Urbanization Drives SOC Accumulation, Its Temperature Stability and Turnover in Forests, Northeastern China

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.