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Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review

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Smallholder farmers struggle to achieve food security in many countries of sub-Saharan Africa (SSA). It is urgently required to find appropriate practices for enhancing crop production while avoiding large increases in greenhouse gas (GHG) emissions in SSA. This review aims to identify common smallholder farming practices for enhancing crop production, to assess how these affect GHG emissions and to identify strategies that not only enhance crop production but also mitigate GHG emissions in SSA. To increase crop production and ensure food security, smallholder farmers usually expand agricultural land, develop water harvesting and irrigation techniques and increase cropping intensity and fertilizer use. These practices may result in changing carbon stocks and GHG emissions, potentially creating trade-offs between food security and GHG mitigation. Agricultural land expansion at the expense of forests is the most dominant source of GHG emissions in SSA. While water harvesting and irrigation can increase soil organic carbon, they can trigger GHG emissions. Increasing cropping intensity can enhance the decomposition of soil organic matter, thus releasing carbon dioxide. Increasing nitrogen fertilizer use can enhance soil organic carbon, but also leads to increasing nitrous oxide emissions. An integrated land, water and nutrient management strategy is necessary to enhance crop production and mitigate GHG emissions. Among the most relevant strategies found, agroforesty practices in degraded and marginal lands could replace expanding agricultural croplands. In addition, water management, via adequate rainwater harvesting and irrigation techniques, together with appropriate nutrient management should be considered. Therefore, a land-water-nutrient nexus (LWNN) approach will enable an integrated and sustainable solution to increasing crop production and mitigating GHG emissions. Various technical, economic and policy barriers hinder implementing the LWNN approach on the ground, but these may be overcome through developing appropriate technologies, disseminating them through farmer to farmer approaches and developing specific policies to address smallholder land tenure issues and motivate long-term investment.
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REVIEW
Challenges and opportunities for enhancing food security
and greenhouse gas mitigation in smallholder farming
in sub-Saharan Africa. A review
Dong-Gill Kim
1
&Elisa Grieco
2,3
&Antonio Bombelli
2,4
&Jonathan E. Hickman
5
&Alberto Sanz-Cobena
6,7
Received: 17 June 2020 / Accepted: 1 February 2021
#International Society for Plant Pathology and Springer Nature B.V. 2021
Abstract
Smallholder farmers struggle to achieve food security in many countries of sub-Saharan Africa (SSA). It is urgently required to
find appropriate practices for enhancing crop production while avoiding large increases in greenhouse gas (GHG) emissions in
SSA. This review aims to identify common smallholder farming practices for enhancing crop production, to assess how these
affect GHG emissions and to identify strategies that not only enhance crop production but also mitigate GHG emissions in SSA.
To increase crop production and ensure food security, smallholder farmers usually expand agricultural land, develop water
harvesting and irrigation techniques and increase cropping intensity and fertilizer use. These practices may result in changing
carbon stocks and GHG emissions, potentially creating trade-offs between food security and GHG mitigation. Agricultural land
expansion atthe expense of forests is the most dominant source of GHG emissions in SSA. While water harvesting and irrigation
can increase soil organic carbon, they can trigger GHG emissions. Increasing cropping intensity can enhance the decomposition
of soil organic matter, thus releasing carbon dioxide. Increasing nitrogen fertilizer use can enhance soil organic carbon, but also
leads to increasing nitrous oxide emissions. An integrated land, water and nutrient management strategy is necessary to enhance
crop production and mitigate GHG emissions. Among the most relevant strategies found, agroforesty practices in degraded and
marginal lands could replace expanding agricultural croplands. In addition, water management, via adequate rainwater harvesting
and irrigation techniques, together with appropriate nutrient management should be considered. Therefore, a land-water-nutrient
nexus (LWNN) approach will enable an integrated and sustainable solution to increasing crop production and mitigating GHG
emissions. Various technical, economic and policy barriers hinder implementing the LWNN approach on the ground, but these
may be overcome through developing appropriate technologies, disseminating them through farmer to farmer approaches and
developing specific policies to address smallholder land tenure issues and motivate long-term investment.
Keywords Sub-Saharan Africa .Smallholder farming systems .Crop production .Greenhouse gas emission .Agricultural land,
water harvesting, irrigation, cropping intensity,fertilizer
*Dong-Gill Kim
donggillkim@gmail.com
1
Wondo Genet College of Forestry and Natural Resources, Hawassa
University, PO. Box 128, Shashemene, Ethiopia
2
Impacts on Agriculture, Forests and Ecosystem Services Division
(IAFES), Foundation Euro-Mediterranean Center on Climate Change
(CMCC), Viterbo, Italy
3
Institute of Bio-Economy (IBE), National Research Council of Italy
(CNR), Via dei Taurini 19, 00185 Rome, Italy
4
Italian National Agency for New Technologies, Energy and
Sustainable Economic Development (ENEA), Rome, Italy
5
NASA Goddard Institute for Space Studies, New York, NY 10025,
USA
6
Escuela Técnica Superior de Ingeniería Agronómica, Alimentaria y
de Biosistemas, Universidad Politécnica de Madrid, Ciudad
Universitaria, 28040 Madrid, Spain
7
Research Center for the Management of Environmental and
Agricultural Risks (CEIGRAM), Universidad Politécnica de Madrid,
28040 Madrid, Spain
Food Security
https://doi.org/10.1007/s12571-021-01149-9
1 Introduction
Agriculture in sub-Saharan Africa (SSA) plays an important
role in livelihood and economic growth through employing
51.6% of the population and generating 20.5% of the gross
domestic product (GDP) of these countries (in 2016) (The
Global Economy 2019). Agricultural production systems in
SSA are largely based on smallholder farming systems, which
are defined by farms covering an area of 2 ha (Lowder et al.
2016;Fig.1). Recent estimates suggest the presence of ap-
proximately 33 million smallholder farms in SSA (IFC 2013),
which contribute up to 90% of the agricultural production in
some SSA countries (Wiggins 2009).
Currently, consumption of self-produced food crops only
covers 20% of the food need of SSA households (Frelat et al.
2016). Thus, food security remains difficult to achieve among
smallholder farmers and they face a large number of chal-
lenges (van Ittersum et al. 2016;Tilmanetal.2011). First,
the agricultural sector is underdeveloped and is characterized
by over-reliance on primary agriculture, minimal use of exter-
nal farm inputs, significant pre- and post-harvest food crop
loss and minimal value addition and product differentiation
(Assefa et al. 2020; van Ittersum et al. 2016; Tilman et al.
2011). All lead to low crop productivity (Singh et al. 2020;
Assefa et al. 2020;Frelatetal.2016; Fig. 2). Second, water
availability is highly affected by droughts in the context of
regional and global climate variability and change (Misra
2014). Third, severe degradation of agricultural soils negative-
ly affects crop yield (Tittonell and Giller 2013). Fourth, SSAs
population is predicted to grow from 1.02 billion in 2017 to
1.4 billion by 2030 and to 2.17 billion by 2050 (United
Nations Population Division 2017). Given population expan-
sion, food demand in SSA will substantially increase; while
cereal demands will most likely triple, current levels of cereal
consumption already depend on substantial imports (van
Ittersum et al. 2016).
