(2003) and Oki and Kanae (2004) estimated global virtual water trade for the year 2000. They assumed a constant global average crop water requirement.
Grote, U., Craswell, E.T. & Vlek P.L.G. (2008). Nutrient and virtual water flows in traded agricultural commodities. In: A.K. Braimoh & P.L.G. Vlek (Eds.), Land Use and Soil Resources. (pp. 121-143). Springer Science+Business Media B.V.
Chapter 7 Nutrient and Virtual Water Flows in Traded Agricultural Commodities Ulrike Grote, Eric T. Craswell, and Paul L.G. Vlek Abstract Globalization and increasing population pressure on food demand and land and water resources have stimulated interest in nutrient and virtual water flows at the international level. West Asia/North Africa (WANA), Southeast Asia, and sub-Saharan Africa are net importers not only of nitrogen, phosphorus, and potassium (NPK) but also of virtual water in agricultural commodities. Nevertheless, the widely recognized declines in soil fertility and problems related to water shortage continue to increase, especially in sub-Saharan Africa. The nutrients imported are commonly concentrated in the cities, creating waste disposal problems rather than alleviating deficiencies in rural soils. And also the water shortage problems continue to contribute to intensified desertification processes, which again lead to increased urbanization and thus water shortage problems in cities. Countries with a net loss of NPK and virtual water in agricultural commodities are the major food exporting countries—the USA, Australia, and some Latin American countries. Understanding the manifold factors determining the nutrient and water flows is essential. Only then can solutions be found which ensure a sustainable use of nutrients and water resources. The chapter ends by stressing the need for factoring environmental costs into the debate on nutrient and water management, and advocates more transdisciplinary research on these important problems. Keywords Nutrient flows, virtual water flows, international trade, environmental degradation
7.1 Introduction Vast quantities of water and nutrients are employed in the production of the food that is traded globally. The term “virtual water” refers to the water volume which is used in the process of producing food (Allan, 1997). For example, it has been found that it takes 500–4,000 l of water to grow 1 kg of wheat depending on the location and the technology. In comparison, the production of 1 kg of beef requires about 10,000 l due to the amount of feed consumed by the animals (de Fraiture & Molden, 2004). 121
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Similarly, it requires different amounts of nutrients to produce 1 kg of wheat, rice, potatoes, or beef, although the regional nutrient requirements vary less based on climate, location, and technologies than compared with water. However, in the case of nutrients, the international transfer in traded commodities is of real not “virtual” nutrients. Changes wrought by humans in nutrient cycling and budgets are complex and vary widely in magnitude across the globe. In the 35 years between 1961 and 1996, nitrogen fertilization increased 6.87-fold and phosphorus fertilizer use increased 3.48-fold as food production increased 1.97-fold, according to Lambin et al. (2003). On the other hand, Vlek et al. (1997) estimate that 230 Tg1 of plant nutrients are removed yearly from agricultural soils, whereas global fertilizer consumption of N, P2O5, and K2O is 130 Tg. In the case of nitrogen the estimated 90 Tg from biological fixation must be added to the nitrogen supply. Developing countries now consume half the global fertilizer production, but the use is uneven since cereal crops grown on the irrigated lands of Asia and cash crops receive most of the nutrients. At the other end of the scale, rainfed areas producing subsistence food crops in the tropics, particularly in sub-Saharan Africa, receive little or no fertilizer. In these areas, the farms of poor smallholders develop negative nutrient balances that render continued crop production unsustainable (Stoorvogel & Smaling, 1990). This exploitation of native soil fertility is coupled with the decomposition and decline in soil organic matter that contributes carbon dioxide to climate change. At the global scale, Miwa (1992) analyzed trends in international trade in food commodities that led to significant negative balances in exporting countries and accumulations in importing countries. Japanese scientists recognized the importance of this problem in their own country which, as a major food and feed grain importer, faces serious nutrient disposal problems due to pollution and eutrophication (Miwa, 1992). Penning de Vries (2006) took this analysis further, describing the environmental problems at both the sources and the sinks of nutrients that move in food commodities. The increasing demand for livestock products in developing countries and the expansion of feed grain use exacerbate the problems (Bouwman & Booij, 1998). Livestock production significantly affects the environment because, as indicated by data for nitrogen, the average efficiency of nutrient conversion from feed to animal products is only 10%, while on efficient dairy farms the range is 15–25% (van der Hoek, 1998). Not only the quality of water is negatively affected through nutrients or antibiotics used in intensive livestock production, but also the total amount of water use as such is immensely high having respective consequences for virtual water contents (Naylor et al., 2005). The environmental impacts of inter- and intra-national nutrient flows through trade commonly concentrate in the burgeoning cities (Penning de Vries, 2006). For _____________________ 1
