Estimating Plant Available Nitrogen in Broiler Litter John Gilmour, Emeritus Professor, University of Arkansas, Fayetteville gilmour@uark.edu. Two-thirds of US broiler production occurs in six states (Alabama, Arkansas, Georgia, Mississippi, North Carolina and Texas). In 2006, Georgia, Arkansas and Alabama produced about 16, 14 and 12% of the total production, respectively. (http://www.nas.usda.gov). Often broiler houses are concentrated on limited land resources with surface waters nearby. Runoff from areas receiving broiler litter can transport P and N to those waters in sufficient quantities to cause increased growth of aquatic flora and fauna. This nutrient enrichment is termed eutrophication. Aquatic weeds, algal blooms and lower dissolved oxygen levels are characteristic of eutrophic ponds, lakes and reservoirs. Phosphorus is usually the nutrient that controls the extent of eutrophication because it is commonly the most limiting nutrient. When broiler litter is land-applied at rates designed to meet the nitrogen (N) needs of a crop accumulation of excess phosphorus (P) occurs in the soil. This is because the litter contains 2 to 3 times as much N as P, but plants typically require 4 to 9 times as much N as P (Sadras, 2005). The situation is exacerbated in the case of broiler litter as all the N is not plant available so application rates are further increased to meet plant N needs. If the litter is surface-applied, rainfall events near the time of application cause large increases in the P concentration in runoff. Several approaches have been taken to limit P transport to surface waters: reusing litter as animal feed or fuel; amending litter with aluminum or iron rich materials to reduce soluble P; removing P from runoff water using buffer strips; soil incorporation of the litter; applying litter during times of low rainfall; and transporting litter to P deficient areas where surface waters are not impacted. Transporting broiler litter to P deficient areas has become more viable as the value of the N, P and potassium (K) in fertilizers has increased. A typical poultry litter has an analysis of 4.1-3.2-2.5 (N-P 2 O 5 -K 2 0). Common fertilizers containing these nutrients are urea (46-0-0), diammonium phosphate or DAP (18-46-0) and potash (0-0- 60). One dry ton of the typical litter contains 82 lbs N, 64 lbs P 2 O 5 and 50 lbs of K 2 0. This is equivalent to 124 lbs of urea, 139 lbs of DAP and 83 lbs of potash. Almost all of the P and K in the broiler litter is plant available, so calculating the value of that portion of the litter simply requires knowledge of the current value of P and K. Only a portion of the N is plant available. Estimating the portion of the plant available N (PAN) in broiler litter has proved challenging because litter is a mixture of bedding (e.g. wheat straw, rice hulls, peanut hulls, sawdust) and manure. In addition, the proportion of bedding to manure changes with the number of flock cycles before cleaning out the broiler house and whether or not the litter is decaked (hard surface layer removed) between flock cycles. The primary nitrogenous materials in litter are ammonium (NH + 4 ), nitrate (NO - 3 ), uric acid and various proteins. Ammonium and nitrate are plant available, while the other N sources must undergo decomposition before becoming PAN. The uricase enzyme breaks down uric acid leading to the production of ammonium. Decomposition of the various proteins increases PAN (N mineralization), while decomposition of bedding which has a high C:N ratio reduces PAN (N immobilization). Studies of 70 days or less 1
typically show a rapid increase in PAN over the first two weeks followed by more erratic PAN values that often do not correspond to the amount of decomposition that has occurred (Gilmour et al., 2004; Gale et al., 2006). Studies of greater than 110 days show that N mineralization far exceeds N immobilization as discussed below. Qafoku et al. (2001) studied potentially mineralizeable N (PMN) in 60 broiler litters incubated in the laboratory at 25 C (77 F) for 112 days at optimum soil moisture. They reported that water soluble organic N was the best measure of PMN. Using their data, PMN was compared to organic N in the litter. A statistically significant linear relationship was found with a slope of 0.57 and an intercept not significantly different from zero (R 2 = 0.56). In a practical sense this means that an average of 57% of the broiler litter organic N was mineralized over the 112 day period. Preusch et al. (2002) presented detailed N mineralization data for two broiler litters in two soils incubated at 25 C and optimum moisture for 120 days. N mineralization followed first order kinetics. Thirty percent of the litter organic N mineralized rapidly followed by another 27% decomposing at about one-fifth the rapid fraction rate. Total N mineralization was essentially the same as found by Quafoku et al. (2001) and the 30% rapid fraction corresponded with the typical uric acid content of broiler litter reported by Eiteman et al. (1994). The rates of decomposition and PMN conversion to ammonium decrease when litter is surface-applied as compared to incorporated