ENVIRONMENT, WELL-BEING, AND BEHAVIOR. Nutrient value of alum-treated poultry litter for land application

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1 ENVIRONMENT, WELL-BEING, AND BEHAVIOR Nutrient value of alum-treated poultry litter for land application M. Guo * 1 and W. Song * Department of Agriculture and Natural Resources, and Department of Chemistry, Delaware State University, Dover ABSTRACT Alum-treated poultry litter has different chemical composition and biological properties than conventional poultry litter. To develop agronomic application rates for this particular organic fertilizer to cropland, the nutrient value (nutrient plant availability) of alum-treated poultry litter needs to be determined. Typical alum-treated poultry litter was collected from a broiler farm and examined for nutrient content, nutrient release kinetics, and nutrient value by leaching the material for 190 d under simulated weathering conditions. Nutrients recovered in the leachate were characterized and treated as the potentially plant-available portion. The artificial leaching revealed that alum-treated poultry litter released 21.4 g of dissolved organic C, 13.8 g of total dissolved N, 0.6 g of total dissolved P, and 34.6 g of K per kilogram into leachate during the 190-d weathering. The predominant nutrient release occurred in the first 5 wk and fit first-order exponential rise-tomaximum models (for dissolved organic C, total dissolved P, total dissolved N, NH 4 -N, K +, Na +, Cl, and SO 4 2 ) and logarithmic equations (for Ca 2+ and Mg 2+ ). The nutrient value of alum-treated poultry litter is estimated at N, 13.8 g kg 1 ; P, 0.75 g kg 1 ; K, 34.6 g kg 1 ; and S, 24.2 g kg 1. The concentration of Al in litter leachate remained below 0.2 mm and thus no Al toxicity should be concerned. Based on these results, it is recommended to apply alum-treated poultry litter at 7.3 t ha 1 for achieving an N supply of 100 kg ha 1 to common field crops while preventing excessive P runoff losses from high test P soils. Key words: alum-treated poultry litter, nutrient release kinetics, nutrient value, weathering, leachate INTRODUCTION Land application is the predominant disposal method for the 12 million tonnes of poultry litter generated annually in the United States (Sharpe et al., 2004). Poultry litter contains high concentrations of C, N, P, K, and other nutrients (Mullins and Bendfeldt, 2001). It is a valuable organic fertilizer for sustaining soil fertility and supporting plant growth. In areas with concentrated poultry production, however, repeated and excessive application of poultry litter to local cropland has resulted in elevated soil nutrient levels (mainly P and N), serious nutrient runoff and leaching losses, and consequently nonpoint source water pollution (Kingery et al., 1993; Vadas and Sims, 1998). For example, the Delmarva (Delaware, Maryland, Virginia) Peninsula generates more than 700,000 t of poultry litter each year as a result of high-density broiler production (Delmarva Poultry Industry, 2008). Application of this huge amount of organic waste and other animal manures to the Peninsula s limited cropland has loaded annually 2009 Poultry Science Association Inc. Received September 20, Accepted April 24, Corresponding author: mguo@desu.edu 2009 Poultry Science 88 : doi: /ps million kilograms of N and 2.0 million kilograms of P via surface runoff to the Chesapeake Bay, causing severe water quality degradation of the Bay (Schroeder et al., 2004; Chesapeake Bay Program, 2006). Different management strategies have been practiced to reduce nutrient runoff losses from land-applied poultry litter, such as soil incorporation (Pote et al., 2003), agronomic application based on crop nutrient demand (NRCS, 1999), litter amendment with Fe-Al by-products (Shreve et al., 1995; Codling et al., 2000), and decreasing litter P content by chicken diet modification (Yan et al., 2000; Maguire et al., 2005). Among these approaches, litter amendment with alum [Al 2 (SO 4 ) 3 14H 2 O] demonstrates extra benefits. Alum is one of the 3 chemical acidifiers [alum as Al+Clear (General Chemical LLC., Parsippany, NJ), NaHSO 4 as Poultry Litter Treatment (Jones-Hamilton Co., Walbridge, OH), and H 2 SO 4 as Poultry Guard (Oil Dri Corp., Chicago, IL] commonly used to treat litter bedding for inhibiting NH 3 volatilization in poultry houses (North Carolina Cooperative Extension Service, 2006). The chemical is typically applied to poultry houses every bird flock at a rate 5 to 10 weight percentage of the overall litter. The application rate is equivalent to 45 to 90 g per bird or 60 to 120 kg per 100 m 2 (Oklahoma Cooperative Extension Service, 2008). Alum amendment at 120 kg per 100 m