In addition, growing concern exists that ongoing practices
for increasing crop yield in SSA may cause increasing green-
house gas (GHG) emissions and further contribute to global
climate change (Leitner et al. 2020; van Loon et al. 2019;
FAO 2018; Tongwane and Moeletsi 2018). Agricultural land
expansion at the expense of forests is expected to continue
(Herteletal.2016; Lambin and Meyfroidt 2011).
Deforestation for agricultural land expansion is a substantial
source of GHG emissions (Grewer et al. 2018; Wanyama et al.
2018; Kim and Kirschbaum 2015) and agricultural intensifi-
cation tends to increase GHG emissions (Grewer et al. 2018;
Kim et al. 2013). These increases can be particularly relevant
Fig. 1 Agricultural production
systems in sub-Saharan Africa are
largely based on smallholder
farming systems. Typical exam-
ple of smallholder farms with
small crop fields located nearby
homesteads in western Ethiopia
(photo courtesy: Dong-Gill Kim)
Year
1960 1970 1980 1990 2000 2010
Crop productivity
(kg N ha
-1
yr
-1
)
0
50
100
150
Crop productivity (1961-2009)
Africa
North America
China
Europe
Fig. 2 Changes of crop productivity in Africa, North America, Europe
and China in 1961 to 2009 (Data source: FAO STAT). The crop
productivity in Africa is very low compared to other regions
Kim D.-G. et al.
when inappropriate agricultural practices, such as severe soil
disturbance or excessive nitrogen (N) fertilizer use, are
adopted (Grewer et al. 2018; Kim et al. 2013). Although emis-
sions of the GHG nitrous oxide (N
2
O), per unit area, may be
low due to the small amount of N fertilizer applied by most
African smallholders (Kim et al. 2016c), N
2
O emissions per
unit of agricultural production (e.g., yield-scaled N
2
O emis-
sions; Kim and Giltrap 2017; Sainju 2016) may be high due to
low productivity (Pelster et al. 2017; Seebauer 2014;Kimaro
et al. 2016). Overall, agricultural GHG emissions in SSA in-
creased by 1.24.7% annually between 1994 and 2014
(Tongwane and Moeletsi 2018), while global agricultural
GHG emissions increased by 1.1% annually between 2000
and 2010 (Tubiello et al. 2013). To sustainably improve agri-
cultural production in SSA, efforts are needed to identify and
implement measures, which can enhance crop yields while
avoiding large increases in GHG emissions (Leitner et al.
2020; van Loon et al. 2019;FAO2018; Tongwane and
Moeletsi 2018).
To enhance crop yields, smallholder farmers in SSA gen-
erally adopt a single approach rather than an integration of
multiple approaches (Thierfelder et al. 2017; Sheahan and
Barrett 2017). However, to enhance crop yield and GHG mit-
igation simultaneously in smallholder crop farming, it is nec-
essary to comprehensively consider different approaches
(Sheahan and Barrett 2017; Zougmoré et al. 2014; Branca
et al. 2013), since adopting a single approach cannot properly
manage the complexity of crop production and GHG mitiga-
tion challenges. The adoption of different approaches can cre-
ate positive synergetic effects beyond the additive effect of
each approach (Sanz-Cobena et al. 2017; Zougmoré et al.
2014;Brancaetal.2013). Even so, due to the lack of on-site
data, further efforts including research and field demonstra-
tions identifying optimal combinations of different ap-
proaches are urgently needed (Sheahan and Barrett 2017;
Thierfelder et al. 2017;Zougmoréetal.2014; Branca et al.
2013).
This review aims 1) to identify the current status and future
potentials of smallholder farming practices for enhancing crop
production, 2) to assess how these practices can affect GHG
emissions, 3) to identify management practices that can both
enhance crop yield and mitigate GHG emissions and 4)
to assess the main barriers to their implementation and
propose potential solutions in smallholder crop farming
systems in SSA.
2 Common practices for increasing crop
production of smallholder farms in SSA
Smallholder farmers adopt various practices to increase crop
production in SSA. For this review, we selected the most
adopted practices by smallholder farmers throughout SSA,
of which the magnitudes of adoption were also relatively well
quantified: 1) land management, exemplified by the expan-
sion of agricultural lands and the increase of cropping inten-
sity; 2) water management, exemplified by the development
of water harvesting and irrigation techniques; and 3) nutrition
management, exemplified by the increase of fertilizer use.
Current status and future potential of these practices are
discussed below.
2.1 Expansion of agricultural lands and increase of
cropping intensity
Expanding agricultural lands is one of the most common land
management practices to increase crop production in small-
holder crop farming in SSA (Nakawuka et al. 2018;
Droppelmann et al. 2017; Heady 2015). Agricultural lands
in SSA have increased from 86.9 × 10
7
in 1993 to 92.0 ×
10
7
ha in 2009 with an average increase rate of 3.2 × 10
6
ha
per year (FAOSTAT 2019). Mainly natural lands, such as
forests, savannahs and wetlands, have been converted to agri-
cultural lands (Gibbs et al. 2010;BrinkandEva2009;DeFries
et al. 2010). In SSA, natural forest decreased from 65.4 × 10
7
in 1993 to 59.9 × 10
7
ha in 2009 with an average deforestation
rate of 3.4 × 10
6
ha per year (FAOSTAT 2019). While overall
agricultural lands have increased in SSA, in most of the land-
constrained countries, such as Ethiopia, Kenya and Malawi,
the farm size of most smallholder farms has been gradually
shrinking. Average farm sizes have been reduced by 3040%
since the 1970s, mainly due to rapidly increasing populations
(Jayne et al. 2014; Headey and Jayne 2014). Expansion of
agricultural lands will likely continue in SSA to meet growing
food demand (Molotoks et al. 2018;Herteletal.2016; OECD/
Food and Agriculture Organization of the United Nations
2015). Alexandratos and Bruinsma (2012) projected that the
area used for crop production in Africa will increase to 266×
10
6
ha in 2030 and 291 × 10
6
ha in 2050. Previous studies
have shown substantial potential to expand agricultural land
in wet savannahs, shrublands and sparse woodlands in SSA
(Chamberlin et al. 2014; Alexandratos and Bruinsma 2012;
Deininger et al. 2011). However, it was found that many coun-
tries in SSA have limited potential for agricultural landexpan-
sion while avoiding deforestation (Jayne et al. 2014;
Chamberlin et al. 2014; Deininger et al. 2011). Except for a
few countries, such as the Democratic Republic of Congo and
Angola, most countries in SSA have less than 6% (0.4 to
5.9%) of non-forested unutilized land available (Jayne et al.
2014). Chamberlin et al. (2014) estimated that potentially ex-
pandable cropland for smallholder farms is only 80 × 10
6
ha in
SSA if forest conversion is to be avoided.
Intensification has been adopted to enhance crop produc-
tion in SSA (van Ittersum et al. 2016; Headey and Jayne 2014;
Mueller et al. 2012), most notably by increasing cropping
intensitythe number of crops grown per a year on the same
Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review
field (Headey and Jayne 2014). As population pressures cause
a gradual shrinking of farm sizes over time (Jayne et al. 2014;
Headey and Jayne 2014), smallholder farmers have been prac-
ticing cultivating their fields continuously, shortening fallow
periods between individual cropping periods and changing the
traditional crop types to high-value mono-species cash crops
(Kim et al. 2016b; Jayne et al. 2014; Headey and Jayne 2014).
