## 2 Materials and Methods
### Soil Sampling, Field Survey and Tree Measurement  
Spot 5 (Earth observation satellite system launched by France) image data collected on 14 September 2010 were geo-rectified and re-projected in ArcGIS firstly. Then, urban forest locations were extracted from the image by the visual interpretation method (ArcGIS). The 306 sites in the present study (12 for 114 years, 90 for 60 years, 57 for 24 years, 41 for 10 years, 106 for 0 years; 31 in the 1 st ring road, 55 in the 2 nd ring road, 75 in the 3 rd ring road, 50 in the 4 th ring road, and 95 in the 5 th ring road, respectively) were sampled according to the stratified random sampling method to allocate plot numbers [30] based on their locations and forest coverage (Figure 1). Field surveys were conducted in July and August 2014, and each plot was about 400 m\({}^{2}\) for tree measurement and soil sampling. At each site, four replicate soil samples were taken undisturbed under the trees down to 20 cm, using a cutting ring (a soil container, 100 cm\({}^{3}\)). The ring-cup was inserted into the soil with a plastic hammer. Then, the intact soil with 100 cm\({}^{3}\) was taken out, and the samples were collected in cloth pockets, transported to the laboratory, and then mixed and fully air-dried at least 2 months until a constant weight for bulk density measurement. Visible plants, stones and rock fragments, and roots greater than 2 mm were removed from soil samples [9] before they were ground and sieved (0.25 mm mesh) for laboratory analysis. For testing purposes, air-dried samples are usually 1% different from oven-dried (105 \({}^{\circ}\)C) soil, and thus utilization of air-dried samples did not bias the total SOC determination. At the same time of soil sampling, all tree species were recorded according to tree name (genus, family) and leaf morphology (conifer and broadleaf). Diameter at breast height (DBH), tree height (TH), height under branch (HUB), and crown size (CS) of each tree in the plot were also recorded. DBH was measured at 1.3 m above-ground level by a DBH tape. TH and HUB were measured using a Nikon Forestry PRO550 laser rangefinder (Nikon, Jackson, MS, USA). Canopy projection was measured on the ground by a regular long steel tape in two directions (North-South; East-West), and these data were used to calculate the CS with the ellipse area formula. These compositional data and tree size data were used to find their possible contribution to SOC patterns in urban-rural gradients. Different land use usually accompanies different SOC dynamics. For finding different forest uses contributing to the urban-rural SOC patterns, the field survey also recorded the forest into five types [20] based on their location, function, and management objectives: (1) roadside forest (RF), trees along railroads, highways, boulevards, roads, and streets; (2) affiliated forest (AF), trees next to buildings in school yards, campuses, hospitals, commercial and business districts, industrial areas and residential areas; (3) landscape and relaxation forest (LF), trees in the public parks, forest parks, historic sites, and scenic areas; (4) ecological and public welfare forest (EF), trees next to river banks and farmlands; and (5) production and management forest (PF), trees in the nurseries, orchards, plantations, and woodlands. SOC, Basal Respiration, \(Q_{10}\),Turnover and Soil Properties: Laboratory Analysis and Parameter Computation  
Soil organic carbon was measured by the heated dichromate/titration method [31]. In this procedure, potassium dichromate (K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\)) and concentrated H\({}_{2}\)SO\({}_{4}\) were added to soil subsamples (0.1 to 0.5 g). Solutions were fully mixed and halted overnight, and tubes were then immersed in an auto-controlled 170-180 \({}^{\circ}\)C oil bath for 5 min. These digestion solutions were titrated with a FeSO\({}_{4}\) solution with an exact concentration, using two to three drops of phenanthroline as a colour indicator (brick-red indicated the endpoint of the titration).