Agriculture Reference
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Toward an attempt to reduce the uncertainty, Huang and Sun (2006) conducted
a metaanalysis of topsoil SOC data from 200 publications, covering 60,000 single
measurements over the period 1993 to 2005, and observed an increase in SOC at
a frequency of over 50% and a decrease of 30%, while the other 20% remained
unchanged. Huang and colleagues reported a semiquantitative estimate of increase
in SOC stock of the topsoil by 0.3 to 0.4 Pg for the period from 1985 to 2005. They
updated the estimates by using soil monitoring survey data available from 146 pub-
lications and calculated the SOC sequestration rate in China's croplands at 21.9 Tg
per year with a total of 0.44 Pg for the period between 1980 and 2000 (Sun et al.
2010). Pan et al. (2009) developed a methodology to estimate the relative increase in
SOC compared with the baseline year when soil monitoring was initiated, and they
performed a metaanalysis of the mean annual rate of SOC change ( Figure 18.5 ) over
different time durations for 1099 single SOC measurements from the soil monitoring
system from the published literature up until 2006. Based on this analysis, Pan and
colleagues estimated a mean annual overall rate of increase in SOC in the topsoil
of 0.06 ± 0.20 g/kg for unirrigated croplands and 0.11 ± 0.24 g/kg for rice paddies.
Therefore, the total annual increase in SOC stock was estimated at 25.5 Tg for the
period between 1985 and 2006. However, there was wide variation in the rate of SOC
increase across different regions of China. Thus, Pan and colleagues concluded that
the topsoil (0-20 cm) of China's croplands had sequestered as much as 0.58 ± 0.38 to
0.65 ± 0.53 Pg C over this period, which could have been a significant contribution
to offsetting the nation's GHG emissions from energy consumption. Summarizing
these indicates the carbon sequestration rates in the range of 20 to 25 Tg per annum
for China's croplands over the two decades between 1990 and 2010.
18.3 SEQUESTRATION OF ORGANIC CARBON IN CROPLANDS:
DRIVERS, POTENTIAL, AND TECHNICAL FEASIBILITY
Topsoil is the most sensitive to climate change and human interference. An analysis
of SOC increase in 966 monitored cropland sites revealed a wide range of SOC
sequestration rates varying with land use and management practices (Xu 2009). Tai
et al. (2011) showed that garden soils had a much higher SOC storage than cropland
soils due to high organic matter (OM) input and solicited management. Thus, soils
of the rice paddies also have topsoil SOC storage higher than those of the dry crop-
lands by almost 10 Mg/ha, because crop C input is higher in rice paddies than in dry
croplands by 170% (Xu et al. 2009; Cheng et al. 2009). A statistical analysis of a
cropland quality survey sponsored by the Ministry of Agriculture of China revealed
a significant change with land use in topsoil SOC storage of a county from Jiangsu
Province for the two decades between 1990 and 2010. This study also indicated a
general SOC decline with the conversion of rice paddies to garden soils, dry crop-
lands, nurseries, and forestlands in the short term but a continuous SOC increase
in the rice paddies since the 1980s (Hou et al. 2007). However, in a case study of a
similar area, maize cultivation in a rice paddy caused loss of topsoil SOC by 30% of
the original SOC stock over 3 to 5 years, and the loss was documented by 13 C isotope
abundance changes (Li et al. 2007b). The loss in SOC by cultivation of maize may
be attributed to enhanced SOC decomposition by plowing for maize production and
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