Cropping intensity in SSA increased 10.6% and 25.4% in low
and high population density countries, respectively, in the
period 19772007 (Headey and Jayne 2014).
2.2 Development of rainwater harvesting and
irrigation
Since more than 90% of cultivated land in SSA is rainfed, crop
production in arid, semi-arid and sub-humid areas in SSA is at
risk from highly variable rainfall, frequent droughts and low
water productivity (Karpouzoglou and Barron 2014; Misra
2014). Rainwater harvesting technologies such as pitting,
contouring, terracing, open ponds, and cisterns have been used
to enhance crop production in certain regions of SSA (Leal
Filho and de Trincheria Gomez 2018; Karpouzoglou and
Barron 2014;Dileetal.2013;Biazinetal.2012). These tech-
nologies have been advanced as essential to achieving water
availability and crop production in these areas (Taffere et al.
2016; Rockström et al. 2010). Indigenous rainwater harvesting
techniques (e.g. spate irrigation) or those modified from tradi-
tional techniques are more common and widely accepted by
smallholder farmers compared to introduced ones (Biazin et al.
2012; Mbilinyi et al. 2005). Studies on the economic costs and
benefits of rainwater harvesting found significant profits in
Ethiopia (Hagos et al. 2012), Tanzania (Senkondo et al.
2004), Kenya (Ngigi et al. 2005) and Burkina Faso (Fox
et al. 2005). Due to substantial rain and currently underexploit-
ed surface and ground water resources, great potential exists for
expanding rainwater harvesting in SSA (Altchenko and
Villholth 2015; Cassman and Grassini 2013; Pavelic et al.
2013). In Ethiopia, Kenya, Uganda and Tanzania, rainwater
harvesting potential was estimated at over 10,000 to
25,000 m
3
rainwater person
1
(Matietal.2006).
Irrigation holds the potential to improve crop production and
mitigate the impacts of climate stress associated with drought
and extreme heat in SSA (Burney et al. 2013). Irrigation has
gradually been expanded in SSA (Altchenko and Villholth
2015; Sheahan and Barrett 2014; You et al. 2011). The average
rate of expansion of irrigated area over the past 30 years is 2.3%
in SSA (You et al. 2011), where the area currently equipped for
irrigation is estimated to be slightly more than 13 × 10
6
ha, mak-
ing up 6% of the total cultivated area (Cassman and Grassini
2013; You et al. 2011). Around 0.2 to 3.5% of smallholder farms
in Ethiopia, Malawi, Niger, Nigeria, Tanzania and Uganda can
access irrigation (Sheahan and Barrett 2014). Despite low irriga-
tion development, irrigated agriculture accounts for nearly 38%
of the economic value of all agricultural output (Svendsen et al.
2009). A field survey of 1554 smallholder farmers in nine SSA
countries showed that gravity-flow, manual-lift and motor-pump
irrigation increased the value of agricultural production per farm-
land size as well as per family worker compared to rain-fed-only
farms (Shah et al. 2013).
There is substantial potential for further irrigation develop-
ment and expansion in SSA (Cassman and Grassini 2013;
You et al. 2011). In SSA, average annual renewable ground-
water availability for irrigation ranges from 692 to 1644 km
3
;
therefore, the total area of irrigable cropland with renewable
groundwater includes between 20.5 to 48.6% of the conti-
nents cropland (Altchenko and Villholth 2015). Xie et al.
(2014) revealed a large potential for profitable smallholder
irrigation expansion in SSA, with irrigation technologies
benefiting between 113 and 369 × 10
6
rural people in the re-
gion by generating net revenues of US $1422 billion yr
1
(Xie et al. 2014). Improving rainwater harvesting and irriga-
tion development in SSA will contribute to enhancing crop
production in smallholder households.
2.3 Increase of fertilizer use
Research demonstrated that the amount of fertilizer applica-
tion in SSA was very low compared to other regions (Fig. 3).
Mean N application rates in SSA were 16 kg N ha
1
in 2009
compared to 169.1 kg N ha
1
in the United States in the same
year (Lassaletta et al. 2014). The low fertilizer use in SSA has
been attributed to low financial capacity of farmers, low avail-
ability of input products in local markets, unfavorable
fertilizer/crop-price ratios (Duflo et al. 2008; Croppenstedt
et al. 2003) and low response rates of crops to fertilizer inputs
Year
1960 1970 1980 1990 2000 2010
Nitrogen input
(kg N ha
-1
yr
-1
)
0
100
200
300
400
China
North America
Europe
Africa
Nitrogen fertilizer application (1961-2009)
Fig. 3 Changes of nitrogen (N) fertilizer application in Africa, North
America, Europe and China in 1961 to 2009 (Data source: FAO
STAT). The amount of N fertilizer application in Africa is very low
compared to other regions
Kim D.-G. et al.
(Roobroeck et al. 2021; Ichami et al. 2019;Riesgoetal.
2016). Some governments in SSA have introduced fertilizer
subsidy programs to increase crop productivity (Koussoubé
and Nauges 2017; Jayne and Rashid 2013). Ten African gov-
ernments spend roughly US$1 billion annually on fertilizer
subsidy programs (Jayne et al. 2014). Recent studies found
that synthetic fertilizer use among smallholders is far more
widespread than commonly assumed (Sheahan and Barrett
2017). Over 75% of all cultivating households in Malawi,
50% in Ethiopia and around 40% in Nigeria use synthetic
fertilizer in some amount in the main growing season
(Sheahan and Barrett 2017). Maize fields receive more syn-
thetic fertilizer than non-maize-dominated plots (Sheahan and
Barrett 2017). Increasinguse of synthetic fertilizer ispredicted
in SSA (Ten Berge et al. 2019; Zhang et al. 2015; Tenkorang
and Lowenberg-DeBoer 2009). The annual growth rate of
synthetic fertilizer demand (20152020) in SSA is predicted
to be 3.1, 1.8 and 1.3 times higher than the global average for
N, phosphate (P
2
O
5
) and potash (K
2
O) fertilizers, respectively
(FAO 2017; Fig. 4). Similarly, N fertilizer use is expected to
increasefrom0.9Mtin2015to1.2Mtin2030inSSA
(Tenkorang and Lowenberg-DeBoer 2009).
3 Impact of the smallholder farming practices
on GHG emissions
Increasing crop production in SSA is an urgent and indubita-
ble necessity. Finding approaches to attaining sustainable crop
production requires an understanding of the environmental
implications of different pathways of agricultural growth.
Here we assess the changes in GHG emissions associated with
the management practices detailed in section 2.
3.1 Expansion of agricultural lands and increase of
cropping intensity
The conversion of natural forest to agricultural land and in-
creasing cropping intensity affect carbon (C) budgets (Kim
and Kirschbaum 2015) due to loss of C stored in standing
woody biomass (Pearson et al. 2017) and degraded SOC
(Wei et al. 2014; Murty et al. 2002). The changes in SOC
and C in vegetation biomass driven by conversion of natural
forest to agricultural land are directly related to changes in the
CO
2
budget, since any loss of biosphere C stocks increases
atmospheric CO
2
(Kim and Kirschbaum 2015). Intensive soil
disturbance caused by increasing cropping intensity can en-
hance the loss of SOC through decomposition of soil organic
matter (Kim et al. 2016a; Jayne et al. 2014; Headey and Jayne
2014), resulting in CO
2
emissions.