2 NUTRIENT VALUE OF ALUM-TREATED POULTRY LITTER 1783 can achieve indoor NH 3 concentrations 20 mg L 1 lower than that (28 to 43 mg L 1 ) in untreated poultry houses (Moore et al., 2000). A supplementary effect of alum treatment is substantial decreases of poultry litter in water-soluble P. In a 7-d laboratory incubation experiment, Moore and Miller (1994) found that the waterextractable P content decreased from >2,000 mg kg 1 to <1 mg kg 1 when poultry litter was amended with alum at 10 weight percentage. When broiler pens were treated with alum at 90 g per bird for 3 consecutive bird flocks, water-soluble P in the poultry litter decreased 30 to 60%, depending on the bird diet (Miles et al., 2003). Poultry litter from 97 commercial broiler houses receiving alum treatment at 90 g per bird for 7 bird flocks had an average water-soluble P content of 596 mg kg 1, only 33% of the average (1,827 mg kg 1 ) found in poultry litter from 97 control houses (Sims and Luka-McCafferty, 2002). Because runoff losses of P from land-applied poultry litter is largely determined by the water-soluble content of P in the waste (Shreve et al., 1995; Hart et al., 2004), land application of alumtreated poultry litter may reduce P runoff losses as much as 87% compared with application of untreated poultry litter (Shreve et al., 1995; Moore et al., 2000; Smith et al., 2004; Warren et al., 2006; Moore and Edwards, 2007). Alum decreases water-soluble P in poultry litter by forming insoluble AlPO 4 hydroxides (e.g., poorly ordered wavellite) and Al(OH) 3 -phosphate surface adsorption complexes (Peak et al., 2002; Hunger et al., 2004). It is doubted that at commonly practiced application rates (i.e., 10 ton ha 1 ), alum-treated litter would be able to supply adequate P to meet crop nutrient requirements. Although Moore and Edwards (2005, 2007) reported that soils receiving alum-treated poultry litter over 7 yr demonstrated slightly higher Mehlich-III test P and tall fescue biomass production than soils fertilized with normal poultry litter, the authors noted that plant tissue P and soil water-extractable P contents in the alum-treated poultry litter treatment were lower. Moreover, soils employed in the study originally had a high Mehlich-III test P content (~160 mg kg 1 ) and the treatment with ammonium nitrate fertilizer alone generated fescue biomass yield comparable with the poultry litter treatments. Thus, for soils with low soil test-plant-available P, crop growth could be restricted by P deficiency if alum-treated poultry litter is the sole P source. Studies have shown that alum treatment restrains mineralization of poultry litter organic P, which accounts for nearly half of the total P (TP; Warren et al., 2008), perhaps further limiting the plant availability of P in alum-treated poultry litter. In addition, alum treatment has little influence on litter N mineralization (Gilmour et al., 2004) but inhibits NH 3 volatilization from poultry litter and increases N supply to plants after land application (Moore et al., 1995, 2000). It is clear that alum treatment could alter nutrient dynamics and supply capacity of poultry litter after land application. Therefore, evaluation of the nutrient value of alum-treated poultry litter for land application warrants systematic studies. Application rate is a key factor in determining nutrient runoff losses from land-applied animal manures (Mueller et al., 1984; Edwards et al., 1994; Harmel et al., 2004). It is recommended that poultry litter be applied at rates matching crop requirements for P (NRCS, 1999), with additional N applied from other sources to meet crop N needs (Harmel et al., 2004). To develop agronomic application rates of alum-treated poultry litter, nutrient value of the organic fertilizer has to be determined. The objectives of this study were 1) to investigate nutrient release kinetics of alum-treated poultry litter after land application, 2) to estimate nutrient supply capacity of the material, 3) to determine its nutrient value in crop production systems, and 4) to recommend agronomic application rates of alum-treated poultry litter for land application. MATERIALS AND METHODS Simulated Weathering Experiments Poultry litter used in the experiments was obtained from a typical Delmarva poultry farm under standard industrial management. Litter crusts near the water and feed lines were decaked and removed from broiler houses shortly after birds were caught for processing. Alum was then spread at approximately 100 kg per 100 m 2 over the floor and incorporated into the top few inches of litter bedding by a tractor-hauled harrow. Poultry litter cleaned out of poultry houses after each bird flock was stored in a shed and later applied to cropland. A bucket of alum-treated poultry litter was collected hours after a between-flock decaking-cleaning and delivered to the laboratory. Litter (originally sawdust) in the poultry house was nearly 18 mo old at the time of sampling. The alum-treated poultry litter was passed through a 6-mm sieve for homogeneity. Its moisture content was measured at 73.8%. The litter was then packed in triplicate polyvinyl chloride columns (15 cm i.d. 10 cm