5. Conclusion

Our results revealed that the mean topsoil soil organic carbon (SOC) contents in 1980, 2000, and 2015 were 13.34 g kg-1, 16.99 g kg-1, and 14.67 g kg-1, respectively. The SOC changes went through two distinct stages, with a net increment of 3.65 g kg-1 in the period of 1980-2000 and a net decrement of 2.32 g kg-1 over the period of 2000-2015. The areas with high SOC contents were identified in the southeast of the study area, while the SOC contents in other parts of the study area were relatively low. The spatial patterns of the SOC changes were significantly different, with an overall SOC increase throughout the study area during the period of 1980-2000, the areas with declines in SOC were mainly distributed in the southeast during 2000-2015. The SOC increase over the first two decades (1980-2000) was largely attributed to the enhanced soil carbon inputs resulting from the increase in fertilizer application (in particular low-cost N fertilizers), while the stagnation of soil carbon inputs associated with the rapid urbanization and economic growth constrained the SOC accumulation in the subsequent 15 years (2000-2015).
With the rapid urbanization and socioeconomic development, significant regional differences in the SOC change between the east and west of the study area occurred in the last 15 years (2000-2015). Thus, we suggest that maintaining the stability of SOC by enhanced the soil carbon inputs is becoming a priority issue and needs to be addressed not only in the scientific realm, but also within the policy arena. Scientific management practices and powerful economic policy can be used as a guide for sustainable potential of SOC sequestration, especially in areas experiencing rapid urbanization processes.