## 2 Materials and Methods
### Soil Sampling, Field Survey and Tree Measurement
Soil samples and a standard soil sample (ASA1, Institute of Geophysical and Geochemical Exploration, Langfang, China) were measured simultaneously in each digestion, and the final SOC content obtained for the reference/standard soil sampled assumed digestion was complete [5]. A 1:5 soil solution (one part soil to five parts distilled water) was used to determine soil pH with an acidity meter (Sartorius PT-21, Shanghai, China). Soil electrical conductivity (EC) was determined with an EC meter (DDS-307, Shanghai Precision Scientific Instruments Co., Ltd., Shanghai, China). Topsoil SOCD (kg/m\({}^{2}\)) within each sample was calculated according to Equation (1) [32]:  
\[\text{SOCD}=\text{BD}\times\text{C}\times(1-\delta)\times\text{h} \tag{1}\]  
where BD is the bulk density (g/cm\({}^{3}\)), C is the SOC content (g/kg), \(\delta\) is the fraction of gravel material, and h is the topsoil depth (0.2 m). Soil basal respiration in laboratory measurement was carried out in the following manner. Dry soil subsamples (100 g) from each of the 306 samples were incubated in two petri dishes (9 cm diameter) under three temperature grades in 3 growth cabinets (HPG-280 HX, Donglian Company, Harbin, China) [33]. Soil subsamples were adjusted to 80% WHC (water-holding capacity) and pre-incubated for seven days at 20 \({}^{\circ}\)C to minimize the initial flush of soil respiration, and then transferred to constant temperature and humidity incubators at 10, 20, and 30 \({}^{\circ}\)C for seven days (a pre-test before the experimental proceedings found that after 5 days of incubation, the soil basal respiration rate was almost steady from day 6 (supplemental Table S1)). Soil basal respiration rates were then measured using a LI-6400 meter (LI-COR, Inc., Nebraska, NE, USA), inserting the sensor head into the incubator and above the petri dishes to improve measurement precision (supplemental Figure S1). Water loss during incubation was examined gravimetrically every day, and it did not exceed 2%. The relationship between temperature and soil respiration was modelled according to the Van't [PERSON] equation [34]. \[\text{R}=R_{0}\text{e}^{\text{bT}} \tag{2}\]  
where R is the above-mentioned soil basal respiration rate (SR) (\(\mu\text{mol}/\text{m}^{2}/\text{s}\)), \(R_{0}\) is a fitting parameter referring to soil respiration rate at 0 \({}^{\circ}\)C, T is the air temperature which was 10 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C respectively, and b is a parameter defining temperature dependence upon soil respiration. Temperature sensitivity (Q\({}_{10}\)) of SOC respiratory decomposition was calculated as the following equation [34]:  
\[\text{Q}_{10}=\text{e}^{\text{10b}} \tag{3}\]  
The SOC turnover time (TT) \(\tau\) (years) can be calculated from the total reservoir size (Ctotal, kg C m\({}^{-2}\)) and the influx or the outflux (kg C m\({}^{-2}\) year\({}^{-1}\)) as the Equation (4) [35]. In this study, the total reservoir size was the carbon stock in soils (SOCD, kg/m\({}^{2}\)), the influx was the carbon uptake through gross primary production (GPP), which we did not consider under the laboratory measurement, and the outflux was the carbon losses, which were represented as the soil annual respiration (AR, kg/m\({}^{2}\)/year) (heterotrophic respiration, in this paper). \[\tau=C_{\text{total}}/\text{flux} \tag{4}\]  
Soil C efflux from AR was calculated according to Function (2) by using whole-year hourly air temperature data (acquired from Changchun Meteorological Station). A total of 20 stations distributed in urban-rural gradients were used in this scaling-up. We made use of inverse distance weight interpolation in ArcGIS 10.3 to obtain the air temperature of the 306 plots, and calculated the soil respiration flux by multiplying the forest soil area and the mean soil respiration rate of the 306 plots. The air temperature data were divided into each gradient in the ring road and urban disturbance time-related urban-rural gradients.