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Abstract. Evidence indicates a strong positive relationship between increases in nitrogen fertilizer use on cropland and nitrate concentrations in shallow "round water. This raises concern about the fate and efficiency of nitrogen fertilizer with current farming practices. Approximately 50 percent of the nitrogen fertilizer applied may be recovered by agronomic crops and 35 percent or less removed in the harvested grain of a crop such as corn. The residual nitrogen is subject to loss by several processes, one being leaching from the crop root zone. Alternative production systems that provide "round water protection must give attention to improved management of nitrogen fertilizer and to practices that minimize the need for nitrogen fertilizer and reduce soil nitrate concentrations. Most important in nitrogen fertilizer management is to more closely match nitrogen availability in the soil with crop needs and to avoid over-fertilization. Nitrogen fertilizer use can be reduced by alternate cropping of low and high nitrogen -demanding crops, use of legumes in the crop rotation to fix nitrogen, an d proper use of manures, crop residues, and other organic wastes. Residual nitrates in soil can be reduced by use of cover crops, nitrogen-scavenging crops in the rotation, and alternating shallow and deep-rooted crops Conservation tillage alone as used with many conventional cropping systems will probably not change the current statue of nitrate leaching Practices used by organic farmers should be carefully studied as possible approaches for "round water protection and adaptation into conservation tillage systems for conserving soil and water resources.
Growing evidence in the U.S. and abroad indicates a strong positive correlation between increases in use of nitrogen fertilizer on cropland and nitrate concentrations in shallow "round water (Hallberg, 1986). This is raisin" major questions about the efficiency and fate of nitrogen fertilizer with current soil and crop management practices. Nitrogen fertilizer use in the U.S. has escalated in recent decades as agriculture has shifted from integrated crop/livestock systems and sod-based rotations to intensive production of high nitrogen-demanding crops such as corn (Zea mays L.), wheat (Triticum aestivum L.), and sorghum (Sorghum bicolor (L.) Moench) (see Fig. 1). Fertilizer nitrogen rates are commonly based on maximum yield goals. At these levers of application, nitrogen recovery by agronomic crops averages about 50 percent (Hallberg, 1986). However, only 35 percent or less of the amount applied is removed in the harvested grain of corn (Hallberg, 1986) and possibly even less for wheat (Power et al., 1986) in the first year. Nitrogen not recovered by the crop is subject to loss by several processes, including leaching below the crop root zone.
Nitrates leach readily, but to be lost from the root zone nitrates must be present in the soil profile and the water in which they are dissolved must percolate below the root zone. The potential for nitrate reduction or denitrification is another important factor controlling the amount of nitrate leaching. Any management practice that alters these conditions will affect the amount of nitrate that moves into ground water.
Problems with nitrate leaching are often associated with heavy fertilization in high rainfall areas, or with irrigation on sandy soils, and sometimes with large feedlot operations (Keeney, 1982). However, nitrate leaching has also been reported in low rainfall areas (Campbell et al., 1983). In the Big Spring Basin in Iowa the increase in nitrate in "round water directly paralleled the increase in the amount of fertilizer nitrogen applied (Hallberg, 1986). From the results of analysis of 124,000 samples of well water, the U.S. Geological Survey developed a national map showing that frequency of high nitrate samples appeared to be associated with irrigation, intense cultivation, high rates of manure usage, confined livestock and poultry operations, high density of septic tanks and geological sources (Madison and Burnett, 1984).
What alternatives are available, either in principle or in actual practice, that will eliminate or minimize the leaching of nitrates below the crop root zone? The solution is efficient management of nitrogen from all sources, whether inorganic fertilizer, biological fixation, manures, crop residues, or soil organic master. Moreover, practices for "round water protection must be compatible with existing goals of soil and water conservation, surface water quality, and profitable production. All considerations must be integrated into the farming system.
