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Agroforestry for Small Landholders of Eastern and Southern Africa

Authors:
237
11 Agroforestry for Small
Landholders of Eastern
and Southern Africa
T.L. Beedy, G. Nyamadzawo, E. Luedeling,
D.-G. Kim, F. Place, and K. Hadgu
CONTENTS
11.1 Introduction ..................................................................................................238
11.1.1 African Smallholders, Soil Management, and Food Security ..........239
11.1.2 Denition and Objectives .................................................................240
11.1.3 Benets and Constraints of Agroforestry .........................................240
11.1.4 Agroforestry Practices and Smallholder Cropping Systems ............242
11.1.4.1 Agroforestry Options .........................................................242
11.1.5 Tree–Crop Interactions .....................................................................247
11.1.5.1 Water Relationships ...........................................................247
11.1.5.2 Weed Suppression ..............................................................248
11.1.5.3 Soil Nutrient Content .........................................................248
11.2 Soil Fertility Replenishment .........................................................................248
11.2.1 Soil Fertility Replenishment with Agroforestry ...............................248
11.2.1.1 Nitrogen Fixation ...............................................................249
11.2.1.2 Other Macro- and Micronutrients ...................................... 250
11.2.1.3 Role of Fertilizer Trees as Nutrient Pumps and Safety
Nets ....................................................................................250
11.2.2 Maize Yield Increases from Improved Soil Fertility ........................251
11.2.2.1 Improved Fallows ...............................................................251
11.2.2.2 Intercrops ...........................................................................253
11.2.2.3 Rotational Woodlots ..........................................................254
11.2.2.4 Parkland Systems in Eastern and Southern Africa ............ 255
11.2.2.5 Faidherbia albida: A Successful Parkland Species ............ 255
11.3 Accumulation of Carbon in Biomass and Soil .............................................257
11.3.1 Biomass Accumulation in Agroforestry in Eastern and Southern
Africa ................................................................................................257
11.3.1.1 Intercropping Agroforestry Systems ..................................257
11.3.1.2 Improved Fallows ...............................................................258
11.3.1.3 Rotational Woodlots and Tree Plantations .........................259
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238 Soil Management of Smallholder Agriculture
11.1 INTRODUCTION
Soil and water resources worldwide are under stress from the accelerated demands
of increasing population. Reduction in soil fertility is driven by increased human
population that has reduced land availability and caused a breakdown of traditional
fallow systems that smallholder farmers relied on for soil fertility replenishment.
Agroforestry is among a suite of sustainable agricultural practices that can rebuild
soil fertility and soil organic matter (SOM), and break the cycle of poverty. Many
agroforestry species are used for biological xation of atmospheric nitrogen (N) into
available N, root uptake, and recycling of nutrients. Nitrogen that accumulates in the
leaves of planted tree fallows and intercrops is released when the biomass decom-
poses after being incorporated into the soil. Because farming practices of African
smallholders tend toward multipurpose mosaics rather than uniform eld manage-
ment, the number of useful combinations of crops with agroforestry is constantly
increasing. New tools such as “Useful Tree Species for Africa” facilitate the choice
of trees within farming systems. Short-term agroforestry species have increased
cereal yields from 10% to 200%, while yield differences under long-term parkland
species such as Faidherbia albida have ranged from slight decreases to doubling
of yields. Parkland systems have long been used by farmers but are now being rec-
ognized by the development community. The multiple sources of the yield benets
under parkland management are currently being documented by researchers. While
rebuilding soil fertility, agroforestry also increases biomass buildup and carbon (C)
sequestration in farming systems. This increase, however, is highly variable through-
out eastern and southern Africa, and the residence time of soil organic carbon (SOC)
is controversial. All agroforestry systems for which data are available accumulate
biomass faster than the natural systems they emulate. The range of C sequestration
by smallholder agroforestry in the tropics has been bracketed between 1.5 and 3.5
Mg C ha1 year−1. Addition of agroforestry species to farming systems has the poten-
tial to either enhance or reduce soil C and greenhouse gas (GHG) emissions. Thus,
the study of GHG emissions with agroforestry practices is critical in describing the
trade-offs between smallholder and ecosystem benets from agroforestry.
11.3.2 SOC in Smallholder Settings ............................................................ 260
11.3.2.1 SOC and Soil Fertility .......................................................260
11.3.2.2 Potential SOC Increases with Agroforestry .......................260
11.3.2.3 Intercrops, Improved Fallows, and Woodlots .................... 261
11.3.2.4 Connection with Carbon Markets ...................................... 263
11.3.3 GHG Emissions ................................................................................264
11.3.3.1 Soil Carbon Dioxide, Methane, and Nitrous Oxide
Emissions ...........................................................................264
11.3.3.2 Emissions of GHGs in Agroforestry in Eastern and
Southern Africa .................................................................. 265
11.3.3.3 Suggested Future Studies ...................................................266
11.4 Conclusions, Challenges, and Future Needs .................................................266
References ..............................................................................................................267
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239Agroforestry in Eastern and Southern Africa
11.1.1 AfricAn SmAllholderS, Soil mAnAgement, And food Security
Africa is the only region in the world where the per capita food production has been
consistently falling (Payne 2010, p. 45). While the number and prevalence of the
undernourished has declined worldwide since 1990, in sub-Saharan Africa the num-
ber of undernourished has increased from 175 million to 239 million people (Food
and Agriculture Organization of the United Nations, World Food Programme, and
International Fund for Agricultural Development 2012). Impoverished populations
in Africa remain concentrated in rural areas. The United Nations Environment
Program (2009) estimated that 80% of the most desperately poor in Africa are sub-
sistence farmers. According to the World Bank (2008), the majority of the poor are
expected to remain in rural areas until at least 2040.
Soil and water resources worldwide are under stress from the accelerated
demands of increasing population (Lal 2010). These stresses are of great concern
in Africa because the rural poor rely most on these soil and water resources. Many
smallholder farmers in Africa are located in areas where rainfall is low, erratic, and
unreliable. Rainfall variability is a major challenge as many smallholder agricultural
systems of eastern and southern Africa are predominantly rainfed and irrigation
systems are not well developed (Camberlin et al. 2009). Most smallholder farm-
ers in Africa practice low-input subsistence farming based on fertility-mining and
extractive practices in which output exceeds input (Lal 2007). As yields decrease,
the pressure to cultivate marginal lands increases. Deforestation and land degrada-
tion accompanying agricultural expansion have fragmented ecosystem provisioning,
regulating, and supporting services previously provided by woodlands. Sustainable
intensication (Figure 11.1) of agricultural systems in sub-Saharan Africa is the key
to reducing food insecurity (Garrity et al. 2010; Payne 2010, p. 45). Agroforestry is
FIGURE 11.1 Mr. Mariko Majoni demonstrates his Gliricidia sepium–maize intercrop near
Chiradzulu, Malawi. (From ICRAF.)
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240 Soil Management of Smallholder Agriculture
among a suite of sustainable agricultural practices that can rebuild soil fertility and
SOM, and break the cycle of poverty (Lal 2004).
11.1.2 definition And objectiveS
Agroforestry is broadly dened as a set of land use practices that deliberately com-
bine trees, shrubs, palms, or bamboo with agricultural crops or animals (Sileshi et
al. 2007). This chapter will describe how agroforestry practices increase soil fertility
and cereal yields for small landholders, as well as increase biomass and soil C accu-
mulation. Emerging data on the increase in GHG emissions with agroforestry will
also be presented, which will inform discussions of the trade-offs between small-
holder households and the ecosystem-wide benets of agroforesty.
The benets of agroforestry are often classied as ecosystem services, which are
conditions and processes through which ecosystems sustain human life (Tallis and
Kareiva 2005). Ecosystem services may be divided into four categories: provision-
ing, regulating, supporting, and cultural services (Carpenter et al. 2009; Sileshi et
al. 2007). Provisioning services of agroforestry/crop combinations include provision
of food from increased soil fertility and improved soil water balance, and timber
and fuelwood from rotational woodlots. Regulating services include erosion con-
trol, improved water inltration, and C sequestration. Supporting services include
biomass production and soil fertility improvement. Finally, cultural services include
spiritual, cognitive, and aesthetic services.
11.1.3 benefitS And conStrAintS of AgroforeStry
For smallholder farmers, the major benet from using the soil-fertility-enhancing
agroforestry practices described above is the increased yield that can lead to greater
food security or additional income. At the same time, few farmers are aware of the
possibility of receiving additional payments for the C sequestration service provided
by the systems. Only a very small number are actually beneting from soil C pay-
ments (the rst ever scheme in western Kenya is very new), and the price paid for
sequestered C is very low. Thus, the fact that agroforestry practices can sequester C
through tree biomass and soil C buildup is not an important factor in farmers’ ex ante
or ex post evaluations of the practices.
What motivates farmers to adopt and manage agroforestry practices for soil fertil-
ity depends on the context, which includes how distinct these practices are from tradi-
tional farming practices. There are some locations where farmers have long practiced
the integration of naturally regenerated trees in their crop elds. The main example
of this is the parkland system in the West African Sahel (Boffa 1999); however, estab-
lishment of similar systems is possible in many dryland areas throughout Africa.
Almost all drylands are important areas for tree regeneration. In typical parkland
systems, trees regenerate naturally from roots or seed, and farmers retain those trees
that are benecial to them. From an economic point of view, the integration of trees
for yield improvement in these types of systems is attractive for the following reasons:
dryland areas have lower labor-to-land ratios than humid and subhumid areas, and
thus labor-saving practices such as tree regeneration and management are compatible
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241Agroforestry in Eastern and Southern Africa
with resources; there is often poor access to markets in sparsely populated areas so
that fertilizer costs are high; and trees offer fodder and other tree products that help
households diversify agricultural enterprises and reduce climatic risks.
A recent study from the Sahel (Place and Binam 2013) found that crop yields were
signicantly higher in elds where mature trees benecial to soils were found than
in treeless plots. This was due to the direct effects of the trees and also an indirect
effect by which farmers were more likely to apply manure and fertilizer on elds
on which such trees were found. In quantitative terms, yields of millet and sorghum
were often found to be 15%–30% higher on elds with an average cover of mature
soil-enhancing trees such as Faidherbia, controlling for other inputs, rainfall and
soil type. A similar nding is reported in the case of mature stands of Faidherbia in
Malawi, where yields of maize (Zea mays) were 15% higher controlling for other fac-
tors (Glenn 2012). Given the low costs involved in establishing and managing trees
through regeneration, the main constraints are related to abiotic and biotic factors
such as aridity, re, grazing, and the lack of germplasm in the soil. However, one
policy constraint is particularly notable in the Sahel. That is the restriction on cut-
ting, pruning, and transport of trees under the forest codes common in West Africa.
These are intended to protect valued indigenous parkland trees; however, now that
many trees are actually on farms, it has the perverse effect of inducing farmers to
remove young seedlings before they mature (Yatich et al. 2013).