The conversion of natural forest to agricultural land and
increasing cropping intensity also affect fluxes of other
GHGs such as methane (CH
4
)andN
2
O(Tate2015;Kim
and Kirschbaum 2015; van Lent et al. 2015). In a global me-
ta-analysis, Kim and Kirschbaum (2015) found that the con-
version of forest to cropland increased net soil CH
4
emissions.
This has been associated with changes in the composition
(Singh et al. 2007,2009) and abundance (Menyailo et al.
2008) of the methanotroph communities driven by changed
soil properties such as soil moisture, N status, and pH (Tate
2015;Levineetal.2011). Global meta-analyses found that the
conversion of forest to agricultural lands tended to increase
soil N
2
O emissions (Kim and Kirschbaum 2015; van Lent
et al. 2015). In general, the effect of the conversion on N
2
O
emissions is related to the increase of N input, changed water-
filled pore space, changed soil management and microclimatic
conditions (Wanyama et al. 2018; van Lent et al. 2015; Smith
2010). Effects of conversion of natural forest to agriculture on
soil GHG emissions have been observed in SSA (Wanyama
et al. 2018; Gütlein et al. 2018; Mapanda et al. 2012). In
Zimbabwe, clearing and converting woodlands to crop lands
increased soil emissions of CO
2
,CH
4
and N
2
O (Mapanda
et al. 2012). In Kenya, converted crop lands receiving N input
emitted higher N
2
O emissions than natural forest (Wanyama
et al. 2018).
Overall, conversion from natural forest to crop lands is
recognized as the largest source of GHG emissions in SSA,
resulting in the release of 0.16 × 10
9
Mg C yr
1
between 1990
and 2009 (Valentini et al. 2014) or a total of 84.2× 10
9
Mg
CO
2
eq between 1765 and 2005 [emission of 7.3 ± 0.6 Mg
CO
2
eq per a converted cropland (ha) per a year; Kim and
Kirschbaum 2015]. These emissions contribute to 14.7% of
global land use change GHG emissions (Li et al. 2017).
Compound annual growth rate
of fertilizer demand (2015-2020)
Sub-Saharan Africa
North America
Latin America & Caribbean
Asia
Europe
Oceania
World
Compound annual
growth rate (%)
0
2
4
6
8
Nitrogen fertilizer
Phosphate fertilizer
Potash fertilizer
Fig. 4 Annual growth rate from 2015 to 2020 (determined as compound
annual growth rate) of synthetic fertilizer (nitrogen, phosphate and potash
fertilizers) demand in different regions (Data source: FAO 2017)
Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review
Assuming that agricultural expansion will continue to be as-
sociated with deforestation, Molotoks et al. (2018)projected
that 11.48 × 10
9
Mg C will be lost in SSA due to agricultural
expansion during 2010 to 2050 (loss of average 0.29 × 10
9
Mg
Cyr
1
). Results overwhelmingly suggest that expanding ag-
ricultural lands to enhance crop production can result in loss of
carbon stocks and increasing GHG emissions in SSA.
3.2 Development of rainwater harvesting and
irrigation
Rainwater harvesting and irrigation can affectSOC. Increased
water supply through water harvesting and irrigation can re-
sult in an increased crop biomass and consequently higher
input of organic matter into soils through litter and fine root
exudates and further decomposition, thus resulting in an in-
crease of SOC (Qiu et al. 2018; Trost et al. 2013;Kochsiek
et al. 2009). On the other hand, water harvesting and irrigation
can enhance microbial activity, resulting in enhanced degra-
dation of SOC (Trost et al. 2013). A global review by Trost
et al. (2013) found that irrigating cropping soils increased soil
Cstocksby90500% in desert climates and 1135% in semi-
arid climates, with the greatest gains in environments with low
initial soil carbon, low precipitation and sparse vegetation. But
in soils with high initial SOC content, the enhancement of
microbial activity can outweigh any increases in biogenic car-
bon inputs, resulting in the lowering of SOC content
(Kochsiek et al. 2009; Jabro et al. 2008; Liu et al. 2008).
Irrigation can also affect other processes leading to GHG
emissions from agricultural soils. The effects of irrigation on
microbial activity and soil physical properties (e.g. soil mois-
ture, temperature, aeration and oxidation status) can affect
methanogenesis, methane oxidation, nitrification, denitrifica-
tion and other microbial processes involved in regulating CH
4
and N
2
O emissions (Trost et al. 2013;Kimetal.2012;
Kessavalou et al. 1998). Some studies found that, especially
at high availability of N, certain types of irrigation strategies
could enhance the rate of soil microbial processes leading to
the production of N
2
O emissions following water application
(Cayuela et al. 2017; Trost et al. 2014;Aguileraetal.2013).
An abrupt increase of soil moisture in dry soil conditions
caused by precipitation or irrigation (often called rewetting)
can also affect GHG emissions. This effect was already re-
portedbyBirch(1958) and updated by other authors
(Congreves et al. 2018; Kim et al. 2012). Increases in CO
2
and N
2
O fluxes following rewetting of dry soils have been
observed in multiple terrestrial ecosystems and various land-
use types including crop land (Guardia et al. 2017;Sanchez
2002). Increased CO
2
(up to 9000%) and N
2
Ofluxes(upto
80,000%) within 6 to 24 h after rewetting has been well re-
ported (Kim et al. 2012). These results suggest that soil
rewetting caused by irrigation can abruptly increase soil CO
2
and N
2
O emissions under conditions when soils are permitted
to dry. However, some studies found no significant effect of
irrigation on N
2
O emissions (Trost et al. 2016; Trost et al.
2014). The existence of only limited field data from SSA
prevents general conclusions on the effect of expanding rain-
water harvesting and irrigation on the amount of GHG emis-
sions (Trost et al. 2013).
3.3 Increase of fertilizer use
Increasing N fertilizer use can affect soil C and GHG emis-
sions. In comparison to unfertilized agricultural fields, in-
creased use of N fertilizer can result in higher plant produc-
tivity and increased organic matter input to soil through roots,
exudates and crop residues, resulting in enhanced soil carbon
sequestration (Peng et al. 2017; Han et al. 2016; Yue et al.
2016). Indeed, a global meta-analysis by Han et al. (2016)
found that N fertilizer application increased SOC (10 to
15.4% or 0.9 to 1.7 C g kg
1
) in agricultural fields compared
to unfertilized agricultural fields. Increasing N fertilizer use
can also increase N
2
O emissions. Assuming that N fertilizer
use will increase from 0.9 × 10
6
Mg in 2015 to 1.2 × 10
6
Mg
in 2030 in SSA (Tenkorang and Lowenberg-DeBoer 2009)
and the IPCC default N
2
Oemissionfactor(EF)of1.0%
(IPCC 2006) is applicable in SSA, 78.6 × 10
6
Mg CO
2
eq
would be produced from 2015 to 2030 in SSA. Closing maize
yield gaps by 75% through increasing N fertilizer application
in SSA will increase N
2
O emissions from currently 255 to
1755 Gg N
2
ON year
1
(increase of 589%) (Leitner et al.