Fertilizer nitrogen, the main concern, is a key input that has enabled U.S. agriculture to achieve its present capability for producing plentiful quantifies of inexpensive food. Modem production technology is heavily dependent on nitrogen fertilizer. Nearly half of the nitrogen that U.S. farmers returned to the soil in 1977 was commercial fertilizer (Power, 1981), and since that time nitrogen has continued to be heavily used (see Fig. 1). Because of the importance of fertilizers in food production, realistic alternatives to reduce nitrates in "round water should not seek to eliminate fertilizer use, but rather to find ways to improve nitrogen efficiency, thereby minimizing losses to the environment as well as costs to farmers.
Nitrate losses from the crop root zone can be reduced in several ways, including improved fertilizer management to match nitrogen availability with crop needs, and the use of crop rotations with legumes to reduce fertilizer requirements for following grain crops. Other methods include the use of cropping practices to more efficiently utilize residual nitrogen and to keep nitrate concentrations in the soil at low levers.
Eliminating excessive nitrogen applications: The accumulation of nitrates in soil is caused primarily by over fertilization in relation to crop needs. One cause of excessive applications is the lack of a reliable soil test for nitrogen. This is true especially for the more humid areas. In most places, fertilizer recommendations are based on yield goals and usually for maximum production. If the expected yield is overestimated or the soil's ability to supply nitrogen is underestimated, nitrate build-up is likely. Hallberg (1986) reported studies in Nebraska and Iowa which suggested that about half of all farmers over-fertilize by 20 to 25 percent, based on the yield they actually obtain. A recent Nebraska study showed that 51 percent of irrigated corn producers surveyed used more than the recommended fertilizer N rates and that the average fertilizer rate on 239 fields could be reduced 88 kg N per ha without reducing corn yields (Schepers, 1982). In our own recent experience in the Pacific Northwest, it is not unusual for farmers' fields to show no response of winter wheat to nitrogen application rates of 110 to 130 kg/ha. Yet fertilizer rates have not decreased even with evidence of considerable residual nitrogen in many fields.
Farmers often tend to over-fertilize as insurance so that lack of nitrogen will not limit yields. At present there is no universally reliable soil test for nitrogen availability. In some of the longer rainfall areas, residual soil nitrate is a useful indicator of fertilizer nitrogen requirement. However, this test is of little value where leaching occurs between sampling and the time that the plant needs the nitrogen. Also lacking is a rapid soil test that provides an estimate of the amount of nitrogen the soil can supply during the growing season.
More research emphasis is needed in developing procedures and methods for accurately predicting the amount of nitrogen fertilizer required for optimum yields. Some corn hybrids are much more efficient than others in utilizing soil nitrates during the grain-fill period an added factor that must be considered Local field experiments which have been properly interpreted by research and extension workers can also help improve estimates of the nitrogen supplying capacity of the soil and of fertilizer needs Farmers also need to be better educated not only on the environmental concerns associated with over-fertilization but on economic losses as well.
Timing of fertilizer applications: Fertilizer efficiency can be improved by making the application near the time of maximum vegetative growth or by making several applications to match crop needs (Stewart et al., 1976; Keeney 1982). Practical examples are summer side-dressing of corn, where the fertilizer is applied several weeks after the crops started to grow, and supplying incremental units of nitrogen in irrigation water. With these practices, fertilizer is used more efficiently and less is required which helps offset added costs of application. Applying fertilizer through the irrigation water has the disadvantage in wet years of requiring more N at times when irrigation is not needed. For some crops such as winter wheat, split applications where part of the nitrogen is applied in the fall and part by top-dressing in the spring provide a better distribution of nitrogen in the soil than a single application. Hence, uptake is more efficient. This works well for winter wheat in the Pacific Northwest, for example, which has a winter rainfall climate.
Fall application of fertilizer should be of concern where there is potential for leaching during the winter. Stewart et al., (1975, 1976) combined a nitrification model with a percolation model to construct detailed maps based on hydrologic characteristics of soil for estimating average nitrate leaching losses of fall-applied and spring-applied ammonia. They emphasize that these maps can serve only as first approximations and must be adjusted for local conditions. Research in central Nebraska shows that most nitrate leaching for irrigated corn occurs during the non-crop period from October to June as a result of normal precipitation.
