Carbon Sequestration: Pathway to Increased Agricultural Productivity and Zero Hunger for Devolping Countries
Carbon sequestration (CS) is the process by which atmospheric carbon dioxide (CO2) is removed from the atmosphere and stored in the ocean, on land surface, or in geological formations and is reported as a rate of carbon (C) storage in units of mass per time such as teragrams (Tg = 1 × 1012) C/year (Sundquist et al. 2008).
The world’s population is projected to hit 9 billion by 2050 (Godfray et al. 2010), and an increase of over 50% in agricultural food supply will be required to meet the growing food demand (Mueller et al. 2012; FAO 2013; Paul et al. 2009; FAO 2009). Recent statistics further indicate growth in per capita world food production of 17%, with per capita food consumption averaging at 2780 kcal day−1 (UNEP-UNCTAD 2008). Whereas modern agricultural practices would spectacularly increase productivity, the majority of the chronically hungry are subsistence small farmers in developing countries and are too poor to purchase inputs and often marginalized from markets (Lal 2016). In the recent decades, the global distribution of hunger has shifted significantly as a result of varied rates of achievement in hunger reduction across regions. However, there are still more than one billion food-insecure people in the world with an additional two billion people prone to hidden hunger or malnutrition caused by the deficiency of micronutrients and protein (FAO 2016). Majority of countries in southern Asia and sub-Saharan Africa (SSA), respectively, account for the first (35.4%) and second (27.7%) largest share of global undernourishment (FAO 2016).
Despite the many efforts directed toward reducing hunger, food demand is anticipated to both grow and shift in the coming decades due to population and economic growth and increase in people’s purchasing power. Similarly, growing urbanization and climate change will encourage people to adopt new diets besides exerting pressure on both land and water resources (Pretty et al. 2006). Challenges such as growing competitions for natural resources, natural disasters, poverty, illiteracy, and diseases would likewise pose threats to food security, thus intensifying the hunger crisis (Wu et al. 2014). Climate change will particularly have immense effects on agriculture and consequently human hunger (Trevors 2009). The transformation and democratization of the world’s food system is the best way to adapt to climate change while simultaneously eradicating hunger and poverty, as the root causes of inequality and environmental degradation are confronted head-on (Altieri et al. 2015).
The nexus of climate change-soil degradation-food security is intricately linked with the Sustainable Development Goals (SDGs) of the United Nations, especially SDG #1 (no poverty), #2 (zero hunger), #3 (good health and wellbeing), #6 (clean water), #13 (climate action), and #15 (life on land) (UN 2015). Consequently, the emphasis on achieving food security, eliminating hunger, and removing poverty has been among important foci of the Agenda 21 (UN 1992), Millennium Development Goals (UN 2000), and the Sustainable Development Goals (UN 2015). The SDG #2 integrates and links food security, nutrition, and a sustainable and climate-resilient agriculture. It has further been shown that advancing sustainable development goals through management of soil health would play a significant role in ending hunger through enhancing the quantity and quality of food (Lal 2016).
Improving and sustaining soil quality and health is an integral step toward achieving the SDGs on zero hunger (Pagliai et al. 2002; Rojas et al. 2016). Through plant growth, soil health is also connected with the health of animals, humans, and ecosystems within its domain. Hayne (1940) stated that “if we feed the soil, it will feed us” and that “only productive soil can support a prosperous people.” Thus, maintaining soil health is essential to human health, ecosystem functions, and nature conservancy. According to CNES (1999), to eradicate hunger, global food production must be increased by 2% year−1, and to eradicate malnutrition and hunger, soil quality must be restored rapidly (by 2050 or sooner) especially in developing countries.