2020).
Initial models of the relationship between N inputs and
N
2
O emissions assumed that N
2
O emissions were a linear
function of N input rate (Dobbie et al. 1999;Bouwman
1996). However, in the last ten years growing evidence sug-
gests that N
2
O emissions often increases as an exponential
function of N input rate (Bell et al. 2016; Shcherbak et al.
2014; Kim et al. 2013; Hoben et al. 2011), though the rela-
tionship is not found universally (Shcherbak et al. 2014). In an
exponential response, emissions increase more rapidly once N
addition rates exceed the ability of plants and microbes to
immobilize it (e.g., >100 kg N ha
1
; Bouwman et al. 2002).
The resulting soil N surplus is available as a substrate for
additional N
2
Oproduction(Kimetal.2013). A study from
western Kenya found an exponential relationship between N
input and N
2
O emissions, with the largest increase in N
2
O
emissions occurring when N inputs increased from 100
to150 kg N ha
1
(Hickman et al. 2015). In addition, low or
non- responsive rates of crop productivity to N fertilizer inputs
have been reported across SSA, ranging from 11 to 69% of
cases in individual farms or field trials (Roobroeck et al. 2021;
Ichami et al. 2019; Shehu et al. 2018; Riesgo et al. 2016). In
soils that exhibit low fertilizer responses, increasing N fertil-
izer use may result in soil N surplus and additional N2O pro-
duction in some regions. The results suggest that increasing N
Kim D.-G. et al.
fertilizer use in SSA should be carefully monitored and man-
aged to avoid its excessive use, especially in intensively cul-
tivated cash crop farming (e.g., sugar cane or bioenergy feed-
stock cultivation). Abruptly increasing N
2
O emissions driven
by increasing N fertilizer use in SSA will otherwise be a great
concern in managing GHG emissions in SSA in the near
future.
4 Strategies to enhance crop production
and GHG mitigation in smallholder farming
systems in SSA
An urgent challenge in SSA is to enhance crop production
while avoiding large increases in GHG emissions from
cropping systems (Leitner et al. 2020; van Loon et al. 2019;
Tongwane and Moeletsi 2018). As potential solutions, ap-
proaches based on land, water and nutrient management and
a land-water-nutrient nexus (LWNN) are presented and
discussed below.
4.1 Land: Improving and utilizing degraded land
The ongoing expansion of agricultural land for enhancing
crop production results in deforestation, habitat degradation
and GHG emissions (van Loon et al. 2019; Valentini et al.
2014; Gibbs et al. 2010). Smallholder farmers in SSA have
limited potential for agricultural land expansion (Jayne et al.
2014;Chamberlinetal.2014; Deininger et al. 2011). Instead
of converting natural land to agricultural lands, it may be
sensible to consider restoring, improving and utilizing degrad-
ed lands such as abandoned and/or unfertile agricultural land
and marginal areas (Foley et al. 2011;Lal2006). Available
estimates suggest that there are 494 × 10
6
ha of human-
induced degraded areas in SSA (Bai et al. 2008). About
40% of grasslands and 12% of croplands have been affected
by land degradation in SSA (Le et al. 2016), which may be
attributedto various factors including deforestation, expanded
agricultural lands in environmentally sensitive areas, low nu-
trient additions, acidification and improper soil management
(CGIAR 2017,Nkonyaetal.2016;Leetal.2016). The annual
costs of land degradation in 2007 were estimated to be US$ 58
billion, which was about 7% of the regions GDP (Nkonya
et al. 2016). In contrast, it has been estimated that the benefits
of restoring degraded lands in SSA would outweigh the costs
by a factor of 7 (ELD Initiative 2015; ELD Initiative and
UNEP 2015). Land degradation is expected to increase further
in SSA due to expansion of agricultural lands and increase of
cropping intensity (Nkonya et al. 2016; Gnacadja and Wiese
2016;Leetal.2016).
One solution to restore, improve and utilize degraded lands
in relatively mesic ecosystems is to practice agroforestry
(Nkonya et al. 2016). Agroforestry can be defined as any
practice to purposefully grow trees together with crops and/
or animals for a variety of benefits and services (Whitney et al.
2018; Jose and Bardhan 2012;Nairetal.2010). Similarly,
another meta-analysis of 94 studies in SSA found that agro-
forestry increased maize yields by 0.72.5 Mg ha
1
(or 89
318%) compared to monocropping systems (Sileshi et al.
2008). Another meta-analysis of SSA studies (Kuyah et al.
2019) found that agroforestry increased crop yields in 77
and 68% of all trials conducted on farms and research stations,
respectively. In addition to the direct benefits of food produc-
tion, agroforestry can provide ecosystem services such as im-
proving soil fertility, enhancing carbon sequestration and mit-
igating GHG emissions (Muchane et al. 2020; Smith et al.
2019;Corbeelsetal.2019; Kim et al. 2016a). A recent global
meta-analysis found that soil N stocks under agroforestry were
46% higher than in monocropping (Muchane et al. 2020).
Similarly, a meta-analysis of SSA studies found that agrofor-
estry increased soil N by 20% (Kuyah et al. 2019). A review
found that the absolute rate of SOC sequestration under agro-
forestry was up to 14 Mg C ha
1
y
1
(0100 cm; Corbeels
et al. 2019). Agroforestry may sequester carbon at an equiva-
lent of 27.2 ± 13.5 Mg CO
2
eq ha
1
y
1
during the early
growth stage (up to an average age of 14 years; Kim et al.
2016a). Assuming 20% of the degraded areas in SSA (494 ×
10
6
ha; Bai et al. 2008) could feasibly be converted to agro-
forestry (Kim et al. 2016a), estimates suggest that doing so
could potentially sequester carbon equivalent to 2.7 × 10
9
Mg
CO
2
eq y
1
, which is 7.7 times larger than annual GHG emis-
sions caused by recent agricultural expansion (0.35 × 10
9
Mg
CO
2
eq yr
1
; Kim and Kirschbaum 2015). Although uncer-
tainty remains in these estimates, the results suggest that
converting degraded land to agroforestry could contribute to
enhancing soil fertility and crop production and mitigating
GHG emissions in SSA. In addition, improving soil fertility
and crop productivity of degraded lands through agroforestry
could reduce the need to convert additional natural land to
agricultural lands, consequently reducing GHG emissions as-
sociated with land-use change (van Loon et al. 2019;Branca
et al. 2013).
4.2 Water: Appropriate rainwater harvesting,
irrigation techniques and water management
The potential for rainwater harvesting and irrigation develop-
ment in SSA is substantial. Further expansion of rainwater
harvesting and irrigation with low cost and appropriate tech-
nologies can contribute to enhancing crop production in
smallholder farms (Rosa et al. 2020;LealFilhoand
Trincheria Gomez 2018; Nakawuka et al. 2018). Evidence
from semi-arid environments also suggests that application
of appropriate irrigation systems may have some potential to
mitigate GHG emissions (Deng et al. 2018; Sanz-Cobena
et al. 2017; Cayuela et al. 2017) following two different
Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review
approaches reviewed below: I. Appropriate rainwater harvest-
ing and irrigation techniques and II. Water management in
paddy soils.
