Plant and Soil

, Volume 335, Issue 1–2, pp 101–115 | Cite as

The opening of Pandora’s Box: climate change impacts on soil fertility and crop nutrition in developing countries

Regular Article

Abstract

Feeding the world’s growing population is a serious challenge. Food insecurity is concentrated in developing nations, where drought and low soil fertility are primary constraints to food production. Many crops in developing countries are supported by weathered soils in which nutrient deficiencies and ion toxicities are common. Many systems have declining soil fertility due to inadequate use of fertility inputs, ongoing soil degradation, and increasingly intense resource use by burgeoning populations. Climate models predict that warmer temperatures and increases in the frequency and duration of drought during the 21st century will have net negative effects on agricultural productivity. The potential effects of climate change on soil fertility and the ability of crops to acquire and utilize soil nutrients is poorly understood, but is essential for understanding the future of global agriculture. This paper explores how rising temperature, drought and more intense precipitation events projected in climate change scenarios for the 21st century might affect soil fertility and the mineral nutrition of crops in developing countries. The effects of climate change on erosion rates, soil organic carbon losses, soil moisture, root growth and function, root-microbe associations and plant phenology as they relate to mineral nutrition are discussed. Our analysis suggests that the negative impacts of climate change on soil fertility and mineral nutrition of crops will far exceed beneficial effects, which would intensify food insecurity, particularly in developing countries.

Keywords

Drought Erosion Food security Precipitation Soil degradation Soil organic carbon Temperature 

Challenges to food security in developing countries in the 21st century

The world in general and developing countries in particular face major challenges of food security in the 21st century. Current estimates suggest that more than 1.02 billion people on our planet are underfed or malnourished including a 10% increase over the last 3 years due to rising food costs (FAO 2009). Food insecurity is increasing and projections are that it will worsen in coming decades. Demand for food is expected to increase 2–5 fold from 1990 to 2030, while per capita arable land area dedicated to crop production continues to shrink because of population growth, urbanization and soil degradation (Daily et al. 1998). Estimates suggest that food production will have to increase by 60% in the coming decades to meet world food demands (Wild 2003). The only way to keep pace with the demand will be to increase crop production by farming marginal lands, or through more intensive production on lands already under cultivation (Lal 2000). The green revolution is an excellent example of how agricultural intensification driven by innovation resulted in exponential increases in crops yields that kept pace with population growth in the mid 20th century (Borlaug 2007). Once again food production in developing countries is being outpaced by rapid population growth. From the supply side, this imbalance is largely driven by edaphic constraints that result from inherently low soil fertility and/or soil degradation from unsustainable farming practices (Lal 2007).

The success of the green revolution was mainly driven by dwarf crop varieties that could respond to fertilizer inputs without lodging. However, yield increases that kept pace with population growth during the green revolution have slowed since the 1990’s (Fig. 1). The green revolution bypassed sub-Saharan Africa as crop yields were heavily constrained by nutrient poor soils and most farmers had little or no access to fertilizers. Africa is the only continent where cereal production per capita has steadily decreased since the early 1960’s (Fig. 1). Much of the research aimed at understanding best farming practices is focused on the challenges and crop species that are relevant to developed countries where the resource base exists for conducting research. Based on these challenges there has been a call for a second green revolution with a goal of enhancing crop yields in developing countries by improving soil fertility through better management practices (Sanchez and Swaminathan 2005) and by breeding crops with greater tolerance to edaphic stresses (Lynch 2007). There is substantial opportunity to improve crop yields since current production is only a fraction of yield potential (Lynch 2007) (Table 1). In addition, significant genetic variation exists for crop traits associated with tolerance to mineral stress, and biotechnological advances are accelerating the process of trait identification and selection (Lahner et al. 2003; Wu et al. 2008).
Fig. 1

Cereal production per capita from 1960–2008. Source: Food and Agriculture Organization of the United Nations (FAO). FAOSTAT 2009 (last updated June 23 2009)

Table 1

Average yield of staple crops from 2003–2005. asource: FAOSTAT, last update January 24, 2006 (Lynch 2007)

 

Average Yield 2003–2005 (metric t/ha)a

Maize

Bean

Rice (paddy)

Wheat

Sorghum

Millet

Africa, developing

1.4

0.6

2.0

2.2

0.85

0.68

Africa, developed

3.1

1.4

2.3

2.2

3.3

0.57

Asia, developing

4.0

0.6

4.0

2.9

1.0

1.0

Asia, developed

15.9

2.0

6.3

3.6

2.9

1.0

Latin America and Carribean

3.3

0.8

3.9

2.6

3.0

1.7

Latin America, developed

10.8

1.9

4.8

4.4

Developed Countries (world)