The management of soil health is however intertwined with the management of soil organic carbon (SOC) pool (Lal 2016). Through supply of macro- and micronutrients, soil health, mediated by SOC dynamics, is a strong determinant of global food and nutritional security. Improvements in soil health, along with increase in availability of water and nutrients, increase soil’s resilience against extreme climate events and impart disease-suppressive attributes (Ogle et al. 2005). Therefore, SOC sequestration and re-carbonization of the agroecosystems are important to advancing the SDGs. In retrospect, sustainable management of world soils (Lal 2004a, b) and biotic resources (Lal 2010c) is critical to mitigating climate change through implementation of initiatives such as the “4 per thousand” (UNFCCC 2015; Chambers et al. 2016), which highlights the importance of sequestering C in soil at the aspirational rate of 0.4%/year to 40 cm depth (Lal 2016). It is an important mechanism of advancing SDG #2. This natural process of CS in soils is beneficial through increasing crop yields, inputs use efficiency, soil biodiversity, and mitigation of climate change through reduction in net CO2 levels (Lal 2010a, b). In this review, it is hypothesized that enhancing the levels of organic matter in soil would lead to better plant nutrition and increased water retention capacity with resultant higher yields and greater agricultural resilience, and hence a vital step on the trajectory to attaining zero hunger in developing Countries.
The objective of this review is twofold: (i) to highlight the contribution of soil CS to SDG #2 – zero hunger – and (ii) synthesize and highlight management options critical in enhancing soil CS for increased agricultural productivity and advancing zero hunger. The paper review has three interrelated sections: management of SOC for increased crop yields; management practices and options for soil CS; and integration of technologies for enhanced CS and agricultural sustainability, the sum total of which climaxes the role of CS in reducing hunger through sustainable agricultural production methods.
Management of SOC for Increased Crop Yields
Crop yields in sub-Saharan Africa (SSA) have stagnated at about 1 t ha−1 for cereals (e.g., sorghum, millet, maize), 3–5 t ha−1 for roots and tubers (e.g., cassava, sweet potato, and yam), and 100 to 200 kg ha−1 for legumes (e.g., cowpeas), because of soil degradation caused by erosion, nutrient mining, and depletion of the SOC pool (Roy 2010). This creates a gap, for instance, in sub-Saharan Africa, of about 16 million Mg in 2001, and that is projected to increase to 52 million Mg by 2015 (Roy 2010). Improving and sustaining soil health, which is entwined with the management of SOC pool, is key to increased agricultural productivity.
SOC plays an important and critical role in increasing crop productivity, improving soil fertility (Tiessen et al. 1994), and reducing atmospheric carbon dioxide (CO2) enrichment (Lal 2004b). This is in addition to providing other ecosystem services, such as maintenance of consistent yields through improvements in water and nutrient holding capacity, soil structure, and biotic activity (Fan et al. 2013).
Adoption of proven soil management technologies, and especially those that enhance SOC content, has been demonstrated to potentially quadruple production of food crop staples in SSA and also improve their nutritional quality. The SOC concentration of between 2% and 3% in the root zone measured across a wide spectrum of soils has been found to be significant in improving agronomic yields of crops and pastures (Loveland and Webb 2003). Globally, adoption of recommended management practices, and of which augment SOC, is projected to enhance average cereal grain yields from 3.4 t ha−1 in 2008 to 4.2 t ha−1 in 2020 (Ingram et al. 2008). Johnson (1986) asserted that an increase in SOC stock could similarly increase crop yield even in high-input commercial agriculture systems (Bauer and Black 1994) and more so in depleted soils. Available literature information collated and synthesized by Lal (2010a) shows a strong relation between crop yields (grain and straw) and other food staples (root crops) and the amount of SOC in the root zone for diverse soils in several countries in Asia and Africa. An increase of 1 ton of SOC, for instance, was found to enhance wheat grain yield by 27 kgha−1 in North Dakota, United States (Bauer and Black 1994), and by 40 kgha−1 in the Semi-arid Pampas of Argentina (Diaz-Zorita et al. 2005), 6 kgha−1 of wheat and 3 kgha−1 of maize in alluvial soils of northern India (Kanchikerimath and Singh 2001), 17 kgha−1 of maize in Thailand (Petchawee and Chaitep 1995), and 10 kgha−1 of maize and 1 kgha−1 of cowpea in western Nigeria (Lal 1981), and for a podzolic loamy sand in Russia, increase in SOC concentration by 1% increased grain yield of wheat by 0.33 Mg ha−1 and that of barley (Hordeum vulgare L.) by 0.29 Mg ha−1. Similarly, Pan et al. (2009), Overstreet and DeJong-Hughes (2009), and Zhang et al. (2016) reported that a 1% increase in SOC content of the topsoil (0–20 cm) could increase cereal yield by 430 kg ha−1 and reduce yield variability by 3.5%. Becker and Johnson (2001) conducting field studies in West Africa similarly reported increased grain yield of upland rice by 0.31 Mg ha−1 with a 1%increase in SOC concentration in the root zone.