Fertilizer source and controlled release: Nitrogen sources should be selected and application timed to avoid nitrate accumulations in the soil when leaching may occur. For example, losses could be substantial if a nitrate source is applied before a rainy period, or if an ammonium source nitrifies and the nitrate is subject to leaching before the crop can use them. Because of potential loss even in the colder climates, fall fertilization of any form of nitrogen for spring crops is now generally discouraged as an environmentally and economically unsound practice (NRC, 1978).
There has been considerable research over the past two decades on controlled release nitrogen fertilizers and nitrification inhibitors as a means of matching nitrogen availability with plant requirements. Sulfur-coated urea is a slow-release fertilizer, since the sulfur coating is slowly soluble. Swoboda (1977) reported that nitrate leaching was decreased by 53 percent where sulfur coated urea was used instead of soluble sources of nitrogen. The concept of controlled release is plausible, but it is difficult to make the nitrogen available according to the plant's needs. Also, carryover of fertilizer nitrogen from sulfur coated urea is almost always greater than from inorganic nitrogen fertilizers (Liegel and Walsh, 1976; Allen et al., 1978). This nitrogen becomes subject to leaching after harvest. The higher cost of slow release fertilizers can be prohibitive. Also, where soil acidification is a problem, use of fertilizer materials such as sulfur-coated urea will hasten the acidification process.
Nitrification inhibitors such as nitrapyrin [2-chloro-6-(trichloromethyl) pyridine (N-Serve)] are also effective in reducing the rate at which ammonium fertilizers are nitrified. Slowing nitrification may provide a better time distribution of nitrogen availability during the crop growth period. The best opportunities for reducing leaching losses with a nitrification inhibitor are on sandy soils with irrigation or where heavy rains occur during the growing season. Added costs and inconveniences in handling the materials and mixing with fertilizers have limited the use of these products. Also, effectiveness of the inhibitor diminishes with time, so concentrations of inhibitor may be insufficient to be effective when needed in some years.
Management practices that reduce the amount of fertilizer nitrogen applied should increase use efficiency and reduce the amount of nitrogen available for leaching. Possibilities include diversified cropping and alternative sources of nitrogen.
Crop rotations and green manure crops: Monocultures of crops such as corn, winter wheat, and sorghum require high nitrogen inputs. It is possible to reduce nitrogen use in a farming system by rotating these crops with spring small grains which require less nitrogen, or legumes, which may require no added nitrogen. This type of crop diversification should result in less nitrogen in the soil profile, and, in general, in less leaching. The potential for using legumes in conservation production systems was recently reviewed at a national conference on this subject (Power, 1987).
Legumes grown as green manure or as a hay crop for several years can supply substantial amounts of nitrogen for subsequent grain crops. According to Voss and Shrader (1984) it is possible for a legume such as alfalfa (Medicago sativa L.) to provide all of the nitrogen for a following corn crop in Iowa and for that corn to outyield fertilized continuous corn.. Their work shows that following a legume the response by corn to fertilizer nitrogen is primarily a function of the yield of the legume. Higher-yielding legumes and more years in the rotation produce higher yields of the corn following the legume. Legumes grown with grass in pastures and haylands can supply nitrogen for the grass and thus reduce the need for fertilizer nitrogen. However, the less the proportion of legume in the mixture, the less the amount of nitrogen supplied for a following grain crop. Estimates of nitrogen provided by a legume meadow to first year corn are equivalent to 155, 112, and 22 kg/ha for meadows consisting of 50 to 100, 20 to 50, and 0 to 20 percent legume, respectively (Voss and Shrader, 1984). However, these amounts will vary depending on climate and soil type.
Evidence shows that legumes in a rotation benefit the yield of succeeding grain crops in other ways besides providing nitrogen. Baldock et al. (1981) summarized data showing that yields of corn immediately following a legume are greater than yields of subsequent years of corn, regardless of the amount of nitrogen supplied. In other words, as the frequency of corn in the rotation is decreased, maximum yields tend to increase. The boost in yields above the nitrogen response is sometimes referred to as the "rotation effect" and adds to the biological nitrogen fixation contribution.