7.8

1.7

6.5

2.9

3.7

1.3

United States

9.4

1.8

7.5

2.9

4.0

1.3

Yield potential with high water and nutrient input

20b

5.8c

10d

10e

4.0f

4.3g

Africa, developed: South Africa; Asia, developed: Japan, Israel; Latin America, developed: Chile; World developed: Albania Armenia, Australia, Austria Republic of Azerbaijan, Belarus, Belgium, Luxembourg, Bosnia and Herzegovina, Bulgaria, Canada, Croatia, Czech Republic, France, Georgia, Germany, Greece, Hungary, Israel, Italy, Japan, Kazakhstan, Kyrgyzstan, Luxembourg, Macedonia, Moldova, Netherlands, New Zealand, Poland, Portugal, Romania, Russian Federation, Serbia and Montenegro, Slovakia, Slovenia, South Africa, Spain, Switzerland, Tajikistan, Turkmenistan, Ukraine, United Kingdom , United States of America, Uzbekistan. bTollenaar and Lee (2002); cBeaver et al. (2003); dPeng et al. (1999); eTripathi et al. (2004); fBaumhardt et al. (2005); gvan Oosterom et al. (2003)

Nutrient impoverished soils contribute to human malnutrition in two important ways. First, they reduce crop yields, causing food scarcity that results in protein-energy malnutrition. Second, crops produced on nutrient poor soils typically have low tissue concentrations of trace elements. Human populations whose diet primarily consists of staple cereal crops (primarily maize, rice, wheat, sorghum, and millet) may meet their protein and energy demands but often suffer micronutrient deficiencies. It is estimated that of the world’s human population, 60–80% are Fe deficient, >30% are Zn deficient, 30% are Iodine deficient and about 15% are Se deficient (White and Broadley 2005). The overwhelming majority of people that suffer from micronutrient deficiencies live in developing countries (Kennedy et al. 2003).

If we are somehow able to clear this first hurdle and increase crop yields and nutrient availability by overcoming soil limitations, global climate change also looms large in determining food sufficiency and quality in the 21st century (Rosenzweig and Parry 1994). Evidence suggests that due to high vulnerabilities and limited resources, developing countries may have limited capacity to implement adaptation measures to achieve food stability in a warmer climate (Kates 2000; Mertz et al. 2009). It is well documented that climate warming, and changes in global precipitation patterns, particularly drought, are already affecting crop production in developing countries (Pandey et al. 2007; Barrios et al. 2008). An important but poorly understood effect of climate change is its influence on soil fertility and nutrient acquisition and utilization by plants (Lynch and St Clair 2004). The first objective of this paper is to provide an overview of what we view as the two most important environmental impediments to food security in developing countries in the 21st century: soil degradation and climate change. We will focus our discussion on the three continents (Asia, Africa and South America) with the largest number of developing countries because that is where the vulnerabilities and knowledge gaps are the greatest. With that foundation, our central objective is to synthesize our current understanding of how climate change is likely to affect crop nutrient acquisition and utilization in soils of the developing world. Interactions between climate and soil resource availability and their influence on crop function and attendant yields are complex and this paper will not attempt a comprehensive exploration of these topics. Instead we will examine how rising temperature, drought and intense precipitation events (the three most important climate variables) are likely to influence nutrient acquisition by crop plants. Finally, we will explore: 1) various adaptation measures that are most likely to be effective in stabilizing crop yields grown under suboptimal soil conditions in future climates; and 2) impediments to their implementation.

Soil limitations to crop productivity in developing countries

Edaphic stresses are so common in soils of developing countries that it has been estimated that on average less than a third of them are free from constraints that significantly reduce crop yields (Lal 2000) (Table 2). Inherently poor soil conditions are a contributing factor to food insecurity and malnutrition, the biggest risk factor for human illness and disease (Sanchez 2002; Sanchez and Swaminathan 2005). Most developing countries exist in tropical or sub-tropical climates with weathered soils including Oxisols, Ultisols, and Alfisols that are characterized by low phosphorus and nitrogen availability, and soil acidity which is often associated with deficiencies of calcium, magnesium and potassium and toxicities of Al and Mn (Sanchez 1976). Soils in the semi-arid and arid subtropics include Aridisols, Inceptisols, Entisols and Vertisols and are prone to deficiencies in P and micronutrient transition metals (Fe, Cu, Mn, and Zn). It is estimated that as much as 50% of irrigated land world wide is affected by salinity stress (Flowers et al. 1997), which reduces crop yields in arid and semi-arid regions.
Table 2

