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| Fig. 1: A wheat field in the sunset. Wheat, rice and corn (maize) make up a large portion of humanity's total caloric consumption. (Source: Wikimedia Commons) |
The human population is increasingly rapidly, and is expected to plateau at 11 billion people worldwide by 2100. The growing human population will require a larger food supply to meet humanity's energy requirement for a healthy and sustainable lifestyle. Humans require a minimum energy consumption of 2100 kilocalories per person per day to sustain bodily functions. The Food and Agriculture Organization (FAO) reported that in the period between 2019-2021, the average worldwide dietary energy supply totaled 2963 kilocalories per person per day. [1] If we wish to preserve this level of food availability in 2100; the total estimated caloric supply amounts to
| 2963 kcal d-1 person-1 × 1.1 × 1010 people | |
| = | 3.3 × 1013 kcal day-1 |
| = | 1.4 × 1017 J day-1 |
Despite our current food supply surpassing the minimum nutritional requirement for humanity; in 2020 an estimated 844.3 million people worldwide were categorized as food-insecure individuals who struggled to consume the minimum nutritional energy requirement. [2] The large number of food-insecure people implies a disparity in dietary energy distribution and significant food waste. Given the ever-increasing human population, we must continuously increase the global nutritional energy supply to maintain a high standard of living for all individuals. Climate change poses challenges to agricultural production and threatens to increase the number of food-insecure individuals. Approximately 30 crop plant species provide 90% of all plant-based human food; furthermore 50% come solely from wheat, rice, maize, and potato. All these crops are susceptible to the ill-effects of climate change, posing a threat to many staples foods. [8] Wheat (see Fig. 1) is an especially important agricultural crop whose production takes place in regions affected by climate change. Here, I will discuss a very specific aspect of this problem, namely the salinization of arable land and potential adaptations to changing soil conditions.
Soil salinization is a growing problem causing the declines in the agricultural yield of significant crops. Soil salinization may occur through a variety of naturally-occurring processes including physical or chemical weathering of parent material from geological formations. [3] However, secondary salinization, introduced by human intervention, is the result of irrigation practices that deposit salts into the soil, and especially impact arid or semi-arid land. [4] Historically, human civilizations have suffered from the consequences of soil salinization, most notably the ancient Mesopotamian civilization. [5] While global estimates of total arable land area that is affected by salinization is unreliable due to the difficulty of obtaining such data, many countries have assessed this issue on regional scales. Regions where soil salinization is particularly well-documented include the Central Valley in California, the Indo-Gangetic Plain in India, and the Yellow River Basin In China.
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| Table 1: Soil classification based on level of salinity as measured by its electrical conductivity. [7] |
Salt content in soils are described by their measured salinity and sodicity. Salinity refers to the salt content in the soil; whereas sodicity refers specifically to the sodium content in the soil. Salinity is measured using electrical conductivity (EC), which is proportional salt concentration and is measured in deci-Siemens per meter (dS m-1). For reference, 1 dS m-1 corresponds to a concentration of 680 mg L-1. [6] Soils can be categorized by their level of salinization as shown in Table 1. For instance, soils with an electrical conductivity within the range 4-8 dS m-1 is considered moderately saline . Sodicity is classified as soils having a pH > 8.5 and a Sodium Absorption Ratio (SAR) > 13. The SAR is a measure of the amount of sodium relative to other alkaline cations and serves as an indicator of the quality of water used to irrigate crops. [7]
Agricultural products exhibit a wide range of salt tolerances. The majority of vegetable crops have low salinity tolerances with significant losses at 2.5 dS m-1; whereas cotton is very salt resistant with yield-loss thresholds at 7.7 dS m-1. Most of these agricultural products experience yield reductions under moderately saline conditions. Table 2 lists common agricultural products and their relative threshold salinities, beyond which the crop will experience yield losses. Fruits and vegetables tend to feature low salinity thresholds, whereas barley and rye have high salinity thresholds. [8] Plenty of research studies have attempted to quantify salinity-induced yield losses via both field studies and mathematical modeling. For instance, Nicholas et al. developed a mathematical model to predict the yield losses of various crops produced in the Central Valley as a function of soil electrical conductivity. [9] Their mathematical model was consistent with field data that showed relative yield losses of various crops. Additionally, the spatial distribution of salt-affected soils were taken into account; as the western part of the San Joaquin Valley is particularly affected by salinity. In these regions, salinity can be as high as 8-16 dS m-1 and suppress tomato growth to 20% of maximum potential yield. While soil salinity mapping is imperfect and data availability is meager, quantification of salt-affected soil is essential in assessing its impact on agricultural production.
