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Unit 7: Agriculture // Section 3: Key Inputs for Photosynthesis

Photosynthesis, in which plants convert and water into plant tissue, is the foundation for agricultural productivity. Before we consider how tools like plant breeding and fertilizer can increase farmers' output, we need to understand this basic process and the constraints on it, including the key roles played by water and nitrogen (N).

Drought is the biggest limit on agricultural productivity because plants need an enormous amount of water. When plants photosynthesize, they use energy from sunlight to convert carbon dioxide (CO2) and water into carbohydrates. As discussed in Unit 4, "Ecosystems," the basic equation for photosynthesis is:

CO2 + H2O + sunlight → (CH2O)n + O2

From this equation it might appear that plants would need equal amounts of water and CO2, but the actual ratio is approximately 400 to 1. Where does all of this water go? More than 98 percent of a plant's water intake passes upward through the plant from roots to leaves and evaporates, exiting the leaf as water vapor through pores in the leaf surface called stomata. Movement of water from the soil to the atmosphere through the bodies of plants is called transpiration. This process serves many important functions: it carries minerals from the soil to the leaves and prevents leaves from overheating. However, the principle reason that plants transpire is to allow uptake of CO2 from the atmosphere.

As water diffuses out of plants' leaves into the surrounding atmosphere, CO2 diffuses in (Fig. 3). The exchange ratio between CO2 intake and water loss is lopsided: diffusion of water molecules out of the leaf is much greater than diffusion of CO2 into the leaf. This happens because the outside atmosphere is much less moist than the interior of a plant: relative humidity is roughly 50 percent outside, compared to 100 percent at the center of leaves, so water diffuses easily out of plants. In contrast, the atmosphere is only about 0.037 percent CO2, so there is a much smaller contrast between CO2 concentrations inside and outside the leaf.

How plants grow

Figure 3. How plants grow
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This simple relationship describing the diffusional exchange of water and CO2 explains why drought is the major factor limiting agricultural yields worldwide. Because the atmosphere is a very dilute CO2 source, plants need to maximize CO2 intake as long as it will not dry out their interiors. Stomata open to promote gas exchange with the atmosphere when water is plentiful, and constrict or close when water is scarce. If stomata must close to conserve water, the plant will not have access to the CO2 it needs to photosynthesize. Therefore, to encourage growth it is essential to supply plants with enough water.

Many farming regions rely on irrigation to increase productivity and ensure consistent yields regardless of yearly fluctuations in rainfall. One-third of global food harvests come from irrigated areas, which account for about 16 percent of total world cropland. Every year, humans divert about 2,700 cubic kilometers of water (five times the annual flow of the Mississippi River) from the global water cycle for crops. Without irrigation, some countries such as Egypt would be able to support only very limited forms of agriculture, and grain production in northern China, northwest India, and the western Great Plains of the United States would fall sharply (Fig. 4).

Irrigation in the heart of the Sahara

Figure 4. Irrigation in the heart of the Sahara
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Source: National Aeronautics and Space Administration. Earth Observatory.

Nitrogen (N), which plants obtain from the soil, is another critical resource for photosynthesis. Natural levels of N availability frequently limit crop yields. Nitrogen is an essential component of proteins, including the enzyme ribulose-bisphosphate-carboxylase-oxygenase (abbreviated as RUBISCO), which catalyzes the incorporation of CO2 into an organic molecule. RUBISCO is thought to be the most abundant protein on Earth, with leaves typically being 2 percent (by dry weight) nitrogen. This is because RUBISCO, from a catalytic point of view, is one of the slowest enzymes known, reflecting an inherent tradeoff between catalytic efficiency (speed) and selectivity (distinguishing between CO2 and O2).

Before dismissing RUBISCO as inefficient (and thus easily improved upon), it is important to realize the constraints under which it operates. From RUBISCO's point of view, CO2 and O2 are quite similar in many ways. RUBISCO is a very large molecule, along side of which CO2 and O2 appear quite similar in size. Furthermore, the two are both uncharged molecules that can react in a similar manner. However, the real challenge is that the ratio of O2 to CO2 in Earth's atmosphere is greater than 500:1. RUBISCO thus is forced to go slowly so that it can maintain a high selectivity for CO2. Like the diffusional uptake of CO2, which makes photosynthesis extremely water-intensive, this tradeoff for RUBISCO between speed and selectivity means that nitrogen plays an important role in natural and agricultural ecosystems.

Prior to World War I the main source of nitrogen fertilizer was organic manure from livestock animals. Explorers also sought out mineral deposits that could be exploited. Chile derived a major share if its gross domestic product from nitrate (saltpeter) mines from roughly 1880 through World War I. Prior to the exploitation of these naturally occurring mineral deposits, guano (seabird droppings) along the coasts of Chile and Peru and on Pacific islands, where seabirds feed on fish in nutrient-rich coastal waters, were a prized source (Fig. 5). In 1856 the U.S. Congress passed the Guano Islands Act, empowering U.S. citizens to take possession of unoccupied islands anywhere in the world that contained guano deposits if the islands were not under the jurisdiction of other governments (footnote 5).

Guano deposits on Gardner Pinnacles, Laysan Island, Hawaii, 1969

Figure 5. Guano deposits on Gardner Pinnacles, Laysan Island, Hawaii, 1969
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Source: Dr. James P. McVey, National Oceanic and Atmospheric Administration, Sea Grant.

In 1908 German chemist Fritz Haber developed the Haber-Bosch process for combining nitrogen and hydrogen gases at high temperatures to produce ammonia (NH3), which can be processed further into nitrate. The process was commercialized and developed on an industrial scale during World War I and World War II to make nitric acid for munitions. It also launched the fertilizer industry. Synthetic fertilizer entered widespread use after World War II, and the increased levels of nitrogen available to support plant growth boosted crop productivity in regions where farmers could afford synthetic fertilizers.

Producing nitrogen fertilizers requires substantial amounts of energy. Although Earth's atmosphere is about 80 percent nitrogen gas (N2), the triple bond of the dinitrogen molecule is so strong that only a small number of prokaryotic organisms can make use of it. Industrial production of N fertilizers takes place at high temperatures and pressures to crack this bond. In addition, the Haber-Bosch process involves oxidizing natural gas (CH4) over an inorganic catalyst to produce hydrogen gas.

Today nitrogen fertilizers are used on a vast scale. World nitrogen fertilizer consumption was approximately 80 million tons in 1999, with as much as 400 kilograms per hectare applied in areas of highly intensified agricultural production. To put this in perspective, the amount of atmospheric (gaseous) nitrogen incorporated in the production of synthetic fertilizers is of the same order as the amount that occurs globally through biological nitrogen fixation and lightning. In contrast, fossil fuel combustion only releases about five percent of the carbon exchange that occurs naturally through photosynthesis and respiration.

Irrigation and fertilizer help farmers ensure that crops will have the basic inputs they need to grow, but these mainstays of modern agriculture can also cause serious environmental damages. In many regions, irrigation depletes normal river flows or contributes to salinization of agricultural lands (for more information, see Unit 8, "Water Resources"). Fertilizer that is not taken up by plant roots (especially nitrogen, which is extremely mobile in its most common form, NO3 -) can wash into nearby water bodies or into ground water, altering the species composition and nutrient balance of downstream ecosystems. This problem is most severe early in the growing season when plants are small and do not have enough root mass to keep water and nutrients from infiltrating into ground water. Mismanaged livestock manure (discussed in section 6) causes similar problems.

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