Soil edaphic conditions and land degradation that place major constraints on crop yields. Source: Food and Agriculture Organization of the United Nations (FAO). TERRASTAT 2009

% Soil w/ major constraints

% Land degradation

Africa

85

77

Asia

70

74

Australia

81

61

Europe

69

91

North America

73

49

South America

81

78

Physical soil constraints also significantly reduce crop yield potential in developing countries. They include poor soil texture, rockiness, compaction (Lal 1987), and slope steepness (Lal 1998). These physical soil characteristics determine the water holding capacity of the soil, the degree of root contact with the soil matrix and cation exchange capacity, all of which influence plant nutrient acquisition (Marchner 1995). The inherent limitations that are common in the soils of developing countries are exacerbated by unsustainable management practices that further degrade soil fertility and function (Table 2). It is estimated that 80% of soil degradation worldwide occurs in developing countries (Lal 2000). Erosion driven by tilling practices and cultivation on sloped terrains is the most ecologically and economically devastating soil degrading process (Ananda and Herath 2003; Meadows 2003). In developing countries erosion can carry away as much as 30–40 t/ha/yr of topsoil (Barrow 1991). Lost with erosion are the nutrients that are abundant in the cation exchange sites and organic fractions of the topsoil. Since most fertilizers are surface broadcast large proportions of fertilizer inputs (20–50%) are also lost through erosive processes (Pimentel 1996). Soil erosion studies in Africa have shown drastic reductions in crop yields (>65%) because the exposed sub-surface soils have much lower fertility and poorer physical characteristics than topsoil (Oyedele and Aina 2006; Salako et al. 2007).

Estimates are that the world has lost 55–90 Pg of soil organic carbon (SOC) through land conversion from natural to agricultural ecosystems (Ruddiman 2003). Soil organic carbon (SOC) is positively correlated with crop productivity (Olson and Janzen 1992). High SOC enhances soil water holding capacity, increases soil fertility through cation exchange and mineralization processes and improves soil structure (Lal 2006). Increasing SOC in degraded soils is therefore a major goal in efforts to renew fertility in degraded soils (Lal 2006). As SOC accumulates it produces a positive feedback in which better functioning soil produces more crop biomass that then increases organic matter inputs to the soil (Lal 2006). Additionally, SOC storage is an important terrestrial sink for CO2, which mitigates climate change (Post and Kwon 2000; Yu et al. 2009). In contrast, when SOC is lost through erosion, tillage and other agricultural processes (Lal 2004), soil function and plant productivity that maintains organic matter in the soil are compromised.

Soil conditions, particularly soil moisture content, temperature and nutrients status have drastic effects on the abundance, infection potential and efficiency of N-fixing bacteria that are vitally important to increasing N in cropping systems that lack fertilizer inputs (Dakora and Keya 1997). Alleviation of soil constraints that are common in Sub-Saharan Africa dramatically increases nodulation, plant growth and crop yields, indicating that soil conditions represent a major limitation to biological N fixation (Dakora and Keya 1997).

Climate change impacts on crop productivity and quality in developing countries

Climate conditions, particularly drought, have had significant impacts on crop yield reductions and food insecurity in Africa over the last 50 years (Barrios et al. 2008). Analyses suggest that this trend will intensify in developing countries during the 21st century (Murdiyarso 2000; IPCC 2007b). During the 20th century Asia, Africa and South America experienced a 0.7–1.0°C increase in temperature (IPCC 2007a). Conservative estimates from climate models suggest that by the end of the 21st century, temperature averages on those continents will have increased by at least another 2–4°C (Salinger 2005; IPCC 2007a). Subtropical regions are likely to experience increases in drought which will be driven by higher vapor pressure deficits and less annual rainfall (Shindell et al. 2006; IPCC 2007a). Variability in precipitation patterns are projected to increase with longer periods of droughts interspersed with more intense rainfall events (Easterling et al. 2000; Groisman et al. 2005; Sun et al. 2007). These changes in temperature and precipitation are expected to have net negative effects on global agriculture and in particular in developing countries (IPCC 2007b). The severe impacts of drought on crop failure that has led to widespread starvation in sub-Saharan Africa during the late 20th century is expected to continue and intensify in the 21st century (Broad and Agrawala 2000; Held et al. 2005). By 2020, yields from rain-fed agriculture in Africa could be reduced by 50%, largely a result of increases in the intensity and duration of drought events (IPCC 2007b). Depending on location, increases in the frequency and magnitude of droughts and floods are expected to have major impacts on agricultural production in Asia (IPCC 2007b). Recent climate models suggest that future patterns of drought in Asia are likely to be most problematic in Asian monsoon regions (AMRs: South Asia and East Asia) and West Asia during the spring and summer months (Kim and Byun 2009). In South America, shifts in precipitation patterns and the disappearance of glaciers are expected to substantially decrease water availability for agriculture. Projected warming and reductions in precipitation expected to occur by 2030 in developing countries, suggests that South Asia and Southern Africa will be the two regions most likely to suffer negative impacts on several crops important to large food-insecure populations (Lobell et al. 2008).