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| Table 2: Threshold salinities from which selected agricultural crops begin to experience yield loss. [8] |
As a case study we will focus on India, which is the second largest producer of wheat and rice, producing 103 million metric tons of wheat and 177 million metric tons of rice in 2019. [10] To demonstrate the significance of India's agricultural production towards its nutritional energy supply; we will estimate the per capita energy availability from its wheat and rice output. We use India's population of 1.4 billion people and an average energy content of 332 kcal per 100 g of wheat as shown here:
| 1.03 × 108 tonnes y-1
× 3.32 × 106 kcal tonne-1 1.4 × 109 people × 365 d y-1 |
= | 669 kcal day-1 person-1 |
India's 2019 wheat production provided 669 kcal per person per day. Similarly given an average energy content of 130 kcal per 100 g of rice; India's rice production provided 450 kcal per person per day. These two staple crops alone provide about half of the bare minimum nutritional energy per individual in India.
Much of India's staple foods come from the Indo-Gangetic Plains, shown in the shaded region of Fig. 2, which span northern states including Uttar Pradesh, Bihar, West Bengal, Punjab, and Rajasthan. However, concerns are mounting over decelerating productivity in rice and wheat due to soil quality degradation. India possesses approximately 141 million hectares (ha) of arable land under cultivation for crops; of which about 7 million ha are salt-affected. Most of these salt affected soils occur in coastal Gujarat and the Indo-Gangetic Plains; and every year more areas become increasingly salt affected. [6] Uttar Pradesh is especially vulnerable with approximately 1.37 million ha of salt-affected land . The average annual wheat yield in Uttar Pradesh lies between 2.7 and 4.3 tons per hectare, below its simulated potential yield of 7.4 tons per hectare. [11,12] While the gap between actual yield and potential yield may not be entirely due to soil salinization, we can get an estimate of the nutritional energy losses resulting from salt-affected soil. Assuming that we only grow wheat on the affected land area, we estimate the minimum food energy losses here:
| (7.4 - 4.3) t ha-1 × 1.37 × 106 ha × 3.32 × 106 kcal t-1 = 1.41 × 1013 kcal year-1 |
The annual nutritional energy losses from sub-optimal yields in Uttar Pradesh is comparable to the total caloric requirement to feed humanity for a single day in 2100. The massive losses in potential food supply can be recovered by improving the conditions for agricultural growth, including the salt-content of soils. Here, we will explore up-and-coming strategies for adapting agricultural practices to withstand the effects of soil salinization.
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| Fig. 2: Map of Indo-Gangetic Plains as marked by purple highlighted region. (Source: Wikimedia Commons) |
Given these concerns about future agricultural production in the Indo-Gangetic plains, active research is ongoing to counteract the adverse effects of soil salinity. For instance, Anandan and Pakash used an hierarchical algorithm to cluster rice varieties based on their salt-tolerant genotypes to facilitate breeding schemes and maximize salt-tolerance. [13] Alternatively, reclamation of salt-affected lands is possible with the growth of salt-tolerant plant species that accumulate excess salts into their biomass. Such plants include Sesbania aculeata which was shown to reduce electrical conductivity by 27% after one year. [8]
Another potential strategy to combat agricultural yield losses from saline soil is the use of plant growth-promoting rhizobacteria (PGPR); a strain of halophilic bacteria that resides in the vicinity of a plant's roots. Halophilic bacteria withstand environments of high salt concentrations, as opposed to glycophytes which are are intolerant to saline conditions. While the mechanism describing how PGPR impart salt-tolerance onto crops is debated; various studies hypothesize that metabolites produced by these bacteria; along with their ability to enhance phosphorus solubility; contribute to plant growth. [14] A study has shown that halophile bacteria isolated from saline regions of coastal Gujarat helped improve grain yield by 58% when compared to a control group of untreated crops. [15] The effectiveness of PGPR inoculation is also evaluated by comparing the weight of biomass produced under saline conditions. For instance, Tiwari et al. found maximum dry matter production in wheat seedlings treated with Bacilus pumilis , a nearly two fold improvement over a control group when grown under moderately saline conditions (EC = 4.6 dS m-1) over thirty days. [16]
While these solutions have yet to be implemented on a larger scale to verify their effectiveness, we can extrapolate findings from research studies to crudely estimate the benefit of using PGPR in adapting to salt-affected soils. Of the total salt-affected area in Uttar Pradesh, approximately 21,000 ha have a salinity > 4 dS m-1, comparable to the soil condition used in Tiwani et al. Lets assume the use of Bacilus pumilis to achieve a two-fold increase in dry matter yield, and the minimum average yield for wheat at 2.7 t ha-1. The gain nutritional energy is given by:
| 2.7 t ha-1 y-1 × 2.1 × 104 ha × 3.32 × 106 kcal t-1 | = | 1.88 × 1011 kcal year-1 |
While the gain in nutritional energy is modest, we can expect these technologies to become more important as irrigation practices will continue to increase the salt content of agricultural lands. Therefore, it is essential that robust solutions be implemented to increase agricultural production of staple crops to feed an ever growing population on Earth.
© Martin Gonzalez. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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