4.2.1 Appropriate rainwater harvesting and irrigation
techniques
Different types of rainwater harvesting and irrigation technol-
ogies have been developed and applied in SSA (Altchenko
and Villholth 2015; Karpouzoglou and Barron 2014;Dlie
et al. 2013). Results from cropping systems in other regions
may be useful to understand the potential effect of these prac-
tices on GHG emissions in SSA. Research carried out under
semiarid conditions in Mediterranean cropping systems sug-
gests that drip irrigation (both surface and subsurface) can
increase the potential to maintain crop yields in the context
of frequent droughts and subsequent water scarcity (Deng
et al. 2018; Sanz-Cobena et al. 2017; Aguilera et al. 2013).
Although N
2
O emission factors in drip-irrigated systems
(0.51 ± 0.26%) were higher than those from rain-fed soils
(0.27 ± 0.21%) in Mediterranean ecosystems, drip-irrigated
systems have on average 44% lower N
2
O emissions than
sprinkler systems (Cayuela et al. 2017). Drip-irrigation com-
bined with optimized fertilization (i.e. fertigation)also showed
a reduction of up to 50% of direct N
2
O emissions compared to
sprinkler systems with non-optimal fertilization rates (Sanz-
Cobena et al. 2017). The results suggest that the development
of rainwater harvesting (Rosa et al. 2020) and low-cost drip
and other irrigation technologies (Kahimba et al. 2015)may
provide an opportunity for smallholders in SSA to boost crop
yield with relatively small additional costs. Although N
2
O
emissions could increase by a factor of two or more compared
to rain-fed Mediterannean systems, the overall emissions per
unit areaand especially per unit productionappear likely
to remain low in the context of global agriculture. Larger-scale
investments in water harvesting and irrigation infrastructure
will be important for increasing crop production and limiting
C losses - or even facilitatingC gains - in agricultural soils.To
avoid large indirect GHG emissions associated with irrigation
infrastructure and pumping, the location of water bodies and
connection with cropping systems, soil characteristics and
landscape morphology should be taken into account for de-
velopment of rainwater harvesting and irrigation technologies.
Significant decreases in crop yields have been reported in
semi-arid conditions when irrigation is suppressed (e.g.
Wriedt et al. 2009;Liu2009). For instance, in Europe, large
negative impacts on crop yields are expected as water deficit
increases (from 4 to 66% decrease for 50 and 150 mm of water
deficit, respectively). In cases of no irrigation, compared to an
optimum water supply, fall in crop yield could be higher than
80% (Wriedt et al. 2009). In SSA, as crop yields are often
damaged by rainfall scarcity and droughts (Karpouzoglou
and Barron 2014;Misra2014), the effect of irrigation on crop
yields is expected to be substantial (Altchenko and Villholth
2015; Cassman and Grassini 2013;Youetal.2011).
Therefore, although certain irrigation systems could enhance
GHG emissions due to increased rates in GHG production
processes mainly associated to rewetting events (e.g. sprinkler
irrigation), the expected growth in crop yields could lead to an
overall decrease in yield-scaled GHG emissions.
4.2.2 Water management in paddy systems
Rice is cultivated in 40 countries in SSA on nearly 10 million
ha (Zenna et al. 2017). Rice is also the fastest growing food
staple in SSA and the second major source of human calories
consumption on the continent (Seck et al. 2012). Water table
management in rice paddies may provide great GHG mitiga-
tion potential in SSA. Studies have found that water manage-
ment practices such as flooding, intermittent drainage,
midseason drainage and alternate wetting and drying treat-
ment were important factors for rice yield and GHG emissions
in paddy fields (Jiang et al. 2019; Meijide et al. 2017;Linquist
et al. 2015). For instance, mid-season drainage of the water
table of a rice paddy in Northern Italy resulted in lower water
use and reduced CH
4
emissions with slightly increased N
2
O
fluxes (Meijide et al. 2017). Alternate wetting and drying
treatments relative to the flooded control treatment in paddies
in Arkansas, USA reduced yields by <113%, but global
warming potential (GWP of CH
4
and N
2
O emissions) was
also reduced by 4590% (Linquist et al. 2015). In central
Japan, compound treatment with a combination of flooding,
midseason drainage and intermittent drainage treatments pro-
duced higher rice grain yield and lower total GHG emissions
compared to continuous flooding or intermittent drainage
treatment (Kudo et al. 2014). Other studies carried out in
SSA have shown that improved water management increased
rice yields (e.g., Materu et al. 2018;Matietal.2011;
Balasubramanian et al. 2007). The reason of observed higher
yields under certain water management practices was attribut-
ed to various mechanisms including altered hormonal levels in
rice plants, greater root biomass in deeper soil and higher root
oxidation activity, an enhancement in carbon remobilization
from vegetative tissues to kernels, and reduction of N loss
through nitrification and denitrification in the early vegetative
growth stages (Yang et al. 2017;Wangetal.2016; Chu et al.
2015). However, a study from rice farms in India suggested
that N
2
O emissions from Indian rice paddies under intermit-
tent flooding might be 3045 times higher than under contin-
uous flooding due to increased denitrification (Kritee et al.
2018). More studies, combining both GHG and yield mea-
surements, are required, but it appears that careful optimized
water management might increase agricultural yields while
reducing GHG emissions in SSA paddies, particularly under
climate change scenarios (van Oort and Zwart 2018).
Kim D.-G. et al.
4.3 Nutrient: Improved soil fertility management with
combined conventional-conservation agriculture
(CCCA) practices
Nutrient management should consider two different aspects
simultaneously. On the one hand, increasing N fertilizer use
is required for resolving problems of depleted soil fertility,
low N fertilization levels and thus low crop productivity in
most smallholders of SSA (van Loon et al. 2019;TenBerge
et al. 2019; Zhang et al. 2015). On the other hand, abruptly
increasing N
2
O emissions driven by increasing N fertilizer use
in SSA could create new challenges for managing GHG emis-
sions in the near future (Leitner et al. 2020;Tongwaneand
Moeletsi 2018). Combined practices of conservation agricul-
ture with conventional agriculture (hereafter combined con-
ventional and conservation agriculture; CCCA) can provide
an appropriate solution for nutrient management. Studies
assessing GHG mitigation potentials of CCCA (Table 1)have
shown the advantage of combining the high crop yield rate of
conventional agriculture with the sustainable soilmanagement
of conservation agriculture (Gram et al. 2020; Droppelmann
et al. 2017;WuandMa2015). Some global meta-analyses
reported GHG mitigation potentials of CCCA (Graham et al.
2017; Charles et al. 2017; Han et al. 2016;Sainju2016).
Nitrous oxide EF of the combined application of composts
and synthetic fertilizers (0.37%) and crop residues and fertil-
izers (0.59%) were lower than N
2
O EF of the sole application
of synthetic fertilizers (1.34%) and the IPCC default N
2
OEF
of 1% for synthetic fertilizers (Charles et al. 2017). Inorganic
fertilizers with straw application and inorganic fertilizers with
manure application increased topsoil organic carbon by 2.0 g
kg
1
(19.5%) and 3.5 g kg
1
(36.2%), respectively (Han et al.
