Plant growth and ecosystem production are limited by nitrogen (N), however, the mechanisms of N limitation in terrestrial carbon (C) sequestration in soil remains unclear. To examine these mechanisms N was deposited at rates of 0, 50, 100, and 150 kg N ha-1 yr-1 for two years in a subalpine Larix principis-rupprechtii plantation. Soil C and N components were measured three times encompassing the entire growing season (spring, summer, and autumn) in the second year of the experiment. Results showed that N-deposition affected soil organic carbon (SOC) in the upper soil layer (0-10 cm) especially in the summer season. Dissolved organic carbon (DOC) played the key role in C loss under the high-N treatment (p < 0.01) with higher N-deposition significantly increasing both DOC and DOC/SOC in summer (p < 0.01). In the summer season when there was sufficient precipitation and higher temperatures, the average DOC across all treatments was higher than spring and autumn. The active C components contributed to SOC sequestration in low and medium N- treatment and DOC, DON dynamics in summer were responsible for the C and N pool loss under the high N-treatment.