There are more opportunities for using agroforestry to improve soil fertility in
the subhumid and humid regions. There, trees are commonly planted as seeds or
seedlings, and this allows farmers much more control over the densities and species
of trees they wish to use. Growing faster in the more humid areas, the trees can also
provide quicker impacts on yields, which is desirable to farmers. For example, an
improved fallow or dense intercrop practice can already raise yields after 2 years due
mainly to the nutrients from leaf fall and incorporation (Sileshi et al. 2008), while soil
physical and biological properties are slower to improve from trees. In both of these
practices, trees are not allowed to reach mature height as they are either removed after
a short time in the case of fallowing or regularly pruned in the case of intercrops.
The economic return from practicing improved fallows has been well studied for
southern Africa and to some extent elsewhere. In Zambia, improved fallows per-
formed much better than continuous maize production without fertilizer (Franzel
2004; Ajayi et al. 2007, 2009). In the recommended 5-year cycle of 2 years fallow
and 3 years cropping, the net prot from unfertilized maize was only US$130 per
hectare against US$269 and US$309 for maize grown under fallows using Gliricidia
(Gliricidia sepium) or Sesbania (Sesbania sesban) species. The use of mineral fertil-
izer for 5 years provided higher net prots; however, returns to labor were similar to
those of improved fallows. Studies of farmer behavior in eastern Zambia revealed
that almost three-quarters of farmers exposed to the practice in the 1990s and early
2000s were still using it by 2009 (Kabwe 2010).
The practice of planting trees for soil fertility is not a traditional farming practice,
probably anywhere in the world. Thus, a major constraint to the practice is awareness
of its potential and understanding of how to put it into practice. A related constraint
is availability of germplasm of trees with soil amelioration benets. Further work
with farmers has found that even if those barriers are removed, other difculties or
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242 Soil Management of Smallholder Agriculture
constraints emerge. These include lack of land in densely populated areas for fallow-
ing, lack of labor for intensive shrub management, lack of water for nursery opera-
tions, and gender considerations when women may manage food production but have
fewer rights to plant trees (Ajayi et al. 2009; Place et al. 2005). Thus, although agro-
forestry has may proven benets, these benets may not be well known to all farm-
ers, and adoption is an involved process of working through constraints.
11.1.4 AgroforeStry PrActiceS And SmAllholder croPPing SyStemS
The cropping systems developed by subsistence farmers depend on the available
natural resource base and the dominant pattern of farm activities (Dixon et al. 2001,
p. 11). Suitable agroforestry technologies vary depending on agroecology and preva-
lent cropping systems.
11.1.4.1 Agroforestry Options
Agroforestry practices may take many forms, and farming practices of African small-
holders tend toward multipurpose mosaics rather than uniform eld management.
Thus, the number of useful combinations of crops with agroforestry is constantly
increasing. To facilitate identication of useful tree–crop combinations, researchers
at the World Agroforestry Centre (ICRAF) and the University of Copenhagen have
developed the “Useful Tree Species for Africa” tool (Figure 11.2) in Google Earth,
mapping the prevalence of tree species and linking to a database of their useful
properties (Lillesø et al. 2011).
The largest smallholder farming system in eastern and southern Africa is the
maize mixed system, although the cereal–root crop mixed system involves a similar
number of farmers (Akinnifesi et al. 2010; Dixon et al. 2001, p. 37). Correspondingly,
FIGURE 11.2 “Useful Tree Species for Africa”—online tool for tree species selection.
(From ICRAF. 2012. Useful Tree Species for Africa. [Online] http://www.worldagroforestry
centre.org/our_products/databases/useful-tree-species-africa; Lillesø, J.-P.B., P. van Breugel,
R. Kindt, M. Bingham, S. Demissew, C. Dudley, I. Friis, F. Gachathi, J. Kalema, F. Mbago,
V. Minani, H.N. Moshi, J. Mulumba, M. Namaganda, H.J. Ndangalasi, C.K. Ruffo, R. Jamnadass,
and L. Graudal. 2011. Potential Natural Vegetation of Eastern Africa, Volume 1: The Atlas.
Forest and Landscape Working Paper 61, University of Copenhagen.)
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243Agroforestry in Eastern and Southern Africa
many critical agroforestry practices for food security in eastern and southern Africa
involve strategic use of fertilizer trees to x N for these annual cropping systems
(Akinnifesi et al. 2010). Agroforestry trees that will tolerate coppicing, such as
Gliricidia, Leucaena (Leucaena leucocephala), and Senna (Senna siamea), are
planted as intercrops in subhumid areas (Table 11.1, Figure 11.1). The advantages
TABLE 11.1
Examples of Common Agroforestry Practices in Eastern and Southern Africa
Agroforestry
Type
Food Crop–
Agroforestry
Combination Function Location (Biome) Rainfall Reference
Intercrop Gliricidia sepium Soil fertility Zomba, Malawi (wetter
Zambezian miombo
woodland) 937 mm
Makumba
etal. 2006
Maize Food
Agropastoral
parkland
Faidherbia albida Wood Adwa, Tigray, Ethiopia
(Afromontane vegetation
and east African evergreen
bushland)a 740–900 mm
Hadgu et al.
2009
Eucalyptus
camaldulensis
Wood
Barley Food
Improved
fallow
Tephrosia vogelli Soil fertility Zomba, Malawi (wetter
Zambezian miombo
woodland) 937 mm
Harawa et al.
2006
Sesbania sesban Soil fertility
Maize Food
Rotational
woodlot
A. crassicarpa Wood Tabora, Tanzania (drier
Zambezian miombo
woodland) 928 mm;
Shinyanga, Tanzania
(Somalia–Masai Acacia
Commiphora deciduous
bushland) 700 mm
Nyadzi et al.
2003
A. julifera Wood
Maize Food
A. nilotica Wood
A. polyacantha Wood
L. leucocephala Soil fertility
Maize Food
Fodder bank Calliandra
calothyrsus
Dairy
fodder
Embu, Kenya (Afromontane
vegetation, Acacia wooded
grassland) 1200–1500 mm
Franzel et al.
2003
Pennisetum
purpureum
Erosion
control
Maize Food
Coffee Cash
Multistorey/
home garden
Enset Food Gedeo, Ethiopia
(Afromontane vegetation)
800–1200 mm
Negash et al.
2012
Coffee Cash
Millettia ferruginea Shade
Cordia africana Shade
Contour
hedgerows
Calliandra
calothyrsus
Erosion
barrier
Embu, Kenya (moderately
sloping land cleared from
Afromontane vegetation)
1200–1500 mm
Angima et al.
2002
Pennisetum
purpureum
Erosion
barrier
Maize Food
a Vegetation types from “Useful Tree Species for Africa” (http://www.worldagroforestrycentre.org/our_
products/databases/useful-tree-species-africa).
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244 Soil Management of Smallholder Agriculture
of the intercrop are the supply of nutrient-bearing leaf biomass directly to the soil,
with no land displaced from food crops. One disadvantage is the difculty of till-
ing the soil near the tree because of woody tree roots. Coppicing species may be
planted in hedgerows for alley cropping where precipitation is ample and the hedge-
rows of trees will not compete with the cereal crop for moisture (Sanchez 1995).
Noncoppicing species, such as Tephrosia (Tephrosia vogelii), Sesbania, and Acacia
angustissima are planted in 2-year improved fallows, to enrich the soil before 2–3
years of maize cropping. Improved fallows also provide N-rich leaf matter directly
to the farmed soil. Although cereal grains behind yields the improved fallow usually
more than repay the time lost to fallow, the most food-insecure farmers may not be
able to take the land out of production to take advantage of the benets.
Fast-growing native and nonnative species have been selected by ICRAF (2013)
and by national agricultural research services (Snapp et al. 2002) to provide rapid
rehabilitation of soils. Tephrosia vogelii has often been used in improved fallows,
and is a common shrub in eastern and southern Africa. A more productive cultivar,
formerly called Tephrosia candida, was selected in India from the original T. vogelii
germplasm, and has been introduced into eastern and southern Africa. Gliricidia
was brought as an intercropping species from central America (ICRAF 2013) to
eastern and southern Africa. Tephrosia and Gliricidia are fast-developing species
that quickly produce nutrient-rich leaf biomass for soil fertility improvement.
In silvopastoral agroforestry (Figure 11.3), trees are either planted or protected
among naturally regenerating populations, and are left scattered in grazing land to
increase the carrying capacity of the grazing system (Hadgu et al. 2009). In some
areas with high population density and rainfall, agroforestry forage species are
planted in fodder banks, and the vegetation is cut and carried to conned livestock
(Figure 11.4), especially dairy stock (Chakeredza et al. 2007).
Change [cereal
grains behind
yields the im-
proved fallow]
to [cereal grains
behind yields
of the impr oved
fallow]?
FIGURE 11.3 Silvopastoral agroforestry near Shinyanga, Tanzania. (From Constance
Neely, Charlie Pye/ICRAF.)
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245Agroforestry in Eastern and Southern Africa
Both traditional and introduced fertilizer trees have been used to improve small-
holder farm productivity. In parkland agroforestry (Figure 11.5), large-statured trees,
such as Faidherbia, have been intercropped with cereal crops across Africa for many
generations (Wood 1992). Faidherbia is a long-lived tree that provides nutrient-rich
biomass to cropping and livestock systems, and requires little space and labor, but is
a slow-growing species that may take a decade after planting to make an appreciable
impact on farming systems.
FIGURE 11.5 Faidherbia albida parkland with maize in Tanzania. (From ICRAF.)
FIGURE 11.4 Smallholder farmer feeding a mixture of rich tree fodder and crop residue to
her dairy animal in Kenya. (From ICRAF.)
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246 Soil Management of Smallholder Agriculture
Rotational woodlots have the advantage of accumulating biomass quickly and
providing much-needed wood for building and cooking fuel. However, where pop-
ulation densities are high, rotational woodlots also compete with food crops for
space in smallholder farming systems. Multistrata (multistorey) agroforestry pro-
tects shade-grown coffee (Figure 11.6) and enset plantings in the highland temperate
areas of Ethiopia (Negash et al. 2012) and other highland areas in eastern and south-
ern Africa. The shade-giving species reduce evapotranspiration and erosion and,
in some cases, x N or provide fruit or timber (Sileshi et al. 2013). Home gardens
provide fruit (Figure 11.7), medicinal products, timber, fuelwood, shade, and other
benets in dense, multistrata plantings around homesteads in humid areas; however,
the shade from the trees prevents planting food crops adjacent to the home, where
they are most easily protected.
FIGURE 11.6 Multistorey (multistrata) coffee production in Ethiopia. (From Dong-Gill Kim,
Wondo Genet College of Forestry and Natural Resources, Hawassa University, Ethiopia.)
FIGURE 11.7 Home garden in southern Ethiopia. (From Dong-Gill Kim, Wondo Genet
College of Forestry and Natural Resources, Hawassa University, Ethiopia.)
