4. Discussion

4.1 Driving factors of spatiotemporal changes in SOC

The differing parent materials or bedrocks of the soil types have a profound influence on SOC levels (Araujo et al., 2017; Angst et al., 2018). Fig. 8a showed that the average SOC contents for the paddy soils (13.62 g kg-1, 17.73 g kg-1, and 14.79 g kg-1 in 1980, 2000, and 2015, respectively) were the highest among the three predominant soil types in the area, followed by those of the yellow-brown soils (13.62 g kg-1, 17.73 g kg-1, and 14.79 g kg-1 in 1980, 2000, and 2015, respectively) and the fluvo-aquic soils (10.31 g kg-1, 12.71 g kg-1, and 13.39 g kg-1 in 1980, 2000, and 2015, respectively). The sorting order of the SOC contents for each soil type remained unchanged among the different sampling dates, indicating that the influences of soil types on the SOC levels in the area were still important. The paddy soils in southern Jiangsu Province were mainly developed from alluvial materials and lacustrine deposits. The high SOC contents were mainly caused by the waterlogged soils in paddy fields where the anaerobic conditions decreased the rate of SOC decomposition (Liu et al., 2006; Li et al., 2018a). Moreover, the physical and chemical properties of the paddy soils were significantly different from those of the other two soil types (fluvo-aquic soils and yellow-brown soils) due to the generally more intensive agricultural activities in the paddy soils (Liu et al., 2006; Liu et al., 2019). During the periodic submergence and drainage, a large amount of amorphous Fe and Al oxides resulting from the frequent rice cultivation significantly enhanced the soil aggregate stability, thereby promoting the accumulation of SOC in the paddy soils (Gong, 1999; Heng et al., 2010; Wei et al., 2017; Xue et al., 2019). However, the fluvo-aquic soils and yellow-brown soils were saturated with water for a relatively short time, and the amorphous Fe and Al oxides concentrations were significantly lower than those in paddy soils (Li et al., 2003; Heng et al., 2010). Therefore, the average SOC in the fluvo-aquic and yellow-brown soils was relatively low. In addition, compared to the fluvo-aquic and yellow-brown soils, the average SOC in the paddy soils differed significantly among the three sampling dates, indicating a stronger influence of agricultural management practices on the SOC changes in the paddy soils in the area.
Agricultural management practices, in particular the application of chemical fertilizers, play an essential role in affecting SOC changes (Lu et al., 2009; Alavaisha et al., 2019). For example, the use of chemical fertilizers has been identified as one of the dominant anthropogenic contributors to SOC accumulation on the North China Plain (Han et al., 2018), in the Yellow River basin (Gong et al., 2011), in the Loess Plateau region (Guo et al., 2011), and on the Huang-Huai-Hai Plains of China (Kong et al., 2013). However, in southern Jiangsu Province, chemical fertilizer inputs to the soils went through two distinct stages over the past three decades. The application of chemical fertilizers, including synthetic N fertilizers, increased continuously before 2000, and declined thereafter (Fig. 8b). An increase in the inputs of chemical fertilizers (in particular synthetic N fertilizers) before 2000 made a significant contribution to the increased SOC because the enhanced crop biomass resulted in proportionally increased soil carbon inputs from the aboveground residues and roots over the first two decades (Li et al., 2013; Zhao et al., 2018). Since 1999, the chemical fertilizer inputs in the area have continuously declined, which was closely related to the decreased cropland areas due to the rapid urbanization process (Lu et al., 2017). Consequently, the importance of the changes in soil carbon input resulting from the land use change should not be overlooked when identifying the dominant driving factors of the SOC change in the area.