Agriculture Reference
In-Depth Information
degraded ecosystems due to land desertifica-
tion and/or climate change impacts. Of course,
such a great loss of SOM could have led to
historical emissions of CO 2 to the atmosphere,
leading to land-use change-induced climate
forcing. Such a large decline in SOM would
also lead to low soil fertility for croplands and
degraded ecosystem services.
overall increase of SOM. However, a con-
tinuous decline may also be observed in
some regions, such as north-east and
south-west China where there is soil erosion
due to intensified agriculture and/or climate
change (Huang and Sun, 2006; Cheng et al .,
2009). SOM levels have been increased by
about 10% over the past few decades, with
increases being greater in rice paddies than
in dry croplands. For the last decade, there
have been an increasing number of studies
addressing the significance of this increase
in soil SOC and the contribution this makes
to the mitigation of CO 2 emissions from agri-
culture. Of the croplands in China, rice pad-
dies had been shown to deliver greater levels
of carbon sequestration due multiple mech-
anisms e.g. physical protection, chemical
binding with oxyhydrates, as well as mo-
lecular stabilization (Pan et al ., 2007, 2009b).
Using the data relating to changes in SOC,
one can infer that a potential saturation of
SOM could be as high as 18 g  kg - 1 and
32 g kg - 1 , respectively, for Chinese dry crop-
lands and rice paddies (Cheng et al ., 2009).
Clearly, most of China's croplands are still a
long way from this saturation level, with pre-
sent levels being far below the general levels
found for EU and US croplands (Pan, 2009).
Increasing SOC is still a challenge for China's
agricultural sector, for which more effective
practices should be pursued, potentially by
the amendment of biochar from crop straw
(Pan et al ., 2011). Nevertheless, the govern-
ment of China has been sponsoring incentives
for enhancing SOM storage through straw
return and combined fertilization using organic
and inorganic fertilizers since 2003. Mean-
while, a national project on conservation tillage
has been initiated, with the aim of extending
conservation tillage to 20% of China's crop-
lands by 2050 (MA-SCDRC, 2009).
The trend of increasing SOM in China's
croplands over the past two decades
There has been much work done to char-
acterize SOM changes with agricultural de-
velopment since the 1980s, motivated by
increasing concerns for soil resilience and
sustainable production, as well as climate
change mitigation through SOC sequestra-
tion. Since the second national soil survey,
which recognized soil fertility and nutrient
status as constraints on China's agriculture,
great efforts have been made to increase
SOM levels and enhance soil resilience
throughout China. Efforts have included
policies to encourage straw return and con-
servation tillage, as well as combined use of
organic and inorganic fertilizers. A series of
long-term experiments has been established
across mainland China for monitoring soil
fertility and fertilizer use efficiency changes
(Pan et al ., 2009b), in particular in the major
crop production regions. Among these is a
national long-term fertilizer experimental
network, which includes 70 sites across the
main agricultural production area of China.
It was initiated in the early 1980s, and in
1990 nine sites, representing the major crop
regions, were updated to a national long-term
monitoring network for soil fertility and fer-
tilizer efficiency. There was also a network
of long-term ecosystem experiments man-
aged by the Chinese Academy of Sciences,
which included 16 sites situated across eco-
logical gradients in China. In addition, the
Chinese Ministry of Agriculture has been
managing cropland fertility and productiv-
ity in 299 sites across the main crop produc-
tion areas of China since the mid-1980s.
As shown in Table 27.1, over the period
1985-2006, the monitoring sites and studies
considered here have shown a trend for the
Benefits of Soil Organic Matter for
Crop Production and Agroecosystem
Functioning
As pointed out by Sohi (2012), SOM, par-
ticularly stable organic matter, may have a
number of benefits for crop production and
 
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