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247Agroforestry in Eastern and Southern Africa
11.1.5 tree–croP interActionS
Several traditional smallholder farming systems in eastern and southern Africa uti-
lize tree–crop interactions for sustainable agricultural production (Buresh and Tian
1998; Kidanu et al. 2004). Trees can explore a relatively large space compared with
crop plants, and they can have the capacity to capture and use aboveground and
belowground resources efciently (Goldberg and Barton 1990; Garcıa-Barrios and
Ong 2004), thereby becoming more resistant to cyclic environmental changes than
annual crops (Hiremath et al. 2002). They can increase available nutrients for crops
by root exudates and leaf drop (Jung 1970; Radersma and Grierson 2004).
Aboveground and belowground resources are partitioned between trees and crops
such that relative interspecic competition is lower than relative intraspecic com-
petition, resulting in niche differentiation (Malézieux et al. 2009). Thus, there can be
a total resource increase in the system or increased resource use efciency (Cannell
et al. 1996; Holmgren et al. 1997). System productivity can also be increased by
reducing nutrient losses through leaching in deep soil, reducing soil erosion, protec-
tion against wind (Malézieux et al. 2008), reduced weed populations (Liebman and
Gallandt 1997), or nutrient capture through N xation and mycorrhizal associations
(Young 1989; Giller 2001). Moreover, trees can add considerable amounts of organic
matter to the soil, improving soil fertility and physical structure, stabilizing soil
structure, and reducing erosion (Young 1997; Roose and Barthes 2001). Thus, trees
and crops are complementary since enhanced soil fertility in the presence of trees
can increase crop productivity in the vicinity of trees (Verinumbe 1987).
11.1.5.1 Water Relationships
A tree can modify and improve the growth of other trees or crops by changing the
biophysical environment (Hunter and Aarssen 1988; Garcıa-Barrios and Ong 2004).
Trees may affect soil water content by increasing it (Bayala et al. 2008; Caldwell
and Richards 1989; Dawson 1993) or by decreasing it (Odhiambo et al. 2001), and
thereby inuence nutrient transport to crop roots and root growth (Radersma et al.
2004). Although trees can increase the potential soil water-holding capacity, they
can also have negative effects on the actual water volume available in the tree–crop
soil system. Trees can reduce soil evaporation by shading crops and reducing air
movement through understories, improve microclimatic conditions by reducing air
temperature and wind speed, and reduce water stress in crops (Monteith et al. 1991;
Vandenbelt and Williams 1992). Trees reduce exposure to heat stress, which mini-
mizes tissue temperature to optimize the phenology and productivity of understory
crops (Monteith et al. 1991; Vandenbelt and Williams 1992) and thus offsets water
losses by evaporation from tree canopies (Ong and Swallow 2003). Depending on the
slope and soil characteristics, trees can also increase inltration (Ong and Shallow
2003). Tree roots can use water accumulated deeper in the soil prole, which can
benet crop growth. Besides, they can use residual available water outside the crop
growing season (Ong et al. 2002; Garcıa-Barrios and Ong 2004). Integrated tree
crop systems have existed for generations and may enhance crop production (Saka
et al. 1994). In general, available water can be used more efciently in a tree–crop
system than a sole crop system owing to favorable microclimate and improved water
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248 Soil Management of Smallholder Agriculture
use efciency. Trees can reduce the unproductive components of the water balance
such as runoff, soil evaporation, and drainage (Ong and Swallow 2003).
11.1.5.2 Weed Suppression
Improved fallows can control Striga (Striga hermonthica), a parasitic weed that
causes large yield losses in many cereal crops (Barrios et al. 1998; Gacheru and Rao
2001). Although the processes are not well understood, it is suspected that the fallow
species excrete substances that cause suicidal early germination of Striga (Jama et
al. 2006). Improved fallows also act as a break crop by smothering weeds, shading
weeds, and outcompeting weeds for nutrients. Increased weed suppression increases
nutrient availability for plant uptake and thus results in increased crop yields.
11.1.5.3 Soil Nutrient Content
System productivity can also be increased by trees through the reduction of nutrient
losses from leaching in deep soil, reduced soil erosion, protection against wind, and
reduction of weed populations and aggressiveness (Leibman and Gallandt 1997).
Trees have the potential to increase overall system productivity by increasing nutri-
ent availability through N xation. Trees with perennial mycorrhizal associations
may improve the efciency of phosphorus (P) cycling because of greater absorptive
area and of increasing pools of organic P (de Carvalho et al. 2010). Deep-rooting
trees may pump nutrients from below the crop rooting zone (Harawa et al. 2006),
both those made available by weathering of the bedrock and those leached down
from the upper layers (Young 1989).
11.2 SOIL FERTILITY REPLENISHMENT
11.2.1 Soil fertility rePleniShment with AgroforeStry
Many smallholder farming areas in southern and eastern Africa have low potential
for agriculture because of poor soil fertility and low SOC content, which also causes
soils to have a low water-holding capacity. The low soil fertility in these soils is
widely recognized as a major factor contributing to low agricultural productivity in
southern Africa (Vanlauwe and Giller 2006). Reduction in soil fertility is driven by
increased human population that has reduced land availability and caused a break-
down of traditional fallow systems that smallholder farmers relied on for soil fertil-
ity replenishment (Jurion and Henry 1969). This has forced people to farm in more
fragile lands. In Zambia, unavailability and inability of most smallholder farmers
to purchase inorganic fertilizers following the removal of government agricultural
subsidies have resulted in a reduction in crop productivity (Howard and Mungoma
1996). A combination of low soil fertility, lack of fertilizer amendments, and poor
rainfall has often resulted in decreased crop production and widespread food short-
ages. This reduction eventually led to a reduction in the ability of most countries in
the region to provide a stable food supply for their people.
Agroforestry is one of the many sustainable options that can assist smallholder
farmers to replenish their impoverished soils (Ajayi et al. 2009). The use of agrofor-
estry technologies has increased crop yields in smallholder farming areas (Kwesiga
K20897_C011.indd 248 7/6/2014 1:57:44 PM
249Agroforestry in Eastern and Southern Africa
et al. 1999; Sileshi et al. 2008). In these systems, soil fertility benets from fallowing
are derived from the use of annual, biannual, or perennial N-xing trees or “legu-
minous fertilizer trees,” which are either planted in rotation (e.g., improved fallows)
or together with crops (e.g., alley or intercropping). Leguminous fallow trees that
have been used successfully include coppicing perennials such as Gliricidia and A.
angustissima and others such as Sesbania and pigeon pea (Cajanus cajan), which do
not coppice well and need to be replanted during the fallowing phase of crop–fallow
rotations.
11.2.1.1 Nitrogen Fixation
Species such as Sesbania, Gliricidia, and Tephrosia replenish soil fertility through
biological xation of atmospheric N into available N, root uptake, and recycling of
nutrients. Nitrogen that accumulates in the aboveground biomass (especially leaves)
of planted tree fallows and intercrops is released when the biomass decomposes
after being incorporated into the soil, and it is utilized by crops during the cropping
phase. Different tree species can x different amounts of N, and the total amount of
N that is released is also dependent on the decomposition rates of the leguminous
tree biomass (Mafongoya and Dzowela 1999). For example, Mafongoya and Dzowela
(1999) reported that biomass from S. sesban decomposed faster than biomass from
A. angustissima under similar eld conditions.
A study in Zimbabwe by Chikowo (2004) showed that total xed N (estimated
using Hyparrhenia spp. as reference plant) in A. angustissima (nonwoody compo-
nents + leaf litter) was 122 kg N ha−1 during the 2-year fallow period, while pigeon
pea, Sesbania, and cowpea xed 97, 84, and 28 kg N ha−1, respectively (Table 11.2).
All [Chikowo
et al. 2004] in
text and tables
were changed to
[Chikowo 2004].
Correct?
TABLE 11.2
Total N Fixed by Different Legumes during the Fallowing Phase
Species Total N (kgha−1) Location Reference
Improved Fallow
A. angustissima 122 Zimbabwe Chikowo 2004
S. sesban 84 Zimbabwe Chikowo 2004
C. cajan 97 Zimbabwe Chikowo 2004
S. sesban, T. vogelii 150 Zambia Ajayi et al. 2005
S. sesban (1 year) 60 Malawi Chirwa and Quinion 2012
S. sesban 128 Kenya Sanchez and Palm (1996)
C. cajan 75–200 Malawi Kumwenda et al. 1996
Rotational Woodlot
A. crassicarpa 78aTanzania Kimaro et al. 2008
A. mangium 87aTanzania Kimaro et al. 2008
A. polyacantha 104aTanzania Kimaro et al. 2008
G. sepium 114aTanzania Kimaro et al. 2008
a Maize uptake during the following 3 years, rather than supply.
Please provide
the full bib-
liographic details
for Chirwa and
Quinion 2012.
Please provide
the full biblio-
graphic details for
Kumwenda et al.
1996.
K20897_C011.indd 249 7/6/2014 1:57:44 PM
250 Soil Management of Smallholder Agriculture
Several other studies have also reported N xation by legume trees, with reported
values ranging from 100 to 200 kg ha−1 (Table 11.2). The benets of the N contribu-
tion of fertilizer trees on a regional scale (Zambia, Malawi, Zimbabwe, Tanzania,
and Mozambique) as of 2003 were estimated to be from $6.27 to $7.13 million per
year in savings on the purchase of mineral fertilizer (Ajayi et al. 2005).
Trees may supply crop nutrients through leaf drop, root exudates, and senescence,
as well as supply timber and fuelwood. In a study in Tanzania, Acacia polyacantha
and Gliricidia leaves had higher nutrient concentrations and lower C-to-N ratios,
reecting higher leaf quality compared with those of Acacia crassicarpa and Acacia
mangium (Kimaro et al. 2008). Nitrogen uptake of maize follows a similar pattern
(Table 11.2), with A. polyacantha and Gliricidia supplying >100 kg ha−1 of N to the
maize and the exotic acacias supplying less N for subsequent crops.
11.2.1.2 Other Macro- and Micronutrients
The ability of agroforestry trees to x N for cereal crops is often emphasized.
However, a closer analysis shows that the benets of agroforestry go beyond N xa-
tion. The use of agroforestry trees has proven to also enhance P availability to sub-
sequent crops (Chikowo 2004; Ayuk and Mafongoya 2002; Ajayi et al. 2005; Jose et
al. 2000) Many soils in southern Africa are P decient, and expensive inorganic P
supplements are needed for crop production. Fertilizer trees can economically close
this gap as they can improve P availability through the secretion of organic acids and
increased mycorrhizal fungi populations in the soil. The fungi that are associated
with increased P availability in agricultural soils are arbuscular mycorrhizal (AM)
fungi (phylum Glomeromycota) (Harrier and Watson 2003). Bagayoko et al. (2000)
reported that N-xing legumes resulted in better colonization of cereal roots and an
increase in AM fungal populations in the soil. Fertilizer trees may act as reservoirs
for AM fungi populations in crop root zones (Ingleby et al. 2007). The mixing of
crops and trees that occurs in agroforestry systems may also result in a more equi-
table occurrence of mycorrhizae throughout the root zone because deep-rooted trees
will distribute the AM fungi to deeper layers and increase the volume of soil from
which nutrients are extracted (Cardoso et al. 2003). Management practices such as
agroforestry that allow a buildup of AM fungi in soils would alleviate P deciency
while enhancing N xation (Houngnandan et al. 2000). AM fungi have been pro-
posed as a partial solution to nutrient deciencies in tropical soils (Cardoso and
Kuyper 2006), and the combination of AM fungi and agroforestry may have a role in
the alleviation of micronutrient deciencies in depleted smallholder soils.
