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.