height) to a depth of 5 cm. The dry mass of poultry litter in each column was 200 g. The columns, with their top open to the atmosphere, were placed in a laboratory at room temperature (23 ± 1 C) and intermittently leached by 18 water-loading events in 190 d. In each rain event, 608 ml of deionized water was continuously pumped at 1.63 ml min 1 to a custommade rain distributor sitting on the top of each column, allowing water to pass evenly through the whole crosssection. The water-loading rate was 5.3 mm h 1, within the intensity range (2.5 to 7.6 mm h 1 ) of moderate rainfall. To simulate the wet-dry cycles that occur in the field, water loading was conducted at 7-d intervals for the first 10 rain events, at 14-d intervals for the 11th to 16th rain events, and at 21-d intervals for the last 2 rain events. Considering that the annual average

3 1784 precipitation on the Delmarva Peninsula is approximately 1,130 mm with relatively even distribution over the year and that the growing season is around 180 d, to simulate the nutrient release process of poultry litter during the first growing season after land application, a total of 600 mm of water was fed through each of the 3 columns in 190 d. Guo and Song The cumulative release of nutrients from the poultry litter columns was calculated by summing the mass of released solutes in each water-loading event from the initiation of the experiments: CR = ( M )/ M, aj j å j= 1 aj PL [2] Leachate Collection and Nutrient Analysis During each water-loading event, leachate from the poultry litter columns was collected in individual 1,000- ml glass beakers. The leachate was immediately recorded for volume and analyzed for ph and electrical conductivity (EC). Aliquots of the leachate were then passed through a 0.45-μm glass fiber filter and stored at 4 C before further chemical analysis. Leachate samples were analyzed for concentrations of dissolved organic C (DOC), total dissolved N (TDN), total dissolved P (TDP), NH + 4, NO 3, NO 2, PO 3 4, K +, Na +, Ca 2+, Mg 2+, Cl, SO 2 4, Fe, and Al. The ph values of leachate samples were measured using an Accumet AB15 ph meter with an Accumet 3-in- 1 ph/atc combination electrode (Fisher Scientific, Suwanee, GA). Electrical conductivity (25 C) was analyzed with an Oakton CON510 conductivity/tds meter (Oakton Instruments, Vernon Hills, IL). Dissolved organic C and TDN contents were determined with a Shimadzu 5000A TOC/TN analyzer (Tokyo, Japan). Total dissolved P contents were analyzed using the phosphomolybdate blue method (Murphy and Riley, 1962) after the leachates were digested with H 2 SO 4 and K 2 S 2 O 8 in an autoclave. Inorganic anions Cl, NO 2, NO 3, SO 2 4, and HPO 2 4 and cations Na +, NH + 4, K +, Ca 2+, and Mg 2+ were measured by ion chromatography using a Metrohm Personal IC 790 system (Metrohm Ltd., Herisau, Switzerland). Dissolved organic N (DON) concentrations were calculated by subtracting NH 4 -N, NO 3 -N, and NO 2 -N from TDN. Dissolved organic P was calculated as the difference between TDP and inorganic PO 4 -P. Data Processing Concentrations of nutrients in poultry litter leachates are reported as means of triplicate measurements. Mass release of nutrients from weathering of poultry litter in a specific water-loading event was calculated as follows: M 1 3 = a å i = ai i 3 1 ( C V ), [1] where M a is the total mass of nutrient a released in the water-loading event, C ai is the concentration of species a in sample i, V i is the volume of sample i (i = 1, 2, 3), and 3 is the replicate number of samples. where CR aj is the cumulatively released mass of nutrient a per unit mass of poultry litter from the onset of the experiments to after the jth water-loading event, M aj is the amount of released nutrient a in the jth waterloading event, j is the number of water-loading events (j = 1, 2,., 18), and M PL is the mass of the poultry litter columns (M PL = 0.2 kg). Nutrient release kinetics were described by fitting the cumulative nutrient release data against the weathering-leaching time using computational models (Sigma- Plot 10.0, Jandel Scientific, San Rafael, CA), employing the Marquardt-Levenberg algorithm in an iterative process. The best fit of kinetic models was determined using paired t-tests (Kirchner and Lauenroth, 1987), r 2 values, and the SE of model parameters. RESULTS AND DISCUSSION Nutrient Content of Alum-Treated Poultry Litter Contents of various nutrients in the alum-treated poultry litter are given in Table 1. The poultry litter was composed of 76.0% organic matter and 24.0% mineral ash. It had organic C (OC), 349 g kg 1 ; total N (TN), 37.3 g kg 1 ; TP, 13.5 g kg 1 ; and K, 35.7 g kg 1. The nutrient contents are typical for Delmarva poultry litter with alum treatment. The lower TP content is a result of phytase modification of bird diet, mandated on the Delmarva since 2006 (DNMC, 2006). Sims and Luka-McCafferty (2002) reported that alum-treated poultry litter from 97 