This chapter utilizes SI units as follows: Mg = 1,000 kg (1 metric ton); Gg = ,000,000 kg (1000 t); Tg = 1,000,000,000 kg (or 1 million t); unless specified as the oxide forms P2O5 or K2O, amounts of P and K are converted to, and expressed as, uncombined elements; the exceptions are where quoted papers expressed combined NPK data in oxide forms that could not be converted to elemental forms. 122
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example, Faerge et al. (2001) estimated that 20,000 Mg of nutrients were annually imported in food into Bangkok, and that large amounts of nutrients were lost, mainly to the waterways. Coping with large inputs of nutrients in the environment is a major problem facing urban administrations, and the problems are likely to get worse as urban populations grow. Similar problems occur in intensive animal production systems. Nutrients can be recycled through the application of solid or liquid wastes to urban and peri-urban crops and forages but, in spite of the obvious benefits, the extent of recycling is limited in most cities; the potential health problems are a constraint, but safe methods of using wastes are available (Keraita et al., 2003). Nutrient outflows from fertilized agricultural lands into coastal zones are a widespread problem, particularly in industrialized and developing countries where urea is used excessively (Glibert et al., 2006). In marginal uplands soil erosion by water (and in some cases by wind) also enriches surface waters with nutrients. Some sediment may be deposited and enrich lowland areas. However, annual net ocean outflows of sediments in Asia are as high as 7,500 Tg, representing a major loss of nutrients to the countries concerned (Milliman & Meade, 1983; Craswell, 2000; Syvitski et al., 2005). Global river flows of dissolved inorganic nitrogen to the oceans have been estimated at 48 Tg per annum, whereas 11 Tg per annum is transported to drylands and inland waters (Boyer et al., 2006). Urban wastewater sources of nitrogen represent 12% of the nitrogen pollution in rivers in the USA, 25% in Europe, and 33% in China (Howarth, 2004). These flows of nutrients are in turn affected by human diversions of surface water, such as dams that collect silt and reduce flows to natural wetlands. The impact of human development on nutrient fluxes in rivers and the seas represent major perturbations to natural terrestrial and aquatic nutrient cycles. In addition, it is important to consider the effects of fertilizers, the production and transport of food, and land transformation for agriculture on fluxes of nitrous oxide because the gas contributes significantly to the greenhouse effect and ozone depletion. Furthermore, nitrates and other nutrients accumulate in groundwater and can affect human health. Nitrogen has been more extensively studied than the other macronutrients, phosphorus and potassium, possibly because human impacts on the global nitrogen cycle extend to industrial perturbations, and atmospheric as well as terrestrial and aquatic phases (Galloway & Cowling, 2002). While trade in nutrients is mainly considered a reason for environmental problems, trade in virtual water is partly seen as a solution to environmental problems. Trading virtual water is expected to ease some of the pressure related to water shortage and desertification, since countries buying food also purchase water resources, thereby saving water they would have needed for producing the food domestically. It is, for example, estimated that globally 8% of the total water needed for food production can be saved through international food trade (Oki et al., 2003). But also at a national level, situations can arise where food trade becomes necessary due to water shortage problems, as it happens in the case of China: North China has become a major food producing area for the south of China although it faces severe problems of water scarcity. Nearly 10% of the water used in agriculture in the north is used to produce food for south China. 123
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To compensate north China for this virtual water flow, a South–North Water Transfer Project is currently being implemented. This leads to the paradoxical situation of planning to transfer huge amounts of water from the south to the north, while exporting substantial virtual water from the north to the south (Ma et al., 2006). In many countries, the transfer or import of virtual water may minimize the need to build dams and diversions thereby reducing the negative social and environmental impacts (see Chapter 6). Our brief review above indicates that a substantial, though fragmented knowledge base is developing on the agricultural, ecological, and environmental aspects of alterations to nutrient and virtual water flows and balances at different scales. Many of the ecological aspects are now better understood, but the economic impacts and implications of these perturbations to nutrient cycling have been relatively neglected. One exception in the area of nutrient balances is the work of Drechsel and Gyiele (1999) who developed a framework for the economic assessment of soil nutrient depletion. They showed that the annual cost of replacing nutrients lost from arable land in countries of sub-Saharan Africa ranges from