11.2.1.3 Role of Fertilizer Trees as Nutrient Pumps and Safety Nets
In cropping systems located in high-rainfall areas, where net N mineralization
exceeds N uptake by crops, inltrating water carries nutrients to the subsoil, result-
ing in a buildup of subsoil N that ranges from 70 to 315 kg ha−1 (Hartemink et al.
1996). This is supported by Chikowo (2004) who reported that up to 45 kg N ha−1
is lost through leaching during the cropping phase of maize fallow rotations. Under
conditions where the soils have substantial anion exchange capacity, e.g., Oxisols and
oxic Alsols, leached N and other nutrients are retained in the subsoil beyond the
reach of most crops and can only be accessed by tree roots (Mekonnen et al. 1997).
K20897_C011.indd 250 7/6/2014 1:57:44 PM
251Agroforestry in Eastern and Southern Africa
Legume trees act as “safety nets” and nutrient pumps as they help in closing the
nutrient cycle by taking nutrients leached into deeper layers up to the surface (Harawa
et al. 2006; Jose et al. 2004). In addition, legume trees break up hardened soil layers and
help increase inltration rates since they have deeper rooting systems (Nyamadzawo
et al. 2008a). The roots also help by bringing water and nutrients to the surface from
deeper soil layers through hydraulic lift (Bayala et al. 2008; Jose et al. 2004).
The soil fertility benets of using fertilizer trees are a result of complex factors
that work together to provide a conducive environment for plant growth. This results
in enhanced plant growth and improved maize yields, due to, for instance, improved
N xation and N availability in fallow crop rotations or tree–crop intercropping.
Increased SOC, a better safety net from deep-rooted tree fallows, increased P avail-
ability from increased soil AM fungi in the soil, and increased weed suppression
all result in improved soil fertility in legume tree–crop intercrops or rotations. We
concluded that the use of fertilizer trees provides a window of opportunity for small-
holder farmers to improve soil fertility and maize yields. The use of fertilizer trees is
a sustainable option that can result in improved food security in households that are
resource constrained, besides providing other goods and services such as fuelwood.
11.2.2 mAize yield increASeS from imProved Soil fertility
Studies from the region have shown that fertilizer trees improve maize yields beyond
continuous maize production without fertilizers or with natural fallows. Controls
used to calculate response ratios in Table 11.3 may be fertilized or unfertilized maize
or natural fallow, and are described in the text. The cereal crop is maize unless noted
otherwise. These studies are not an exhaustive list, but are given to illustrate the
scope of possible yield benets and to illustrate important issues in management of
these combined systems. For a more detailed treatment of maize yield increases with
agroforestry, see Sileshi et al. (2008).
11.2.2.1 Improved Fallows
Maize yields obtained after fallowing are highly variable. For example, Nyamadzawo
et al. (2012) reported maize yields that ranged between 10% and 250% higher after
fallows compared with continuous maize cropping without fertilizers (Table 11.3).
They reported yields of 1.8 and 0.7 Mg ha−1 for A. angustissima and Sesbania under
conventional tillage (CT), while no tillage had even lower yields (1.3 and 0.8 Mg
ha−1) for A. angustissima and Sesbania, respectively, in the rst year of cropping
after 2 years of fallowing. However, in the second year of cropping, the maize yields
were 1.6 and 0.5 t ha−1 for Sesbania and A. angustissima, respectively, under CT,
while the maize yields were 1.5 and 0.3 t ha−1 for Sesbania and A. angustissima,
respectively, under no tillage. Maize yields are also affected by additional factors
such as pests, diseases, and competition for water. In the rst year after fallowing
in the same study by Nyamadzawo et al. (2012), maize after Sesbania was infested
by cutworms and this resulted in lower crop yields, while in the second cropping
season, there was a mid-season drought that affected the maize crop in coppicing A.
angustissima. This resulted in lower crop yields as a result of competition for water
and not because of N deciencies.
K20897_C011.indd 251 7/6/2014 1:57:44 PM
252 Soil Management of Smallholder Agriculture
TABLE 11.3
Cereal Yield Increases with Agroforestry Compared with Unfertilized or
Fertilized Maize or Grass Fallow
Species
Fallow
Years
Crop
Years
Yielda
(Mgha−1)
Responseb
Ratio Location Reference
Improved Fallow
A. angustissima 2 2 3.1c,d 2.5 Zimbabwe Nyamadzawo 2012
S. sesban 2 2 1.5d1.1 Zimbabwe Nyamadzawo 2012
S. sesban 2 2 5.6d2.8 Zambia Ayuk et al. 2002
T. vogelii 2 2 5.5d 3.1 Zambia Ayuk et al. 2002
T. diversifolia 1 1 5.5 1.2 Kenya Niang et al. 2002
S. sesban 1 1 7.1 1.6 Kenya Niang et al. 2002
T. vogelii 1 1 6.2 1.4 Kenya Niang et al. 2002
Intercrop
G. sepium 1 1 0.8 1.6 Malawi Harawa et al. 2006
S. sesban 1e1 0.7 1.4 Malawi Harawa et al. 2006
T. vogelii 1e1 0.9 1.8 Malawi Harawa et al. 2006
G. sepium 1 1 5.2 3.4 Malawi Makumba et al. 2006
G. sepium 1 1 4.2 4.3 Malawi Akinnifesi et al. 2007
Rotational Woodlot
A. crassicarpa 5 2 4.0d 1.3 Tanzania Kimaro et al. 2008
A. mangium 5 2 4.7d 1.6 Tanzania Kimaro et al. 2008
A. polyacantha 5 2 5.9d 2.0 Tanzania Kimaro et al. 2008
G. sepium 5 2 6.2d 2.1 Tanzania Kimaro et al. 2008
A. crassicarpa 4 1 2.0 3.3 Tanzania Nyadzi et al. 2003
A. julifera 4 1 1.75 2.9 Tanzania Nyadzi et al. 2003
A leptocarpa 4 1 1.1 1.8 Tanzania Nyadzi et al. 2003
S. siamea 4 1 0.75 1.3 Tanzania Nyadzi et al. 2003
Parkland
F. albida Decades 1 2.2 0.95 Malawi Saka et al. 1994
F. albida Decades 1 2.0 1.5 Malawi Saka et al. 1994
F. albida Decades 1 1.9 1.76 Ethiopia Poschen 1986
F. albida Decades 1 1.6g1.36 Ethiopia Poschen 1986
F. albida Decades 1 1.4f1.5 Ethiopia Hadgu et al. 2009
F. albida Decades 1 1.4f1.4 Ethiopia Hadgu et al. 2009
F. albida Decades 1 3.0 2.0 Ethiopia Dechasa 2010
F. albida Decades 1 3.8g1.9 Ethiopia Dechasa 2010
F. albida Decades 1 1.0h1.3 Ethiopia Dechasa 2010
F. albida Decades 1 0.7i1.3 Ethiopia Dechasa 2010
a Yield of the agroforestry treatment.
b Yield of the agroforestry treatment divided by the control.
c Yield gures are for maize except where otherwise indicated.
d Combined yield over 2 cropping years following the agroforestry rotation.
e Relay intercrop.
f Barley.
g Sorghum.
h Wheat.
i Tef.
Please provide
the full biblio-
graphic details for
Ayuk et al. 2002,
cited twice in
table.
K20897_C011.indd 252 7/6/2014 1:57:44 PM
253Agroforestry in Eastern and Southern Africa
Ayuk and Mafongoya (2002) reported a successive decline in maize yields
with the number of cropping years after fallow termination. After fallowing with
Sesbania, maize yields were 3.6, 2.0, and 1.6 Mg ha−1 for the rst, second, and third
year, respectively, after fallow termination, while after Tephrosia maize yields were
3.1, 2.4, and 1.3 Mg ha−1 for the rst, second, and third year, respectively, after fal-
low termination. Mafongoya et al. (2003) reported that fallow species had a positive
N balance in the rst year of cropping after the fallow phase. In the second year of
cropping, the N balance in the fallow systems decreased, partly due to large offtake
in the large yields and partly due to leaching through the soils and other associated
losses of N due to immobilization, volatilization, and denitrication. These obser-
vations were supported by Mafongoya and Dzowela (1999), who suggested that the
postfallow cropping phase should be restricted to 2 years.
In Kenya, Niang et al. (2002) tested Tithonia (Tithonia diversifolia), Tephrosia,
and Sesbania in a single year of fallow, followed by two rainy season maize crops
in the following year. The fallow species produced 1.2, 1.4, and 1.6 times as much
maize as the sole maize control. Tithonia was propagated from cuttings, Tephrosia
was direct-seeded in the maize, and Sesbania seedlings were transplanted into the
elds.
Improved fallows usually follow a 2-year fallow plus 2-year cropping pattern, so
that the yields in the cropping years must be more than double the yields under sole
maize for the fallow to be worthwhile for the smallholder. In Table 11.3, all of the
2-year yield increments save one in Zimbabwe are well above two, so that the prac-
tice produces worthwhile yield gains for the smallholders, while at the same time
improving soil and ecosystem health.
The addition of half the recommended fertilizer dose to tree fallow plots may
provide higher yields than fertilizer alone or tree fallows alone. In Zambia, cumula-
tive maize yields were 10.7 Mg in fallows with half of the recommended fertilizer
rate, compared with 10.4 Mg for sole fallow over a 3-year period (Ajayi et al. 2005).
Similar results were also reported by Chikowo (2004) and Makumba et al. (2006),
who reported better maize yields in plots where half the recommended fertilizer
rates were applied compared with sole fertilizers or unfertilized fallows. From these
observations, it was concluded that at certain levels of fertilizer use, there is some
synergy between mineral fertilizers and improved fallow species such as Sesbania
and Tephrosia (Ajayi et al. 2005).
11.2.2.2 Intercrops
Harawa et al. (2006) found yields to be 1.6 times higher with Gliricidia intercrop,
compared with the sole maize control, across all slope positions. Maize yields were
1.4 and 1.8 times those in the sole maize control with Sesbania and Tephrosia relay
intercrops in the same study. In the relay intercrop, the fallow species are planted
annually directly following maize planting. Leaf biomass developed during the dry
season is incorporated annually during land preparation.
Each of the agroforestry species ts into specic agroecological niches in cereal-
based cropping systems. Harawa et al. (2006) tested Sesbania, Tephrosia, and
Gliricidia on upper-slope, mid-slope, and bottom-slope positions near Zomba in
southern Malawi. Sesbania was found better adapted for the bottom-slope position,
K20897_C011.indd 253 7/6/2014 1:57:44 PM
254 Soil Management of Smallholder Agriculture
while Gliricidia was best placed in the well-drained middle slope positions. Tephrosia
yields were similar across slope positions. However, Brewbaker (1990) has demon-
strated that Sesbania may successfully establish in various soils and environments.