broiler houses on the Delmarva Peninsula demonstrated average OC, TN, and TP contents of 323, 42.4, and 20.1 g kg 1, respectively. The Al content for the present litter was 10.7 g kg 1, close to the average value of 11.5 g kg 1 reported by Sims and Luka-McCafferty (2002). Total Ca, Mg, and Fe in the poultry litter were 22.9, 6.5, and 0.65 g kg 1, respectively. The poultry litter exhibited a neutral ph of 7.1 and a high salinity of 23.6 ds m 1 (1:5 solid:water extracts). Water-soluble OC, N, and P in the litter were 37.2, 17.6, and 0.39 g kg 1, respectively. The water-soluble P was less than 3% of the TP. In poultry litter without alum treatment, water-soluble P averages at 1.83 g kg 1 and accounts for more than 8% of the TP (Sims and Luka-McCafferty, 2002). This result confirms that alum treatment can significantly reduce poultry litter water-soluble P content and subsequently P runoff

4 NUTRIENT VALUE OF ALUM-TREATED POULTRY LITTER 1785 losses from poultry litter-fertilized land. Of the watersoluble P, 57.7% was inorganic P (PO 4 -P) and 42.3% was organic P. The water-soluble N comprised 71.8% NH 4 -N and 29.2% organic-n. Little NO 3 and no NO 2 were detected. Nearly all K in the poultry litter existed in water-soluble forms (Table 1). Despite the high total Al content, alum-treated poultry litter was rather low in water-soluble Al (0.021 g kg 1, Table 1). Sims and Luka-McCafferty (2002) reported an average water-soluble Al content of g kg 1 for alum-treated poultry litter from 97 houses. Water-soluble Ca, Mg, Na, Cl, and SO 4 2 in the poultry litter were 3.96, 2.94, 3.58, 11.65, and g kg 1, respectively. Leachate Fluxes of Poultry Litter Columns A total of 600 mm of water was applied to each poultry litter column in 18 simulated rain events each at 33.3 mm during a period of 190 d (Figure 1). On average, 461 mm of leachate was received from each of the columns. The balance was held by the litter material and subsequently lost to the atmosphere via evaporation. The poultry litter initially exhibited a water-holding capacity of 1.82 g g 1 (oven dry mass basis). With leaching out of fine particles in the litter columns, water retention decreased slightly over time and stabilized at 1.55 g g 1 after 11 wk (Figure 1). Field soils generally have a water-holding capacity less than 0.6 g g 1. Fertilization with poultry litter will evidently improve soil water-holding capacity. Concentrations of Nutrients in Poultry Litter Leachate Leachate from the alum-treated poultry litter columns had ph values ranging from 6.8 to 8.4. The ph was the lowest for the initial leachate. It increased to 8.0 in the second water-loading event and fluctuated between 7.6 and 8.4 afterward (data not shown). The leachate alkalinity is likely attributed to carbonate salts in chicken excreta derived from the broiler feed components (e.g., CaCO 3, K 2 CO 3, and NaHCO 3 ). A ph in the basic range implicates that application of alum-treated poultry litter will not aggravate soil acidification. Electrical conductivity of the leachate ranged from 0.4 to 19.5 ds m 1, decreasing as the weathering process progressed (data not shown). The initial high EC value suggests that salinity toxicity could potentially affect seed germination and seedling development if poultry litter is applied at excessively high rates. The poultry litter leachate contained high concentrations of dissolved organic matter (DOM). Its DOC content was initially as high as 5,430 mg L 1. The DOC decreased abruptly to 940 mg L 1 in the second rain event and then gradually to 34 mg L 1 at the end of the leaching experiments (Figure 2). Leachate DOM is a readily available C and N source for soil organisms. The solution phase of top soils in agricultural land has Figure 1. Water loading (dashed line) and leachate flux (solid bar) time series measured from alum-treated poultry litter columns. Error bars represent SD of the treatment mean. DOC typically lower than 10 mg L 1 (Guo et al., 2001). The relatively constant DOC level at 34 mg L 1 in leachate after 6 mo of simulated weathering indicates that release of DOM from land-applied poultry litter Table 1. Nutrient contents of alum-treated poultry litter Parameter Value Moisture content (%) 73.8 ± 1.7 Organic matter content (g kg 1 ) 760 ± 1 Ash content (g kg 1 ) 240 ± 1 Total organic C (g kg 1 ) 349 ± 1 Total N (g kg 1 ) ± 0.85 Total P (g kg 1 ) ± 0.36 Total K (g kg 1 ) ± 1.04 Total Ca (g kg 1 ) ± 0.48 Total Mg (g kg 1 ) 6.45 ± 0.39 Total Al (g kg 1 ) ± 0.21 Total Fe (g kg 1 ) 0.65 ± 0.03 ph 7.0 Electrical conductivity 1 (ds m 1 ) ± 1.02 Water-extractable components 2 (g kg 1 ) Dissolved organic C ± 1.21 Water-soluble P 0.39 ± 0.00 Water-soluble N ± 0.14 NO ± NO 2 ND 3 Cl ± SO ± PO ± NH ± 0.13 K ± 0.17 Na ± 0.05 Ca ± 0.07 Mg ± 0.03 Fe ± Al ± Measured with poultry litter-water mixtures at 1:5 dry mass:water ratio. 2 Extracted the poultry litters at 1:10 dry solid:water ratio for 8 h. 3 ND = nondetectable.