Animal manures, crop residues and other organic materials: Animal manures, crop residues, urban wastes, and organic by-products of various industries are valuable sources of cheap nutrients that can substitute for commercial fertilizer. Most of the nitrogen is in the organic form. It becomes available at a rate that depends on the composition of the material, the rate, method, and time of application, the soil type and climate, and the cropping system. In most cases, the release is slow, which should reduce the possibility of nitrate build-up in soil, especially if the amount and timing of the application are consistent with the crop's need for nitrogen. Recovery of nitrogen in non-leguminous crop residues by the following crop is typically less than 20 percent (Power and Papendick, 1985). Animal manures, which have a higher nitrogen content than crop residues, decay much faster. In some manures, such as swine manure, up to 90 percent of the total nitrogen may be available the first year (Power and Papendick, 1985). Availability of nitrogen in other materials, such as municipal refuse, sewage sludge, waste waters, and industrial by-products, varies widely but most are similar to crop residues and animal manures. However, some wastes may contain high levels of heavy metals or other undesirable substances, which may require special considerations for land application.
With proper management, the return of organic materials to soil tends to increase the soil organic matter, microbial biomass, and level of biological activity, and to keep more of the nitrogen in the organic form, which does not leach and is released more slowly to the crop. For example, Hooker et al. (1982) showed that burning or removal of wheat or sorghum residues decreased the soil organic carbon and increased leaching of nitrates. By adding crop residues, manures or other organic materials the farmer to some extent is able to control the size of the nutrient reservoir and the rate of nitrogen release, and hence the potential loss by leaching. Recent research in Nebraska showed that for continuous corn, while crop residues maintained on the soil surface provided little N directly to the next crop, surface residues maintained more favorable soil, water, and temperature regimes and greatly enhanced mineralization and uptake of the native soil organic N (Power et al., 1986). They also found that much of the N in soybean residues was utilized by the next crop.
Often it is impossible to avoid some accumulation of nitrates in the normal rooting zone after crop harvest. Also nitrate commonly leaches below the root zone of some shallow-rooted crops during the growing season and accumulates in the layers below. This nitrate can be leached by winter or early spring rains. Winter cover crops and certain cropping sequences offer possible ways to scavenge this nitrogen to prevent its escape to ground water.
Winter cover crops: A winter cover crop reduces the potential for nitrate leaching by absorbing nitrates that remain after harvest of the main crop, and by extracting water that is available to move through the profile during the regular non-crop period (Stewart et al., 1975). Some of this absorbed nitrogen will be available to the following crop when the cover crop is returned to the soil. Non-legumes such as oats (Avena sativa L.), rye (Secale cereale L.), wheat or timothy (Phleum pratense L.) extract nitrogen efficiently and can grow during the fall and even in winter in many areas of the U.S.
Cover crops need to be established early enough so that there is adequate growth before dormancy in cold weather. Possible approaches are to interseed the cover crop by ground or air seeding before harvest of the main crop. Soil disturbance by harvesting equipment and residue cover may provide enough soil coverage of seed to ensure satisfactory stands. Other possibilities include no-till seeding after the main crop is well established or after harvest in climatic areas that receive enough rain and warm weather.
Crop sequence: Proper crop sequencing can sometimes be used to improve fertilizer nitrogen efficiency and reduce nitrate leaching. For example, Johnson et al. (1975) showed that unfertilized soybeans (Glycine max (L.) Merr.) can scavenge large quantities of residual fertilizer nitrogen from a previous corn crop. Nitrates that escape the root zone of a shallow rooted crop such as potatoes (Solanum tuberosum L.) may be utilized by a following deeper rooted crop such as corn or wheat.
Residual nitrogen can also be readily immobilized by an early season, rapid growing grass or cereal crop. Other possibilities include double cropping systems which alternate deep and shallow rooted crops, or intercropping of legume and non-legume crops. For example, alfalfa can supply most if not all of the nitrogen requirement in an alfalfa-grass hay crop. Furthermore, the deep-rooted alfalfa can scavenge nitrate from deeper levels than most crops can (Stewart et al., 1968).
