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Agriculture and the Nitrogen Cycle
Assessing the Impacts of Fertilizer Use on Food Production and the Environment
By Arvin R. Mosier, J. Keith Syers, John R. Freney ISLAND PRESS
Copyright © 2004 Scientific Committee on Problems of the Environment (SCOPE)
All rights reserved.
ISBN: 978-1-59726-743-4
CHAPTER 1
Nitrogen Fertilizer: An Essential Component of Increased Food, Feed, and Fiber Production
Arvin R. Mosier, J. Keith Syers, and John R. Freney
Nitrogen (N) fertilizer has made a substantial contribution to the tripling of global food production over the past 50 years. World grain production was 631 million tons in 1950 (247 kg person-1) and 1840 million tons in 2000 (303 kg person-1); per capita grain production peaked in 1984 at 342 kg person-1.
Since 1962 annual production of N fertilizer has increased from 13.5 to 86.4 Tg (1 Tg = 1012 g) N in 2001 worldwide (FAO 2004). Unfortunately, the distribution of fertilizer N use is not uniform globally; so in some areas of the world, sub-Saharan Africa (SSA), for example, little fertilizer N is used (in 2001 only 1.1 kg person-1 compared with 22 kg person-1 in China), and local food production has not kept up with the increase in human population. As a consequence the protein supply per person in SSA is only 10 g day-1 compared with 100 g day-1 for people in developed countries. The limited availability of fertilizers in SSA has contributed to the decline in soil fertility through the loss of soil organic matter (Greenland 1988; Syers 1997).
In other areas of the world (e.g., Europe), excessive fertilizer N is sometimes used. Excessive use of N can lead to numerous problems directly related to human health (e.g., respiratory diseases induced by exposure to high concentrations of ozone and fine particulate matter) and ecosystem vulnerability (e.g., acidification of soils and eutrophication of coastal systems) (Cowling et al. 2001, Boyer and Howarth 2002, Galloway et al. 2002b, Mosier et al. 2002).
Little new land is suitable for crop production; therefore, the output per unit area must increase to meet an expected world population of 8.9 billion people by 2050 (FAO 2004). If the efficiency of nitrogen use (NUE) is not improved, marginal lands, including those on steep slopes, will be brought into production to help meet rising food needs, and the result will be increasing land degradation. Because of the limitation on arable land area and the need to minimize the pollution of waters and the atmosphere, the efficiency of the use of fertilizer N must be improved to sustain land quality to feed the growing population (Cassman et al. 2002).
Global Nitrogen Fertilizer Consumption
The global demand for N fertilizer is dictated largely by cereal grain production (Cassman et al. 2002). From 1995 to 1997, about 65 percent of the global N fertilizer consumed was for producing cereal grains (IFA/FAO 2001). IFA and FAO project that the relative amount of N fertilizer used by 2015 will remain unchanged but that total N consumption in cereal production will increase by about 15 percent. The increased demand for cereal production, and thus N fertilizer, is fueled mainly by human population growth but also by increased consumption of animal products on a per capita basis (Boyer et al., Chapter 16; Roy and Hammond, Chapter 17; Wood et al., Chapter 18, this volume). During the 40 years between 1961 and 2001, the human population of the world doubled from 3078 to 6134 million persons (FAO, 2004); grain production, meat production, and N fertilizer consumption, however, increased by 140, 230, and 600 percent, respectively. On a per capita basis, the respective increases were 21, 67, and 254 percent during this period.
Fertilizer N has contributed an estimated 40 percent to the increases in per capita food production over the past 50 years (Brown 1999; Smil 2002). This global figure does not reflect local and regional differences in food supply and demand. It also does not reflect the varying efficiencies of fertilizer N use in crop production across regions. For example, in 2001, on a per capita basis, N fertilizer consumption in the United States was 38 kg person-1, 11 kg person-1 in India, but only 1.1 kg person-1 in SSA. There are a variety of reasons for the inequities in fertilizer N distribution around the globe. In some parts of Asia, Europe, and North America, fertilizer is relatively inexpensive and available to farmers. In SSA and in parts of Asia, the cost is high (as much as five times the global market price; Roy and Hammond, Chapter 17, this volume) and supply is limited.
As a result of the high cost and the limited availability of fertilizer, grain production in SSA was limited to 124 kg person-1 compared with 237 kg person-1 in India, where fertilizer is more readily available, and 1136 kg person-1 in the United States, where fertilizer is both inexpensive and readily available (Palm et al., Chapter 5, this volume; FAO, 2004). In regions like North America, people consume near-double maintenance levels of both protein (114 g day-1 total) and calories (3700 kcal day-1 total), whereas many people within SSA have lower-than-needed protein and calories available for consumption.