Recent analyses suggest that uncertainty in understanding crop responses to future climate change is greater for temperature than precipitation (Lobell and Burke 2008). The influence of warmer temperatures on crop yields will somewhat depend on moisture availability. In areas where precipitation is plentiful or irrigation is available, warmer temperatures may positively influence yields by: 1) increasing rates of physiological capacity (Taiz and Zeiger 2006); 2) lengthening the growing season (Juin et al. 2004); and 3) reducing the incidence of frost damage to crops in temperate climates (Moonen et al. 2002). In semi-arid and arid regions, the largest impact of warmer temperatures on agriculture will be exacerbation of soil moisture deficit (drought) driven by increased rates of evapotranspiration (Biggs et al. 2008). Interestingly, increases in both daily maximum and minimum temperatures (which will continue to rise during the 21st century) have been shown to negatively impact crop yields, by altering phenology (Mitchell et al. 1993; Peng et al. 2004) and through heat stress at more extreme temperatures (Spiertz et al. 2006).

Soil moisture is the master environmental variable because its availability integrates climate and soil conditions, and because plants and soil microbes are so responsive to its availability. Drought is an important selection force on biological organisms (McDowell et al. 2008) and can drastically alter plant community structure and function (Ciais et al. 2005; Holmgren et al. 2006). One of the important innovations in human agriculture was irrigation which mitigated the negative impacts of water deficit on crop growth. However, because of the extent and magnitude of projected climate change, dwindling quality and quantity of freshwater resources, and poor irrigation infrastructure, adaptation to drought in developing countries appears to be limited in the future (Kates 2000; De Wrachien and Feddes 2004).

In addition to the effects of climate on crop growth potential and yield, temperature extremes and water deficit can have varying effects on the nutritional quality of harvested products. Elevated temperature and drought tend to reduce grain yield and starch content, while increasing protein content (Gooding et al. 2003, Erekul and Kohn 2006). Temperature is positively correlated with grain micronutrient concentrations (Karami et al. 2009) and frost damage reduces grain filling and typically has negative effects on grain quality (Cromey et al. 1998; Allen et al. 2001). Elevated CO2 generally increases grain size but reduces protein and mineral nutrient concentrations (Hogy and Fangmeier 2008), which may result from tissue dilution and/or reductions in transpiration-driven mass flow of nutrients (Lynch and St.Clair 2004). Post-harvest fruit and grain losses can be as high as 20% in developing countries (Aidoo 1993). Fruit and grain spoilage is particularly problematic in tropical areas where climate conditions are optimal for microbial growth. The effects of climate change on postharvest storage of crops is poorly understood but could be substantial in some regions.

Key climate-nutrient interactions in cropping systems of developing countries

Brouder and Volenec (2008) pointed out that: “implicit in discussions of plant nutrition and climate change is the assumption that we know what to do relative to nutrient management here and now but that these strategies might not apply in a changed climate.” In other words, the rate and magnitude of changes in precipitation and temperature, anticipated in the coming century have the potential to fundamentally alter our understanding and strategies for the nutrient management of crops.

Drought effects on nutrient acquisition

Crop yields on soils in developing countries decrease exponentially with increasing aridity (Lal 2000). Soil moisture deficit directly impacts crop productivity but also reduces yields through its influence on the availability and transport of soil nutrients (Table 3). Drought increases vulnerability to nutrient losses from the rooting zone through erosion (Gupta 1993). Because nutrients are carried to the roots by water, soil moisture deficit decreases nutrient diffusion over short distances and the mass flow of water-soluble nutrients such as nitrate, sulfate, Ca, Mg, and Si over longer distances (Mackay and Barber 1985; Barber 1995). Roots extend their length, increase their surface area and alter their architecture in an effort to capture less mobile nutrients such as phosphorus (Lynch and Brown 2001). Reduction of root growth and impairment of root function under drought conditions thus reduces the nutrient acquisition capacity of root systems (Marchner 1995).
Table 3

