however, it remains in the field for longer – 0.5, 1, 2 and 3 years, depending on the
soil fertility status and farm household (HH) cash flow requirements. Cassava is
usually grown when the soil fertility is depleted after the several maize cropping
seasons, and is considered a kind of fallow crop by the farmers (Lippe et al. 2011).
In addition, farmers occasionally use maize-cassava intercropping and maizecassava based agroforestry systems during the transition from more to less
fertile soils.
For this study, maize and cassava samples from several fields within the Chieng
Khoi watershed were collected to assess ABG biomass and C stocks (Table 3.4).
The maize fields varied in terms of their distance from the homestead, whereas the
cassava fields were randomly selected based on the length of the growth cycle.
There was a gradient found in terms of yield potentials and corresponding C
accumulations based on increasing distance from the homesteads, which clearly
mirrors the land use geography and cropping history at this site. Maize fields further
away from homesteads usually have a younger cropping history and hence a higher
yield potential than fields closer to homesteads (Boll et al. 2008). The total ABG
biomass produced by maize varied between 11.7 and 23.4 Mg ha
À1 for fields with a
longer and more recent cropping history respectively. The biomass produced was
equivalent to an annual C accumulation of 4.7–9.4 Mg ha
À1 .
Table 3.4 Biomass and C stocks for the main upland crops in the Chieng Khoi watershed; maize
data collected in 2008 (Boll 2009) and cassava data in 2009 (Rathjen 2010)
Biomass (DM)
(Mg ha
À1
)
C-accumulation
(Mg ha
À1 year
À1
)
C sequestration (entire
rotation) (Mg ha
À1
)
Maize
a
Field 0
*
Grain yield
5.0
2.00
2.00
Total ABG
11.7
4.68
4.68
Field 500
**
Grain yield
4.2
1.68
1.68
Total ABG
11.7
4.68
4.68
Field 1000
***
Grain
9.7
3.88
3.88
Total ABG
23.4
9.36
9.36
Cassava
b
1-year-old
Tuber yield
4.8
2.24
2.24
Total ABG
4.4
2.20
2.20
Weeds
0.9
0.47
0.47
2-year-old
Tuber yield
4.4
2.05
4.10
Total ABG
5.7
1.43
2.86
Weeds
2.1
0.53
1.06
3-year-old
Tuber yield
12.2
1.90
5.69
Total ABG
11.6
1.93
5.80
Weeds
2.1
0.34
1.03
*
>35 years under cultivation; field is 0 m distant from homestead
**
maize cropping since 1999; field is 500 m from homestead
***
maize cropping since 2004; field is 1,000 m from homestead
a
Calculation of maize C stocks based on own data
b
Calculation of cassava C stocks estimated using the IPCC default value (IPCC 2007)
138
H.L. Fro ¨hlich et al.
soil fertility status and farm household (HH) cash flow requirements. Cassava is
usually grown when the soil fertility is depleted after the several maize cropping
seasons, and is considered a kind of fallow crop by the farmers (Lippe et al. 2011).
In addition, farmers occasionally use maize-cassava intercropping and maizecassava based agroforestry systems during the transition from more to less
fertile soils.
For this study, maize and cassava samples from several fields within the Chieng
Khoi watershed were collected to assess ABG biomass and C stocks (Table 3.4).
The maize fields varied in terms of their distance from the homestead, whereas the
cassava fields were randomly selected based on the length of the growth cycle.
There was a gradient found in terms of yield potentials and corresponding C
accumulations based on increasing distance from the homesteads, which clearly
mirrors the land use geography and cropping history at this site. Maize fields further
away from homesteads usually have a younger cropping history and hence a higher
yield potential than fields closer to homesteads (Boll et al. 2008). The total ABG
biomass produced by maize varied between 11.7 and 23.4 Mg ha
À1 for fields with a
longer and more recent cropping history respectively. The biomass produced was
equivalent to an annual C accumulation of 4.7–9.4 Mg ha
À1 .
Table 3.4 Biomass and C stocks for the main upland crops in the Chieng Khoi watershed; maize
data collected in 2008 (Boll 2009) and cassava data in 2009 (Rathjen 2010)
Biomass (DM)
(Mg ha
À1
)
C-accumulation
(Mg ha
À1 year
À1
)
C sequestration (entire
rotation) (Mg ha
À1
)
Maize
a
Field 0
*
Grain yield
5.0
2.00
2.00
Total ABG
11.7
4.68
4.68
Field 500
**
Grain yield
4.2
1.68
1.68
Total ABG
11.7
4.68
4.68
Field 1000
***
Grain
9.7
3.88
3.88
Total ABG
23.4
9.36
9.36
Cassava
b
1-year-old
Tuber yield
4.8
2.24
2.24
Total ABG
4.4
2.20
2.20
Weeds
0.9
0.47
0.47
2-year-old
Tuber yield
4.4
2.05
4.10
Total ABG
5.7
1.43
2.86
Weeds
2.1
0.53
1.06
3-year-old
Tuber yield
12.2
1.90
5.69
Total ABG
11.6
1.93
5.80
Weeds
2.1
0.34
1.03
*
>35 years under cultivation; field is 0 m distant from homestead
**
maize cropping since 1999; field is 500 m from homestead
***
maize cropping since 2004; field is 1,000 m from homestead
a
Calculation of maize C stocks based on own data
b
Calculation of cassava C stocks estimated using the IPCC default value (IPCC 2007)
138
H.L. Fro ¨hlich et al.