Further, in a survey across 57 countries covering 37 ha (3% of the cultivated area in developing countries), Pretty et al. (2006) reported an average crop yield increase of 79% (geometric mean 64%) with increase in SOC. All crops showed water use efficiency gains, with the highest improvement in rain-fed crops. These findings have further been confirmed through use of several models to project crop yields and soil CS (Jenkinson and Rayner 1977; Coleman and Jenkinson 1995; Parton et al. 1994; Izaurralde et al. 2001, 2002; Gijsman et al. 2002; FAO 2004b), and in all cases significant increase in crop yields with increase in SOC was reported.
Management Practices and Options for Soil Carbon Sequestration
There are a plethora of management practices and possibilities for capturing CO2 from the atmosphere and storing it in the soil (Kane 2015) and thereby creating a positive carbon budget. The overall strategy is to increase SOC density, distribution of SOC in the subsoil, aggregation, and formation of secondary carbonates. The SOC density can be improved by increasing C input into the soil and decreasing losses by erosion, mineralization, and leaching. Restoration of degraded soils and ecosystems, erosion control, and conversion of agriculturally marginal soils to a restorative land use are important options for SOC sequestration (Kaur et al. 2000, 2002a, b).
Similarly, soil C can be enhanced through expanding the amount of land under permanent grassland or forest vegetation, protection of trees on farmland, mixing trees with crops, and emphasizing mixed stands of trees, which have been shown to be more efficient for CS than monoculture stands (Montagnini and Nair 2004). Use of crop residues and manure to enhance soil biodiversity, especially earthworm activity, is an important strategy for increasing SOC concentration and soil quality (Bhadauria and Ramakrishnan 1996). The other more specialized and highly recommended management practices for enhancing CS include but not limited to organic farming, conservation agriculture, agroforestry, biochar, and climate-smart agriculture.
Because of its methodological approach that incorporates the use of organic fertilizers and amendments, e.g., growing leguminous crops or adding compost, animal dung, or green manures (Pretty et al. 2008), organic farming directly contributes to improvement of the natural capital through increased SOC concentration in the soil. Consequently, many improvements to the natural environment, including reduced soil erosion combined with improved organic matter in soils, lead to better CS (Ostrom 1998; Pretty et al. 2003).
Several field studies have proved the positive effect of organic farming practice on soil carbon pools (Küstermann et al. 2008; Pimentel et al. 2005). In Switzerland, a long-term trial biodynamic system showed a stable carbon content, while a carbon loss of 15% in 21 years was measured for the compared conventional system. In the USA, a field trial showed a fivefold higher carbon sequestration in the organic system (i.e., 1218 kg of C ha yr.−1) in comparison with conventional management (Hepperly et al. 2006). The potential of CS rate by organic farming for European agricultural soils has been estimated at 0–0.5tC ha yr.−1.
With enhanced SOC concentration, significant improvements in the water table - as a result of increased water holding capacity of the soil - and soil properties are achieved and manifest in healthier soils. This soils are; able to sustain plant growth, higher in nutrient content and more stable thus enabling the farmers to grow crops for longer periods with higher yields, and when conditions are marginal, thus making a major impact on reducing food insecurity of a region (Pretty et al. 2003).
