Environmental Safety of Genetically Engineered Crops

Environmental Issues Associated with Agricultural Production Systems

15 Environmental Issues Associated with Agricultural Production Systems Kurt D. Thelen Agriculture is one of the predominant uses of land worldwide. Approximately 40% of the world’s land area, at least 1.5 billion hectares, is under cultivation, and over two-­ thirds of human water use is for agriculture (FAO 2010; Millennium Ecosystem Assessment 2005). Agricultural production systems impart a significant environmental footprint on the surrounding ecosystems. The quality of soil, water, and air is affected by agricultural activities occurring on the surrounding landscape. Many of these activities adversely affect environmental quality, contributing to desertification, soil degradation, deforestation, global warming, and water scarcity (FAO 2010; Millennium Ecosystem Assessment 2005). However, improved agricultural management practices minimize the adverse environmental impacts associated with agricultural production, and in some cases can even improve the quality of soil, water, and air. Soil Degradation of soil has been an issue since the onset of agriculture. Tillage disturbance of soil for seedbed preparation, weed control, and the addition of nutritional amendments is an integral part of agriculture. These practices, though historically essential for food production, have resulted in tremendous soil erosion and loss of soil productivity on a global basis. Lal (2003) estimates that water and wind erosion affect 1,094 and 549 million hectares annually, leading to loss of soil on-­ site. Erosion is particularly problematic in regions with sloping land, and around 45% of the world’s agricultural land has slopes of greater than 8% (FAO 2002). Erosion also leads to off-­ site damage. In the United States, the annual off-­ site damage from soil erosion caused by water recreation, water storage, navigation, flooding, and municipal water treatments is estimated at $8.8 billion (CTIC 2006). Cropland erosion in the United States is estimated at 4 billion tons of soil lost from 160 million hectares of farmland annually at an estimated cost of $20 billion for replacement nutrients and $7 billion for lost water and soil depth (Pimentel et al. 1995). Adverse soil effects also include nutrient depletion and salinization. Farmers often use insufficient fertilizers to replace lost nutrients, leading to nutrient mining and lost soil productivity (FAO 2002). Salinization can result from the use of irrigation in arid and semiarid 16| Kurt D. Thelen environments where rainfall is inadequate to provide leaching needed to remove excess salts from the soil. It has been estimated that 40% of the world’s arable land is subject to this problem (Smedema and Shiati 2002). Heavy irrigation also can cause waterlogging of the soil, which brings soil-­ borne salts closer to the surface. High evaporation rates remove the water and leave behind the salt, eventually leading to a salt crust on the soil surface. Water Use and Quality Use of water for irrigation results in lowered water tables, increased river diversions to agricultural fields, and salinization of some water tables in arid or semiarid regions. In some areas of India and China, water tables are dropping by 1–­ 3 meters per year because of heavy withdrawals of water for agricultural production (FAO 2010). Agriculture is already the primary use of water, and demands for agricultural use, and possible associated shortages, are expected to increase over the next decades (FAO 2010). Agriculture is the leading source of pollutants in assessed streams, rivers, and lakes in the United States, as determined by the Environmental Protection Agency (US EPA). As an industry, agriculture adversely affects 18% of assessed streams and rivers and contributes 48% of the water quality problems in the rivers and streams classified by the US EPA as having impaired water quality. Agricultural pollution problems affect 18% of lakes assessed by the US EPA and contribute 41% of the reported quality problems in lakes characterized as impaired. Leading pollutants attributable to agriculture include siltation, dissolved solids, bacterial pathogens, habitat alterations, oxygen-­ depleting substances, carried nutrients, and thermal modifications. Of these, siltation, sometimes referred to as sedimentation, and carried nutrients are generally identified as having the most extensive effect. Sediment moving from agricultural fields through erosion finds its way to streams, rivers, and lakes. Once in aquatic ecosystems, sediment can cause algal blooms and eutrophication from carried nutrients; reduce available habitat where fish lay eggs; deplete available oxygen through the biological oxygen demand of microorganisms; degrade the organic fractions carried with sediment; block sunlight penetration, reducing growth of beneficial aquatic plants; and in severe cases, abrade gills of indigenous fish. As algae die, they sink to the bottom, where microbial degradation consumes oxygen, resulting in the suffocation of fish...