In Malawi, Makumba et al. (2006) found that an 11-year Gliricidia intercrop produced
an average of 2.6 times as much grain as sole maize, while a combination of Gliricidia
plus 48 kg N ha−1 produced 3.5 times as much maize as unfertilized sole maize. The per-
formance of sole maize plus 48 kg ha1 N was similar to the Gliricidia intercrop with no
N added. In two of the study years, Makumba at al. (2006) noted a statistically signicant
interaction of the Gliricidia intercrop with the N fertilizer. Akinnifesi et al. (2007) also
found 4.3 times as much maize yield with the Gliricidia intercrop.
Yield data reported by Harawa et al. (2006), Makumba et al. (2006), and
Akinnifesi et al. (2007) differ considerably. The rst study was done in farmer’s
elds with a combination of farmer and researcher management. The yields reported
here were obtained after 1 year of intercropping. This study contrasts species perfor-
mance at different slope positions, but also illustrates the decit in SOM and nutri-
ents that is typical in smallholder agriculture in this region. The second and third
studies were done at Makoka Agricultural Research Station in southern Malawi. The
yields given in these studies were averaged over multiple cropping seasons, during
which Gliricidia leaf biomass was incorporated each year. The research station, with
ample means for inputs, land, and labor, is expected to have a much greater yield
over a longer period, while the means of the smallholder to provide inputs are more
restricted and variable across years.
11.2.2.3 Rotational Woodlots
Although intercrops and improved fallows provide some fuelwood and poles to the
farmstead, smallholders with relatively large landholdings may choose to rotate 0.5–
0.8 ha into a woodlot to provide additional wood for the household (Kimaro 2009).
These woodlots are of longer duration than improved fallows. Maize is intercropped
with the woodlot trees for the rst 2–3 years; then the trees are left to develop for 1
or 2 years. The trees are then harvested, and maize is planted to take advantage of
the nutrients in the decomposing leaf litter and root mass.
Total maize yields over 2 years after the woodlot rotation were higher (Kimaro
et al. 2008) after A. polyacantha and Gliricidia than after A. crassicarpa and A.
mangium (Table 11.3). Yields after the Australian acacias were only 1.3–1.6 times
those after natural fallow, while yields after A. polyacantha and Gliricidia were
more than double the yields compared with the natural fallow. This difference prob-
ably reects higher soil fertility improvement during the fallow period (Kimaro et al.
2008). Yields after A. polyacantha and Gliricidia were also similar to fully fertilized
maize, while unfertilized sole maize yields were similar to those after natural fallow.
In contrast, the amount of aboveground biomass produced was much greater in A.
crassicarpa at 51 Mg ha−1 than in the other three species, with A. mangium and A.
polyacantha producing 38 and 36 Mg ha−1, respectively, and Gliricidia producing 29
Mg ha−1. Thus, in choosing species for this rotation, the smallholder must consider
the trade-offs between timber and food production.
Nyadzi et al. (2003) reported maize yields on a single cropping year after a 4-year
fallow. The yields were lower than those in Kimaro et al. (2008), while the response
K20897_C011.indd 254 7/6/2014 1:57:44 PM
255Agroforestry in Eastern and Southern Africa
ratios were higher. This may be due to the very low initial SOM (0.4–0.8 g kg−1) in
Nyadzi et al. (2003), and to the fact that response ratios tend to be higher in the rst
year of cropping than in the second year.
Although short- or long-lived species may be used for agroforestry practices, the
management and structure of the tree component is very different in parklands com-
pared with improved fallows, intercrops, or rotational woodlots. While the agrofor-
estry component in the short-term systems is coppiced or harvested in 1- to 5-year
cycles, parkland trees are allowed to develop into mature large-scale specimens,
with crops and livestock beneting in various ways from the understory location.
Parklands are integrated into various crop and livestock systems across Africa from
north to south, and from east to west.
11.2.2.4 Parkland Systems in Eastern and Southern Africa
Parkland tree species are usually naturally occurring tree species (Maranz 2009),
which are also protected and managed by farmers (farmer managed natural regen-
eration—FMNR) (Haglund et al. 2011). The main parkland tree species available
in eastern and southern Africa include F. albida, Cordia africana, Croton macro-
stachyus, Acacia tortilis, Moringa stenopetala, Terminalia brownii, Acacia senegal,
Acacia seyal, Ziziphus mauritiana, Balanites aegyptiaca, Ficus sur, and Millettia
ferruginea (Hailu et al. 2000; Kassa et al. 2010).
Some studies reported that parkland trees do not provide short-term crop yield
benets, which could be attributed to the competition for resources between trees
and crops (Bayala et al. 2012). However, parkland trees are protected by farmers
for their sustainable and long-term multiple benets, including direct products such
as fodder, fruit, fuelwood, medicinal, or vegetable products. They are also valu-
able for long-term ecosystem benets, such as C accumulation, reduction in soil ero-
sion, maintenance of soil structure and fertility, improvement of crop microclimate,
reduction of wind incidence, and provision of shade (Dechasa 2010).
11.2.2.5 Faidherbia albida: A Successful Parkland Species
Among parkland tree species, the potential of Faidherbia, an indigenous African
acacia, is well recognized. It is widespread on millions of farmers’ elds through-
out the eastern, western, and southern regions of the continent. It is highly com-
patible with food crops because it is usually dormant during the rainy season
(see Figure 11.5). Thus, it exerts minimal competition with annual crops, while
enhancing crop yields and soil health (Barnes and Fagg 2003). In eastern and
southern Africa, most smallholders cannot afford to buy inorganic fertilizers,
often because of cash constraints. Faidherbia creates a unique opportunity for
increasing smallholder productivity by input of high-quality leaf residue for
increased soil fertility (Garrity et al. 2010), reducing the need for inorganic N
fertilizer. As many smallholders are engaged in both crop and livestock agricul-
ture and their available fodder resources are often inadequate (Giller et al. 2009),
Faidherbia also increases livestock production through supplying high-quality
fodder. It also enhances C storage in farmed landscapes. Faidherbia is considered
a keystone species for climate-smart (evergreen) agriculture in much of Africa
(Garrity et al. 2010).
K20897_C011.indd 255 7/6/2014 1:57:44 PM
256 Soil Management of Smallholder Agriculture
Studies in Africa have documented increases in maize grain yield (Table 11.3)
under Faidherbia (Barnes and Fagg 2003). In Malawi, 50% maize yield increases have
been recorded under Faidherbia trees compared with sole maize (Saka et al. 1994).
Thirty percent to 70% yield increases have also been observed under Faidherbia
on most staples in Ethiopia, including maize, sorghum, and wheat (Poschen 1986;
Dechasa 1989; Hadgu et al. 2009). Yield increments of 100%, 70%, 40%, and 10%
for sorghum, maize, wheat, and teff, respectively, were also reported (Dechasa 2010).
Improvements have been made to traditional agroforestry practices with
Faidherbia. This practice has resulted in signicant contribution to food security
as farmers who have adopted Faidherbia produced 1.5 Mg more maize per hectare
than conventional practice (Haggblade and Tembo 2003). In this regard, Zambia
has taken a lead in systematically utilizing the potential of Faidherbia, releasing
a national recommendation to plant Faidherbia trees at a density of 100 trees ha−1
in crop elds as a permanent canopy to increase soil fertility and crop productivity
(Garrity et al. 2010). The density may later be reduced to 25–30 trees ha−1 as the trees
mature. To date, >200,000 families have adopted this practice (Garrity et al. 2010).
The government of Ethiopia has recently launched an initiative to plant 100 million
Faidherbia trees. Alongside promoting natural regeneration of this important tree in
farmers’ elds, planting Faidherbia at such a scale on smallholder farms is expected
to have signicant economic and environmental benets and add climate resilience
to farming systems, particularly in semiarid areas of Ethiopia.
Faidherbia is a slow-growing tree, often taking 20 years or more to contribute
to crop growth (Poschen 1986). Yield increases also differ with location (Saka et al.
1994). Combining fast-growing multipurpose shrubs such as Gliricidia and Sesbania
with parkland trees should speed soil fertility increases and also increase wood and
fodder availability. Integrating fast-growing shrubs along with natural regeneration
of Faidherbia may alleviate the concern of some smallholders regarding the slow
establishment of Faidherbia seedlings.
Moreover, trees should be spatially and systematically congured (Figure 11.8) to
match species to sites and farmer circumstances. Hadgu et al. (2009) noted a pattern
of land use intensication in Tigray, northern Ethiopia, which reduced the density of
Faidherbia, and was accompanied by an increase in timber cultivation and a decline
in barley yields.
(Note in Figure 11.8 the loss in ecosystem services associated with cutting
Faidherbia within a eld.) The effect of the trade-off between barley production and
timber on food security should be considered at the farm level. More data are also
required on other promising parkland tree species such as Moringa stenopetala,
Balanites aegyptiaca, Millettia ferruginea, Cordia spp., Croton spp., and Ziziphus
spp., with regard to their effects on crop productivity. Studies similar to those on
Faidherbia should also be done with such promising tree species.
Agroforestry practices used in appropriate agroecological niches hold great prom-
ise for recuperating soil fertility while providing greater food and feed production in
agricultural landscapes. Intercrops are more appropriate in areas with high popula-
tion densities, while improved fallows and rotational woodlots are more appropriate
where land can be allocated to fallow and/or wood production. Faidherbia parklands
have mostly been developed by farmer-managed natural regeneration in river valleys
K20897_C011.indd 256 7/6/2014 1:57:44 PM
257Agroforestry in Eastern and Southern Africa
and lowland lakeshore areas in eastern and southern Africa, but are increasingly
being planted in uplands, especially in Zambia.
Use of different agricultural technologies to improve food security has been a
perennial subject in sub-Saharan Africa. However, managing the fate of C in food,
energy, and climate systems has become increasingly difcult (Lal 2010). It is
increasingly important to manage cropping systems with both food security and C
balance in mind.
11.3 ACCUMULATION OF CARBON IN BIOMASS AND SOIL
11.3.1 biomASS AccumulAtion in AgroforeStry
in eAStern And Southern AfricA
Biomass buildup in agroforestry systems throughout eastern and southern Africa is
highly variable. This variation is partly explained by edaphic and climatic site condi-
tions, but it depends at least as strongly on the type of agroforestry that is practiced.
Management of the system is also an important factor. Unmanaged natural regenera-
tion of miombo woodlands, a typical vegetation type of eastern and southern Africa,
occurs slowly, with annual biomass increments estimated at 1 Mg ha−1 in Zambia
(Stromgaard 1985) and 0.43 Mg ha−1 in Tanzania (Aune et al. 2005).
