Long-term storage of manure is an important step in improving nutrient utilization and reducing N loss in animal production systems. Storage allows more timely application of manure nutrients. When N is applied just before seeding or when the crop is actively growing, it is better utilized by the crop. Applying large amounts of manure within a short time period also provides more convenience for rapid incorporation of the nutrients into the soil. With rapid incorporation, volatile N loss in the field and the potential loss in surface runoff are reduced.
A number of storage methods are used, which can greatly affect the amount and type of N lost. A major factor affecting N loss is manure DM content. Storage methods can be categorized as solid, slurry, and liquid storage, and typical DM contents are greater than 15%, 7 to 15%, and less than 7%, respectively. Other manure characteristics that affect N loss include total N concentration, ammoniacal N concentration, and pH. With a higher concentration of N in the manure and, in particular, ammoniacal N, the rate of loss and the potential total loss of N increase. The transformation and loss of ammonia are very sensitive to manure pH; relatively low loss occurs below a pH of 6 and very high loss occurs when the pH exceeds 8 (Muck and Steenhuis, 1982). Environmental factors, such as ambient temperature, wind velocity, and solar radiation, also affect the rate of loss from open storages (Sommer, 1997). These effects on loss and methods for reducing loss will be addressed for each storage type.
Solid manure is produced when large amounts of bedding are used to absorb manure moisture or when the manure is partially dried. Solid manure can be stored in a stack or heap. During storage, some level of aerobic decomposition or composting will occur. The amount of decomposition and the N loss that occurs is largely related to the manure DM content, the carbon-to-N ratio, and the amount of aeration that the manure stack receives. A stack of undisturbed poultry manure at a DM content of 50% or more is reasonably stable, with relatively small N loss (Table 3; Rodhe and Karlsson, 2002; van Horne et al., 1998). Undisturbed stacks of cattle and swine manure typically have a lower DM content and a greater range in N loss compared to poultry manure (Table 3). In general, less N loss occurs as the DM of cattle and swine manure decreases, but manure pH and environmental conditions also have an effect (Petersen et al., 1998).
Most of the N loss from manure stacks is in the form of ammonia volatilization (Martins and Dewes, 1992
; Eghball et al., 1997
; Petersen et al., 1998
; Sommer and Dahl, 1999
; Sommer, 2001
). Covering stacks with peat reduced this loss by 80 to 90%, but a straw cover showed no benefit (Rodhe and Karlsson, 2002
). Loss due to N leaching from the stack should be less than 10% of the total loss, but greater loss can occur. Nitrous oxide emissions will occur, but this loss should be less than 5% of the total N loss. When aerobic decomposition or composting is promoted through aeration, N losses increase (Table 3
). Although a wide range in N loss is reported, most manure composting studies show a total loss of about 40% of the initial N.
Slurry storage in a tank or earthen pond is commonly used on dairy farms and in some other animal production systems. A combination of feces, urine, and wash water provides slurry with a DM content normally in the range of 7 to 12%. These wastes can be pushed onto the surface of stored manure or pumped into the bottom of the storage unit. Nitrogen loss from slurry storage is primarily a function of the amount of mixing that occurs throughout the storage period. With top surface loading, more mixing occurs and relatively fresh manure remains on the surface. Ammonia volatilizes more readily from this fresh manure, which leads to greater N loss. With bottom loading, a crust of dried manure and bedding material can form on the surface. This crust greatly restricts volatile loss (Table 3; Muck et al., 1984). Due to the anaerobic conditions of slurry storage, essentially all of the N loss is in the form of volatilized ammonia.
With a top-loaded slurry tank or pond, the rate of N loss varies widely as influenced by manure pH, ambient temperature, wind speed, and loading rate (Muck and Steenhuis, 1982; Olesen and Sommer, 1993). Little loss occurs at temperatures below freezing or with a manure pH less than 6, but the rate of loss increases rapidly with increasing temperature and pH (Muck and Steenhuis, 1982). When averaged over the weather conditions typical of the northern United States, a loss of about 30% of the initial N stored can be expected (Table 3).
A number of covering techniques have been tested to reduce N loss and odor from slurry storages. The most effective cover is a permanent lid. When a good seal is maintained, this cover nearly eliminates storage loss (Sommer et al., 1993). Other options include the use of a floating foil or plastic cover, or floating layers of peat, Leca, oil, straw, or polystyrene spheres (Sommer et al., 1993; Hartung and Phillips, 1994). Any of these covers can reduce N loss from slurry storage by 80 to 90%, if a complete cover can be maintained throughout the storage period. When surface cracks develop or the covering material sinks into the slurry, the effectiveness is greatly reduced. Overall, the development of a natural crust appears to be about as effective as the use of other covering materials.