Potential interactions of global change variables with mineral stress discussed in the text

Process

Global change variables

Interaction with mineral stress

Erosion

heavy precipitation, drought

general losses of soil nutrients, SOC and fertilizer

Transpiration-driven mass flow

drought, temperature, RH, CO2

NO3, SO4, Ca, Mg, and Si

Root growth and architecture

drought, soil temperature, CO2

All nutrients, especially P and K

Mycorrhizas

CO2

P, Zn (VAM) N (ecotomycorrhizas)

Soil microbes (N cycling)

drought, soil temperature

N

Biological N Fixation

drought, soil temperature

N

Soil redox status

flooding

Mn, Fe, Al and B

Soil leaching

heavy precipitation

NO3, SO4, Ca, Mg

Plant phenology

temperature

P, N, K

Soil organic carbon status

soil moisture, soil temperature, CO2

all nutrients

Salinization

precipitation, temperature

Na, K, Ca, Mg

Drought also disrupts root-microbe associations that are a principal strategy for nutrient capture by plants. Reductions in both carbon and oxygen fluxes and nitrogen accumulation in root nodules under drought conditions inhibit nitrogen fixation in legume crops (Gonzalez et al. 2001; Ladrera et al. 2007; Athar and Ashraf 2009). Drought alters the composition and activity of soil microbial communities which determine the C and N transformations that underlie soil fertility and nutrient cycling (Schimel et al. 2007). For example, soil moisture deficit has been shown to reduce the activity of nitrifying bacteria by slowing diffusion of substrate supply and through cytoplasmic dehydration (Stark and Firestone 1995). Less is known about how drought influences mineralization and decomposition in agricultural systems but it likely slows these processes. Studies suggest that the root-mycorrhizal symbiosis is not overly sensitive to moderate soil moisture deficits (Entry et al. 2002; Garcia et al. 2008). There is a large literature documenting the beneficial effects of mycorrhizal fungi in crops plants experiencing drought conditions (Wu and Chang 2004; Boomsma and Vyn 2008). Part of the benefit provided by mycorrhizae under drought conditions is associated with increase in nutrient transfer to the roots (Goicoechea et al. 1997; Al-Karaki and Clark 1998).

Effects of Intense precipitation on nutrient acquisition

Excessive precipitation can reduce crop yields (Paul and Rasid 1993; Kawano et al. 2009) (Table 3). Intense rainfall events can be a major cause of erosion in sloped cropping systems and where soil instability results from farming practices that have degraded soil structure and integrity (Meadows 2003). Surface erosion during intense precipitation events is a significant source of soil nutrient loss in developing countries (Tang et al. 2008; Zougmore et al. 2009). Because of its high mobility in soil, nitrate leaching following intense rainfall events can also be a significant source of N loss in agriculture (Sun et al. 2008).

Agricultural areas with poorly drained soils or that experience frequent and/or intense rainfall events can have waterlogged soils that become hypoxic. The change in soil redox status under low oxygen can lead to elemental toxicities of Mn, Fe, Al and B that reduce crop yields (Setter et al. 2009), and the production of phytotoxic organic solutes that impair root growth and function (Marchner 1995). Hypoxia can also result in nutrient deficiency since the active transport of ions into root cells is driven by ATP synthesized through the oxygen dependent mitochondrial electron transport chain (Drew 1988; Atwell and Steer 1990). Significant nitrogen losses can also occur under hypoxic conditions through denitrification as nitrate is used as an alternative electron acceptor by microorganisms in the absence of oxygen (Prade and Trolldenier 1990; Marchner 1995).

Effects of temperature on nutrient acquisition

Soil warming can increase nutrient uptake from 100–300% by enlarging the root surface area and increasing rates of nutrient diffusion and water influx (Ching and Barber 1979; Mackay and Barber 1984) (Table 3). Water soluble nutrients including nitrate, sulfate, Ca, Mg primarily move towards roots through transpiration-driven mass flow (Barber 1995). Since warmer temperatures increase rates of transpiration, plants tend to acquire water soluble nutrients more readily as temperature increases. Temperature increases in the rhizosphere can also stimulate nutrient acquisition by increasing nutrient uptake via faster ion diffusion rates and increased root metabolism (Bassirirad 2000). However, any positive effects of warmer temperature on nutrient capture are dependent on adequate soil moisture. If under dry conditions higher temperatures result in extreme vapor pressure deficits that trigger stomatal closure (reducing the water diffusion pathway in leaves) (Abbate et al. 2004), then nutrient acquisition driven by mass flow will decrease (Cramer et al. 2009). Temperature driven soil moisture deficit slows nutrient acquisition as the diffusion pathway to roots becomes longer as ions travel around expanding soil air pockets (Brouder and Volenec 2008).