11.3.1.1 Intercropping Agroforestry Systems
All agroforestry systems for which data are available accumulate biomass faster than
the natural systems they emulate. Montagnini and Nair (2004) bracket the range
of C sequestration by smallholder agroforestry in the tropics between 1.5 and 3.5
Mg C ha−1 year−1. Most published studies of biomass accumulation in simultaneous
E3
E2
E1
0 III T3 II T2 I T1
c
b
Added ecosytem service
a
a:
b:
c:
Ecosystem service
loss by cutting trees
from inside of a field
Ecosystem service
loss by cutting tree
lines at the edge of a field
Ecosystem service
loss by cutting trees
from corners of a field
FIGURE 11.8 Theoretical model for added ecosystem services of increasing tree density
(e.g., F. albida) on barley yield at farm level where E1, E2, and E3 refer to increasing barley
yield levels for three spatial density congurations of a tree on the corner, edge, and within
agricultural elds. (From Hadgu, K.M., L. Kooistra, W.A. Rossing, and A.H.C. van Bruggen,
Food Sec., 1, 337, 2009. With permission.)
K20897_C011.indd 257 7/6/2014 1:57:44 PM
258 Soil Management of Smallholder Agriculture
agroforestry systems, i.e., farming systems where trees and crops are grown simul-
taneously on the same plots, conrm this range. In a study on intercropping of
Faidherbia with beans and maize in the Morogoro region of Tanzania, Okorio and
Maghembe (1994) reported biomass production in trees ranging between 2.1 and 4.7
Mg ha−1 year−1 on average during 6 years, depending on tree spacing. Intercropping
of Gliricidia with maize during 10 years in Malawi produced a total of 20.5 Mg ha−1
of tree prunings for incorporation into the soil, corresponding to 2.9 Mg ha−1 year−1
(Makumba et al. 2007).
Results from a 7-year trial in the same study with slightly different tree man-
agement showed substantially lower biomass accumulation in tree prunings of only
around 1.1 Mg ha−1 year−1 (Makumba et al. 2007). Considering that destructive sam-
pling of trees at the end of the trial also revealed 17 Mg ha−1 in tree stumps and struc-
tural roots, the total biomass accumulation per year should nevertheless have been
around 2.7 Mg ha−1 year−1, on average, so that this trial also falls within the range
estimated by Montagnini and Nair (2004).
Much lower biomass accumulation rates have been reported from agroforestry
practices in western Kenya (Henry et al. 2009). In this study, aboveground biomass
was measured on 35 farms across two administrative districts. Assuming biomass
levels on all farms could be raised to reach the third quartile of the distribution of
measured biomass for the respective district, aboveground biomass accumulation
potentials were estimated for different land use types. With the exception of wind-
rows and rotational woodlots, all agroforestry systems evaluated had biomass buildup
potentials below the range specied by Montagnini and Nair (2004). Converting
gures from Henry et al. (2009) from C stocks to biomass stocks (at 45%–50% C in
biomass), individual trees in home gardens were expected to raise biomass levels by
0.3 and 0.6 Mg ha1 year−1. Individual trees in food crops had biomass accumulation
potential between 0.2 and 0.3 Mg ha−1 year−1, individual trees in cash crops between
0.1 and 0.4 Mg ha−1 year−1, and individual trees in pastures around 0.1 Mg ha−1 year−1.
Windrows had higher potential at 2.9–3.5 Mg ha−1 year−1, and biomass buildup in
rotational woodlots was substantial at 2.3–12.2 Mg ha−1 year−1. Two scenarios of
intensifying hedgerow biomass had potential of accumulating between 0.2 and
0.5 Mg ha−1 year−1 of biomass.
11.3.1.2 Improved Fallows
Several studies have estimated biomass buildup in improved fallow systems. Albrecht
and Kandji (2003) tabulated data from several studies using a range of tree species,
in which aboveground biomass stocks ranged from 7.0 to 21.0 Mg ha−1 year−1 after
12months, from 19.8 to 31.0 Mg ha−1 year−1 after 18 months, and from 27.0 to 43.4 Mg
ha−1 year−1 after 22 months. These values appear on the high side of what is realistic,
and are in contrast to modeling results by Walker et al. (2008) for biomass buildup in
the IMPALA project, in which some of the gures in the table were generated. They
estimated biomass buildup of only 10–32 Mg ha1 after 10 years, corresponding to an
average accumulation rate of only 1.0–3.2 Mg ha1 year−1 (Walker et al. 2008).
Kaonga and Coleman (2008) measured aboveground C inputs in coppiced fallows
in Zambia. These were in the range of 2.6–3.2 Mg C ha1 year−1 for multiple species
K20897_C011.indd 258 7/6/2014 1:57:44 PM
259Agroforestry in Eastern and Southern Africa
(L. leucocephala, Gliricidia, Calliandra [Calliandra calothyrsus], and S. siamea),
corresponding to around 5.2–7.1 Mg biomass ha−1 year−1. For eastern Zambia,
Kaonga and Bayliss-Smith (2009) quantied C stocks in improved fallows at 2.9–9.8
Mg ha−1, which corresponds to between 5.8 and 21.8 Mg biomass ha−1.
11.3.1.3 Rotational Woodlots and Tree Plantations
Much higher biomass accumulation rates are possible in the tree phases of rotational
woodlots. After 7 years of growth, woodlots in Tanzania had between 26.0 and 57.6
Mg biomass ha1, corresponding to mean accumulation rates of 3.7–8.2 Mg ha−1
year−1 (Nyadzi et al. 2003). The best performance was obtained from Acacia lepto-
carpa, followed by A. crassicarpa, Acacia julifera, Senna, and Leucaena pallida.
These data also formed some of the basis for assumptions by Palm et al. (2010), who
estimated that woodlots in Mbola, Tanzania, accumulated 5.3 Mg ha−1 year−1 during
a 5-year rotation.
Carbon accumulation in woodlots in Morogoro, Tanzania, ranged between 2.3
Mg ha−1 year−1 under Acacia nilotica and 5.1 Mg ha−1 year−1 under A. crassicarpa
(Kimaro 2009). These rates correspond to between 4.6 and 11.3 Mg ha−1 year−1
of biomass accumulation. Aune et al. (2005) found that the amount of C seques-
tered during a 4-year tree phase of rotational woodlots in Uganda was between
4.9 Mg ha−1 year−1 for Eucalyptus camaldulensis and 3.9 Mg ha−1 year−1 for Alnus
acuminata, corresponding to biomass increments by 7.8–10.9 Mg biomass ha−1
year−1. For rotational woodlot systems, biomass accumulation is naturally much
higher during the tree phases, during which all the above-mentioned studies were
conducted, than during the crop phases, before which essentially all tree biomass
is removed.
Even higher biomass accumulation rates than in rotational woodlots are achieved
in tree plantations. Ståhl et al. (2002) found that plantations of Sesbania, Calliandra,
eucalyptus (Eucalyptus saligna), and grevillea (Grevillea robusta) produced 31.5,
24.5, 32.5, and 43.5 Mg aboveground biomass ha−1, respectively, during 22 months
in the highlands of eastern Kenya. Palm et al. (2010) used this source to assume a
biomass accumulation rate of 12.2 Mg ha−1 year−1 for 5-year-old woodlots in Sauri,
Kenya. A plantation of Pinus patula in Tanzania was shown to build up 5.86 Mg C
ha−1 year−1, corresponding to 11.7–13.0 Mg biomass ha−1 year−1.
Thus, while aboveground biomass buildup in the natural miombo forest in this
region ranges from 0.43 to 1 Mg biomass ha−1 year−1, agroforestry species added to
smallholder cropping systems in improved fallows and parklands usually produce
1–5 Mg biomass ha−1 year−1. Rotational woodlots may add up to 8 Mg biomass ha−1
year−1 when the gures include the cropping phase. Plantations may produce twice
the biomass of rotational woodlots, but require the land to be removed from cropping
for long periods. Although building up aboveground biomass in farmed landscapes
is an important component of the global biotic C pool, the soil C pool is 4.5 times the
size of the biotic pool (Lal 2004), and thus may serve as an important sink for C, in
addition to offering advantages in soil fertility and cropping system yield described
above. Building up SOC offers important benets on both the smallholder and eco-
systems scales.
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260 Soil Management of Smallholder Agriculture
11.3.2 Soc in SmAllholder SettingS
11.3.2.1 SOC and Soil Fertility
Improved fallows, intercrops, and woodlot rotations increase the labile fractions of
SOM, which supply nutrients to crops following fallows (Barrios et al. 1997; Beedy
et al. 2010; Kimaro et al. 2011), and can also contribute to improving soil structure,
buildup of SOM, and C stocks, thus contributing to C sequestration. Buildup of SOM
is critical to soil productivity and generally corresponds to nutrient exchange capac-
ity. Release of N from SOM may contribute most of the 40 kg N ha−1 taken up by the
average maize crop of 1 Mg ha−1 (Sanchez and Palm 1996; Makumba et al. 2006).
SOC increases the cation exchange capacity (CEC) of the surface soil, which is espe-
cially important for nutrient storage in kaolinitic soils and other light-textured soils
with low CEC. Increasing SOM can reduce P xation in soils with high iron and
aluminum oxide contents, thus making the P available for plant uptake. High SOC
in fallows results in reduced rates of nutrient leaching due to reduced mineralization
rates (Nyamadzawo et al. 2009). Agboola (1994) reported that 80% of CEC, and
available P, K, Mg, and Ca were highly correlated with SOM levels in some West
African Alsols. Under fallow systems, the microbial biomass has been shown to
be higher (Nyamadzawo et al. 2009), the microbial community more diverse, and
the rate of plant material decomposition much faster than in nonfallowed systems
(Sarmiento and Bottner 2002), thus ensuring nutrient recycling and timely release of
N and other nutrients. Leguminous fertilizer trees increase SOC levels of soils and
thereby indirectly improve soil fertility. Fernandes et al. (1997) suggested that our
greatest opportunity is that SOM is a renewable resource whose level can be replen-
ished by additions of organic inputs.
11.3.2.2 Potential SOC Increases with Agroforestry
Agroforestry land use systems have been reported to sequester more C than other
forms of agriculture. The amounts of biomass and SOC additions vary with tree
species, soil type, rainfall, and environmental conditions. The extent to which
agroforestry practices will build soil C is controlled by the ability of the tree–crop
combination to produce biomass residue to be transformed into SOM. Plant bio-
mass residues may be deposited on the soil surface for decomposition, incorporated
by tillage, or added to the soil prole at varying depths by root exudates and root
decomposition. Carbon sequestration from cropping system residues varies with soil
temperature and moisture, litter quality, and root dynamics (Post and Kwon 2000).
Albrecht et al. (2003) also specify re, decomposition, leaching, and erosion as four
major avenues of SOC loss. Burning of crop residues and tillage oxidize soil C.
Erosion transports soil C offsite, sometimes into rivers and lakes. Leaching trans-
ports soil C downward in the soil prole and sometimes removes it into streams and
rivers via runoff from elds.
The ability of a given soil to retain organic matter varies strongly with soil tex-
ture (Albrecht et al. 2003). SOC oxidizes more quickly in sandy soils and those
with weak aggregation. Organic matter that is not adsorbed to clay colloids or pro-
tected within soil aggregates is quickly decomposed by soil microorganisms (Six
et al. 2000). In ne-textured soils, SOC is adsorbed to clay colloids, protecting it
Please provide
the full bi blio-
graphic details for
Albrecht et al.
