Benefits of SOM in Agroecosystems: The Case of China - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 321

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9

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0 1 2 3 4 5

Soil organic matter concentration (%)

Fig. 27.4. Total seasonal N 2 O emission across the SOM level from dry croplands of China.

straw and manure than those receiving
chemical fertilizers only (Han et al ., 2009).
A further study which carried out a labora-
tory incubation with topsoil samples from
different fertilized plots, revealed an en-
hanced degradation of spiked pyrene from
the soil with organic amendments, which
showed a significant linear correlation
with both SOM content and soil microbial
abundance (Han et al ., 2009). Another
study by Wang et al . (2009) reported re-
duced extractability of soil PAHS with in-
creasing SOM content for the same experi-
ment. This is the first strong indication
that SOM accumulation in soils helps to
build up a healthy ecosystem.
Over the past decade, an increasing
amount of evidence has been gathered to
suggest that SOC sequestration can lead to
a healthy soil microbial community. With
SOC increase, enhanced microbial abun-
dance and gene diversity were observed under
combined organic/inorganic fertilization in
a rice paddy (Zhang et al ., 2004). One of our
recent studies on the changes in microbial
activity with rice cultivation in sediment-
derived wetland soils from the Yangtze River
Valley demonstrated an enhancement of the

soil microbial community and diversity,
together with an increase in soil enzyme
activity with an increase in SOM levels in
the rice paddy soils (Huang et al ., 2006). It
could be the case that SOC accumulation en-
hances soil productivity through the build-
up of an active and healthy soil microbial
community in rice paddies.
Changes in soil respiration, and in turn
efflux of CO 2 (and CH 4 in rice paddies), with
SOM accumulation have been widely ar-
gued (Zheng et al ., 2007, 2008). A healthy
soil may have high microbial abundance
but not necessarily high respiration rates,
especially the respiration intensity of SOM.
In a case study by Zheng et al . (2006), in a
situation of SOC accumulation under good
fertilization, the metabolic quotient of the
soil microbial community and the respiratory
quotient of SOC were both reduced, while
total soil CO 2 evolution was higher. In many
field studies, the correlation coefficient of soil
respiration with SOC content in rice soils
without N being limiting was generally low.
Instead, soil respiratory activity and respired
CO 2 flux were often shown to be lower in
plots rich in SOC under a well-designed fertil-
izer scheme when compared with relatively