2016). In a separate meta-analysis, GHG intensity (net global
warming potential per unit crop yield) was found to be 70 to
87% lower under the improved combined management that
included no-till, crop rotation/perennial crop and reduced N
rate than under traditional management such as conventional
till, monocropping/annual crop and recommended N rate
(Sainju 2016). Studies comparing GHG emissions in conven-
tional practices and CCCA in SSA (Kurgat et al. 2018;
Kimaro et al. 2016;Nyamadzawoetal.2014)demonstrated
that yield-scaled N
2
O emissions were 19 to 88% lower in
CCCA practices compared to conventional practices
(Table 1). In Mali, pearl millet (Pennisetum glaucum)fields
treated with both manure and inorganic fertilizer urea emitted
significantly less N
2
O than plots receiving only urea fertilizer
(Dick et al. 2008). The lower N
2
O emissions in soils amended
with manure were attributed to the initial slow release and
immobilization of mineral N and the consequently diminished
pool of N available to be lost as N
2
O (Nyamadzawo et al.
2014; Mapanda et al. 2011; Dick et al. 2008). The results
suggest that CCCA has a greater potential to increase soil
fertility while avoiding abruptly increasing N
2
O emissions
driven by increasing N fertilizer use. In addition, improving
soil fertility through CCCA could lead to a consequent in-
crease of crop productivity and decrease of the need to convert
additional land to agriculture, thereby reducing associated
GHG emissions (van Loon et al. 2019; Branca et al. 2013).
4.4 Land-water-nutrient Nexus (LWNN) approach
To achieve the goal of enhancing crop production while
avoiding abruptly increasing GHG emissions in smallholder
crop farming in SSA, it is strategic to implement comprehen-
sive approaches resulting in beneficialland, water and nutrient
management interactions (Sheahan and Barrett 2017;
Thierfelder et al. 2017;Zougmoréetal.2014; Branca et al.
2013). Research conducted in Kenya and Tanzania found that
the combination of water harvesting techniques (ex. tie-
ridges) with manure or inorganic fertilizer resulted in higher
maize or cowpea yields than when these factors were applied
separately (Githunguri and Esilaba 2014;Miritietal.2011;
Itabari et al. 2004). In semi-arid West Africa, stone bunds, zaï
and half-moon techniques combined with the application of
organic and/or mineral fertilizers increased agricultural pro-
ductivity and carbon sequestration (Zougmoré et al. 2014).
Differences in the current status of land, water and nutrient
depending on the climate and land use history in different
regions may exist. Accordingly, different schemes are needed
to deal with each of the land, water, and nutrient components
and their nexus (Fig. 5).
A simplified hypothetical example of a LWNN approach
would be based on applying suitable agroforestry practices
combined with CCCA and appropriate rainwater harvesting
and irrigation technologies in degraded lands. This approach
can restore soil fertility, produce food and enhance carbon
sequestration; also improving soil quality, including soil or-
ganic matter, a critical factor for increasing yield response to N
input in SSA (Maman et al. 2018; Kihara et al. 2016;Jayne
and Rashid 2013; Tittonell and Giller 2013). Since irrigation
or CCCA practices can increase yields, this approach could
also help to limit N
2
O emissions due to an increased plant
demand and uptake for N, which would reduce its availability
for conversion to N
2
O (Kim and Giltrap 2017). Therefore,
through the LWNN approach, it may be possible to enhance
crop production and GHG mitigation.
In order to evaluate co-benefits and trade-offs and identify
optimized LWNN schemes, measures accounting for both
crop production and GHG mitigation are necessary. In many
previous studies, agricultural yield was not well accounted for
in GHG budgets and mitigation strategies (Kim and Giltrap
2017; Rosenstock et al. 2013;Linquistetal.2012). To address
the issue, studies use the concept of yield-scaled GHG emis-
sions (GHG emissions per unit agricultural yield) to account
for both crop yields and GHG emissions in various regions
including SSA (Ortiz-Gonzalo et al. 2017; Kim and Giltrap
Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review
Table 1 Summary of comparing conventional agriculture practices and combined conventional-conservation agriculture (CCCA) practice in sub-Saharan Africa
No Country Crop type Conventional practice CCCA practice Effects of CCCA Reference
1 Zimbabwe Maize (Zea mays L.) N fertilizer (NH
4
NO
3
-N;
120 kg N ha
1
)
N fertilizer (NH
4
NO
3
-N;60kgNha
1
)&
composted manure (60 kg N ha
1
)
Yield-scaled N
2
O emission mitigation (48%) Mapanda et al. 2011
2 Zimbabwe Rape (Brassica napus) N fertilizer (NH
4
NO
3
-N;
120 kg N ha
1
)
N fertilizer (NH
4
NO
3
-N;60kgNha
1
)&
manure (65 kg N ha
1
)
Yield-scaled N
2
O emission mitigation (88%) Nyamadzawo et al.
2014
3 Zimbabwe Maize (Zea mays L.) N fertilizer (NH
4
NO
3
-N; 120 kg N ha
1
) N fertilizer (NH
4
NO
3
-N;60kgNha
1
)&
manure (60 kg N ha
1
)
Yield-scaled N
2
O emission mitigation (19%) Nyamadzawo et al.
2014
4 Zimbabwe N fertilizer (urea, 120 kg N ha
1
) N fertilizer (urea, 120 kg N ha
1
)&crop
residues (Maize, 4 Mg C ha
1
)
N
2
O mitigation (56%) Gentile et al. 2008
5 Zimbabwe N fertilizer (urea, 120 kg N ha
1
) N fertilizer (urea, 120 kg N ha
1
)&crop
residues (Maize, 4 Mg C ha
1
)
N
2
O mitigation (49%) Gentile et al. 2008
6 Ghana N fertilizer (urea, 120 kg N ha
1
) N fertilizer (urea, 120 kg N ha
1
)&crop
residues (Maize, 4 Mg C ha
1
)
N
2
O mitigation (103%) Gentile et al. 2008
7 Kenya N fertilizer (urea, 120 kg N ha
1
) N fertilizer (urea, 120 kg N ha
1
)&crop
residues (Maize, 4 Mg C ha
1
)
N
2
O mitigation (72%) Gentile et al. 2008
8 Kenya Vegetables N fertilizer (diammonium Ph
osphate; 40 kg N ha
1
)
N fertilizer (diammonium Phosphate; 20 kg
Nha
1
) & manure (15 kg N ha
1
)
N
2
O emissions intensity (N
2
OI) mitigation
(50%)
N
2
O emissions economic intensity (N
2
OEI)
mitigation (45%)
Kurgat et al. 2018
9 Tanzania Maize (Zea mays L.) Conventional cultivation Reduced tillage & N fertilizer (urea,
100 kg N ha
1
)
Yield-scaled global warming potential
(GWP)
mitigation (62 to 71%)
Kimaro et al. 2016
10 Mali Pearl millet
(Pennisetum
glaucum)
N fertilizer (urea, 50 kg ha
1
) N fertilizer (urea, 50 kg ha
1
)&manure
(8000 kg dry matter ha
1
)
Yield-scaled N
2
O emission mitigation (52%) Dick et al. 2008
Kim D.-G. et al.
