial vegetation, as well as the differences between ecosystem types and fertilization regimes. We analyzed the different responses of plant biomass allocation to N enrichment to investigate the potential mechanisms of vegetation C gain response to chronically elevated atmospheric N deposition. Our results showed that N addition significantly enhanced the net photosynthetic rate and C storage in the Chinese terrestrial vegetation, and the responses of plant biomass allocation to N addition differed considerably between the different ecosystem types or fertilization regimes. The N concentration in the leaves of terrestrial plants significantly increased, and hence significantly reduced the C/N ratios in the leaves and litter, but did not significantly affect the C/N ratio in fine roots. N addition significantly increased plant net photosynthetic rate, but decreased photosynthetic nitrogen-use efficiency. On average, the above-ground biomass, litter mass, as well as root biomass increased significantly by 38%, 17%, and 18%, respectively. The magnitude of response of shoot to N addition was higher than that of the root in general. However, the responses of above- and below-ground biomass allocation to N addition were inconsistent in different ecosystem types. We found that the N addition significantly increased above-ground biomass but non-significantly increased root biomass in N-limited temperate forests and grasslands. Alternatively, N addition significantly increased root biomass rather than above-ground biomass in N-rich subtropical forests in China. The results of regression analysis showed that above-ground biomass did not increase linearly with the increase of root biomass, the response ratio of above-ground biomass firstly rising and then falling with the response ratio of root biomass. In addition, the net photosynthetic rate and photosynthetic nitrogen-use efficiency of plants also firstly increased and then decreased with the increase of leaf N concentration. It implies that the plant C partitioning strategies under the increased atmospheric N deposition are evolutionary development. With the continuous exogenous N inputs, plants would invest more C to promote root growth to acquire other resources, instead of preferentially investing more C to shoot to promote the increase of above-ground biomass. In summary, the nonlinear relationship between plant above- and below-ground biomass may affect predictions of C gain in terrestrial ecosystems, and future studies should focus on the changes in plant C partitioning strategy under long-term elevated atmospheric N deposition.