Adopting organic fertilization, one of the extensively adopted practices under organic farming, is widely found to have positive effects on the yields. For example, Hine and Pretty (2008) showed that maize yields increased by 100% (from 2 to 4 t ha−1) in Kenya in 2005; Parrott and Marsden (2002) reported that millet yields increased by 75–195% (from 0.3 to 0.6–1 t ha−1) and groundnut by 100–200% (from 0.3 to 0.6–0.9 t ha−1) in Senegal in 2001. Scialabba and Hattam (2002) further showed that potato yields increased by 250–375% (from 4 to 10–15 t ha−1) in Bolivia between the early 1980s and 2000.
Various researchers have similarly demonstrated that food security is significantly higher for organic farmers because of the production of a diverse range of products at reduced input cost (IFAD 2005). Organic farmers have considerably higher on-farm diversity, growing on average 50% more crops than conventional farmers, better soil fertility, less soil erosion, increased tolerance of crops to pests and diseases, and better farm management skills (Altieri et al. 2015).
The FAO conference on organic agriculture and food security, held in May 2007, concluded that organic agriculture has the potential to improve food security in developing countries, particularly among small-scale farmers who are often among the poorest and least food secure (Scialabba 2007).
Conservation agriculture (CA), which is based on minimum tillage, permanent soil cover, and crop rotations, is promoted as a remedy for the problems of poor agricultural productivity and soil degradation in sub-Saharan Africa (Maetz et al. 2011). CA is claimed to offer benefits of increased soil organic matter, improvements in water harvesting, reduction in the risk of crop failure, increased and stabilized yields, reduction in soil erosion, improvement in soil structure, reduced pests and diseases, reduced weed pressure, increased productivity, and enhancing food security (Derpsch et al. 2010; Li et al. 2011; Marongwe et al. 2011). A meta-analysis of the long-term effects of CA on maize yields revealed that with crop rotations, soil cover, and high-input use, maize yields under CA generally increase over time in low-rainfall areas (Rusinamhodzi et al. 2011).
Results from on-station trials at Monze Farmer Farmer Training Centre (MFTC) in Zimbabwe and Zambia, comparing conventionally plowed and conservation agriculture systems, showed generally higher infiltration and soil moisture on CA plots with residue retention. Higher available soil moisture on CA fields especially during critical crop development stages resulted in higher maize grain yield at MFTC in 2005/2006 thus showing higher rainfall-use efficiency (Thierfelder and Wall 2009).
Agroforestry systems, encompassing a wide range of land use practices (e.g., farming with trees on contours, bush and tree fallows, establishing shelter belts, and riparian zones/buffer strips with woody species), can increase C storage in the vegetation and in the soil and may also reduce soil C losses stemming from erosion (Paustian et al. 2004; Lal 2003). Through contributing to soil organic matter and mulch material, trees help increase water infiltration into the soil, improve moisture retention, and reduce evaporation – effectively increasing crop resistance to drought. The benefits of agroforestry with leguminous trees in southern Africa have been attributed to substantial amounts of N2 fixation and P recovery, greater CS, and provision of a more resilient agroecosystem with stable crop yields (Sanchez et al. 1997a, b; Vanlauwe et al. 2005; Sileshi et al. 2012). For example, Sharma (2000) and Parrott and Marsden (2002) reported yield increases of 175% on farms in Nepal between 1999 and 2000. Soto-Pinto et al. (2000) studied outputs from shade-grown coffee production in Mexico and found that shaded groves had higher yields (23–38%) than conventional production, based on surveys conducted between 1996 and 1999. Systems involving use of trees or shrubs to delimit fields (live fences) were also found to increase yields, e.g., Ellis-Jones and Mason (1999) reported increased yields from13.5 to 31.7 tha−1 of cassava between 1996 and 1999.
Apart from boosting crop yields, advances in agroforestry have many links with improving the health and nutrition of the rural poor. The expansion of fruit tree cultivation on farms can have a significant effect on the quality of child nutrition. This is particularly important as indigenous fruit tree resources in local forests are overexploited (Garrity 2004).
