Manure provides a valuable resource when applied to crop or grassland. Organic matter and nutrients are returned to the soil to help rebuild soil texture and biological activity and to provide nutrients required for growth of the next crop. Nitrogen losses can be very high following application, reducing the fertilizer value of the manure and adding to the degradation of the environment. Management used during and following application can have a large impact on the N losses that occur.
Nitrogen losses during this phase of manure handling include ammonia volatilization, nitrate leaching, N runoff, and N emissions that result from nitrification and denitrification processes. The greatest loss normally occurs through ammonia volatilization. When manure is surface-applied through broadcast spreading, most of the ammonium N can be lost within a few days of application. Of the total N in manure, roughly half is in an ammonium form or another form that can easily transform into ammonia. Loss occurs rapidly following slurry application, with 30 to 70% of the total loss occurring within the first 6 to 12 h (Sommer and Hutchings, 1995; Meisinger and Jokelo, 2000). The rate of loss then slows due to a lower ammonium N concentration, a drop in manure pH, infiltration into the soil, and formation of a surface crust. For application of drier manure, such as poultry litter, the initial loss rate is lower and more constant through time (Meisinger and Jokelo, 2000). Thus, initial loss from solid manure may be less than half that of slurry. However, when manure is not incorporated into the soil, total loss over time can be similar between manure types.
The rate and amount of ammonia loss is related to the weather conditions and characteristics of the manure and the soil to which it is applied. The rate of loss is very low at 0°C, but it increases exponentially with increasing temperature (Sommer et al., 1991). Loss increases with wind speed up to a speed of about 2.5 m/s. Air humidity may have an effect, but this effect appears to be small. Precipitation soon after application can greatly suppress ammonia emission by moving the ammonium N into the soil (Meisinger and Jokelo, 2000). Soil conditions including the moisture content, texture, cation exchange capacity, pH, and plant or residue cover all affect the rate and amount of ammonia loss (Meisinger and Jokelo, 2000).
Manure characteristics that affect ammonia loss include ammonium N content, DM content, and pH. Loss generally increases in proportion to the amount of ammonium in the manure. A lower DM content allows the manure to absorb into the soil more quickly, thus reducing loss (Meisinger and Jokelo, 2000). Ammonia emission is reported to increase by 5 percentage units of ammonium N for each 1% increase in slurry DM content between 1 and 9% DM (Chambers et al., 1999). At DM contents greater than 12%, there is little effect. Manure pH can have an effect, but this effect is normally small because the pH of slurry increases rapidly after application (Meisinger and Jokelo, 2000).
Nitrogen can be lost through runoff when manure is surface-applied before or following frozen soil conditions. This loss should be relatively small, around 3% of the total N applied, but runoff losses up to 10% of applied N are reported (Gangbazo et al., 1995). When manure was incorporated into the soil, runoff loss of N was found to be no greater than that from unfertilized land (Gangbazo et al., 1995).
Organic and ammonium N incorporated into the soil can be taken up by a crop, lost through nitrate leaching, or lost through denitrification. There is considerable interaction among these processes, which makes it difficult to predict the fate of the N. Normally, leaching loss is relatively small when manure is applied to a growing crop or just before the establishment of a crop. Loss of less than 2% of total N can be expected (Stout et al., 2000). When manure is applied to fallow land in the autumn, excessive leaching loss of 10 to 30% of applied N can occur (Carey et al., 1997; Beckwith et al., 1998; Weslien et al., 1998; Di and Cameron, 2002a). Compared to fallow soil, seeding a cover crop, such as winter rye, can reduce N leaching loss by 40 to 80%, depending on the precipitation patterns over the winter and soil characteristics (Beckwith et al., 1998). A cover crop reduces N loss by reducing the moisture drained through the soil profile and by taking up nitrate N moving through the profile.
Nitrification and denitrification processes in the soil cause N emissions in various forms, with the primary forms being N2O and N2. The amount of denitrification that occurs is a function of the amount of total N in the soil, available carbon in the soil, and the anaerobic conditions within the soil (Carey et al., 1997). Although total denitrification loss of N can be large (Carey et al., 1997), limited data indicates that N2O loss is normally less than 2% of the total N applied (Weslien et al., 1998; Sherlock et al., 2002).