in the soil. This is because decomposition decreases in the surface-applied case. Shomberg et al. (1994) studied the differences in plant residue decomposition for surface-applied versus incorporated application methods. Applying these factors to broiler litter, the rapid and slow fractions of surface-applied litter will decompose at about one-third and two-thirds of the rate of incorporated litter, respectively. Parallel declines in the rate of PAN formation likely also occur. For surface-applied litter, ammonium can volatilize as ammonia (NH 3 ). As temperature and wind speed increase, ammonia volatilization increases. In a field study on conservation tillage land, Sharpe et al. (2004) reported that 23% of the total N volatilized in about a week when daily temperatures ranged from 77 to 86 F and wind speed was from 10 to 20 miles per hour. When daily temperature ranged from 63 to 90 F and wind speed was from 2 to 10 miles per hour, volatilization was 5.3% of total N. In the latter case, a rainfall event (about 0.7 inch) essentially stopped volatilization 2 days after litter application. When daily temperature ranged from 27 to 59 F and wind speed was from 2 to 16 miles per hour, volatilization was 3.7% of total N occurring only during the warm portion of the day. Marshall et al. (1998) found that ammonia volatilization was small for a two year study at three locations. They reported that 4% of the total N volatilized when broiler litter was applied to tall fescue (Festuca arundinacea Schreb.) pastures. Overall, it appears that ammonia volatilization will be less than 5% of the total N in the litter unless the weather is warm and windy and a rainfall event does not occur soon after application. With the information described above, it is possible to estimate how much PAN in lb N/dry ton of litter is available for a given location using the equation shown below, PAN = (PMN/100 %organic N + % inorganic N - %total N volatilized/100 %total N) 20 2
where organic N is total N minus inorganic N (ammonium plus nitrate N). Estimates can be made for individual months or for the entire growing season. If the litter is incorporated %total N volatilized is set to zero. It is also zero for any month after the month the litter is applied. The rate of PMN formation varies with location due to temperature and soil moisture differences and method of application (surface-applied versus incorporated). Litter amendments like alum do not markedly affect PMN (Gilmour et al., 2004), while proper composting does (Preusch et al., (2002). Properly composted litters have very low PMN values. Percent total N volatilized varies with weather immediately after application for surface-applied litter. To test the validity of the equation for estimating PAN, data from Gale et al. (2006) were used. The mean total N and inorganic N from twelve litters and average monthly temperature for the application site were inputs. PMN was estimated using a simple computer model where rapid and slow fractions decomposed simultaneously and monthly temperature was used to adjust kinetics from Preusch et al. (2002). The percent PAN estimated using the above equation was 53%. The mean full season field %PAN reported by Gale et al. (2006) was 43%. Gale et al. (2006) applied the broiler litter about one month before planting the crop, so some loss of PAN during the initial month likely occurred. The same approach was used to estimate PMN for broiler litter applied in April for the 16 states with the largest broiler production. Mean monthly air temperatures were obtained from http://www.weather.com. Soil moisture was assumed to be adequate for decomposition and N mineralization. The results are presented in the table below. Incorporated Broiler Litter PMN State April May through 1 st Year 2 nd Year August Alabama 28 32 72 13 Arkansas 26 32 67 18 Delaware 26 34 69 16 Florida 31 33 77 8 Georgia 31 33 77 8 Kentucky 24 31 63 18 Louisiana 31 34 76 9 Maryland 26 34 69 16 Mississippi 29 32 74 11 Missouri 25 32 65 18 North Carolina 27 31 68 17 Oklahoma 28 34 72 13 Pennsylvania 23 33 64 18 South Carolina 31 33 76 9 Texas 31 36 78 7 Virginia 26 31 66 18 Mean 28 33 71 14 3
First month (April) PMN ranged from 24 to 31% of the organic N in broiler litter. The next four months (May to August) ranged from 31 to 36% of the organic N in the broiler litter. The mean for the April through August period was 61% of the organic N in broiler litter. Mean values for first and second year annual PMN were 71 and 14%, respectively. States with lower annual PMN the first year had correspondingly higher PMN the second year. As pointed out earlier, broiler litter properties can be quite variable making PAN estimation difficult. As a first approximation, the PAN estimates using the equation for PAN and the data in the table offers a reasonable approach. For example, using mean %organic N and %inorganic N of 3.0% and 0.5%, respectively, and the mean values in the table above, PAN for litter incorporated in April would be: April PAN = (28/100 3.0 + 0.5-0) 20 = 26.8 lb N/dry ton April PAN is 38% of the total N (70 lb N/dry ton in this example) in the litter. This example illustrates