## 4 Discussion
### Possible Reasons for the Urbanization-Induced Forest SOC Changes  
The most important reasons for SOC accumulation under urbanization were the forest type-related land use alternations and EC-related soil property changes, and this is an important finding of this paper. It was reported that the forest floor C stocks are directly affected by litterfall C inputs [19]. Compared with other forest types, LF is mainly located in the semi-natural environments (such as public parks, forest parks, historic sites, and scenic areas), and the larger litterfall in LF might increase the C inputs over other forest types (RF, AF, PF and EF) for their large areas where people cannot easily disturb. Also, human activities in LF potentially increase human-related C (such as the food and rubbish people bring in) inputs into soils, as these places are attractive sites for sightseeing and physical activities [46]. Therefore, the close correlations between LF proportion and SOC-related parameters indicates that land uses of LF are a driver of SOC accumulation as shown in urban-rural gradients (Tables 2 and 3). EC value was positively associated with SOCD and the changing trend was similar to SOCD (Tables 2 and 3). EC is a proxy for soil salinity [22] and a decrease in salinity would improve the accessibility of soil organic matter to the soil microbial community [55]. In Northeast China, heavy snow in winter is usually accompanied with large amounts of snow-melting salt utilization. This salt utilization probably increases soil EC [56], and it is likely a pattern that urban central regions had more utilization of these salts. Therefore, the high EC value in urban centres was another driver of SOC accumulation. Our study also showed that only the proportion of Pinaceae and conifer/broadleaf trees had associations with TT, and the compositional changes had marginal associations with SOCD, basal respiration and Q\({}_{10}\) (Table 3), and this is different from those in non-urban sites. In non-urban forests, species differences were important drivers for SOC changes [19]. Conifer forests showed much lower SOC accumulation capacity than broadleaf forests in Natural reserves in NE China [24]. This kind of leaf phenological differences-related SOC changes did not contribute to urban-rural SOC patterns, which were manifested in this study too (Table 3). During urbanization processes, heavy human disturbance including litter removal, watering and fertilization, etc., are possibly changing the natural-based patterns. These changes may include low quality of litter within or near urban cores [57], microbial diversity in rural, suburban, and urban stands contributing to organic material decomposition [23; 57], microclimate differences, nutrient inputs, and changes in soil fauna [58]. Some more detailed studies related to inter-species differences should be done both in urban and natural regions to identify underlying contributions of different species to soil conditions and processes. Understanding the potential factors affecting urban forest SOC accumulation under urbanization, serves to improve the capacity of carbon sinks of urban forests in the future. For example, an effective method was to increase the LF proportion in an urban region, although such activity is difficult in the city owing to limited green space available in urban areas. In Changchun city, the green space coverage decreased from 45.42% to 30.65% from 2006 to 2014 [59]. Secondly, chemical snow-melting salt utilization could enhance the salinity of soil, with many disadvantages on surface water, ground water, vegetation and animals [56]. However, SOC accumulation should be one good effect for such salt utilization, which has been shown by the positive relations between EC and SOCD in this paper. Finally, urbanization-induced forest SOC accumulation should be fully considered in terrestrial C balance analysis, given that the background of urbanization has become the first driver of land use changes in today's world [1].