There has been a strong trend in recent years toward less tillage to reduce mechanical disturbance of the soil and maintain crop residues on the surface. The purpose is to control runoff and erosion and use energy more efficiently. Conservation tillage is designed to leave a minimum of 30 percent of the soil surface covered with residues. Practices used range from a few tillage operations for weed control and seedbed preparation to one-pass no-till planting. It mainly excludes conventional moldboard plowing and other intensive cultivation systems that invert the soil.
Minimizing or eliminating tillage is a radical departure from the long-established conventional practice and can cause some marked changes in the physical, chemical, and biological properties of the soil. With crop residues left on the surface, soils are slower to warm in the spring, and they remain wet much longer than with clean tillage. Nitrogen fertilizer applied on the surface in the conservation tillage system could be immobilized in the short term by the surface residues and organic matter that accumulate in shallow layers. This may reduce nitrogen availability for crops. However, equipment is now available that can place fertilizer below the high organic matter zone.
The fertilizer nitrogen response in notill and plow systems has been compared extensively. In most studies, the cropping systems were intensive grain production, as is traditional with conventional farming. The most commonly reported effect of tillage on nitrogen response has been lower crop nitrogen uptake or yield for no-till at low nitrogen rates, but equal yields at higher nitrogen rates (Fox and Bandel, 1986). The second most commonly reported response has been the same as above for low nitrogen rates, but higher yields with no-till than plow tillage at higher nitrogen rates. Other responses have been highly variable (Fox and Bandel, 1986). Explanations for the different nitrogen responses in the two tillage systems include differences in immobilization, mineralization , denitrification , water conservation, and leaching. With present cropping systems, there is a tendency to apply as much or even more fertilizer nitrogen with no-till as with conventional plow tillage.
Baker and Johnson (1983) report that conservation tillage reduces the volume of runoff by an average of 25 percent, but the degree of reduction is highly variable and site-specific. This means that under the same conditions more water moves through the soil profile with notill than with conventional tillage. A simulation study of the water balance in the wheatlands of eastern Washington showed that during a year with 645 mm of precipitation, deep drainage accounted for 97 mm of water loss from a bare soil and 270 mm of loss from a residue-covered soil (Bristow et al., 1986). Increased infiltration with conservation tillage is attributed to improved structure of the surface layers, micro-ponding and physical retardation of rate of water movement across the soil surface, and continuity of macrochannels between the surface and the subsoil. Tunnels bored by insects and earthworms also increase infiltration.
Fox and Bandel (1986) concluded that on the basis of studies conducted thus far there is a greater potential for nitrate leaching in no-till than in tilled soils. They cited extensive research in Kentucky with continuous corn that showed this. The greater loss in no-till was explained on the basis of increased downward water movement in the undisturbed soil. However, other studies indicate longer nitrate concentrations with no-till than in tilled soil as a result of great denitrification potential or immobilization (Doran, 1980; Fox and Bandel, 1986; Rice and Smith, 1984). This compensating effect would offset the greater leaching potential with notill. As a result there may be little longterm difference in nitrate leaching between conventional tillage and no-till systems with other practices remaining the same.
There are several thousand farmers in the U.S. operating commercial farms with little or no use of fertilizer nitrogen. They are often referred to as organic farmers, and in many cases their operations rely exclusively on organic sources of nitrogen (USDA, 1980). On many such farms, legumes supply most or all of the nitrogen needs for the entire crop rotation. Any nitrogen deficit is reduced further through the use of green manures, effective erosion control that minimizes loss of nitrogen in organic master and soil, and recycling of crop residues, animal manures, and other organic wastes (Papendick and Elliott, 1984). In some cases, the organic nitrogen sources are supplemented by relatively low additions of commercial inorganic fertilizer. Many of the practices employed by organic farmers are similar to those designed to minimize nitrate leaching as discussed in the sections "Reducing the need for nitrogen fertilizer" and "Reducing soil nitrate concentrations".
Organic farmers are apparently able to control availability and release of nitrogen through various techniques of soil management. With their methods, nitrates appear not to be in excess, except possibly in some localized situations where heavy manure or sludge applications are made on a frequent basis. According to the USDA (1980) study, the yield levers and cropping systems used would indicate that nitrogen supply on most organic farms is on the "lean" side. This, together with the use of organic forms of nitrogen, should greatly minimize chances of nitrate leaching from the root zone of most crops.