5 1786 Figure 2. Dissolved organic C (DOC) contents of leachate from alum-treated poultry litter columns. Error bars represent SD of the treatment mean. Guo and Song may last a long time (i.e., until all organic matter is mineralized). The leachate also contained high concentrations of inorganic salts. Concentrations of Na +, K +, Ca 2+, Mg 2+, Cl, and SO 4 2 in leachate ranged from 0.2 to 37, 0.5 to 157, 1.0 to 14, 1.3 to 34, 0.0 to 76, and 0.3 to 116 mmol L 1, respectively (Figure 3). Tracking the trend of leachate EC, these inorganic ions were initially abundant in the leachate, decreased rapidly in the first 6 wk of leaching, and became dilute afterward. The temporary increase of Ca 2+ and Mg 2+ concentrations at 21 d (Figure 3) was likely a transformation result of litter Ca-Mg phosphate minerals by Al: free or organic-complexed Al had stronger affinity for PO 4 3 and could displace Ca-Mg 2+ in phosphate minerals to form variscite (AlPO 4, Brady and Weil, 2008). The data indicate that release of inorganic nutrients from weathering of alum-treated poultry litter would occur predominantly within the first 2 mo after field application if adequate precipitation is provided. When flushed out, these inorganic salts may pose salinity toxicity to vulnerable plants. In a greenhouse trial, significant inhibition on soybean germination was observed when the poultry litter was spread over potted soil at above 12 t ha 1. The inhibition was attributed to the high salt content of the poultry litter. The concentration of dissolved Al in the leachate was consistently below 0.2 mmol L 1, although alum treatment resulted in a high content of total Al (10.73 g kg 1, Table 1). The highest Al concentration (0.17 mmol L 1 or 4.6 mg L 1 ) appeared in the initial leachate; it decreased to 0.05 mmol L 1 (1.4 mg L 1 ) in the second water-loading event and became undetectable after the 12th water-loading event (data not shown). At ph above 6.8, dissolved Al in leachate existed mainly in organic complexes. Aluminum toxicity is not a concern when soil ph is greater than 5.0 or Al 3+ in soil solution is less than 4 mg L 1 (Troeh and Thompson, 1993). Evidently, alum-treated poultry litter will not pose Al toxicity to crops after field application. In field studies with continuous application of alum-treated poultry litter for 8 yr, Moore and Edwards (2005) did not observe any Al toxicity on forage crops or promotion in soil Al availability. Similar to Al, Fe was present in the leachate at a low concentration, less than 0.21 mmol L 1 and decreasing with the weathering time. Total dissolved N content in leachate from the alumtreated poultry litter columns ranged from 32 to 2,324 mg L 1 (Figure 4). The major forms of N were DON and NH 4 -N. At the early stage, DON dominated in the Figure 3. Concentrations of inorganic salt nutrients in leachate from alum-treated poultry litter columns. Error bars represent SD of the treatment mean.

6 NUTRIENT VALUE OF ALUM-TREATED POULTRY LITTER 1787 leachate, whereas NO 3 -N and NO 2 -N were undetectable. As the weathering-leaching time progressed, NO 3 -N and NO 2 -N took the place of NH 4 -N in the leachate. The original poultry litter contained little NO 3 -N (Table 1). Leachate NO 3 -N and NO 2 -N were products of microbial activity. As fine particles were leached out of the columns, poultry litter was improved in water drainage. Less water was held in the litter columns (Figure 1), allowing oxygen to diffuse into the litter matrix. Nitrifying bacteria became active and oxidized NH 4 -N to NO 2 -N and then to NO 3 -N. In the late stage of the leaching experiments, NO 3 -N became the predominant form of TDN and NH 4 -N was nearly undetectable (Figure 4). At the end of the weathering experiments, TDN in the leachate remained as high as 38 mg L 1, suggesting that the release of N from poultry litter weathering would continue for an extended period. Preusch et al. (2002) found that only 42 to 64% of poultry litter organic N was mineralized after 120 d of incubating fresh poultry litter with loam soils at 7.9 to 8.7 g kg 1 and 25 C. Qafoku et al. (2001) reported a similar result of net N mineralization (24 to 74%, mean 51%) for 60 poultry litters after 112 d of laboratory incubation. Total dissolved P observed in the poultry litter leachate was between 8 and 55 mg L 1 (Figure 5). Organic P was the predominant P form found in the first 5 water-loading events, accounting for 54 to 74% of the TDP. When leachate DOC decreased to below 250 mg L 1, inorganic P became dominant. The portion of inorganic P increased to >85% of TDP when leachate DOC became less than 100 mg L 1 after 90 d of weathering. It further increased to >97% of TDP as the DOC content decreased to below 70 mg L 1 after 120 d (Figure 5). Unlike other nutrients, leachate TDP did not simply reduce its concentration with weathering time. Three concentration peaks occurred during the 190-d leaching process. In addition to the initial peak, another 2 peaks appeared 