Numerous methods are used to apply and incorporate manure, and N loss varies widely depending on the method used. The major methods include irrigation, broadcast spreading, band spreading, and some form of injection into the soil (Table 4). The largest N loss usually occurs through irrigation. During the irrigation process, a portion of the N is volatilized or otherwise made airborne before contacting the plant or soil surface. Very high values of 15 to 43% of the total N applied have been reported for this loss (Safley et al., 1992), but more typical values appear to be in the range of 5 to 10% (Sommer and Hutchings, 1995; Meisinger and Jokela, 2000). Ammonia volatilization continues from the field surface, causing an additional loss of up to 35% of total applied N (Meisinger and Jokela, 2000). Loss is influenced by ambient temperature, precipitation, and other weather conditions. Due to wet soil conditions following irrigation, rapid incorporation of the manure is difficult, which contributes to greater loss.
Losses with broadcast spreading are very variable and again depend on the manure, soil, and weather conditions. During the spreading operation, less than 1% of the total N applied is lost (Sommer and Hutchings, 1995
; Meisinger and Jokela, 2000
). Broadcast spreading on grassland or heavy crop residue increases N loss by 30 to 50% (Meisinger and Jokela, 2000
) compared to application to bare soil. Incorporation of surface-applied manure with a tillage operation stops ammonia volatilization. Since a large portion of the ammonia emission occurs within a few hours of spreading, rapid incorporation of slurry is important to reduce N loss. The initial rate of ammonia emission is not as high from poultry litter, so rapid incorporation is somewhat less important (van Horne et al., 1998
; Rodhe and Karlsson, 2002
A number of techniques have been used to apply manure slurry in bands (Meisinger and Jokela, 2000). This technique is primarily used on grassland or other established crops. When bands are well formed and maintained on the surface, N loss can be reduced 30 to 70% compared to broadcast spreading on the same crop. Other studies have shown band application to be less effective, particularly when the band spreads, covering more of the field surface.
Direct injection of slurry into the soil is the most effective method for reducing N loss during and following application. With deep injection (>10 cm in depth), so that the soil totally covers the manure, ammonia N loss is less than 5% of the total N applied (Table 4). A problem with deep injection in grassland or other established crops is that the injector causes root damage that can reduce crop growth (Mattila and Joki-Tokola, 2003). More shallow injection techniques (5 cm in depth) allow less root damage. These are not as effective in stopping ammonia emissions, but they are more effective than broadcast or band spreading (Table 4). Since injection conserves more of the manure N, greater leaching and denitrification losses can be expected from the soil, but this depends on the timing of the application and crop, soil, and weather conditions following application (Dosch and Gutser, 1996; Weslien et al., 1998).
Management to Reduce Application Losses
A number of management strategies or techniques can be used to conserve N during and following field application. From an N conservation perspective, irrigation of manure should be avoided (Table 4). Broadcast spreading of slurry on grassland or on heavy crop residue without incorporation should also be avoided. Injection techniques provide the best option in cropping systems where they can be practically used. Rapid incorporation of manure within a few hours of application can provide substantial benefit. Band spreading with a trailing hose or similar device can provide some benefit particularly when slurry is applied to an established crop or grassland where incorporation is not possible. Tilling the soil before irrigation, broadcast, or band application may also improve the infiltration of liquid or slurry manure and thus reduce loss slightly.
Application rate and slurry DM content can be manipulated to reduce N loss. A higher application rate may reduce the fraction of the applied N that is lost, but this depends on competing forces of adsorption into the soil and volatilization (Meisinger and Jokela, 2000). Less loss would occur with one heavy application as compared to two or more lighter applications that provide the same total applied N. Because slurry with a low-DM content is absorbed more rapidly, dilution can reduce loss (Sommer and Hutchings, 1995). This strategy is not usually practical because a 50% reduction in DM content will double the amount of manure handled. Aeration of slurry has been used to reduce DM content by up to 16% (Leinonen et al., 1998). About 10% of the total N can be lost during the aeration process, which offsets any reduction in application loss obtained through the decrease in DM content.
Dropping the manure pH below 7 before it is applied can reduce ammonia emission (Sommer and Hutchings, 1995; Meisinger and Jokela, 2000). Nitric or sulfuric acid treatments used to drop the pH to 6.5 have reduced ammonia loss by up to 75%. Use of amendments, such as alum or ferrous sulfate, can also have an acidifying effect on manures, which may reduce N loss.
The best strategy to reduce leaching and denitrification losses is to apply the right amount of manure close to the time the nutrients are needed by the crop (Beckwith et al., 1998). Autumn application, particularly on fallow soil, can lead to large losses. About 50 to 70% of the nitrate accumulated in the soil profile by late autumn is leached during the winter (Di and Cameron, 2002a). Likewise, excess N beyond crop needs will move through the soil profile and be lost. Large amounts of N in the soil also increase denitrification loss. Nitrification inhibitors have been used to reduce the N lost by denitrification with mixed results. Use of an inhibitor almost eliminated an increase in denitrification following the injection of cattle slurry into grassland soil (de Klein et al., 1996). In a study of N2O loss in different crop rotations, use of an inhibitor had little effect (Stoeven et al., 2002). Use of an inhibitor also had small and inconsistent effects on the amount of N leached through the soil profile over winter (Beckwith et al., 1998).