Nevertheless, there are little or no hard data available on leaching loss of nitrates on organic farms. Lack of such data makes it difficult to quantitatively assess the impact on nitrates in "round water that would occur on a macroscale with a shift to organic farming practices. Other trade-offs would also have to be considered, such as changes in cropping systems and livestock feeding practices, and social and economic impacts.
The solution to the nitrate-ground water problem lies in improved nitrogen management on cropland. In the future, there must be a better balance between crop production objectives and the need to conserve naturel resources and protect the environment. This can only be accomplished by use of practices which integrate the goals of profitable crop production with those of soil and water conservation, surface water quality, and "round water protection.
There is no "quick fix" for the nitrate ground water problem. However, much can be clone now through both short and long-term management strategies that would be practical and effective in minimizing nitrate seepage from the root zone.
Agriculture should make every effort to eliminate over- fertilization, particularly where there is a leaching hazard. The main approach should be to develop mechanistic methods and procedures for accurately predicting nitrogen needs for a given crop based on climate, soil, and management factors. Soil and plant testing are sound concepts for determining nutrient availability, but these have not been perfected on a widespread basis for nitrogen. Accurate prediction of crop nitrogen needs should rank as a high national research priority.
At the same time, attention should be given to improved application methods such as the timing, placement, and form of the fertilizer to enhance crop uptake and to minimize nitrate leaching. Use of nitrification inhibitors and slow release materials should be encouraged wherever these products can be shown to reduce nitrate loss from the root zone.
Other approaches for discouraging over-fertilization include farmer incentive programs, government regulations, and education. For example, farmers would be less likely to over-fertilize as a form of insurance against below optimum yields if they could be compensated in some way for any yield loss due to nitrogen deficiency from a conservative application. However, subsidies and restrictions on application amounts should be considered only as very short-term solutions for "round water protection.
Farmers and the public also need to be better educated on the potential adverse environmental and health consequences resulting from nitrate pollution of "round water, and the relation between pollution and nitrogen management practices. Farmers in particular should be made fully aware of the economic loss of wasted fertilizer and how this loss is accentuated by deterioration of "round water quality. If nitrates leach to the "round water, it is likely that various pesticides have also leached, resulting in additional (and possibly more serious) health implications.
The long-term goal should be to develop and implement production systems that do not result in nitrate pollution of "round water. For the present, these should bear on some of the concepts presented in this paper for reducing the hazard of nitrate leaching. Practices used by experienced organic farmers should be carefully studied for possible adoption or adaptation into systems that can be used by more farmers. Possibly some of the concepts used by organic farmers to conserve nitrogen can be incorporated with some of the best features of conservation tillage for protection of soil and water resources. More research is needed on poorly understood crop yield losses during the transition phase in shifting from, conventional farming with high nitrogen use to organic practices.
Another long-term need is to develop specific cropping systems that make more efficient utilization of biologically fixed nitrogen. Since nitrogen fertilizer has come into widespread use, research on legumes for nitrogen fixation has declined markedly. Especially needed is research on legume cultivars that fit well in rotation with grain crops. There is also a need for legumes for specialized uses such as cover crops and for fixing nitrogen during the noncrop season to reduce the need for fertilizer nitrogen. For example, if a legume could be developed that would grow rapidly and fix 40 to 50 kg N/ha in early spring, it could be chemically killed and direct seeded into a fate season cereal such as sorghum. This would not only reduce the need for commercial fertilizer, but would provide protection against soil erosion as well. Other long-term approaches include engineering major crops such as wheat and corn to fix nitrogen to meet crop needs.
Disclaimer. Trade names and company names are included for the benefit of the reader and do not imply endorsement or preferential treatment of the product by the U. S. Department of Agriculture.
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Citation : Papendick Robert I., Lloyd Elliot F., Power James F., 1987, "Alternative production systems to reduce nitrates in ground water", Vol. 2, No. 1, pp. 19-24.
Copyright © 1987 Reprinted with permission.
Reprinted with permission.
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