<Fig. 10>

4.2 Challenge for soil carbon sequestration

Our results, together with the existing studies in the area, provide more details on the regional differences of the changes in SOC during the period of 2000-2015 (Fig. 11), which may provide more insights into the future C sequestration in the area. Fig. 11a showed that a continuous decline in SOC during the period of 2000-2015 mainly occurred in the eastern part of the study area (i.e., Changshu, Wujiang), where the reduction in cropland area was more severe than that in the western parts, although the SOC levels in the west decreased first and then increased steadily (i.e., Jintan, Yixing) (Fig. 11b). The trend of the SOC changes in the western parts of the study area can be attributed to the large-scale implementation of the crop straw/stover return policy since 2000 (Zhao et al., 2018).
Moreover, the SOC stock in croplands of the study area increased from 30.95 t ha-1 in 1980 to 42.12 t ha-1in 2000, and then decreased to 37.53 t ha-1 in 2015, which was highly consistent with the changes in SOC contents (Table 1,Text S2 , and Figure S3). The increase rate of SOC stock over the past 35 years was 0.19 t ha-1yr-1, which was slightly higher than the estimate of Jiangsu Province (0.16 t ha-1 yr-1) by (Liao et al., 2009) and the estimate of Chinese cropland soils (0.14 t ha-1 yr-1) by (Zhao et al., 2018), indicating a considerable SOC sequestration potential in the study area. However, although cropland SOC accumulation can be achieved through enhancing the C input (i.e., improving the straw/stover management), future SOC sequestration in the area is still facing big challenges, in particular the balance between agricultural development and the urbanization process for ensuring soil carbon input to maintain the SOC levels. A high SOC level not only benefits the biodiversity of agroecosystems (Wiesmeier et al., 2019), but also improves key soil functions, such as the capacity for acid buffering (Ritchie & Dolling, 1985), which are especially important for areas with accelerated soil acidification, e.g., this study area (Xie et al., 2019). Therefore, management practices and policies for enhancing the soil carbon input should still be given the top priority for maintaining the stability of SOC levels.