表层土壤对植物生长及大气二氧化碳固定等多方面具有重要的作用。土壤碳库由多种复杂的碳组分构成，不同来源的碳组分具有截然不同的稳定性和空间分布规律，可能在未来全球变化背景下产生强烈的反馈作用。快速城市化进程造成了一系列城市环境变化，深刻影响了城市森林土壤碳循环过程，进而重塑了土壤碳在不同空间尺度上的分布格局。研究城市森林表土碳组分在不同空间尺度上的分布格局及影响因素，有助于深入理解城市森林土壤碳的稳定机制，对于准确预测气候变化及人类活动双重干扰下的城市土壤碳库动态也具有重要参考价值。本研究以城市森林为研究对象，结合野外采样及数据整合分析，揭示了多尺度下（局地-区域-全球）城市森林表层土壤（0-20 cm）碳组分的空间格局，探讨了不同尺度下城市森林表层土壤碳组分空间变异的影响因素，区分了各种因素的相对贡献，对比了城市与自然森林表层土壤碳组分特征及空间格局异同。本研究取得的主要结论如下：

（1）在局地尺度上，自北京市城区过渡到郊区，城市森林表土无机碳（SIC）浓度线性降低，表土有机碳（SOC）及其物理组分颗粒态有机碳（POC）和矿物结合态有机碳（MAOC）浓度则均呈先降低后增加的非线性趋势。城-郊梯度上的土壤SOC组分始终以POC为主，可能同城市土壤的发育时间相对较短有关。此外，POC对SOC的贡献在城区减小，而MAOC对SOC的贡献在城区增大。土壤质地、pH和城市公园年龄是解释表土碳组分浓度在城-郊梯度上空间变异的重要因素，相比之下气候因素（年均温和年降水）的作用却并不明显。总之，在局地尺度上，人为因素比气候因素对解释城市森林土壤碳组分的空间变异更加重要。

（2）在区域尺度上，中国东部城市森林表土碳组分浓度在气候梯度上显示出明显的纬度格局，表土POC及MAOC浓度在相对湿润的高纬度及低纬度地区更高，而在相对干旱的中纬度地区较低，相反，表土SIC浓度在相对干旱的中纬度地区更高。三种土壤碳组分在纬度梯度上的变化规律同自然森林大致相似。年均温是解释表层土壤POC浓度在纬度上空间变异的最重要因素，随着年均温增大，表土POC浓度显著降低。年均温对MAOC在纬度上空间变异的影响却并不显著，反映了MAOC相对稳定的性质。土壤质地是解释表土MAOC纬度变异的最重要变量，土壤粘粉粒含量越高，MAOC浓度越高。年降水是解释SIC浓度在纬度上空间变异最重要的因素，较干旱地区的土壤能够积累更多的SIC。表土POC及MAOC浓度均随着城市公园年龄的增加而增加。此外，与自然森林不同，城市森林表土有机碳库以植物源主导的POC组分为主。总之，在区域尺度上，自然因素（气候和土壤）主导着中国东部城市森林表土碳组分的空间变异，人为因素影响的城市公园年龄也具有一定作用。

（3）在全球尺度上，城市森林表土SOC密度具有明显纬度格局，并随着年均温、城市绿度指数和人均GDP的增加而显著增加。估算得到全球平均SOC密度为53（43-63）（Mg C ha^-1），比所有自然土壤的全球平均值（43 Mg C ha^-1）高23 %。全球城市森林中的总SOC储量（0-20 cm）大约为1.40（1.14-1.66）Pg C，相当于全球陆地植被总SOC的2.5 ‰。在国家尺度上，挪威城市森林表土SOC密度最高且SOC密度排名前十的国家都是高收入国家。美国城市森林表土SOC储量最大，约占全球总量的三分之一。总之，在全球尺度上，自然和人为因素共同影响城市森林表土SOC的纬度格局。

综上所述，城市森林表土碳组分在不同空间尺度上具有不同的变化规律及主导因素。人为因素在小尺度上的影响更大而自然因素在大尺度上的影响更大。与自然森林相比，城市森林土壤以不稳定的有机碳组分为主，暗示着温度升高可能加速城市森林土壤碳的损失。因此，在未来的城市森林管理中应考虑采取相应措施提高城市森林土壤的“保碳”能力。

