Chapter 16
Fertility Management Effects on Runoff Losses of Phosphorus 1
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H . A . Torbert and Κ . N. Potter
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National Soil Dynamics Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 4116 Donahue Drive, Auburn, A L 36832 Grassland, Soil and Water Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 808 East Blackland Road, Temple, T X 76502
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Most agricultural soils are deficient in at least one of the essential nutrients for plant growth, or the complex processes of nutrient cycling interact to limit their availability. These nutrients are commonly added to soil as fertilizers i n either inorganic or organic forms. However, poor application of fertilizers (both inorganic and organic) can lead to nutrient loss from agricultural land by runoff and leaching. These potential non-point source nutrient losses can contribute to environmental degradation, eutrophication of surface waters, and possible human health risks. These concerns have been stimulated by blooms of the toxic dinoflagellate algae (Pfiesteria piscicidia) that have caused fish kills and human illnesses and by reports of a hypoxic area (low dissolved oxygen) in the Gulf of Mexico (20,000 km ) (1), which have been attributed to pollution from excess nutrients (2). Nonpoint source pollution from agriculture has been identified as the leading source of water quality reduction by the U S E P A (3), with estimates that agriculture affects the degradation of 60% of river miles, 50% of lake acres, and 34% of estuaries acres. While these impairments are not nutrient specific, both Ν and Ρ have been implicated in causing accelerated eutrophication. The role of nutrients in the eutrophication process is very complex, but in general, lake eutrophication is associated with Ρ, Ν is associated with ocean waters, and both Ν and Ρ are associated with estuaries (4, 5). Factors affecting non-point loss of nutrients from agricultural soils are numerous and complex. Many of the factors that affect nutrient losses can be greatly impacted by agricultural management practices. Scientists have continued to research and develop methods to improve agricultural practices to reduce nutrient losses. 2
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U.S. government work. Published 2004 American Chemical Society In Environmental Impact of Fertilizer on Soil and Water; Hall, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Rainfall Simulators One method to help study factors affecting nutrient losses from agriculture is the rainfall simulator. Rainfall simulators reproduce conditions consistent with storm events that generate water runoff. Because of numerous advantages, the use of rainfall simulators for scientific studies of erosion and infiltration have been utilized for many years (6, 7, 8). The scientific validity of using rainfall simulation for research work to evaluate nutrient losses in runoff has also been well documented (9, 10,11, 12, 13, 14). Several different types of rainfall simulators have been described in the scientific literature, including simulators used in laboratory studies (15), as well as both microplot and mesoplot scale setups (16) used in field research. For example, Sharpley and Moyer (15) described a rainfall simulator used to produce rain for soil columns in a laboratory study, while Sumner et al. (16) described a rainfall simulator for mesoplot runoff studies, in which rainfall was simulated in a 600 m area. Most rainfall simulators use the microplot size study area and have been used for a variety of scientific studies of non-point nutrient losses from agriculture (12). For example, in the studies described by Torbert et al. (17,18) a rainfall simulator was used similar to that described by Miller (19), using a Spraying Systems Wide Square Spray 30 W S Q nozzle at a nominal rate of 125 mm h" , producing a drop size of 2.5 mm, and kinetic energy of 23 J m" mm" . In that case, an 1-m area on 2-3% slope was used as the plot size by surrounding the plot with a metal frame driven 0.1 m into the soil to define the study area. Rainfall application was also made to a 10-m region around the study site which was surrounded by a tarp curtain to prevent interference of drops by wind. Similar systems were used to collect most of the data described here. 2
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Conservation Tillage One effective means of reducing non-point source pollution from crop land is the use of conservation tillage systems. These systems are known to be very effective in reducing erosion and limiting the amount of nutrients that leave the field in sediment (20,21,22,23,24, 25). Potter et al.(