the importance of management the first month after broiler litter application. Incorporation and timely planting maximizes plant N uptake and minimizes losses to ground and surface waters. If the litter had been allowed to remain on the soil surface for a few days during warm, windy weather, April PAN could be reduced by as much as 25% of the total N or 18 lb N/dry ton. If the crop is not planted near the time of litter application, similar losses are possible. For May through August, PAN would be: May to August PAN = (33/100 3.0) 20 = 19.8 lb N/dry ton May through August PAN is 28% of the total N. Note that no inorganic N or volatilized total N were included in the equation after the first month. The same PMN estimates were made for surface-applied litter for the top 16 states in broiler production. The results are presented below. First month (April) PMN ranged from 7 to 13% of the organic N in broiler litter. This is about one-third the amount for incorporated broiler litter. The next four months (May to August) ranged from 37 to 43% of the organic N in the broiler litter or nearly 20% more than the incorporated value. Overall, the mean April through August PMN values were smaller in the surface-applied case as compared to incorporated broiler litter and the distribution by month was different. Mean values for first and second year annual PMN were 59 and 17%, respectively, for the surface-applied litter. First year values were 12% less than those for incorporated litter. PAN estimates using the equation for %PAN and the data in the surface-applied table were then evaluated for the surface-applied case. Again, using mean %organic N and %inorganic N of 3.0% and 0.5%, respectively, the %total N volatilized is set at 5%, and the mean PMN values from the surface-applied table, PAN for litter incorporated in April would be: April PAN = (11/100 3.0 + 0.5 5/100 3.0) 20 = 13.1 lb N/dry ton 4
Surface-Applied Broiler Litter PMN State April May through 1 st Year 2 nd Year August Alabama 11 40 60 18 Arkansas 9 40 56 17 Delaware 10 41 58 17 Florida 13 41 66 17 Georgia 11 40 60 18 Kentucky 9 38 53 16 Louisiana 13 41 65 17 Maryland 10 41 58 17 Mississippi 12 40 62 18 Missouri 9 39 55 16 North Carolina 10 39 58 17 Oklahoma 11 41 60 18 Pennsylvania 7 37 49 15 South Carolina 13 41 65 17 Texas 13 43 67 17 Virginia 9 39 56 16 Mean 11 40 59 17 April PAN is 19% of the total N (70 lb N/dry ton in this example) in the litter as compared to 38% for the incorporated case. If the weather had been hot and windy all the PAN could have been lost as maximum values for ammonia volatilization are about 25% of total N. For May through August, PAN would be: May to August PAN = (40/100 3.0) 20 = 24.0 lb N/dry ton May through August PAN is 34% of the total N or about 21% larger than the incorporated case. Again, note that inorganic N or volatilized total N were not included in the equation after the first month. Finally, the approach described herein is useful from a planning standpoint where broiler litter PAN determines the rate of litter application or the amount of supplemental N fertilizer required for the desired yield. Once, the litter application rate is known, the value of plant available N, P and K in the litter can be assessed and compared to the cost of the applied litter. If that calculation is favorable, both agriculture and the environment benefit. Approved CEU Credits: 2.0 Nutrient Management 1.0 Soil & Water Management 5
Literature Cited Eiteman, M.A., R.M. Gordillo and M.L. Cabrera. 1994. Analysis of oxonic acid, uric acid, creatine, allantoin, xanthine, and hypoxanthine in poultry litter by reverse phase HPLC. Fresenius Journal of Analytical Chemistry. 38:680-683. Gale. E.S., D.M. Sullivan, C.G. Cogger, A.I. Bary, D.D. Hemphill and E.A. Myhre. 2006. Estimating plant-available nitrogen release from manures, composts, and specialty products. J. Environ. Qual. 35:2321-2332. Gilmour, J.T. M.A. Koehler, M.L. Cabrera, L. Szajdak and P.A. Moore, Jr. 2004. Alum treatment of poultry litter: Decomposition and nitrogen dynamics. J. Environ. Qual. 33:402-405. Marshall, S.B., C.W. Wood, L.C. Braun, M.L. Cabrera, M.D. Mullin and E.A. Guertal. 1998. Ammonia volatilization from tall fescue pastures fertilized with broiler litter. J. Environ. Qual. 27:1125-1129. Qafoku, O.S., M.L. Cabrera, W.R. Windom and N.S. Hill. 2001. Rapid methods to determine potentially mineralizeable nitrogen in broiler litter. J. Environ. Qual. 30:217-221. Preusch, P.L., P.R. Adler, L.J. Sikora and T.J. Tworkoski. 2002. Nitrogen and phosphorus availability in composted and uncomposted poultry litter. J. Environ. Qual. 31:2051-2057. Sadras, V.O. 2006. The N:P stoichiometry of cereal, grain legume and oilseed crops. Field Crops Res. 95:13-29. Schomberg, H.H., J.L. Steiner and P.W. Unger. 1994. Decomposition and nitrogen dynamics of crop residues: residue quality and water effects. Soil Sci. Soc. Amer. J. 58:372-381. Sharpe, R.R., H.H. Schomberg, L.A. Harper, D.M. Endale, M.B. Jenkins and A.J. Franzluebbers. 2004. Ammonia volatilization from surface-applied poultry litter under conservation tillage management practices. J. Environ. Qual. 33:1183-1188. 6