Emerging evidence suggests that warmer temperatures have the potential to significantly affect nutrient status by altering plant phenology (Nord and Lynch 2009). The duration of plant developmental stages is extremely sensitive to climate conditions and is particularly responsive to temperature (Cleland et al. 2007). Experimental warming was shown to shorten phenological stages in wheat that resulted in a 9% yield decrease per 1°C increase in temperature (Mitchell et al. 1993). Nord and Lynch (2008) found that genotypes with shorter vegetative growth phases (shortened phenology) had ∼30% decreases in reproductive tissue and seed production in soil with low phosphorus availability because of reduced P acquisition and utilization. This interaction between warming and P acquisition through shifts in plant phenology like other climate-nutrient interactions likely operate at a global scale (Fig. 2).
Fig. 2

Important climate-nutrient interactions often occur at a global scale. For example, this diagram illustrates that greater than 50% of the vegetated land area predicted to have significant increases in warming during the 21st century overlaps with low P soils. Recent studies suggest that climate warming hastens plant phenology, which is likely to exacerbate P deficiency in plants. Data from Jaramillo and Lynch, unpublished

Because of the important role of SOC in enhancing soil moisture retention, fertility and structure, it has a disproportionately large impact on food security in developing countries (Lal 2006). Soil organic carbon stocks are the sum of soil organic inputs driven by plant productivity (root exudates, root and shoot turnover) and soil organic losses via heterotrophic respiration and erosion. Warmer temperatures can increase or decrease crop productivity and yield depending on crop type and agricultural zone (Singh et al. 1998). In the tropical and sub-tropical climates of most developing countries a 2–3°C increase in temperature is expected to diminish crop productivity (Easterling and Apps 2005). Simulation models predict large losses in agricultural SOC over the 21st century resulting from lower crop productivity (inputs), and higher rates of heterotrophic respiration in response to climate warming (Jones et al. 2005; Smith et al. 2009). There is evidence that drier soils under warmer temperatures will also increase SOC losses via higher rates of wind erosion (Lee et al. 1996).

Climate warming contributes to the degradation of freshwater quality and availability (IPCC 2007b). A major way in which this happens is through salinization, an important limitation to agriculture productivity in semi-arid regions that is expected to intensify through the 21st century (Yeo 1999). Increases in demand for irrigation in semi-arid regions such as the Indo-Gangetic plain of northern India and Pakistan and parts of Africa driven by population growth and climate warming are expected to increase the extent of salinization in agriculture (Yeo 1999). Climate warming can increase agricultural salinization by increasing the demand for irrigation and increasing rates of surface water evaporation. Warmer temperature can also increase salt accumulation in crops via increased transpiration rates (West and Taylor, 1980). Theoretically there are reasons to believe that elevated CO2 may mitigate salt accumulation but empirical evidence does not support that conclusion (Yeo 1999, Nicolas et al. 1993). In addition, rising sea levels driven by climate warming is expected to contribute to seawater intrusion of coastal aquifers (Don et al. 2006; Antonellini et al. 2008).

Adaptation/mitigation strategies and limitations

The trends outlined above describe a dire situation that is likely to worsen in the short term. To address this challenge it is urgent that the fertility and productivity of agro-ecosystems in developing regions be maintained and even improved to keep pace with population growth. The conceptual framework for this effort should be integrated soil fertility management (ISFM), consisting of three primary components: 1) judicious use of fertilizers and soil amendments, 2) soil conservation to reduce erosion, maintain soil organic matter, and enhance water and nutrient bioavailability, and 3) cultivation of crop species, genotypes and cropping systems that make optimal use of soil resources for food production while conserving soil fertility.

Technically, the simplest solution to many of the fertility problems in low input agro-ecosystems would simply be to use fertilizers particularly in African nations where soil fertility is low and fertilizer use has been minimal (Fig. 3). Indeed, the first Green Revolution consisted mainly of fertilizers and genotypes that could respond to them without lodging. Although many tropical soils have chemical characteristics that make fertilizer use problematic, the basic technologies for fertilizing tropical soils have been known for decades (Sanchez 1976), and demonstration plots have shown sustained yield improvements in response to liming and the application of chemical fertilizers (Sanchez et al. 1983; Fearnside 1987). In recent years private foundations have devoted considerable resources to the improvement of fertilizer availability and use in sub-Saharan Africa as a key element of their programs to reduce world hunger.