Topsoil provides major amounts of nutrients for plant growth and also plays an important role in atmospheric CO₂ fixation. Soil carbon consists of various complex carbon fractions which have significantly different stability which can generate strong feedback in the context of future global changes. Rapid urbanization has caused significant changes of urban environments and deeply affected the soil carbon cycle in urban forests. As a result, urbanization strongly reshaped the spatial patterns of soil carbon in urban forest at different spatial scales. A better insight in the spatial patterns and drivers of topsoil carbon fractions in urban forests at different scales can deepen our knowledge of the soil carbon stabilization mechanisms in urban forests. Besides, it is also of great reference value for accurately predicting the dynamics of urban soil carbon pools under the dual interference of climate change and human activities. In this study, we investigated the spatial patterns of topsoil (0-20 cm) carbon fractions and the predominant drivers in urban forests at multiple scales (local-regional-global). Moreover, we compared the characteristics and patterns of soil carbon fractions between urban and natural forests. Main findings of this study are as follows:

(1) At the local scale, our results indicate a linear decrease of topsoil inorganic carbon (SIC) concentrations in urban forests from the urban core to the rural area in the Beijing Metropolitan, China, while topsoil organic carbon (SOC) concentrations, particulate organic carbon (POC) and mineral associated organic carbon (MAOC) all show a decrease up to 40 km from the urban core and then an increase to the rural area. Topsoil SOC fractions along the urban-rural gradients are dominated by POC, which may be related to the relatively short development time of urban soils. In addition, topsoil POC contributes less to SOC in urban areas, while MAOC contributes more to SOC in urban areas. Soil texture, pH, and the urban forest park age are important factors in explaining the spatial variation of topsoil carbon fractions in urban forests, while the role of climate plays a limited role. Overall, at the local scale, anthropogenic factors may play a more critical role than climatic factors in regulating the spatial variations of topsoil carbon fractions in urban forests.

(2) At the regional scale, our results indicate a strong latitudinal pattern of topsoil carbon fractions concentrations in urban forests across nine large cities in eastern China. Topsoil POC concentrations and MAOC concentrations are higher in relatively humid high- and low-latitude areas and lower in relatively arid mid-latitude cities, while the SIC concentrations is higher in relatively arid mid-latitude cities. Spatial variations of soil carbon fractions in urban forests along the latitudinal gradients is similar to natural forests. Mean annual temperature (MAT) is the most important factor explaining the spatial variations of topsoil POC concentrations along the latitudinal gradients. Specifically, topsoil POC concentrations significantly decreases as MAT increases. However, MAT plays a limited role in explaining spatial variations of MAOC concentrations, indicating the relatively stable characteristic of MAOC. Soil texture is the most important explanatory variable affecting the spatial variations of topsoil MAOC concentrations, with higher clay and silt contents resulting in higher MAOC concentrations. Spatial variations of topsoil SIC concentrations were mainly controlled by mean annual precipitation (MAP). We also found that both topsoil POC and MAOC concentrations increase with urban park age. Furthermore, different from natural forests, the soil organic carbon pool in urban forest is dominated by POC. In summary, at the regional scale, natural factors (climate and soil) shape the spatial variations of topsoil carbon fractions in urban forests. Urban Park age, which is controlled by human activity, also plays an important role.

(3) At the global scale, we indicate that topsoil SOC in urban forest increased significantly with higher mean annual temperature, urban greenness index, and GDPP across global cites. We mapped surface-layer SOC density in global urban forests and estimated an average SOC density of 53 (43-63) (Mg C ha-1), which is 23 % higher than the global average of all natural soils (43 Mg C ha-1). The total SOC stock in global urban forests was estimated to 1.40 (1.14-1.66) Pg C to a 20 cm depth, which is 2.5 ‰ of the corresponding global total in terrestrial biomes. Specifically, Norway had the highest SOC density in urban forests (89.9 Mg C ha-1) and the top ten countries were all high-income countries. The United States has the largest SOC stocks in urban forests (0.41 Pg C), accounting for nearly one-third of the global total. Our findings first estimate the share of urban SOC in the global terrestrial C cycle budget and provide a baseline for C management in developing countries to increase SOC stocks in urban forests.

In conclusion, topsoil carbon fractions in urban forest show different patterns and dominant drivers at different scales. Human factors are more important at the local scale while natural factors are more important at the large scale. Compared with natural forests, urban forest soils are dominated by unstable organic carbon fractions, implying that global warming may lead to significant carbon loss in urban forest soils. Thus, corresponding measures should be considered to improve the carbon conservation capacity of urban forest soil for the future urban forest management.