Concomitant with CS, biochar is rapidly being promoted as a “multiple win” technology for small farmers in Africa and beyond (Leach et al. 2011). It is intended to improve soil properties and functions relevant to agronomic and environmental performance (Lehmann and Joseph 2009; Woolf et al. 2010). Biochar-based strategies are thus being seen to offer valuable routes to building sustainable agricultural futures – not least for resource-poor farmers for whom soil fertility and water availability are seen as key constraints on crop production and food security. In relation to agriculture, it is argued that the burial of biochar provides a potentially powerful method for enhancing soils, helping them to retain nutrients and water, enabling increases in agricultural productivity without, or with much reduced, applications of inorganic fertilizer. (Leach et al. 2011). When applied to sandy acidic soils in southwestern USA, biochar increased SOC and improved overall fertility (Novak et al. 2009). Rondon et al. (2006) reported improved biological nitrogen fixation by bean crops with application of biochar, and improved B and Mo availability.
In SSA, studies have also reported yield increases following biochar application (e.g., Kimetu et al. 2008). In Zambia, maize yield increases of between 80% and over 400% were observed on biochar-amended soil relative to the control (Cornelissen et al. 2013). On a degraded tropical soil in Kenya, Kimetu et al. (2008) demonstrated that biochar had the capacity to restore soil quality and crop productivity, resulting in about 2.9 tons more yield on biochar-amended soil than control plots with fertilizer but no biochar. These yield increases are also associated with increases in nutrient and water use efficiencies (Utomo et al. 2011; Liang et al. 2006). Enhancing resource use efficiency is particularly important in smallholder agroecosystems, where losses of both water and nutrients via runoff and deep drainage are often high. In Ghana, Yeboah et al. (2009) reported up to 5% increase in N recovery when biochar was applied to maize fields on a sandy soil. An increase in nutrient uptake of 100 kg K ha−1, 10 kg Mg ha−1, and 5 kg Ca ha−1 was observed in maize in Columbia (Major et al. 2010). This was accompanied by a progressive maize yield increase from 28% to 140% over 4 years (Major et al. 2010). In a study by Lehmann et al. (2003), a 70% increase in cowpea biomass production was noted compared to control with no biochar application. These increase in crop yields were attributed to the liming effect of the biochar, increased availability of Ca and Mg, proliferation of N-fixing bacteria in leguminous crops, and nutrient and water retention and bioavailability (Lehmann et al. 2003; Liang et al. 2006; Major et al. 2010). Chan and Xu (2009) reported yield increases of R. sativus with the application of 10, 25, and 50 t ha−1 of biochar produced from poultry manure. In summary, given that carbon in biochar is not directly taken up by plants, the impact of biochar on crop productivity is largely through improvements in soil physical, chemical, and biological properties.
CSA technologies and practices and their impact on crop yields
Details of the practices
Impacts on crop yields
Improved agronomic practices
Higher yields due to reduced on-farm erosion and reduced nutrient leaching (e.g., Kaumbutho and Kienzle 2007; Pretty 2000; Altieri 2001)
Higher yields when cropped, due to increased soil fertility (e.g., Kwesiga et al. 2003)
Use of legumes in the rotation
Integrated nutrient management
Increased efficiency of N fertilizer, organic fertilization, legumes and green manure, organic manure
Incorporation of residues
Higher yields through increased soil fertility, increased water holding capacity (e.g., Lal 1981)
Higher yields over long run, particularly where increased soil moisture is valuable (e.g., Hine and Pretty 2008)
Higher yields, greater intensity of land use (e.g., Khan 2005)
Bunds/zai, tied-ridge system
Terraces, contour farming
Higher yields (e.g., Ellis-Jones and Mason 1999)
Various agroforestry practices
Potentially greater food production, greater yields on adjacent croplands from reduced erosion in medium-long term, better rainwater management, and where tree cash crops improves food accessibility (e.g., Sharma 2000 as cited by Parrott and Marsden (2002); Soto-Pinto 2000; Verchot et al. 2007)
A few examples of actual yield increases (summarized in Table 1) under CSA practices are demonstrated by a study from Kenya in which maize yield increased from 1.2–1.8 to 2.0 t ha−1 (Kaumbutho and Kienzle 2007) and Benin where farmers attained maize yields equivalent to the application of 130 kg N ha−1 (Pretty and Hine 2001) with the use of mucuna (Mucuna puriens) cover crop. The use of sunn hemp (Crotalaria juncea) and cowpea (Vigna unguiculata) as green manures in Cuba provided the equivalent of 175 kg N ha−1 to squash; in addition, the green manures improved the physical and chemical characteristics of the soil (Altieri 2009). The CSA practices and technologies have the potential for increased carbon sequestration, therefore contributing to climate change mitigation and agricultural production sustainability.