These efforts notwithstanding, it is not obvious that application of chemical fertilizers will be sufficient or even successful over the broad spatial scales they are needed. Low input farmers typically do not have money to buy fertilizers, which are often considerably more expensive in poor countries than in wealthy nations because of poor transportation infrastructure and greater distances to the source of manufacture (Sanchez 2002). Even farmers that can afford fertilizers typically have poor access to distribution markets and limited information about how to properly use them. Local markets for goods and services are often dysfunctional because of corruption and lack of competition. In recent years, fertilizer costs have increased substantially along with energy costs, since fossil fuels are needed for fertilizer production and distribution. In the medium to long term, the cost of concentrated P fertilizer will increase as readily available phosphate ore deposits are depleted (Herring and Fantel 1993). Given the magnitude of these challenges, and the current trends in energy prices and sociopolitical stagnation in the poorest nations, it is doubtful that on a global scale, resource-poor farmers will be able to fertilize their way to higher yielding, more sustainable production systems in the next 10 or 20 years, a critical period in terms of resource degradation and hunger alleviation.

The use of locally available fertility amendments is a more feasible strategy for many poor farmers. These include minimally processed rock phosphate for acid soils, locally available liming materials, biological nitrogen fixation (see below), agroforestry systems, and other nutrient sources with low production and transportation costs. The use of rock phosphate is especially promising since low P availability is a primary constraint in weathered soils characteristic of many poor countries, and sources of rock phosphate are found in many developing nations (Smyth and Sanchez 1982; Gichuru and Sanchez 1988). These are promising solutions for farmers with some access to credit, transportation, and markets, as in many developing regions of Latin America and Asia. In subsistence agriculture characteristic of the poorest and most food insecure regions of Africa, even these inputs may be out of reach. A larger problem is that many poor farmers do not own their land, and therefore lack incentives for investing in soil fertility improvements.

The second leg of ISFM is soil management to conserve and enhance soil fertility, including erosion control, maintenance of soil organic matter (Fernandes et al. 1997), and enhancing nutrient bioavailability by promoting beneficial root symbionts, most notably via biological nitrogen fixation (Hubbell 1995) and mycorrhizal associations (Plenchette et al. 2005). Although the critical importance of these management tools has been known for decades, other than nitrogen fixing food legumes they are rarely used in the poorest countries. Indeed, many poor farmers use soil resources abusively, without apparent regard for the longer-term consequences of practices such as deforestation, residue burning, and cultivation of steep slopes without runoff barriers. In some cases this is due to ignorance, compounded by disruption of traditional cropping practices by war, disease, and migration. Soil resources may be devalued because of the transitory nature of the cropping system, such as in the ‘slash and burn’ agriculture at retreating forest margins, or because the land is not owned by the farmer. In other cases soil fertility may be recognized as a resource but valued less than competing imperatives such as labor requirements for fuel wood collection, or the need to maximize food production in the short term. Many of the poorest farmers have little access to technical information or government services, and lack the exogenous incentives for soil conservation as a public good enjoyed by farmers in rich countries. The capability of this leg of ISFM to address the soil fertility crisis in the third world is therefore problematic, because of poor diffusion and adoption.

The third component of ISFM is the cultivation of crop species, genotypes and cropping systems that make optimal use of soil resources for food production while conserving soil fertility. Although crop species and cropping systems vary substantially in their ability to produce food in marginal soils, the adoption of new crop species and cropping systems is subject to some of the same socio-economic barriers noted above for soil conservation practices, with the added obstacle of cultural attachments to specific foods. For subsistence farmers, the crops they grow and consume are a dominant feature of their daily life. Strong preferences may exist for specific grain types within a crop species, regardless of yield advantages to introduced types. The cultivation of more nutrient-efficient crop species and cropping systems will probably be an important element of the adaptation of third world agriculture to global climate change, but these changes will likely be slow and difficult adjustments for traditional agricultural communities, made as a last resort. The prospect of cultivating new genotypes of existing crops with greater productivity in infertile soils is considerably more promising, especially if such genotypes have agronomic and grain characteristics that are similar to traditional landraces. Substantial genotypic variation exists in crop species for tolerance to Al toxicity (Kochian et al. 2004), Mn toxicity (Gonzalez and Lynch 1999) and low P (Lynch and Brown 2001), and this variation has been deployed through crop breeding programs in Africa, Asia, and Latin America (Lynch 2007). Selection for root traits that increase the acquisition of limiting nutrients such as phosphorus in crop plants (Fig. 4) is an increasingly important objective of breeding programs in developing countries (Yan et al. 1995). Successful genotypes can be adopted and disseminated through informal seed exchange networks, requiring no new additional information, credit, or social resources. Indeed, genotypes with greater productivity at suboptimal soil fertility are ‘scale neutral’ in that they would benefit producers at all resource levels—improving yield with low inputs and reducing input costs in intensive systems.
Fig. 3