Liquid storage in large lagoons is a common practice on large livestock, swine, and poultry production operations. Large amounts of water are added to the manure to obtain a DM content of about 5% or less. Some type of liquid and solid separation is often used to reduce the solids content of the manure entering the lagoon. A flush system is normally used to remove manure from the housing facility where recycled lagoon effluent provides most of the flush water. With this approach, N loss is normally very high (Table 3). High loss occurs because a surface crust does not form, and wave action provides constant mixing throughout the lagoon.
When liquid and solid separation is used, about 25% of the total manure N is removed in the manure solids with the remainder in the liquid portion (Sutton et al., 1994). If these solids are applied to cropland, the crop can use a large portion of this organic N. Often these solids are used as bedding material, which recycles this N back into the manure. To reduce the potential spread of disease organisms, these solids may be processed through composting. As discussed earlier, about 40% of this N may be lost during composting.
Most of the N entering the lagoon is lost to the atmosphere or recycled in the manure flush system. Normally, a series of lagoons are used where the effluent from the first becomes the influent of the next. Nitrogen concentration and N volatilization decrease as the fluid flows through each successive lagoon. Nitrogen emission rates are correlated with N concentration, pH, and surface temperature of the lagoon liquid and the wind speed across the lagoon (Harper et al., 2000; Aneja et al., 2001; de Visscher et al., 2002). At least 70% of the N entering a series of lagoons is lost to the atmosphere (Sutton et al., 1994; van Horne et al., 1998). Harper et al. (2000) found that less than 1% of the initial N entering the first lagoon was recovered from the final lagoon and applied to cropland. Most of the effluent from the final lagoon was used as flush water, thus carrying remaining N back through the barn and into the first lagoon. With this type of recycling, nearly 100% of the N is emitted into the atmosphere from some point in the cycle. When a single or two-stage lagoon system is used, where a greater portion of the liquid is applied through irrigation, the N loss from the lagoon is expected to be less, probably around 50%.
Of the large amounts of N emitted, about half of this N is in the form of ammonia (Harper et al., 2000). Most of the remaining half seems to be in the form of N2, with a small emission of N2O. Since N2 emission is not detrimental to the environment, the large N loss is not as great a problem as may be first assumed. Ammonia losses, however, are still high from large lagoon systems, which can adversely affect the environment of the region.
A number of additives have been promoted or tested to reduce the ammonia emission from stored manure (McCrory and Hobbs, 2001). Additive types include digestive additives, acidifying additives, and adsorbents.
Digestive additives consist of selected microbial strains and/or enzymes that are intended to enhance the biodegradation of manure. Such microbial treatments provide a promising solution due to their regenerative nature, but those evaluated thus far have not provided consistent or substantial benefit (McCrory and Hobbs, 2001). Current products appear to have been developed without a thorough understanding of the microbiological processes occurring in livestock manure. Although current products are not recommended for routine use, potential for the development of effective products exists. More research is needed to investigate known strains of bacteria or enzymes with known modes of action. Effective organisms must be able to grow, reproduce, and become a dominant or at least a major part of the indigenous community.
Acidifying additives that reduce the pH of the manure can greatly reduce ammonia volatilization. Potential treatments include acids, base precipitating salts, and labile carbon (McCrory and Hobbs, 2001). A number of acids can be used to decrease manure pH, but problems that deter their use include high cost, corrosiveness, and hazards to animal and human health. Base precipitating salts, such as chloride and nitrate salts of magnesium and calcium, can also decrease pH, but they are less effective than acids in reducing and sustaining low pH levels. They offer a treatment for short-term reductions in ammonia volatilization. Labile carbon treatments, such as sucrose and potato starch, have reduced pH by stimulating indigenous anaerobic microorganisms to produce organic acids. Tests with these treatments have shown a reduction in ammonia volatilization of 42 to 98% (McCrory and Hobbs, 2001). Large quantities of these additives are required though, which makes current treatment options uneconomical.
A variety of additives can be used to adsorb the ammonia and/or ammonium N in manure. The most common of those tested or used are clinoptilolite and peat (McCrory and Hobbs, 2001). Clinoptilolite can be applied as a feed additive, but it is more effective when applied directly to the manure. Application to broiler litter has reduced aerial ammonia concentrations by 35%, but total reductions in N loss have not been documented. Peat can adsorb 2.5% of its dry weight in ammonia N. Covering heaps of solid manure or floating a layer of peat over stored slurry can reduce N loss by up to 80%. A major challenge with this technique is maintaining a complete cover throughout the storage period.