Cassava produced 4.4, 5.7 and 11.6 Mg ha
À1 of ABG dry matter and 4.8, 4.4 and
12.2 Mg ha
À1
of tubers during 1, 2 and 3 years of cropping respectively, though there
was no weed control applied in cassava fields as the crop is considered a fallow.
Hence, an additional amount of biomass of 0.9–2.1 Mg ha
À1 was collected from
cassava fields. The annual ABG C accumulation in the cassava fields was 2.7, 2.0 and
2.3 Mg ha
À1 in the 1-year-old, 2-year-old and 3-year-old cassava stands respectively,
while C accumulations in the tubers were in the range 1.9–2.2 Mg ha
À1 year
À1
.
The main reason for this gradient can be attributed to land use. Most of the soils
in the Chieng Khoi catchment are Alisols and Luvisols, derived from sandstone,
with varying soil fertility levels (see also Chap. 2). Once degraded by cultivation
practices, these soils will not recover, even after more than 50 years of fallow. As a
result of inappropriate land use, soils on the middle and lower slopes have been
affected by severe soil erosion, whereas the foot slope soils have instead suffered
from an accumulation of eroded infertile subsoil material, as well as stagnic
conditions. The unsustainable land uses practiced at upslope landscape positions
has had a severe impact on downslope areas, affecting the long-term productivity of
these sites (Clemens et al. 2010). This has been mitigated by the introduction of new
varieties and the increased application of external inputs such as fertilizers. (Lippe
et al. 2011).
Maize and cassava are cash crops sold to middlemen; therefore, substantial
amounts of C stored in maize grains and cassava tubers leave the catchment area
after the harvest, in the range 1.68–3.88 Mg ha
À1 . The remaining ABG biomass is
often used to feed cattle or buffaloes, representing an additional source of C export
from the fields.
3.5.3 Carbon Stocks at the Watershed Level
Perennial land use systems displayed higher C stocks than annual cash crops in the
watershed. Nguyen (2009) used remote sensing, ground truthing and participatory
methods and approaches to reconstruct the land use history of the Chieng Khoi
watershed, quantify biomass and C stocks of the perennial vegetation, and evaluate
communal CO 2 sequestration potential. The protected natural forest area was
3,189 ha before 1954 and 929 ha in 2007, and even though forest cover decreased
by ‘only’ 36 % over this time, the results showed a decrease of 61 % in total
communal C stocks.
Annual time-averaged C accumulation by both annual and perennial crops was
equal to or sometimes higher than that of natural forests; however, ABG structure,
rooting patterns, the time span of C storage, soil cover during the wet season, litter
fall and residue remaining on-site, differed strongly among LUTs (Nguyen et al.
2010a). In particular, the shift from natural vegetation to annual crops led to a C
depletion over the course of time, although the C accumulation for maize and
cassava was quite high during crop growth (Ha ¨ring et al. 2010). Most of the
biomass produced; however, is exported as harvest product or serves as fodder
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
139
À1 of ABG dry matter and 4.8, 4.4 and
12.2 Mg ha
À1
of tubers during 1, 2 and 3 years of cropping respectively, though there
was no weed control applied in cassava fields as the crop is considered a fallow.
Hence, an additional amount of biomass of 0.9–2.1 Mg ha
À1 was collected from
cassava fields. The annual ABG C accumulation in the cassava fields was 2.7, 2.0 and
2.3 Mg ha
À1 in the 1-year-old, 2-year-old and 3-year-old cassava stands respectively,
while C accumulations in the tubers were in the range 1.9–2.2 Mg ha
À1 year
À1
.
The main reason for this gradient can be attributed to land use. Most of the soils
in the Chieng Khoi catchment are Alisols and Luvisols, derived from sandstone,
with varying soil fertility levels (see also Chap. 2). Once degraded by cultivation
practices, these soils will not recover, even after more than 50 years of fallow. As a
result of inappropriate land use, soils on the middle and lower slopes have been
affected by severe soil erosion, whereas the foot slope soils have instead suffered
from an accumulation of eroded infertile subsoil material, as well as stagnic
conditions. The unsustainable land uses practiced at upslope landscape positions
has had a severe impact on downslope areas, affecting the long-term productivity of
these sites (Clemens et al. 2010). This has been mitigated by the introduction of new
varieties and the increased application of external inputs such as fertilizers. (Lippe
et al. 2011).
Maize and cassava are cash crops sold to middlemen; therefore, substantial
amounts of C stored in maize grains and cassava tubers leave the catchment area
after the harvest, in the range 1.68–3.88 Mg ha
À1 . The remaining ABG biomass is
often used to feed cattle or buffaloes, representing an additional source of C export
from the fields.
3.5.3 Carbon Stocks at the Watershed Level
Perennial land use systems displayed higher C stocks than annual cash crops in the
watershed. Nguyen (2009) used remote sensing, ground truthing and participatory
methods and approaches to reconstruct the land use history of the Chieng Khoi
watershed, quantify biomass and C stocks of the perennial vegetation, and evaluate
communal CO 2 sequestration potential. The protected natural forest area was
3,189 ha before 1954 and 929 ha in 2007, and even though forest cover decreased
by ‘only’ 36 % over this time, the results showed a decrease of 61 % in total
communal C stocks.
Annual time-averaged C accumulation by both annual and perennial crops was
equal to or sometimes higher than that of natural forests; however, ABG structure,
rooting patterns, the time span of C storage, soil cover during the wet season, litter
fall and residue remaining on-site, differed strongly among LUTs (Nguyen et al.
2010a). In particular, the shift from natural vegetation to annual crops led to a C
depletion over the course of time, although the C accumulation for maize and
cassava was quite high during crop growth (Ha ¨ring et al. 2010). Most of the
biomass produced; however, is exported as harvest product or serves as fodder
3 Water and Matter Flows in Mountainous Watersheds of Southeast Asia:. . .
139