<Fig. 8>
Crop-derived carbon is the major source of carbon input in cropland soils (Wiesmeier et al., 2014; Wang et al., 2015b). Fig. 9a showed that the average soil carbon inputs in the major croplands in the area went through two distinct stages over the past three decades. The soil carbon inputs from the crops (straw/stover returned to the soils plus roots) increased continuously before 2000, and fluctuated greatly thereafter. The rapid increase of carbon inputs before 2000 was the significant contributor to SOC accumulation over the first two decades because the initial SOC content (13.34 g kg-1) in 1980 was still relatively low. In 2000, the SOC levels in the area reached a relatively high level (16.99 g kg-1). However, the cropland areas have decreased significantly since 2000 due to the rapid urban expansion, resulting in the strong fluctuations in the soil carbon inputs in the area. The mean soil carbon inputs of 1.44 t ha-1 yr-1 after 2000 may still be insufficient for maintaining the high SOC levels in 2000. The soil carbon inputs after 2000 differed significantly among the 14 county-level cities (Fig. 9b), with increased carbon inputs occurring in the west (Danyang, Liyang, Jintan, Yixing), while the areas with declining carbon inputs mainly surrounded Taihu Lake (Wujiang, Kunshan, Wuxian). This regional difference in soil carbon inputs was closely related to the rapid urbanization process in the study area (Fig. 10a).
Urban expansion may lead to fragmentation of the cropland landscape, which was a dominant factor limiting soil carbon inputs (Liu et al., 2011; Hao, 2012; Xia et al., 2017). Fig. 10b showed that the ratio between urban land area and cropland area was negatively correlated with the soil carbon inputs during the period of 2000-2015, indicating that the cropland soil C inputs in the areas with higher urbanization rates were low. Moreover, socioeconomic development actively pushes forward the marketization process of land use (Bao et al., 2019), particularly for industries and tertiary industries, resulting in the reduction of agricultural investment. For example, the number of people employed in agriculture in the area declined significantly during 2000-2015 (from 2.85 million to 1.42 million), while the employees in the tertiary industry increased by two times, and was almost 6 times higher than those in agriculture in southern Jiangsu Province (Jiangsu Bureau of Statistics, 2001-2016), implying that staple food agriculture in the area was becoming less important. This also caused poor management of the cropland soils. Therefore, although the increasing soil C input resulting from the enhanced crop productivity due to chemical fertilizers benefited the SOC accumulation over the first two decades, the stagnant soil carbon inputs associated with rapid urbanization and economic growth constrained the SOC accumulation in the subsequent 15 years (2000-2015).