Agriculture Reference
In-Depth Information
Table 1.4
Estimates of organic carbon in wetland and other soils
compared with other global carbon pools
Pool
Area
(Mha)
Carbon
stocks
(Pg)
Annual
change
(Pg year
−
1
)
Soil
Wetland mineral
a,b
670
380
Wetland organic
a,b
350
260
Wetland rice
c
109
10
Forest
d
4200
790
Grassland
a
3000
500
Arable
a
1400
140
Desert
3750
10
Total
13 500
2090
Others
e
Atmosphere
770
+
3
Land
Short-lived biota
130
Long-lived biota
700
−
1to
−
2
Litter
60
Sea
Surface water
725
Deep water
37 675
Fossil fuel
5000-10 000
−
5
10
8
Sediments
Source
:
a
Scharpenseel (1997).
b
Armentano and Verhoeven (1990).
c
IRRI (2002) (2% C to 30 cm depth).
d
Dixon
et al
. (1994).
e
Bolin and Cook (1983) (atmosphere adjusted to 2003).
in most landscapes and maintain the widest range of redox conditions of any
ecosystem, they have a central role in the global nitrogen cycle. Nearly 20% of
natural N
2
fixation occurs in wetlands (Table 1.5) because of the favourable water
and nutrient status for N
2
fixing organisms (Buresh
et al
., 1980; Bowden, 1987).
Wetlands are also important sinks for nitrate which under anoxic conditions is
reduced to N
2
by microbes in denitrification:
5CH
2
O
+
4NO
3
−
+
4H
+
−−−→
2N
2
+
5CO
2
+
7H
2
O
Table 1.5 shows the importance of denitrification in wetlands on a global scale.
Further, agricultural wetlands are important sources of NH
3
which is emitted by
volatilization of ammoniacal-N in the floodwater:
NH
4
+
−−−→
NH
3
+
H
+
