###### Abstract  
Global urbanization is a vital process shaping terrestrial ecosystems but its effects on forest soil carbon (C) dynamics are still not well defined. To clarify the effects of urbanization on soil organic carbon (SOC) variation, 306 soil samples were collected and analyzed under two urban-rural gradients, defined according to human disturbance time and ring road development in Changchun, northeast China. Forest SOC showed a linear increase with increasing human disturbance time from year 1900 to 2014 (13.4 g C m\({}^{-2}\) year\({}^{-1}\)), and a similar trend was found for the ring road gradient. Old-city regions had the longest SOC turnover time and it increased significantly with increasing urbanization (\(p\) = 0.011). Along both urban-rural gradients SOC stability toward temperature variation increased with increasing urbanization, meaning SOC stability in old-city regions was higher than in new regions. However, none of the urban-rural gradients showed marked changes in soil basal respiration rate. Both Pearson correlation and stepwise regression proved that these urbanization-induced SOC patterns were closely associated with landscape forest (LF) proportion and soil electrical conductivity (EC) changes in urban-rural gradients, but marginally related with tree size and compositional changes. Overall, Changchun urbanization-induced SOC accumulation was 60.6-98.08 thousand tons, accounting for 12.8-20.7% of the total forest C biomass sequestration. Thus, China's rapid urbanization-induced SOC sequestration, stability and turnover time, should be fully estimated when evaluating terrestrial C balance. urbanization; forest soil organic carbon; soil basal respiration; temperature stability; carbon turnover; global change +
Footnote †: journal: Journal of  
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the largest terrestrial organic carbon pool, containing twice as much carbon as the atmosphere or vegetation before its release into the atmosphere, and forests are the largest terrestrial ecosystem for carbon sequestration [3]. A small change in soil carbon may radically alter the balance of terrestrial ecosystems and the global carbon cycle [4]. It is important to scientifically understand the effects of urbanization on forest carbon dynamics, especially the underground soil organic carbon (SOC) changes and underlying mechanisms related with soil turnover, temperature stability and respiration. Urbanization effects are typically evaluated along urban-rural gradients, which have been used in studies considering ecosystem structure [5], biodiversity [6], and forest soil heavy metals [7]. In a city with clear development history, classification of regions with urban settlement history could be used as urban-rural gradients with exact development time. In China, cities are usually sprawled through ring road development, i.e., a first ring road region will have had the longest urbanization history, while an outer ring road will have had a shorter urbanization time. On the other hand, ring roads are much more related with lifestyle and government regulations as a sign of urbanization. It is well known among citizens that many government regulations are issued depending on the ring road numbers. In Changchun city, for example, the use of fireworks is now strictly prohibited within the 4 th ring road since the government's 16 th executive meeting. Also, heavy-polluting vehicles are now forbidden within the 4 th ring road from 6 a.m. to 8 p.m. since 1 January 2015. Both two urban-rural gradients related to urban history and ring road-sprawl have been used in the analysis of aboveground biomass [8; 9], however, their reliability on SOC changes still needs to be testified in a study. Laboratory experiments set to determine soil basal respiration can also indicate SOC decomposability and turnover [10], and the factor by which soil respiration rate increases when temperature increases 10 \({}^{\circ}\)C (Q\({}_{10}\)) can indicate the temperature stability of SOC decomposition [11].