## 4 Discussion  
### Chungchun SOCD and Its Related Character  
Many studies have considered carbon storage capacity of forests or soil carbon in urban environments in relation to different land uses or land covers. The average SOCD in the topsoil of Changchun forests was 5.66 kg m\({}^{-2}\) (0-20 cm), which was smaller than that found in Leicester (9.9 kg m\({}^{-2}\)) (0-21 cm) [39] and Beijing (7.33 kg m\({}^{-2}\)) (0-20 cm) [40] but larger than the average SOCD found in Guiyang (3.95 kg m\({}^{-2}\)) (0-15 cm) [41], Changsha (3.87 kg m\({}^{-2}\)) (0-15 cm) [42], Nanjing  
Figure 5: Comparison of urbanization-induced SOC accumulation, biomass C accumulation and ecosystem carbon sink capacity, for Changchun city of China. (**A**) Annual scale C-accrual of NPP, net ecosystem exchange and urbanization C accrual; (**B**) whole city C sequestration in biomass C and urbanization-related SOC accrual estimated from urban history gradients and ring road gradients. Notes: The NPP C accrual and biomass C were cited from reference 27, NEE was calculated by NPP and annual respiration, and urbanization C accrual value (1-m depth) was calculated as the 2-fold of 20-cm depth SOCD [37; 38].  
(2.18 kg m\({}^{-2}\)) (0-20 cm) [43] and Chuncheon (1.39 kg m\({}^{-2}\)) (0-30 cm) [44]. One reason for such differences might be the relatively low temperature in Changchun [37]. This city is located in northeast China, at a higher latitude than the other cities, and its low temperatures might induce a slower decomposition rate of soil organic matter [45]. In addition, Changchun belongs to the north-eastern old industrial base, and its evolution and forest development were accompanied by heavy industrial development. This increased soil organic matter input from environmental pollution, including human-induced organic matter accumulation under trees cleaned by sanitation workers and drainage accumulation from the city [46; 47], led to the down-regulation of SOC decomposition [45] for about 100 years. Similarly, Beijing, which is the largest city in China and presents heavy urbanization and industrialization, had the largest soil carbon content, almost identical to the value registered in Atlanta, United States (7.7 kg m\({}^{-2}\)) [33].  
The Q\({}_{10}\) value obtained for Changchun (1.92 \(\pm\) 0.03) was within the range reported for temperate forests (1.8-4.2) [48]. Mostly, excluding root respiration, natural forests have a larger Q\({}_{10}\) value [34] than that registered in urban forests. Globally, the average Q\({}_{10}\) value for soil respiration in urban forests, excluding root respiration, is above 3.5 [18], reaching only 2.81 in northeast China [27]. Thus, urban forests became less affected by temperature changes than natural forests. Contrary to the trend observed in SOCD, Q\({}_{10}\) decreased with increasing urbanization (Figures 2C and 3C), indicating that forest soil basal respiration was less sensitive with temperature change under urbanization pressure, which might have led to SOCD accumulation (Figures 2A and 3A).  
An average turnover time of 41.92 years has been reported for SOC in Chinese forests' toposoil [49]. In northeast China, a SOC turnover time of 13 years has been reported for the topsoil of temperate forests [50]. In the present study, the average SOC turnover time in the urban forest topsoil was 14.57 years, and it was much lower than average values determined for China but comparable to natural forests at the same latitude. However, our data were obtained under one settled condition in the laboratory, assumed as heterotrophic basal respiration, and the underestimation of respiration may lead to an overestimate of carbon turnover time. An increase in soil SOC accumulation and turnover time was evidenced by their positive correlations (Figure 4C). Thus, the increase in SOC turnover time observed from 1900 to 2014 indicated that the urbanization-induced SOC accumulation in soil was greater than the increase in basal respiration during the same period.