21 to 63 and 147 to 190 d after the experiments started, implying different pools of P in poultry liter. Phosphorus may exist in poultry litter in various forms with different water leachability. Dou et al. (2000) reported that H 2 O, 0.5 M NaHCO 3, 0.1 M NaOH, and 1.0 M HCl sequentially extracted 49, 19, 5, and 25% of the TP in poultry litter. At the end of the 190-d weathering experiments, leachate TDP was still higher than 10 mg L 1. It implies that release of P from land-applied poultry litter may last longer than 1 growing season. In a 4-yr field study with continuous annul application of poultry litter (TP 15.6 g kg 1 ) at 9.4, 18.8, and 37.6 t ha 1 to pasture plots, Johnson et al. (2004) reported that annual removal of the applied P by forage ranged from 6.6 to 10.7%. Supply of N and P nutrients from poultry litter applied in the previous year should be determined and considered in developing an environmentally sound nutrient management plan. Nutrient Release Kinetics Cumulative nutrient release from the alum-treated poultry litter columns was calculated as a function of weathering time. Release of DOC is described by a firstorder exponential rise-to-maximum (ERM) equation: CR = CR + CR ( -e -kt ), [3] t 1 0 W where CR t is the cumulative mass of DOC released per unit mass of poultry litter in the columns, CR 0 is the DOC that is resident and could be initially leached out per unit mass of litter, CR W is the mass of DOC that Figure 4. Concentrations (Conc.) of organic N (black bar), NH 4 -N (white bar), NO 3 -N (striped bar), and NO 2 -N (gray bar) in leachate from alum-treated poultry litter columns. The sum of these forms of N is total dissolved N. Error bars represent SD of the treatment mean. Figure 5. Concentrations (Conc.) of inorganic (white bar) and organic (black bar) P in leachate from alum-treated poultry litter columns. The sum of inorganic and organic P is total dissolved P. Error bars represent SD of the treatment mean.

7 1788 Guo and Song Figure 6. Release kinetics of dissolved organic C (DOC), total dissolved P (TDP), inorganic P (Inorganic P), total dissolved N (TDN), NH 4 -N, and NO 3 -N from weathering of alum-treated poultry litter. Curves are obtained by fitting the data with mathematical models. Regression equations and coefficients of determination are shown. could be potentially released as a result of weathering per unit mass of poultry litter, k is a first-order rate constant, and t is weathering time (d). The fitted curve precisely describes the cumulative release patterns of DOC from the litter columns, with a coefficient of determination (r 2 ) greater than (Figure 6, DOC). The model predicts that under the described conditions, 21.3 g of DOC would potentially be released per kilogram of the alum-treated poultry litter, accounting for 6% of the total OC and for 57% of the water-extractable OC in the original litter (Table 1). It is true that all OC in poultry litter will be eventually mineralized. However, merely 6% would be recovered in the leachate; the rest would be transformed into CO 2 and dissipate in the atmosphere. When alum-treated poultry litter was spread over a silt loam soil in a mason jar and incubated in the laboratory at 25 C, Gilmour et al. (2004) found that 50% of the litter OC was mineralized into CO 2 within 70 d. In the present study, more than 85% of the leachate DOC was collected during the initial 35 d. As leachable OC diminished in the poultry liter, the DOC release rate (indicated by the curve slope) decreased rapidly with the weathering time (Figure 6, DOC). Conversion into CO 2 further reduced the amount of DOC that might be leached by water. As a result, the leachate DOC remained low (<50 mg L 1 ) at the late stage of the experiments (Figure 2). Cumulative release of TDP also followed an ERM t model: CR = + -e ( 1 ) (Figure 6, TDP TDP). The small rate constant (0.0129) suggests a long-lasting process for P release from poultry litter weathering. Unlike DOC, nearly all releasable P would be recovered in the leachate. The release kinetics predict that 0.63 g of TDP would be released per kilogram of the alum-treated poultry litter. The potential release was 1.6 times higher than the water-extractable P in the original poultry litter but 21.8 times lower than the TP (Table 1). Most of the P in alum-treated poultry litter exists as insoluble precipitates (e.g., AlPO 4 hydroxides). The released P in leachate can be viewed as the plant-available portion. To ensure adequate crop P supply, the application rate has to be adjusted accordingly if alum-treated poultry litter is the sole P source. Compared with DOC, TDP was released at much lower rates (Figure 6). After 35 d, only 46% of the potentially leachable TDP was released from the alum-treated poultry litter. It would require 167 d for 90% of the potentially leachable TDP to be released from the poultry litter. Because inorganic forms of P were dominant in poultry litter leachate (Figure 5), the release kinetics