Agriculture Reference
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soil degradation occurring in a range of landforms. Ren et al. (2006) showed a SOC
loss of over 30% in degraded limestone sloping lands compared to a preserved terrain
from Guizhou, Southwest China. Climate change could be another driver for SOC loss
in Northeast China, where organic soils were very common before the 1960s, either in
the peatlands of Heilongjiang province or in the grass-meadow lands of the northeast
great plain. Warming and drying have been common features since the 1980s in this
area, which accelerated the SOC decomposition and loss of originally SOC-rich crop-
lands. In addition, cultivation of wetlands for rice and maize/soybean production in
the lower reach of Sanjiang valley of Heilongjiang could have resulted in a significant
loss of SOC stock in China, which was considered as a major cause for the national
total loss of 1.5 Pg C from cultivated wetlands (Zhang et al. 2008). Therefore, soil deg-
radation and climate change are important factors in aggravating SOC sequestration
in China's croplands. In other words, protection of soils against desertification and
adaptation to climate change would be a win-win strategy for attaining a high SOC
sequestration potential and for ensuring the ecological safety of China (Lal 2002).
SOC stock and its sequestration capacity are key parameters for evaluating the
climate change mitigation potential of soils of some regions. Thus, several studies
on SOC dynamics focused on the magnitude of the SOC sequestration potential.
However, the technical attainability should also be addressed in soliciting practical
options for SOC sequestration in mitigating climate change as well as ensuring food
production in agriculture (Lal 2004).
As estimated by the FAO, the SOC sequestration capacity of global cropland soils
may be as much as 20 Pg, with an expected mean annual sequestration rate of 0.9 ±
0.3 Pg in the first 25 years (Lal 2004). As first proposed by Lal (2002), China's soils
could have an overall C sequestration capacity of 11 Pg until 2050, including 105 to
198 Tg/year for SOC and 7 to 138 Tg/year for soil inorganic carbon (SIC). Recently,
other workers estimated the total sequestration capacity to be in the range  of 2 to
2.5 Pg C up to 2050 (Wu and Cai 2007). However, Li (2004) argued for a sequestra-
tion potential of 500 Tg by reclamation of low-yielding croplands over a long time.
There are several approaches to assess an acceptable estimation of topsoil SOC
sequestration of China's croplands in terms of both biophysical and technological
attainability. First, an estimation of the sequestration capacity could be assessed by
determining the recovery of SOC losses from cultivation, the maximum or ultimate
biophysical potential for SOC sequestration in croplands. By this means, a SOC seques-
tration capacity of as high as 2 Pg C could be potentially reached in the long run when
good management and conservation are maintained. A second approach may be by
synthesizing the SOC sequestration rates against the initial SOC level, where the gap
between the attainable level and the present level could be considered as the sequestra-
tion potential (Xu 2009). This approach led to an estimation of total SOC sequestration
potential of China of 0.8 ± 0.2 and 1.2 ± 0.5 Pg, respectively, for rice paddies and dry
croplands (Cheng et al., revision submitted). Alternatively, the sequestration potential
of croplands may also be considered as the capacity to store all of C the input, which
would be the case with the adoption of conservation tillage with full straw return,
already known as the best means for enhancing SOC sequestration in croplands of
China (Wang et al. 2009). Thus, the gap between the attainable SOC saturation level
under the best management practice (BMP) scenario of conservation tillage with straw
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