Substantial N losses from volatilization, leaching, and denitrification occur from manure deposited by grazing animals, and the loss processes are somewhat different from those of slurry or solid manure application. Fecal and urinary deposits normally occur at different times and locations in the field. Most fecal N is organic and is thus relatively stable following deposition. About 5% of the fecal N is lost by volatilization (Ryden, 1986). Most of the excreted N (55 to 75%) is in the urine, in which higher levels are associated with the overfeeding of protein (Jarvis et al., 1989). Urinary N (urea) is rapidly hydrolyzed to form ammonia, which is then nitrified at a slower rate. Ammonia volatilization loss can be very high, but rapid absorption of the urine into the soil surface can reduce this loss. Reported losses vary from 5 to 66% of the total urinary N with greater loss under hot and dry weather and soil conditions and relatively low loss under cool and moist conditions (Ball and Ryden, 1984; Jarvis et al., 1989). Average total loss is approximately 10% of the excreted N (Table 4; Oenema et al., 2001b). Ammonia emission is greatest during and immediately after a grazing event. Rain, poor drying conditions, and low wind all help reduce this emission rate (Ryden, 1986). Limited data on outdoor swine production indicate that N loss may be twice that found with grazing cattle (Eriksen et al., 2002).
Leaching loss of N can be much higher under grazing conditions than occurs for spread manures. Nitrogen concentrations under a urine patch are very high, equivalent to an application rate of 300 to 1,000 kg N/ha. Much of this N is in excess of crop needs and is leached down through the soil profile. Reported loss ranges from about 10 to 60% of the urinary N deposited (Garwood and Ryden, 1986; Stout et al., 1997; Silva et al., 1999; Di and Cameron, 2002b). Factors affecting this loss are soil type or texture, rainfall following deposition, and the time of the year the deposit is made. Urine N deposited in the spring is more likely to be taken up by a growing crop and thus provides about half the loss of that deposited in the fall (Stout et al., 1997). Leaching loss from fecal N is small, about 2% of that deposited (Stout et al., 1997). Combined leaching losses are 10 to 30% of the total N excreted on the pasture (Table 4).
Runoff loss of N also occurs from pastures, but this loss is small. Preliminary data from Ahmed et al. (2002) indicated an average loss of about 1% of the excreted N on continuously grazed pasture. Rotational grazing provided up to an 80% reduction, and the use of a vegetative filter strip reduced this loss by 50% or more. On poorly drained soils, though, runoff loss may be much greater, with less leaching (Garwood and Ryden, 1986).
Denitrification losses from pastures can also be substantial, particularly under urine deposits. Reported losses range from about 5 to 30% of the applied urinary N (Garwood and Ryden, 1986; Fraser et al., 1994; Di et al., 2002). Most of this loss appears to be in the environmentally benign form of N2, but some portion will be in the form of N2O. Available data indicate that less than 8% of the applied N will be lost as N2O with a typical loss around 2% (Ryden, 1986; Clough et al., 1996; Oenema et al., 2001b).
Management can be used to reduce N loss from grazing animals, but the benefit of these changes may be small and implementation may be impractical. One practical step that should always be considered is to feed supplemental protein feeds efficiently, and thus reduce urinary N excretion. Overstocking of animals along with a large import of forage and other supplemental feeds should also be avoided. Movement of watering and supplemental feeding areas will improve nutrient distribution, thus increasing plant uptake and reducing loss. Volatile loss may be reduced by irrigating the paddock immediately after grazing to wash the N into the sod and soil. Spreading of crushed manure natural zeolites has been suggested to increase the cation exchange capacity at the base of the sward (Ryden, 1986). Leaching loss can best be reduced by avoiding grazing in the late autumn or winter when plant uptake of N is low. Removing the autumn growth through silage harvest can help reduce the accumulation of excess soil nitrate, which at that time of the year will likely be lost by leaching (Stout et al., 1997). Less use of N fertilizer with greater use of clover and other legumes to supply needed crop N can also reduce soil N levels and leaching loss (Garwood and Ryden, 1986). Di and Cameron (2002c) decreased leaching loss 60% and decreased denitrification loss by 82% by applying a nitrification inhibitor, but practical application of this technology would be difficult.