Root traits of importance in adaptation to phosphorus deficiency include shallower root architecture, aerenchyma formation, longer and denser root hairs, greater root exudate and phosphatase production and mycorrhizal associations (Lynch 2007)

Fig. 4

World fertilizer use in 2007. Source: Food and Agriculture Organization of the United Nations (FAO). FAOSTAT 2009

There is a significant effort in the scientific community to biofortify crops with trace elements to alleviate micronutrient malnutrition (Tanumihardjo et al 2008). Promising biofortification solutions include micronutrient enrichment of fertilizers (Cakmak 2009), intercropping of dicot and gramineous species (Zuo and Zhang 2009) and using molecular breeding and biotechnology to produce genotypes with root traits that increase the acquisition of limiting micronutrients (Zhu et al. 2007; Mayer et al. 2008). Based on known climate impacts on nutrient acquisition by crops (reviewed in this paper), projected climate changes in the 21st century are very likely to have net negative effects on trace element acquisition of crops in developing countries. Biofortification is a potentially powerful tool in offsetting edaphic and climate constraints to trace element acquisition by crops. However, to be successful, scientists will need to understand how climate conditions impact biofortification strategies.

Adoption of more nutrient-extractive genotypes without additional interventions may lead to accelerated nutrient mining in some systems (Henry et al. 2009). In upland systems where soil erosion causes major losses of soil fertility, greater crop biomass through increased nutrient extraction may actually enhance the sustainability of the system by reducing topsoil loss (Lynch 1998). In many low-input systems, increased productivity might permit the farmer to climb out of the poverty trap of low inputs and low yields, subject to the accessibility of additional fertility inputs as discussed above. Improved genotypes may represent the leading edge of technical intervention in low input systems, because of the relatively few barriers to their adoption, as well as the large impact they can have on crop yields. Genotypes selected for synergies with other fertility enhancing technologies, such as legume genotypes with superior utilization of rock P, leading to greater biological nitrogen fixation, or genotypes with greater soil cover that reduced soil erosion, may represent the leading edge of technology packages that could substantially improve and sustain the productivity of marginal lands.

Presently, the poorest nations confront a critical lack of trained people to implement this vision. Agricultural training has actually been de-emphasized in recent decades by development agencies and donors, and what training has occurred has often been directed to trendy fields such as biotechnology that have limited utility in the poorest countries. There has been inadequate attention to the complexity of these problems, including agro-ecological as well as socio-cultural factors, in favor of searches for technical solutions of limited scope, perhaps informed by the dramatic success and technical simplicity of the first green revolution. Thus despite the renewed emphasis on global food security by research donors, especially for Africa, it is not clear that sufficient progress will be achieved to avert a human disaster of epic proportions, as food insecure people are further marginalized by climatic shifts and the social disruptions that are likely to accompany them.

Conclusions

Although the interactions of global climate change and crop nutrition are not well understood, it is probable that the net effects of these changes will be negative for agricultural production in poor nations. Drought induced by higher temperatures and altered rainfall distribution would reduce nutrient acquisition, biological nitrogen fixation, and may disrupt nutrient cycling. More intense precipitation events would reduce crop nutrition by causing short-term root hypoxia, and in the long term by accelerating soil erosion. Increased temperature will reduce soil fertility by increasing soil organic matter decomposition, and may have profound effects on crop nutrition by altering plant phenology. Since soil fertility is already a primary constraint to food security in many developing regions, and crop production is already marginal, these stresses may be disastrous for vulnerable populations. Social adaptation to changing conditions is possible, although most of the technical options face serious obstacles of diffusion and adoption. An urgent effort is required to improve crop nutrition and soil fertility management in poor nations, integrating agro-ecological and socio-cultural aspects of the problem, to avert worsening of a situation that is already desperate.

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© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  1. 1.Department of Plant and Wildlife SciencesBrigham Young UniversityProvoUSA
  2. 2.Department of HorticultureThe Pennsylvania State UniversityUniversity ParkUSA

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