of inorganic P from litter weathering followed patterns similar to those of TDP (Figure 6, Inorganic P). The fitted ERM equations estimate that 0.53 g of inorganic P would be potentially released per kilogram of the alum-treated poultry litter. Overall, inorganic P accounted for 84% of the TDP released from the alum-treated litter. Release of TDN from the alum-treated poultry litter could also be described by an ERM model: t CR = + -e ( 1 ) (Figure 6, TDN). The TDN potential TDN release is predicted at g kg 1, accounting for 77.0% of the water-extractable N and 36.3% of the TN in the litter (Table 1). Relative to TDP, TDN was released at significantly higher rates. Within the first 35 d, more than 90% of the leachable TDN was recovered in the leachate. Water-extractable N is directly leachable. The fact that TDN recovered in the leachate was merely three-quarters of the waterextractable N suggests significant N losses via NH 3 volatilization. Laboratory incubation of alum-treated poultry litter spread over a silt loam soil for 42 d resulted in 20 to 23% of litter organic N converting to NH 4 -N (Gilmour et al., 2004). Field studies demonstrate that up to 24% of TN in surface-applied poultry

8 NUTRIENT VALUE OF ALUM-TREATED POULTRY LITTER 1789 litter would be lost through NH 3 volatilization (Sharpe et al., 2004). Ammonium-N was the major form of the leached TDN (Figure 4). The NH 4 -N release pattern is also well described by an ERM model: t CR = ( 1 -e )(Figure 6, NH NH4 - N 4 -N). The leachate-recoverable NH 4 -N was 10.4 g kg 1 of litter, accounting for 77% of the leachable TDN. Within 35 d, 94% of the potentially leachable NH 4 -N was released. The leachable NH 4 -N is even lower than the corresponding water-extractable fraction in the original litter (12.6 g kg 1, Table 1). Evidently, N losses via NH 3 volatilization were significant during weathering of alum-treated poultry litter. Release of NO 3 showed a pattern distinct from other nutrients (Figure 6, NO 3 -N). Because fresh poultry litter contained negligible NO 3 (Table 1), leachate NO 3 was essentially a product of oxidation of NH + 4 by nitrifying microbes. Due to the anoxic conditions initially developed inside the poultry litter columns as a result of high water-holding capacity (Figure 1), activities of nitrifying bacteria (requiring O 2 as an electron acceptor) were inhibited and little NH + 4 was oxidized, as indicated by the absence of NO 3 in leachates collected at the early experimental stage (Figure 4). Drainage of the poultry litter columns was improved with the decomposition of labile C and the leaching out of fine particles, allowing oxygen to diffuse into the columns at the middle stage of the leaching process. The best model to describe the cumulative release of NO 3 -N is a first-order sigmoidal logistic equation: CRmax CR =, t k æ ö t 1 + èç t12 / ø [4] where CR max is the maximum amount of leachable NO 3 -N in the poultry litter columns, t 1/2 is the length of time required to release 50% of the CR max, and k is the release rate constant. The fitted curve, which has r 2 > 0.995, suggests that if O 2 and NH 4 + are available, the activity (and presumably the biomass) of nitrifying bacteria increase sigmoidally with time, resulting in a sigmoidal increase in NO 3 -N release. Approximately 0.76 g kg 1 of NO 3 -N would be released from the alum-treated poultry litter. Half of the potential NO 3 -N release occurred 132 d after the leaching experiments started. Relative to NH 4 -N and organic N, NO 3 -N was much less significant in TDN release. Release of K +, Na +, Cl, and SO 4 2 from weathering of alum-treated poultry litter can also be described by models similar to those for DOC and P release (Figure 7). Potentially leachable K +, Na +, Cl, and SO 4 2 in the litter were 34.6, 8.8, 12.1, and 85.6 g kg 1, respectively. The leachable contents of these nutrients were slightly higher than their corresponding water-extractable fractions (Table 1), implying that these components existed in poultry litter predominantly as water-soluble salts. Release of these nutrients from poultry litter was rapid. Within 35 d, 87 to 98% of the potentially leachable ions were released. Unlike other salt nutrients, releases of Ca 2+ and Mg 2+ from weathering of poultry litter were unique. Release of these 2 nutrients can be described using the logarithmic equation: CR a k t t t = + ln( + ), [5] 0 where a, k, and t 0 are constants. The model fits the experimental data well (r 2 > 0.99; Figure 7, Ca 2+, Mg 2+ ). The release rate decreased gradually with time. During the duration of this study (>6 mo), 3.97 g of Ca 2+ was released per kilogram of the alum-treated poultry litter. This is rather close to the water-extractable frac- Figure 7. Release kinetics of K +, Na +, Ca 2+, Mg 2+, Cl, and SO 4 2 from leaching of alum-treated poultry litter. Curves are obtained by fitting the data with mathematical models. Regression equations and coefficients of determination are shown.