Integration of Technologies for Enhanced CS and Agricultural Sustainability
SDG #2 aims to “End hunger, achieve food security and improved nutrition and promote sustainable agriculture.” The approaches, discussed herein, used to enhance soil organic carbon (SOC), which refers to containing C in soil organic matter (SOM), and subsequently soil CS in their entirety are concurrently geared toward promoting sustainable agriculture production. Sustainable agriculture centers on the need to develop technologies and practices that do not adversely affect environmental goods and services and that lead to improvements in food productivity (FAO 1989). The capacity for soils to sequester carbon hinges upon SOM content. Aside from CS itself, increasing SOM also enhances structural stability, improves water holding capacity and aeration, promotes nutrient exchange, supports buffering capacity and besides is an important indicator of soil quality and agricultural sustainability (Swift and Woomer 1993). Since increasing SOC may also be able to mitigate some local environmental problems, it will be necessary to have integrated soil management practices that are compatible with increasing SOM management, controlling soil residual nutrients, and enhancing plant growth and agronomic productivity (Tiessen et al. 1994; Reeves 1997), and promoting agricultural sustainability. Sustainable agriculture is thus crucial in the delivery of outcomes within the 17 SDGs including but not limited to no poverty and zero hunger (Lal 2008).
Agriculture is key to attaining the UN Sustainable Development Goal on zero/eradicating hunger (SDG #2) and securing food for a growing world population, estimated to reach 9–10 billion by 2050. This population would require an increase in global food production of between 60 and 110% (Foley et al. 2005; IAASTD 2008; Tilman et al. 2011; Pardey et al. 2014). The SDG #2 broadly integrates and links food security, nutrition, and a sustainable and climate-resilient agriculture. Advancing SDG #2 through management of soil health has been touted as a panacea to ending hunger through enhancing the quantity and quality of food (Lal 2016). According to CNES (1999), to eradicate malnutrition and hunger, soil quality must be restored rapidly toward 2050, and its management is closely connected with the management of SOC pool and CS. Enhancing the SOC concentration (through, e.g. mulch farming and cover cropping, complex rotations including agroforestry, integrated nutrient management in conjunction with use of crop varieties that produce large root biomass and fix biological nitrogen and recycling of plant nutrients fortified by rhizobial and mycorrhizal inoculations, fertigation with drip subirrigation, and creating disease-suppressive soils through improvement of rhizospheric processes, increasing the amount of land under permanent grassland or forest vegetation, protection of trees on farmland, mixing trees with crops and emphasizing mixed stands of trees) was variously reported to play an important role toward increasing crop productivity and by extension reducing hunger. The CS that is similarly enhanced through increasing crop yields, inputs use efficiency (e.g., fertilizer, irrigation) and soil biodiversity is thus an important mechanism for advancing SDG #2. Frequently advocated technologies that enhanced carbon sequestration and mitigated climate change were inter alia reported as organic farming, conservation agriculture, agroforestry, biochar initiatives, and climate-smart agriculture. Agricultural management practices that enhance soil carbon storage were thus found to serve multiple roles including climate change mitigation, improving soil health, and the overarching benefit of sustainable agricultural productivity. With sustainable and resilient agricultural productivity, hunger and poverty would be addressed and thus contributing to the realization of SDG #2.
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