Soil organic carbon dynamics,
functions and management in
West African agro-ecosystems
Bationo A., Vanlauwe B ., Kihara J. and
Kimetu J.
Outline
• Introduction
• Variability of soil organic carbon content at agroecosystem and farm level.
• Effect of soil and crop management on soil organic
carbon
• Role of organic amendments on land productivity
• Future research challenge with emphasis on organic
matter quantity and quality
• Conclusion
The Vicious and Virtuous cycle
Land
Degradation
Improved
Livelihoods
Lack of
Knowledge
Vicious
cycle
Lack of
Resources
Improved
Knowledge
Virtuous
cycle
Improved Soil
Management
Introduction…
The growth rate for cereals grain yield is about 1%
while population growth is about 3%. During the last
35 years, cereals production per capita has
decreased from 150 to 130 kg/person, whereas in
Asia and Latin America an increase from about 200
to 250 kg/person have been observed. Both labor
and land productivity are among the lowest of the
world.
Per Capita food production declined by about 30%
and cereal self-sufficiency from 85 to 65%
Introduction…
Annual cereal deficit in sub-Saharan Africa
amounts to 100 million tons
Food imports increased by about 185% between
1974 and 1990, food aid by 295%
The food gap (requirements minus production) is
widening
The average African consumes only about 87% of
the calories needed for a healthy and productive
life
Introduction …
 16% of Africa’s current arable land base is so eroded
that it cannot be useful any longer agriculturally
 70% of deforestation is caused by farmers who in
their quest for food have no incentive to ponder longterm environmental consequences
 Increase in area under food crop in sub-Saharan
Africa was mainly due to use of marginal lands
hence further environmental degradation through soil
erosion and nutrient mining
Increase in yield has been more due to land expansion
than to crop improvement potential
Crops
Area
Yield
Production
Cassava
2.6
0.7
3.3
Maize
0.8
0.2
1.0
Yam
7.2
0.4
7.6
Cowpea
7.6
-1.1
6.5
Soybean
-0.1
4.8
4.7
Plantain
1.9
0.0
2.0
Based on three-year average for 1988-1990 and 1998-2000. FAO database.
Growth rate of millet 1979-1994
Country
Area
(%)/year
WA
4.7
Yield
(%)/
year
- 0.4
Mali
5.1
Niger
Production
(%)/year
Production/ hbt
(%)/year
4.2
1.2
- 1.0
4.0
2.3
3.9
- 1.0
2.8
- 1.3
Nigeria
7.7
- 2.3
5.2
2.2
B.F
3.8
2.0
5.9
3.0
Percentage chances in soil fertility parameters in
farmers’ fields as a result of 50 years of cultivation in the
Savannah zones
Zones
Exchangeable cations
Ca
Mg
K
pH
Sudan
21
32
25
4.0
N. Guinea
19
27
33
3.8
S. Guinea
46
51
50
10.0
Source: Balasubramanian et al. 1984
Macronutrient loss versus
consumption in Africa
Loss
Consumption
Million tons per year
5.0
4.5
4.4
4.0
3.5
3.0
3.0
2.5
2.0
1.5
0.8
1.0
0.5
0.3
0.5
0.2
0.0
N
P
Nutrients
K
Biophysical and economic parameters related to
household resource endowment
Indicator
Units
Farm resource endowment
-1
-1
Soil C balance kg ha yr
Soil erosion
-1
-1
t ha yr
-1
Farm returns $ yr
Low
Medium
High
–400
–318
190
5.6
5.5
2.1
70
545
3
-1
Household income$ yr
454
1,036
3,127
(Shepherd & Soule, 1998)
Effect of depth of soil mechanical de-surfacing
at Mbissiri, Cameroon
Maize yield (t ha-1)
3
2
1
0
0
5
7.5
10
Scoured depth (cm)
12.5
15
Cumulated erosion in 1989, 90, 91 (t ha-1)
Influence of mulch cover on erosion
200
150
100
50
0
0%
20%
24%
35%
50%
Mulch cover in 1989, 1990 and 1991
100%
Maize yields in 1992 (t ha-1)
Influence of cumulated erosion for a three year
banana mulch on grain yield of next maize crop
No input
5
Manure
4
Manure+NPK
3
Manure+NPK+dolomite
2
1
0
0
50
100
150
Cumulated erosion in 1989, 1990, 1991 (t ha-1)
Variability of soil organic carbon
content at agro-ecosystem and
farm level
Total System C (t ha-1)
100
80
40
Vegetation and Land Use
(Casamance)
Transition
14.1o N
Sudanese-Guinean
(New Groundnut Basin)
14.8o N
Wetter Sudanese Woodland
(Old Groundnut Basin)
15.2o N
Drier Sudanese Woodland
16.1o N
Sahalian-Sudanese Transition
(Sylvo-pastoral Zone)
Sahelian Shrubby Grassland
60
Senegal River Valley
120
16.5o N
13.1o N
20
0
Source: Woomer 2003
Carbon stocks and other fertility indicators of granitic
soils in different agro-ecological zones in West Africa
AEZ
Depth
OC
Total N
Total P
(cm)
pH
H 2O
g/kg
g/kg
mg/kg
Equatorial
Forest
0-20
5.3
24.5
1.6
628
Guinea
Savanna
0-20
5.7
11.7
1.39
392
Sudan
Savanna
0-20
6.8
3.3
0.49
287
Source: Windmeijer and Adriesse 1993
Carbon stocks of different subsystems in a typical
upland farm in the Sudan-savanna zone
AEZ
pH
H2O
OC
g/kg
Total N Available P Exchangeable
K (mmol/kg)
g/kg
mg/kg
Home
garden
Village
field
6.7-8.3
11-22
0.9-1.8
20-220
4.0-24
5.7-7.0
5-10
0.5-0.9
13-16
4.0-11
Bush
field
5.7-6.2
2-5
0.2-0.5
5-16
0.6-1
Source: Prudencio et al
Use of organic resources within a farm for various
farmer typologies
Correlation (r) between selected soil (0-20 cm)
fertility parameters and average annual rainfall
Ca
CEC
Corg
pH KCl 0.62***a 0.64*** 0.65***
Ca
0.98*** 0.88***
CEC
0.86***
Corg
Total N
Clay
Total N Clay
0.62*** -0.02
Rainfall
0.25**
0.92*** 0.36*** 0.31***
0.91*** 0.40*** 0.36***
0.97*** 0.46*** 0.42***
0.44*** 0.34***
0.40***
** and *** indicate statistical significance at the 0.05 and 0.001
level, respectively.