The fact that N fertilizer is not used efficiently is in part responsible for these issues. On average the crop takes up only 20 to 50 percent of the N applied to soil for cereal crop production. Although N fertilizer use is low in many parts of the world, the NUE may be lower than in areas where consumption is higher. Low efficiency of N is typically caused by an insufficiency of other required nutrients (e.g., P, K, and secondary and micronutrients, Aulakh and Malhi, Chapter 13, this volume), which limits plant growth along with N. In rice production, NUEs of 30 percent or lower are typical in many regions, whereas efficiencies approaching 70 percent are not uncommon in areas of intensive maize production (Dobermann and Cassman, Chapter 19, this volume). Even in these high-efficiency regions, losses of N occur, exacerbating water-quality problems both locally and downstream of crop production areas.
Agricultural Nitrogen Cycle
Fertilizer supplies about 50 percent of the total N required for global food production. In 1996 global fertilizer N consumption totaled 83 Tg N (Smil 1999), and consumption has increased little since then, for example, 84.1 Tg N in 2002 (FAO 2004). Therefore, Smil's estimates of the global N flows are probably still appropriate and are used here. The other annual inputs into crop production — biological N-fixation (~33 Tg; 25–41 Tg), recycling of N from crop residues (~16 Tg; 12–20 Tg) and animal manures (~18 Tg; 12–22 Tg) (Figure 1.1), atmospheric deposition, and irrigation water (not shown in Figure 1.1) — provide an additional ~24 Tg (21–27 Tg) (Smil 1999). Of the ~170 Tg N added, about half is removed from the field as harvested crop (~85 Tg). The remainder of the N is incorporated into soil organic matter or is lost to other parts of the environment for which global estimates of individual loss vectors are highly uncertain. Leaching, runoff, and erosion account for ~37 Tg of the annual N losses; ammonia volatilization from soil and vegetation contributes ~21 Tg yr-1. Denitrification losses as gaseous dinitrogen (N 2) amount to ~14 Tg yr-1, and N2O and NO from nitrification /denitrification contribute another ~8 Tg N to the total loss (Smil 1999; Balasubramanian et al., Chapter 2; Peoples et al., Chapter 4; Goulding, Chapter 15; Boyer et al., Chapter 16, this volume). Van der Hoek (1998) also estimated that more than 60 percent of the annual N input into food production was not converted into usable product. Thissurplus N, defined as the difference between input and output, is either lost to the environment or accumulates in the soil. Agricultural soils in the United States (and probably most of those in Western Europe) are considered to be at near steady state for soil accumulation of N; thus, all inputs not removed from the field in crops are likely to be lost to the atmosphere or aquatic systems (Howarth et al. 2002).
The relative inefficiencies of animal protein production exacerbate the inefficiencies of N utilization. Larger N losses from global food production are likely in the future as the human population and the demand for animal protein increase (Galloway et al. 2002a). The increase in consumption of animal products worldwide, except for regions within SSA, has been accompanied by an intensification of animal products in some regions, particularly North America. Because of the centralization of livestock production in regions that produce relatively little animal feed, the areas of crop production located close to the intensive animal-production systems are not adequate to carry the load of animal waste input. As a result, the remainder of the N is stored in lagoons or solid piles (Smil 1999) or distributed elsewhere, partly through NH3 volatilization, surface runoff, leaching, and wind erosion. Most of the volatilized NH3 is deposited near the feedlot, but significant amounts can be converted to aerosols and transported 1000 km or farther. Much of the remaining "unused" N eventually finds its way into ground and surface waters. These losses can contribute to environmental and human health problems (Peoples et al., Chapter 4, this volume).
Environmental and Human Health Impacts
One of the most important impacts of N on the environment is that on water quality. Because N is frequently the nutrient most limiting biological productivity in estuaries (Vitousek et al. 1997), inputs of soil and fertilizer N from agricultural land can be a major contributor to N-induced eutrophication. The excessive growth of algae and macrophytes, the resulting oxygen depletion, and the production of a range of substances toxic to fish, cattle, and humans are now major pollution problems worldwide (Howarth et al. 1996). In contrast, low levels of N in soil can be a causative factor in soil erosion, which is a major contributor to land degradation. An insufficient amount of plant-available N can limit plant growth, resulting in reduced canopy interception of rainfall and less soil-binding by plant roots, both of which result in increased soil loss and can have major impacts on water quality through sedimentation and the release of N and P, causing excessive growth of aquatic nuisance plants.
According to Townsend et al. (2003), increases in reactive N in the environment have some clear and direct consequences for human health; air pollutants, primarily nitrogen oxides (NOx) and dietary nitrate, have been issues of concern. In the case of dietary nitrate, much confusion and controversy remain (McKnight et al. 1999; Peoples et al., Chapter 4, this volume).
Almost 60 years ago, high nitrate (which can be reduced to nitrite in the intestine) concentrations in drinking water drawn from local wells (Comly 1945) were implicated in the incidence of infantile methemoglobinemia ("blue baby syndrome"). In recent years this view has been challenged, and strong evidence now exists that endogenous nitric oxide/nitrite production, triggered by intestinal infection rather than exogenous dietary nitrate intake, is responsible (McKnight et al. 1999; L'hirondel and L'hirondel 2002). This condition now appears to be rare in the developed world, where nitrate levels in drinking water are higher than they previously were and for the most part are increasing; in less-developed countries, ingestion of contaminated water, and its associated gastroenteritis, appears to be a more likely cause of methemoglobinemia (Leifert and Golden 2000).
