<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.