Source: Manu et al., 1991
Soil organic carbon losses are more related to clay
and slit contents than rainfall
Effect of soil and crop
management on soil organic
carbon
Carbon losses (kg ha-1 yr-1) by erosion, runoff and
leaching in the topsoil (30cm) in runoff plots
station
erosion
Adiopodoume (2100 mm rainfall)
Sub-equitoral forest
13
Cereals
1801
Korogho (1300 mm rainfall)
Sudanian savanna
6
Maize, with fertilizers
65
Saria (800 mm rainfall)
Sudano-sahelian savanna
9
Cereals
150
C losses
Total
runoff
leaching
1
65
74
7
88
1873
2
18
13
3
20
84
1
5
2
0.3
11
115
(Adopted from Roose E and Barthes B , 2001)
Annual loss rates of soil organic carbon
measured at farm level in WASAT
Site
Clay + Annual losses
silt (%)
(K) (%)
Bambay
3
7
Saria (non eroded)
12
2
Saria eroded
19
6
Relationship between silt+clay content and
80
silt+clay
associated carbon for different systems
g silt + clay C kg-1 soil
60
Forest
2:1 clays
Grassland
40
1:1 clays
Cultivated
20
0
0
20
40
60
80
100
silt + clay content (%)
Source: Six et al., 2002
Organic carbon changes under continuous
cropping and under fallow in an ultisol
Bare
12
Fallow
NPK+R
NPK-R
Organic C (g/kg)
10
8
6
4
2
0
1
2
3
4
5
Year
6
8
9
10
Adapted from Kang 1993
Carbon content (g kg-1)
Evolution of carbon content in the 0-10cm horizon, as affected
by time and treatment in runoff plots of Mbissiri station,
Cameroon
Savanna
Plowed, bare
Plowed, cropped
Zero-tillage, cropped
8
6
4
2
0
1990
1991
1992
1993
1994
0.10
Organic Carbon (%)
0.20
0.30
0.40
0
Soil depth (cm)
20
Management
Control
Crop residue
40
Fertilizer
Crop residue & fertilizer
Fallow
60
Se(±)=0.019
80
Effect of different management on soil organic matter content,
Sadore, Niger. Rainy season 1997
Pearl millet grain yield (kg.ha-1)
1200
S.E. = 38
1000
800
Millet rotated with cowpea
600
Kg P/ha
Continous millet
400
0.0
6.5
13.0
Effect of phosphorus and cropping systems on pearl millet
grain yields, Sadoré, Niger, rainy season 1992-1995.
Sorghum-groundnut rotation in Burkina Faso shows good
crop as opposed to Continuous Sorghum crop (inset)
0.30
Soil organic carbon (%)
0.28
S.E=0.02
0.26
C-M
M-M
M/C-C
0.24
M/C-M/C
0.22
0.20
0.0
6.5
Phosphorus applied (kg P/ha)
13.0
Effect of Phosphorus and cropping system on soil organic carbon, Sadore, Niger, 1995.
Role of organic amendments
on land productivity
Maize grain yield (t/ha)
5
4.5
4
3.5
Compound
fields
3
2.5
2
Long distance
1.5
1
0.5
0
Carsky et al., 1998
0
1
2
Soil Organic C (%)
3
Phosphorus use efficiency (kg grain/kg P) in village
(non degraded) and bush (degraded) fields for
pearl millet production, Niger
Nutrient
P applied
(26 kg P/ha)
Years
Degraded
Non degraded
2000
42
79
2001
47
79
3000
Control
2500
Fert + Manure
Grain yield (kg/ha)
Fertilizer
2000
1500
1000
500
0
1960
1970
1980
1990
Sorghum grain yield as affected by
mineral and organic fertilizers over time.
