Benefits of SOM in Agroecosystems: The Case of China - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 319

capacity to protect N. Data collected from
ten provincial demonstration farms of high
rice productivity in Jiangsu in the early
1990s suggested a high level of total soil N
capacity of 1-2 g kg - 1 under SOM contents
over 10 g kg - 1 ( Fig. 27.2 ). In a study of soil
fertility changes with different fertilization
from a long-term trial from the Tai Lake
region in Jiangsu, topsoil N was found to
increase significantly with SOM accumu-
lation. Total N was found to increase by
0.2 g kg - 1 of topsoil and 0.1 g kg - 1 of topsoil at
a depth of 0-5 cm and 5- 15 cm under com-
bined fertilization of organic/inorganic fertil-
izers compared to under inorganic fertilizer
alone, in accordance with an increase in SOC
by almost 1.5 and 0.8 g kg - 1 , respectively
(Qiu et al ., 2005). The relationship between
changes in soil phosphorus (P) retention
and SOM accumulation are less well docu-
mented. Generally, P becomes more mobil-
ized in OM-rich soils, a phenomenon of
organically activated mobilization. However,
soil aggregation with SOM accumulation
may offer physical protection for P released
from dispersed soil particles. One of our
earlier studies on P balancing at the plot

scale showed less P loss from soils with
high SOM contents (accumulated under
combined organic/inorganic fertilization
with straw return or pig manure) (Jiao et al .,
2007). Thus, SOM accumulation may also
have benefits for P retention in agricultural
soils, as P is generally deficient worldwide.
The question of whether N 2 O emission
would be increased with increase of soil N is
still poorly understood. Although N 2 O emis-
sions from croplands are dependent on soil
moisture regimes rather than solely on soil
N contents, the emission factor of N 2 O from
N fertilizers is generally smaller in soils high
in SOM as compared to those low in SOM.
In our database of soil GHG emissions from
China's croplands, an increase in N 2 O emis-
sion from croplands with increasing SOM
contents was not visible, nor of CH 4 ( Figs 27.3
and 27.4). This indicates that soil C seques-
tration with SOM accumulation would not
necessarily lead to increased GHG emissions,
a concern voiced by Schlesinger (2010) and
Powlson et al . (2011). This finding also sug-
gests possible changes in soil (micro-)bio-
chemical processes with SOM accumulation
in croplands.

2.5

y = 0.074 x + 0.3772
R 2 = 0.7287

2

1.5

1

0.5

0 0

5 10 15 20

Soil organic mattter content (g kg -1 )

Fig. 27.2. Correlation of soil total N with organic matter content in plots from ten provincial high-yielding
demonstration farms in Jiangsu in 1992. (Data collected from farm archives, unpublished.)