(2003). This was
cited four times
in text.
K20897_C011.indd 260 7/6/2014 1:57:45 PM
261Agroforestry in Eastern and Southern Africa
from decomposition. After an improved fallow of 1–1.5 years with various species in
Kenya, Albrecht et al. (2003) found an increase in SOC in the top 30 cm by 1.69–2.15
Mg ha1 C in coarse soils, and by 2.58–8.34 Mg ha−1 C in ne-textured soils. Because
agroforestry increases biomass addition to SOC, it increases soil aggregation, while
woody cover and leaf litter from agroforestry protect topsoils from erosion, increas-
ing the SOC that remains in the soil prole.
11.3.2.3 Intercrops, Improved Fallows, and Woodlots
Smallholders in eastern and southern Africa manage agroforestry species as inter-
crops with food crops such as maize or as improved fallows to replenish soil fertil-
ity and/or provide timber and fuelwood (Akinnifesi et al. 2010). Three of the most
widely used management patterns for agroforestry in eastern and southern Africa are
intercrops, improved fallows, and rotational woodlots.
Intercropping systems are most appropriate for smallholders in relatively popula-
tion-dense areas with small landholdings, as no land has to be removed from cereal
production to include the agroforestry species. Intercrop populations will vary across
different cropping systems and ecological conditions. Gliricidia, for example, was
planted in alternating planting ridges, at 0.9 m distance between trees within a ridge,
and 0.75 m distance between ridges in Malawi, with maize planted in the ridges at
44,400 plants ha−1 (Makumba et al. 2007). These Gliricidia populations may also
be used in minimum tillage systems. Makumba et al. (2007) reported 123 and 149
Mg ha−1 of soil C (Table 11.4) in 10 and 7-year intercrops in Malawi (Makumba et
al. 2007). Soil C in the intercrop was roughly double that in unfertilized sole maize,
at 64 and 73 Mg ha−1 after 7 and 10 years, respectively. The 7-year study also accu-
mulated 1.2 times as much soil C under the maize–Gliricidia intercrop as under an
adjoining grass fallow. Unfertilized maize in this study represents the loss of soil C
when such soils are cropped continuously to maize with no inputs.
A 10-year intercrop in Zambia with three agroforestry intercrop species ranged
from 225 to 245 Mg ha−1 of soil C (Kaonga and Bayliss Smith 2009), similar to the
245 Mg ha−1 found in natural fallow, but 1.3–1.6 times as much soil C as found in
fertilized maize and miombo treatments. These differences may be, in part, due to
a decline in soil C in the fertilized maize treatment. Thus, including agroforestry
intercrops in a cropping system can maintain and increase soil C compared with
the natural miombo vegetation and to cropping systems with only mineral fertil-
izer added. In another study from the same location, Kaonga and Coleman (2008)
reported an increase from an initial soil C of 26.2 Mg ha−1 up to 37.4 Mg ha−1 during
10 years in the upper 20 cm of the soils.
Improved fallow agroforestry is appropriate for smallholders in low-population-
density areas, where landholdings are larger and land can be spared for improved
fallows. The trees are established and left for 1.5–2 years to develop leaf biomass that
is incorporated into the soil before the crop planting phase. Kaonga and Coleman
(2008) reported that Tephrosia, pigeon pea, and Sesbania were established at 1 m by
1 m spacing, and maintained for 2 years. During land preparation for the third year,
the trees were cut to 10 cm, the stems and branches harvested for fuelwood, and the
leaves and twigs incorporated into the soil. Maize then replaced the trees in a 2-year
cropping phase.
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262 Soil Management of Smallholder Agriculture
Between the rst and second years of the cropping phase in Kaonga and Coleman
(2008), soils were sampled to 20 cm and analyzed for total SOC (Table 11.4). The
total soil C after 1 year of cropping was 1.0 and 1.2 times that of the initial soil C.
SOC to 200 cm depth (Kaonga and Bayliss-Smith 2009) was comparable to the
150Mg ha−1 C found in nearby natural miombo vegetation and more than nine-
tenth of the 165 Mg ha−1 C found in an adjacent natural fallow. Nyamadzawo et al.
(2008) reported that fallows of A. angustissima and S. sesban accumulated 26.3 and
25.4 Mg ha−1 in SOC (Table 11.4) after 2 years of fallowing, and fallowing resulted
in 3.7–9.1 Mg ha−1 more SOC compared with continuous maize cropping. After an
improved fallow of 1–1.5 years with various species in Kenya, Albrecht et al. (2003)
found an increase in SOC in the top 30 cm of 1.69–2.15 Mg ha−1 C in coarse soils,
and from 2.58 to 8.34 Mg ha−1 C in ne-textured soils.
Rotational woodlots differ from improved fallows in that the tree species may be
selected for wood production rather than N xation. However, rotational woodlots
also usually increase SOC and promote increased yields in subsequent cereal crops.
Tree and crop components are established at the same time, and the intercropping
continues for 2–3 years with crop yield declining because of competition from the
Please indicate
Nyamadzawo
et al. (2008) as
either 20 08a or
2008b.
TABLE 11.4
Development of Soil Organic Carbon with Agroforestry Practices
Woody Species
Depth
(cm)
SOCa
(Mg ha−1)
Period
(Years) Country Reference
Intercrop
G. sepium 200 123 b 10 Malawi Makumba et al. 2007
G. sepium 200 149 10 Malawi Makumba et al. 2007
G. sepium 200 225 10 Zambia Kaonga et al. 2009
L. leucocephala 200 245 10 Zambia Kaonga et al. 2009
S. siamea 200 245 10 Zambia Kaonga et al. 2009
Various 20 30.5–37.4 10 Zambia Kaonga and Coleman 2008
Improved Fallow
C. cajan 200 149 4 Zambia Kaonga et al. 2009
S. sesban 200 150 4 Zambia Kaonga et al. 2009
T. vogelii 200 155 4 Zambia Kaonga et al. 2009
Various 20 27.3–31.2 4 Zambia Kaonga and Coleman 2008
A. angustissima 120 26.3 2 Zimbabwe Nyamadzawo et al. 2008
S. sesban 120 25.4 2 Zimbabwe Nyamadzawo et al. 2008
Rotational Woodlot
A. polyacantha 15 21.6 5 Tanzania Kimaro 2011
G. sepium 15 18.8 5 Tanzania Kimaro 2011
A. crassicarpa 15 15.8 5 Tanzania Kimaro 2011
A. mangium 15 25.6 5 Tanzania Kimaro 2011
A. nilotica 15 22.7 5 Tanzania Kimaro 2011
a SOC, soil organic carbon.
b Comparative values from these studies are found in the text.
Please provide
the full bib-
liographic details
for Kaonga et
al. 2009, c ited
several times in
table.
Please indicate
Nyamadzawo
et al. (2008) as
either 20 08a or
2008b
Please indicate
Nyamadzawo
et al. (2008) as
either 20 08a or
2008b
Please provide
the full bi blio-
graphic details for
Kimaro 2011.
K20897_C011.indd 262 7/6/2014 1:57:45 PM
263Agroforestry in Eastern and Southern Africa
trees. This is followed by a 2–3-year tree fallow period during which little or no
management is required to maintain the trees. After this, the woodlot is cleared to
supply wood for household use, such as building poles, rewood, and tobacco curing.
Subsequently, crops are grown between tree stumps to benet from the ameliorated
soil conditions (Kimaro 2009).
In Kimaro et al. (2011), two Australian acacias (A. crassicarpa, A. mangium), two
African acacias (Acacia nilotica, A. polyacantha), and one fertilizer tree (Gliricidia)
were compared at Morogoro, Tanzania. After 5 years, all of the tree fallows devel-
oped more total SOC than the 13.0 Mg ha−1 measured under continuous maize, and
A. mangium had double the SOC of the continuous maize, probably because of
declining C in the control. The natural grass fallow comparison had 17.8 Mg ha−1
SOC, and the SOC under the tree fallows ranged from 0.89 to 1.4 times the SOC
compared with the grass fallow.
11.3.2.4 Connection with Carbon Markets
The studies cited in Table 11.4 fall broadly within the area of southern and eastern
Africa dominated by open, dry miombo woodlands, which occupy about 10% of
the African land mass (Malmer and Nyberg 2008). According to Williams et al.
(2008), soil C stocks in a Mozambican site had a narrower range (21–74 Mg C ha−1)
in the top 0.3 m on abandoned land than in the miombo woodland soils (18–140 Mg
C ha−1). A growing proportion of miombo woodlands have been cut for fuelwood
and converted to smallholder agriculture, which in very few years of extractive
cropping reduces SOM and nutrients such that maize production becomes unsus-
tainable. Each of the agroforestry interventions described increases SOC, allowing
for production of food crops and fuelwood with less soil degradation. These tech-
nologies allow the restoration of some of the ecosystem services formerly provided
by the miombo forest, while increasing the provisioning services to provide for the
increasing human population resident on the land. Although these practices hold
great promise, their use in smallholder settings to generate C credits comes with
several constraints. SOM is generally lower and more variable on smallholder land
than on larger landholdings or at research sites, decreasing payments and increas-
ing monitoring costs among smallholder households. The residence time of SOC is
controversial (Davidson and Janssens 2006; Lal 2004), and studies are especially
needed in the area of belowground C cycling and GHG evolution within different
cropping systems.
Two ongoing projects operating in Kenya show the difculties in establishing a
price at the farm level. The TIST program (TIST 2011) is paying 1 shilling per sur-
viving tree per year (similar to about $12 per year per hectare) but hopes to be able
to have a larger payment in later years. The Vi Agroforestry soil C program plans
to pay $11 per Mg of C sequestered, which it estimates may be about 2.25 Mg ha−1
during a 20-year period (which, therefore, is just over $1 per hectare per year) (Vi
Agroforestry Strategy 2013–2015). The actual price will, however, depend on the
actual C sequestered. Given these limitations, the major impetus for the promotion
and adoption of agroforestry practices remains the potential to increase food and
fuelwood productivity for smallholder households, and reduce land, watershed, and
ecosystem degradation.
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264 Soil Management of Smallholder Agriculture
11.3.3 ghg emiSSionS
Decomposition of SOM in cropping systems releases nutrients for crop production,
but also returns carbon dioxide (CO2) to the atmosphere. At the same time, some
of the C from decomposition of organic matter is retained in soil aggregates and
adsorbed to soil colloids, some of which will later be eroded (Lal 2004). Addition of
agroforestry species has the potential to either enhance or reduce soil C storage (Kim
2012) and GHG emissions. Thus, the study of GHG emissions is critical to describ-
ing the trade-offs between smallholder and ecosystem benets from agroforestry.