2017;Sainju2016; Kim et al. 2016c;Kimaroetal.2016). For
instance, in maize and winter wheat (Triticum aestivum L.)
fields in Zimbabwe, yield-scaled N
2
O emissions was used to
compare the application of inorganic fertilizer (ammonium
nitrate, NH
4
NO
3
-N) with manure and sole application of in-
organic fertilizer (Nyamadzawo et al. 2014). These studies
suggest that yield-scaled GHG emissions may be an alterna-
tive means to account for food security and GHG mitigation
(Kim and Giltrap 2017;Sainju2016; van Kessel et al. 2013).
Therefore, instead of separately considering agricultural yield
and GHG emissions, yield-scaled GHG emissions may iden-
tify optimal LWNN schemes.
Barriers and their potential solutions for enhancing crop
production and GHG mitigation in smallholder farming sys-
tems in SSA.
Inextricably linked, technical, economic and policy barriers
to adopting integrated approaches (e.g. LWNN) for enhancing
crop production and GHG mitigation may exist. From the
technical perspective, the most challenging barrier for small-
holder farmers may be the lack of relevant knowledge and
experience in applying agroforestry (Mbow et al. 2014;
Rioux 2012; Place et al. 2012), rainwater harvesting, irrigation
and water management (Leal Filho and Trincheria Gomez
2018;Nakawukaetal.2018) and soil fertility management
practices (Brown et al. 2018b; Masso et al. 2017;Vanlauwe
et al. 2015). Technology transfer remains a challenge in the
smallholder context. Limited institutional and human capacity
or infrastructure supporting extension programs generally ex-
ist in SSA (Brown et al. 2018a; Wheeler et al. 2017;Ajayi
et al. 2009). From an economic perspective, initial financial
and labor investments can be very high, representing a critical
barrier to adopting new methods for smallholder farmers.
Returns on investment are not immediate since trees may take
years to grow and bear benefits (e.g., timber, firewood, fruit,
etc.). It also takes time for farmers to realize that after adopting
these new approaches, their lands demonstrate improved soil
fertility, which in turn brings significant increases to yields
(Place et al. 2012; Schlecht et al. 2006). Investment in new
technologies and capacity building are costly and need to be
addressed by strong policy. From a policy perspective, land
tenure questions may introduce an additional challenge, as
there may be reduced incentives for farmers to make the nec-
essary investments in labor and finances if they cannot rely on
the future returns of their investments (Higgins et al. 2018;
Holden and Otsuka 2014). The intersectional nature of inte-
grated practices for enhancing crop production and GHG mit-
igation may introduce structural challenges to the develop-
ment of national policies, since intersectional planning and
resource sharing are very rare at the national level in SSA
(Place et al. 2012). Additionally, with limited resources, gov-
ernments must juggle multiple priorities including health, ed-
ucation, and the development of clean water and road infra-
structure, which may create a particular challenge for intro-
ducing practices whose primary purpose is GHG mitigation.
Furthermore, GHG mitigation strategies need to be planned
by national policies in response to international commitments
made by the Intergovernmental Panel on Climate Change, like
the Paris Agreement (UNFCCC 2015).
These challenges are far from trivial, but various efforts
may improve the chance of smallholder farmers adopting the
LWNN approach. Successful technologies will be those with
low barriers to entry, reliable returns on investment and
Water
Rainwater
harvesng,
irrigaon & water
management
Nutrient
Combined
convenonal-
conservaon
agriculture pracc e
Land
Improving degraded
land through
agroforestry
Enhancing
crop yield & GHG migaon
Soil & biomass carbon
Crop yield
GHG emission
Crop yield
Soil carbon
GHG emission
Crop yield
Land-Water-Nutrient Nexus
Fig. 5 Land-Water-Nutrient
Nexus (LWNN) approach to en-
hancecropyieldandmitigate
greenhouse gas (GHG) emission
in smallholder crop farming sys-
tems in sub-Saharan Africa. :
increase and : decrease
(Produced by authors)
Challenges and opportunities for enhancing food security and greenhouse gas mitigation in smallholder farming in sub-Saharan Africa. A review
appropriate and appealing design and implementation. Taking
advantage of locally available knowledge, experience and re-
sources to develop appropriate technologies and disseminat-
ing new information and technologies through the farmer to
farmer approach may improve rates of adoption and technol-
ogy transfer (Brown et al. 2018a,b; Kiptot and Franzel 2015;
Kiptot et al. 2006). Lessons must be taken from past successes
and failures to develop socioeconomic incentives for adoption
and maintenance of sustainable agricultural technologies
(Long et al. 2016;Arslanetal.2014). Micro-financing tied
to carbon trading schemes such as REDD+ can be used to
support investment and development among smallholders
(Gizachew et al. 2017; Mbow et al. 2014; Minang et al.
2014). Policy for smallholder farmers to secure land tenure
and encourage long-term investment is urgently needed.
5 Conclusion
Smallholder farmers in SSA have commonly practiced expan-
sion of agricultural land, increase of cropping intensity, and
development of water harvesting and irrigation to enhance
crop production. However, these practices may result in cre-
ating trade-offs between enhancing cropproduction and GHG
mitigation. To enhance crop production while avoiding
abruptly increasing GHG emissions, interrelated land, water,
and nutrient management strategies such as those offered by
the LWNN approach require consideration. While technical,
economic and policy barriers may hinder implementing the
LWNN approach on the ground, these may be overcome by
developing appropriate technologies, disseminating informa-
tion and technologies through the farmer to farmer approach,
applying small spatial and long-term temporal scale trials and
developing specific policies for smallholder farmers.
Throughout this study, serious data gaps were identified in
the effects of different land, water and nutrient management
strategies on SOC and GHG emissions. The effect of rainwa-
ter harvesting and irrigation on SOC and GHG emissions has
especially not been well studied and deserves further investi-
gation. The data gaps hinder further in-depth assessments of
the trade-offs between enhancing crop production and miti-
gating GHG emissions caused by smallholder farmerspast
and future practices. Further studies are urgently needed for
addressing these data gaps and developing viable options for
applying the LWNN approach proposed herein.
Acknowledgements We are grateful for the numerous researchers and
technicians who provided invaluable data. It is impossible to cite all the
references due to the limited space allowed and we apologize for the
authors whose work has not been cited. The authors are grateful to Lutz
Merbold and Maria Vincenza Chiriacò for constructive and valuable
comments in the earlier manuscript. Tanya Kreutzer Sayyed edited the
final draft of the manuscript and contributed to Fig. 5. Financial support
was provided by International Atomic Energy Agency (IAEA)
Coordinated Research Project (CRP D15020) Developing Climate
Smart Agricultural Practices for Mitigation of Greenhouse Gasesand
the European Commission through the project Supporting EU-African
Cooperation on Research Infrastructures for Food Security and
Greenhouse Gas Observations(SEACRIFOG; project ID 730995).
Author contributions D.-G.K. designed and led the study and D.-G.K.,
E.G., A.B., J.E.H, and A.S.-C. carried out the study and drafted and
revised the paper.
Funding Financial support was provided by International Atomic Energy
Agency (IAEA) Coordinated Research Project (CRP D1.50.20)
Developing Climate Smart Agricultural Practices for Mitigation of
Greenhouse Gasesand the European Commission through the project
Supporting EU-African Cooperation on Research Infrastructures for
Food Security and Greenhouse Gas Observations(SEACRIFOG; project
ID 730995).
Data availability No new data was generated from study.
Declarations
Conflict of interest The authors declared that they have no conflict of
interest.
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