<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).