The changing situation with regard to dietary nitrate and gastrointestinal cancer is equally interesting. Early thinking called for restrictions on nitrate levels in food because of the formation of carcinogenic nitrosamines by nitrosation of amines in the gastrointestinal tract (McKnight et al. 1999); however, not only is the incidence of gastric and intestinal cancers reduced in groups who consume vegetables high in nitrate (Corella et al. 1996), but there is also a worldwide decline in the incidence of gastric cancer (Correa and Chen 1994) at the same time the nitrate content and intake of green vegetables are increasing (McKnight et al. 1999). Epidemiologic studies point toward a possible protective effect of nitrate (L'hirondel and L'hirondel 2002). These studies suggest that dietary nitrate, which determines the production of reactive nitrogen oxide species in the stomach, is an effective host defense against gastrointestinal pathogens and can have beneficial effects against cancer and cardiovascular diseases.
The nitrate–human health issues remain controversial, and a thorough reevaluation is timely. This area is an important one for further work, given that nitrate levels in groundwater in Europe are sometimes larger than the currently recommended safe levels.
Prospects for Increasing Nitrogen Use Efficiency
As pointed out in several chapters of this volume, fertilizer N has a low efficiency of use in agriculture (10–50 percent for crops grown in farmers' fields; Balasubramanian et al., Chapter 2, this volume). One of the main causes of low efficiency is the large loss of N by leaching, runoff, ammonia volatilization, or denitrification (Raun and Johnson 1999), with resulting contamination of water bodies and the atmosphere. With the limitation on arable land area and the need to minimize the pollution of waters and the atmosphere with reactive N derived from N fertilizer, the only way to continue to feed the increasing population is to increase the efficiency of use of fertilizer N (Cassman et al. 2002).
It is important to know the forms and pathways of N loss and the factors controlling them so that procedures can be developed to minimize the loss and increase the NUE. Investigations have shown that the predominant loss process and the amounts lost are influenced by ecosystem type, soil characteristics, cropping and fertilizer practices, and prevailing weather conditions. As a consequence, losses can vary considerably over small distances within a field because of soil variability, from region to region because of differing cropping practices, and with time over a growing season because of climate. In Europe, where nitrate forms of fertilizer dominate, nitrate leaching and denitrification are the main loss pathways; in the rest of the world, where urea is the main fertilizer used, ammonia volatilization tends to be more important (Goulding, Chapter 15, this volume).
Volatilization of added N as ammonia from fertilized grassland (13 percent of added N), upland crops (18 percent), and fertilized rice (20 percent) in developing countries exceeds that lost in developed countries (6, 8, and 3 percent, respectively; IFA/FAO 2001). The largest losses overall and the lowest NUEs, however, tend to occur in the developed world (Goulding, Chapter 15; Dobermann and Cassman, Chapter 19, this volume). The low efficiency in the developed world occurs because farmers often apply excess N as insurance against low yields. The relatively low cost of fertilizer N compared with the value of the crop product lost in the developed world has led to its misuse and overapplication. The same does not usually hold true in the developing world, where access to fertilizer is limited (Hubbell 1995).
Many approaches have been suggested for increasing fertilizer NUE, including the optimal use of fertilizer form, the rate and method of application, matching N supply with crop demand, optimizing split application schemes, supplying fertilizer in the irrigation water, switching from urea to calcium ammonium nitrate to limit ammonia loss, minimizing application in the wet season to reduce leaching, applying fertilizer to the plant rather than to the soil, changing the fertilizer type to suit the conditions, and using slow-release fertilizers (Balasubramanian et al., Chapter 2, this volume). The genetic variation in both acquisition and internal-use efficiencies indicates potential for further increases in NUE through plant selection (Giller et al., Chapter 3, this volume).
In addition, agronomic practices that improve early crop growth, reduce competition for N uptake by weeds, reduce pest incidence, and improve irrigation and drainage will increase the NUE. Dobermann and Cassman (Chapter 19, this volume) provide an example of how such external factors, in addition to N management, can increase the NUE. The factors involved in increasing this efficiency in corn production in United States from 42 to 57 kg grain kg N-1 were (1) greater stress tolerance of modern maize hybrids; (2) improved management (conservation tillage, better seed quality, higher plant densities, weed and pest control, balanced fertilization with other nutrients, irrigation); and (3) improved matching of the amount and timing of applied N to the indigenous supply and crop demand.
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Excerpted from Agriculture and the Nitrogen Cycle by Arvin R. Mosier, J. Keith Syers, John R. Freney. Copyright © 2004 Scientific Committee on Problems of the Environment (SCOPE). Excerpted by permission of ISLAND PRESS.
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