11.3.3.1 Soil Carbon Dioxide, Methane, and Nitrous Oxide Emissions
Carbon dioxide (CO2) is the dominant pathway of C loss in most terrestrial ecosys-
tems, as well as the most important GHG in the atmosphere (Forster et al. 2007). Soil
CO2 is produced primarily by both heterotrophic (i.e., decomposer organisms) and
autotrophic activity (i.e., living roots and mycorrhizae) (Raich and Schlesinger 1992;
Schlesinger and Andrews 2000). Soil CO2 efux amounts to 75–80 Pg of CO2 per
year globally (Raich and Potter 1995; Raich et al. 2002) and made up 20%–40% of the
total annual input of CO2 into the atmosphere in the 1990s (Raich and Schlesinger
1992; Schimel 1995). Soil temperature, soil moisture, soil C content, litter quality,
root dynamics, and plant photosynthesis or growth are known control factors for soil
CO2 ux (e.g., Raich and Schlesinger 1992; Rustad and Fernandez 1998; Davidson et
al. 2000; Vargas and Allen 2008).
Methane (CH4) has the second-largest radiative forcing of the long-lived GHGs
after CO2 (Forster et al. 2007). The net CH4 ux is the result of the balance between
the two offsetting processes of methanogenesis (microbial production under anaero-
bic conditions) and methanotrophy (microbial consumption) (Dutaur and Verchot
2007). Methanogenesis occurs via the anaerobic degradation of organic matter by
methanogenic archaea within the archaeal phylum Euryarchaeota (Thauer 1988).
Methanotrophy occurs through methanotrophs metabolizing CH4 as their source of
C and energy (Hanson and Hanson 1996). In anoxic soils, emergent vegetation also
inuences CH4 ux to the atmosphere, as plants enable oxygen transport to the rhi-
zosphere, through aerenchymateous tissue, and through the production of labile sub-
strates via root exudation (Joabsson et al. 1999). In general, CH4 production rates are
controlled by the availability of suitable substrates, alternative electron acceptors for
competing redox reactions (i.e., sulfate reduction), the nutritional status of the eco-
system (i.e., bog vs. fen), water table position or soil moisture content, temperature,
and soil salinity (Hanson and Hanson 1996; Dutaur and Verchot 2007).
Atmospheric nitrous oxide (N2O) contributes to both the greenhouse effect (Wang
et al. 1976) and ozone layer depletion (Crutzen 1970). Nitrous oxide has a relatively
high global warming potential (i.e., 298 times greater than CO2 in a 100-year time
horizon; Intergovernmental Panel on Climate Change [IPCC] 2006; Forster et al.
2007) and agricultural soils provide 3.5 Tg N2O-N year−1 of total anthropogenic
N2O emissions (5.7 Tg N2O-N year−1) (IPCC 2006). Use of N fertilizers and ani-
mal manure are the main anthropogenic N2O sources, which together account for
roughly 24% of total annual emissions (Bouwman 1996; Forster et al. 2007). The
main processes that produce N2O in soils are nitrication, the stepwise oxidation of
K20897_C011.indd 264 7/6/2014 1:57:45 PM
265Agroforestry in Eastern and Southern Africa
NH3 to nitrite (NO2
) and then to nitrate (NO3
) (Kowalchuk and Stephen 2001), and
denitrication, the stepwise reduction of NO3
to NO2
, NO, N2O, and ultimately N2.
In denitrication, facultative anaerobic bacteria use NO3
as an electron acceptor in
the respiration of organic material under low oxygen (O2) conditions (Knowles 1982).
In nitrier denitrication, which is carried out by autotrophic NH3-oxidizing bacte-
ria, NH3 is oxidized to nitrite NO2
, followed by the reduction of NO2
to nitric oxide
NO, N2O, and molecular nitrogen (N2) (Wrage et al. 2001).
11.3.3.2 Emissions of GHGs in Agroforestry in Eastern and Southern Africa
Nitrogen-xing tree and crop intercropping systems can be a sustainable agrofor-
estry practice in eastern and southern Africa (Makumba et al. 2006; Akinnifesi et al.
2010), and they can also contribute to mitigation of climate change through enhanced
soil C sequestration. Makumba et al. (2007) reported soil C and soil CO2 emissions
in a 7-year-old Gliricidia and maize intercropping system, and a sole maize cropping
site in southern Malawi. They estimated that while soil C in the intercropping system
was about double that in the sole maize cropping site, soil CO2 emissions from the
intercropping system were up to three times higher. The increased soil CO2 emis-
sions in the intercropping system could be due to increased SOM and enhanced root
respiration in extended root systems from the intercropping system (Makumba et al.
2007). Using the data provided in Makumba et al. (2007), a C loss as soil CO2 emis-
sions (51.2 ± 0.4 Mg C ha−1) was estimated, amounting to 67.4% of the sequestered
soil C (76 ± 8.6 Mg C ha−1 in 0–2 m soil depth) for the rst 7 years in the intercrop-
ping system (Kim 2012). These results suggest the need to account for the C loss as
soil CO2 emissions in assessing the overall impact of the agroforestry system on soil
C dynamics.
Maize yields in the Gliricidia and maize intercropping systems without addi-
tional synthetic N fertilizer input were similar to the yields in the sole maize crop-
ping with 48 kg N ha−1 year−1 fertilizer applied in southern Malawi (Makumba et
al. 2006). These results support the premise that additional N is provided to the
crop through N xation by Gliricidia (e.g., Makumba et al. 2006; Akinnifesi et al.
2010). These results also suggest that up to 48 kg N ha−1 year−1 of fertilizer could be
reduced in the intercropping system. Globally, 1% of applied N fertilizer converts to
N2O emission (IPCC 2006), and it was observed that 0.25%–4.1% of applied N fer-
tilizer converts to N2O emission in sub-Saharan Africa (Kim et al. 2012). Therefore,
the reduced N fertilizer use through the intercropping system may result in reduced
N2O emissions. In contrast, N2O emissions in the intercropping system may not be
lower than in the conventional cropping system where N fertilizer is applied. Soil
collected under N-xing tree species produced signicantly more N2O than soil
collected under non-N-xing trees and N-xing crop species in Senegal (Dick et
al. 2006). Nitrous oxide emissions from a maize eld that previously had a 2-year
fallow of A. angustissima and Sesbania were signicantly higher than those from
an unfertilized maize eld in Zimbabwe (Chikowo 2004). Increased soil organic C
and N in the intercropping system can enhance the denitrication process, one of
the major processes that produce N2O gas in soil (e.g., Knowles 1982). It is there-
fore important to consider N2O emissions to better understand the contributions of
agroforestry to N2O dynamics.
K20897_C011.indd 265 7/6/2014 1:57:45 PM
266 Soil Management of Smallholder Agriculture
Soils have been shown to both produce and consume CH4 (Topp and Pattey 1997;
Le Mer and Roger 2001). It is well known that forest soils are the most active sink
for CH4, followed by grass lands and cultivated soils, and that the CH4 uptake poten-
tial of many upland soils is reduced by cultivation and application of ammonium-N
fertilizer (e.g., Topp and Pattey 1997; Le Mer and Roger 2001; Dutaur and Verchot
2007). These results suggest that synthetic N fertilizers used in conventional crop-
ping systems may increase CH4 emissions in sub-Saharan Africa. By contrast, the
intercropping system uses less or no synthetic N fertilizer and may have the potential
to mitigate CH4 emissions. Overall, GHG emissions from the agroforestry systems
either reduce benets gained from enhanced soil C sequestration or add new benets
from reduced N2O and CH4 emissions. However, there is little data on GHG emis-
sions from agroforestry systems in eastern and southern Africa.
11.3.3.3 Suggested Future Studies
GHG emission in agroforestry has not been well understood, although it is recog-
nized that agroforestry can be a source of GHG emissions or mitigate GHG emis-
sions. First, studies quantifying the source and the mitigation capacity of GHG in
various agroforestry systems in eastern and southern Africa are urgently needed.
Especially, careful comparison of GHG emissions in agroforestry with monocrop-
ping will provide a better understanding of the contribution of agroforestry to miti-
gating GHG emissions. It is worth noting that eld measurements of GHG emissions
in eastern and southern Africa should accurately observe peak GHG emissions fol-
lowing rewetting of dry soils (e.g., start/onset of the rainy season), since several
reports indicate that peak GHG emissions occur following soil rewetting in the areas
(e.g., Makumba et al. 2007; Dick et al. 2006), and these peak emissions may sig-
nicantly affect annual GHG budgets as has been shown in other areas (e.g., Lee et
al. 2004; Goldberg et al. 2010; Kim et al. 2012). These peaks could be measured by
using an automated measurement system (e.g., Wolf et al. 2010; Kim et al. 2010a) or
by increasing the frequency of manual chamber measurements during these periods
(e.g., Beare et al. 2009; Kim et al. 2010b). An area of signicant promise involves
combining microbial community analyses and/or stable isotope techniques with ux
measurements. Models are promising tools for evaluating the importance of GHG
emissions in agroforestry systems. Initially, simple linear regressions and empirical
models can be developed on the basis of the relationships between environmental
factors, including soil moisture and/or soil temperature and soil GHG uxes. With
improved understanding of C and N biogeochemistry and hydrological dynamics in
agroforestry systems, process-based models can be developed to more accurately
simulate GHG ux. It is critical to enhance the communication between eld scien-
tists and the modeling community, as models can be used to generate hypotheses to
be tested in the eld and laboratory (Kim et al. 2012).
11.4 CONCLUSIONS, CHALLENGES, AND FUTURE NEEDS
Including agroforestry species in smallholder cropping systems has well-documented
benets in reduced land degradation and increased food production. Agroforestry
also has the potential to increase carbon storage in soils and aboveground wood
K20897_C011.indd 266 7/6/2014 1:57:45 PM
267Agroforestry in Eastern and Southern Africa
biomass. However, the effects of smallholder soil/agroforestry management deci-
sions on carbon-related agroecosystem processes are not well documented and need
further study.
The continuing use of agroforestry practices that are biotically compatible with
farming systems depends not only on benets to food security and the environment
but also on compatibility with farming practices already in place, positive policy
context, awareness among farmers of the management practices and potential ben-
ets, and constraints to seed supply, land, and labor.
Use of carbon credits to recapitalize soil fertility is developing very slowly in
eastern and southern Africa. Further research is needed, especially in the area of soil
carbon cycling and the effects on carbon cycling of different management decisions.
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Estimation of aboveground tree biomass and carbon in mixed maize/tree parklands by nondestructive means requires the development of allometric equations from readily measurable variables such as diameter at breast height and tree height. Equations of this type have not been well developed for Faidherbia albida in eastern and southern Africa. In this study, F. albida trees were characterized in block plantings and in naturally regenerating parklands at six sites in Malawi. Allometric equations were developed for block planted and parkland management regimes. Forty-five intact trees with diameters ranging from 5 to 38 cm were sampled in the block planting while in parklands thirty-eight trees with diameters ranging from 5 to 116 cm were sampled. Destructive sampling was used to measure volumes and collect wood samples. Diameter at breast height, tree height and crown areas were used as predictors for dry weight of the above-ground biomass. Comparing the estimated equations to previously published data shows that these local species-specific equations differ slightly and that both can be used in the estimation of biomass in F. albida trees. Individual trees in parklands stored more biomass and carbon while block-planted trees stored more biomass per hectare. In parklands, F. albida crown area cover per hectare was 17.8 %, but could feasibly be increased under natural regeneration to as much as 23.1 %.
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