Effect of cattle dung and urine on millet grain and total
above ground bio-mass, Sadore Niger
Manure Dung
+
Application
Grain
Urine
-
Biomass Grain
Urine
Biomass
Kg/ha
Cattle
0
-
-
80
940
2990
580
4170
320
2170
6080
1150
7030
470
3850
7360
1710
9290
560
3770
s.e.m
175
812
109
496
Adapted from Powell et al., 1998
Effect of fertilizer application and crop
residue on maize grain yield
8
7
NPK+CR
NPK-CR
CR-NPK
Control
Yield after 10yrs, plot burnt
Yield after 10yrs, plotmulched
Maize yield (t/ha)
6
5
4
3
2
1
0
1
2
3
4
5
6
Year
7
8
9
10
Adapted from Kang 1993
Pearl millet total dry matter yield as affected by long-term
application of crop residue and fertilizer
10000
Millet total dry matter yield (kg ha-1)
Control
Crop Residue (CR)
8000
Fertilizer (F)
CR + F
6000
s.e. = 307
4000
2000
0
Source: Bationo et al,
1998.
1983
1985
1987
1989
1991
Years
1993
1995
1997
Incremental millet grain and stover yield due to
fertilization in sadore, Niger
Year
Treatment
1985
CR
Fertiliser
CR + Fertiliser
CR
Fertiliser
CR + Fertiliser
1986
Fertiliser effect
Grain
Stover
----- kg per kg P applied ----
67
137
57
112
188
427
184
359
Source: Bationo et al., 1995
ORGANIC INPUTS FOR N MANAGEMENT
Decision Guide
Yes
Yes
Lignin < 15%
Polyphenols < 4%
Incorporate directly
No
Mix with N fertilizer or
high quality organic matter
Yes
Mix with N fertilizer
or add to compost
No
Apply at the soil surface
%N
> 2.5
No
Lignin < 15%
FEs for Different Organic Materials
150
% Fertilizer equivalent
y = 65.345x - 148.75
2
R = 0.6486
100
y = 25.721x - 74.13
2
R = 0.7362
50
0
0.00
1.00
2.00
3.00
4.00
5.00
Plant materials, low PP, W Africa
-50
Plant materials, low PP, E+S Africa
Plant materials, high PP, W Africa
Calliandra, high PP, E+S Africa
-100
N content (%)
IMPROVEMENT IN SOIL C (KG/HA/YR)
TYPE OF INPUT AFFECTS SOM COMPOSITION
300
MICROBIAL C
PARTICULATE C
MINERAL-ASSOCIATED C
200
100
0
Fertilizer +
Stover
0
Manure 0
(Kapkiyai et al., 1996)
+
+
0
0
+
0
0
0
+
+
0
+
0
+
+
+
+
+
Base saturation and pH (water) for soil
experiments in Saria, Burkina Faso
Treatment
Base
saturation
0.63
pH
Chemical fertilizer
0.37
4.6
Crop residues 5t ha-1
0.7
5.2
Control
5.2
(Source: Pichot et al 1981)
Soil pH as affected by soil depth and management
practices. Sadoré, Niger, rainy season, 1996
pH (KCl)
3.9
4.1
4.3
4.5
4.7
0.0
0.2
Soil depth (m)
Control
Crop residue (CR)
Fertilizer (F)
0.4
CR + F
Fallow
s.e. = 0.06
0.6
0.8
4.9
Maximum phosphorus sorbed as affected by soil depth
and management practices, Sadoré, Niger, 1999
Maximum P sorbed (mg P kg-1)
30
50
70
90
110
130
150
0.0
Control
0.1
Crop residue (CR)
Fertilizer (F)
CR + F
Soil depth (m)
0.1
0.2
0.2
0.3
0.3
Limitations
Availability of crop residue
• Optimum rate : 2 t ha-1
• Farmers field : 200kg ha-1
Farmers’ doses combined with the use of
small quantities of P fertilizers can boost
crop biomass.
Utilisation de micro-dose de P (4kg/ha)
+P
-P
Limitations
– Manure use is part of
internal flow and does not
add always nutrients from
outside the farm
– Limited quantities, low
nutrient content and often
high labor demands for
processing and application
– Potential livestock transfer of
nutrients in W. Africa is 2.5
kg N and 0.6 kg P per hectare
of cropland
Limitations
– 5-20 tons recommended but less
than 700kg is available in semiarid W. Africa
– Can only increase yield by 2%
per year
– Need between 10-40 ha of
grazing land to maintain yield
on 1 ha of cropland
Future research challenge with
emphasis on organic matter
quantity and quality
Future research challenges
• Focus more on whether the organic resource quality concept is
also useful for predicting different degrees of stabilization of
applied organic C in one or more of the organic matter pools
• Increasing the dual purpose grain legume component for
improvement of soil organic carbon and for a better integration of
crop-livestock production systems
• Improvement of nutrient use efficiency in order to offer costeffective mineral fertilizer recommendations to the small-scale
farmers
• Use of decision support systems, modelling, and GIS for the
extrapolation of research findings