Organic Carbon Sequestration Potential and the Co-Benefits in China’s Cropland - Principles of Sustainable Soil Management in Agroecosystems - page 505

18.2 STATUS OF SOC STORAGE OF CHINA'S CROPLANDS

18.2.1 B ackground Soc S tock in the 1980 S
The multiple types of terrestrial ecosystems existing in large areas of China have a
significant role in preserving C stocks and reducing greenhouse gas (GHG) emis-
sions through land use and land cover change. There have been numerous studies on
estimating total SOC stock among land use types. A total SOC stock to 1-m depth
for soils of China has been estimated at 90 Pg by a national panel conference on
SOC (Xiangshan Science Conference Series No. 362 in 2004). Of the total stock,
15 Pg C is in China's croplands. Detailed data of topsoil SOC measurements from
the second national soil survey allowed a reliable quantitative assessment of back-
ground topsoil SOC stock. A statistical analysis by Song et al. (2005) estimated that
China's cropland soils contain 5.1 Pg of SOC stock to 0.2-m depth, of which 1.3 Pg
is contained in soils of rice paddies cultivated on 30 Mha, and another 3.8 Pg in soils
of dry croplands cultivated on about 100 Mha. In comparison, soils under China's
forestland contain 5.9 Pg C in topsoil in an area of 1.42 Mha, and soils under China's
grassland contain merely 1.15 Pg with a land area of 3.31 Mha (Fang et al. 2007). In
addition, there exists a relatively small stock of 0.9 Pg in wetlands covering an area
of 0.4 Mha (Zhang et al. 2008). The distribution of SOC stocks in China's croplands
is presented in Table 18.2.
As a vital part of soil, topsoil is rich in SOM, which supports soil fertility and
biodiversity and is crucial to ecosystem functioning. In agriculture, the SOM con-
tent of topsoil largely determines the cropland productivity. China's croplands were
depleted of SOM stock because of intensive cultivation (Song et al. 2005), wide-
spread degradation (Lal 2002), and improper management (Feng et al. 2011). An
official report released by the Ministry of Agriculture of China in 2004 indicated
that topsoil SOC content was as low as 1% in 65% of China's croplands and was
no higher than 1.5% in croplands from all the major production regions including
Henan, Hebei, Shandong, Hubei, Hunan, Jiangxi, and Sichuan provinces. In contrast,

TABLE 18.2
Estimates of the Baseline SOC Stock in China's Croplands in the
Early 1980s

SOC Stock (Pg)

Land Use Type Whole Soil Topsoil Topsoil SOC (Mg C/ha)

Uncultivated 75 33 49.84 ± 46.69

Cultivated soil 15 5.1 35.87 ± 32.77

Rice paddy / 1.3 43.98 ± 19.07

Dry cropland / 3.8 33.44 ± 36.89

Source: Recalculated from the data by Song et al. (2005). Pan et al. (2003). Topsoil refers
to the depth of 0-20 cm for croplands and of 0-30 cm for uncultivated soils,
while whole soil implies a depth of 0-100 cm.