It has been reported that soil constitutes the largest carbon pool in terrestrial ecosystems. Soil organic carbon (SOC) in forest ecosystems is considered a major stock of carbon sequestered by vegetation biomass production. Soil total nitrogen (STN) is typically closely coupled with SOC and contributes to ecosystem productivity. Studies on soil carbon and nitrogen in the forests of Northwest China are relatively few. In order to provide a more accurate assessment of the carbon storage and accumulation characteristics in the forests of this region, we investigated the representative natural secondary forests of Quercus aliena var. acuteserrata in the Xiaolongshan Mountains of Gansu Province. These broadleaved forests are typically found at elevations of 1400-1800 m in this area, the western part of the Qinling Mountains. We selected forest stands of Q. aliena var. acuteserrata at three age stages (mid-aged, pre-mature, and mature), and three representative plots (30 m×20 m) were established for each age stage. Soil samples were taken at three points diagonally across the plot using a soil auger (6-cm internal diameter) at depths of 0-10 cm, 10-20 cm, 20-30 cm, 30-50 cm and 50-100 cm. The cored samples from the same depth in each plot were mixed and all samples were then transferred to the laboratory for further analyses. They were air dried and ground to pass through a 0.25-mm sieve prior to the analyses of carbon and nitrogen. In each plot, we also dug a soil profile (1 m depth) and sampled undisturbed soil, with two replicates for each soil layer, using a cutting ring (100 cm³). These soil samples were weighed and their moisture was measured to calculate the bulk density of each layer. The SOC content was determined by the potassium dichromate-sulfuric acid oxidation method. STN content was determined using a FOSS-8400 full-automatic Kjeldahl apparatus. Both SOC and STN densities over a forest area were calculated using their contents multiplied by the soil bulk density at each profile depth and soil profile thickness. The results showed the following. (1) Changes in SOC and STN contents along the soil profiles were consistent among the forests at different age stages. The surface soil layers had the highest SOC and STN contents, and they gradually decreased with increasing vertical gradient. (2) Both SOC and STN densities in the 1-m layer increased with increasing forest age. The SOC densities in mid-aged, pre-mature, and mature forests were 122.92, 242.21, and 280.53 t/hm², respectively, with the difference being statistically significant (P < 0.05). The STN densities in the three age stages were 10.37, 18.94, and 24.76 t/hm², respectively, and the differences were significant (P < 0.05). SOC and STN stored in the top 0-20 cm of soil accounted for large proportions of the whole 1 m soil, reaching 37%-56%. (3) SOC and STN contents showed significant positive linear correlation (P < 0.0001). In addition, both SOC and STN showed significant negative relationships with the soil bulk density. (4) The accumulation rates of SOC and STN varied with growth periods, and the accumulation rate of SOC (10.84 t hm^-2 a^-1) and STN (0.78 t hm^-2 a^-1) during the relatively young period (from mid-aged to pre-mature stages) were higher than the rate of SOC (1.92 t hm^-2 a^-1) and STN (0.29 t hm^-2 a^-1) during the mature period (from pre-mature to mature stages).