9 1790 tion (3.96 g kg 1 ) yet far lower than the total content (22.85 g kg 1, Table 1) of Ca 2+ in the litter material. The cumulative release of Mg 2+ was 4.53 g kg 1. It is lower than the total content (6.45 g kg 1 ) of Mg 2+ in the poultry litters but almost twice the water-extractable fraction (2.94 g kg 1, Table 1). It suggests that supply of Ca 2+ and Mg 2+ from poultry litter weathering is long lasting and the water-extractable content can be used to estimate the portion available to plants. Supply Capacity-Based Nutrient Value of Alum-Treated Poultry Litter Masses of nutrients recovered in leachate from weathering of alum-treated poultry litter for 190 d were calculated from leachate flux and solute concentration data. The results are reported in Table 2. For comparison, potentially leachable nutrients predicted by the release kinetics models are also included. Within 190 d, a ton (mg) of the alum-treated poultry litter released, as recovered in leachate, approximately 13.8 kg of N, 0.59 kg of P, and 34.6 kg of K. Of the leached N, 19.3% was DON, 75.3% was NH 4 + -N, and 5.3% was NO 3 -N. Of the released P, inorganic orthophosphate P accounted for 62.7%. The actual amounts recovered in leachate of all test nutrients except for PO 4 -P, Ca, and Mg agree well with the potentially leachable amounts in poultry litter as predicted by the release kinetic models (Table 2), suggesting that the nutrient-releasing process was virtually complete and that the nutrient supply capacity was reached. Release of P, however, would continue until the PO 4 -P supply capacity of 0.53 g kg 1 of litter was fulfilled. Nutrients recovered in leachate from weathering of poultry litter can be treated as the plant-available portion. It is true that in field applications, nutrients released from weathering of animal manure either surface broadcast or soil incorporated are subject to several Table 2. Released nutrients and the predicted release potentials from weathering alum-treated poultry litter Nutrient Practically released (g kg 1 of litter) Release potential (g kg 1 of litter) DOC ± ± 0.27 TDN ± ± 0.35 NH + 4 -N ± ± 0.30 NO 3 -N 0.73 ± ± 0.02 NO 2 -N ± TDP ± ± 0.02 PO 4 -P 0.37 ± ± 0.05 Na ± ± 0.19 K ± ± 0.80 Ca ± 0.21 Mg ± 0.26 Cl ± ± SO ± ± 2.69 Fe ± Al ± DOC = dissolved organic C. 2 TDN = total dissolved N. 3 TDP = total dissolved P. Guo and Song processes. In addition to plant uptake, the nutrients may be lost to the atmosphere via volatilization and to aquatic systems through runoff and leaching; they may also be fixed by microorganisms or transformed into precipitates by chemical reactions and become plantunavailable. Soil incorporation may promote poultry litter decomposition and reduce NH 3 volatilization. Meanwhile, soluble phosphate may react with Ca 2+, Mg 2+, Al 3+, and Fe 3+ in soil and form crop-unutilizable precipitates. Soil microorganisms will also tie up some of the released N and P. Overall, the data on nutrient release from weathering of poultry litter and subsequent leachate recovery under the described conditions can be used to predict the nutrient supply capacity and nutrient availability of land-applied poultry litter, especially in conservation tillage systems. Based on the 190-d weathering-leaching experiments, it is estimated that the nutrient value of alum-treated poultry litter is N, 13.8 g kg 1 ; P, 0.75 g kg 1 ; K, 34.6 g kg 1 ; and S, 24.2 g kg 1. The P value includes the leachable inorganic phosphate in poultry litter. It is clear that only a small portion of TN and P in alumtreated poultry litter is plant-available. Studies have shown that 21 to 64% of N in poultry manure is available to crops (Bitzer and Sims, 1988; Preusch et al., 2002). When poultry litter was applied at 9.4 to 37.6 t ha 1 annually to pasture plots for 4 yr continuously, Johnson et al. (2004) determined that 6.6 to 10.7% of the applied P was uptaken by forage plants. For the alum-treated poultry litter employed in the present study, 37.0% of TN, 5.5% of TP, and 96.9% of total K are plant-available. Livestock manure should be applied at optimum rates that ensure sufficient crop nutrient supply while minimizing environmental threats. Manure nutrient value, soil nutrient status, and crop nutrient requirements have to be incorporated in application rate development. Common fertilization rates for field crop farming are N, 100 kg ha 1, and P, 25 kg ha 1. To match the popularly practiced rates, alum-treated poultry litter should be applied at 7.3 dry tonnes ha 1, with supplemental chemical P fertilization at 19.5 kg of P ha 1. Considering that most cropland in regions with concentrated poultry production already demonstrates agronomically excessive soil test P (>25 mg kg 1 ) due to historic overapplication of conventional poultry litter (Vadas and Sims, 1998; Codling et al., 2000), supplemental P fertilizer application is generally unnecessary. Alumtreated poultry litter applied at 7.3 dry tonnes ha 1 will supply 100 kg of N ha 1 and 5.5 kg of P ha 1 to crops in 1 growing season. As a matter of fact, this recommended rate is still lower than the currently practiced rate (10 dry tonnes ha 1 ) of conventional poultry litter application. Conclusions A variety of nutrients including OC, N, P, K, Ca, Mg, Cl, and SO 4 were released from weathering alum-treated

10 NUTRIENT VALUE OF ALUM-TREATED POULTRY LITTER 1791 poultry litter. The vast majority of these released nutrients were recovered in poultry litter leachate. Significant portions of OC and N were lost to the atmosphere via CO 2 emission and NH 3 volatilization. The poultry litter leachate was neutral to slightly alkaline, with ph ranging from 6.8 to 8.4. Initially, the leachate demonstrated a high salinity, with EC up to 20 ds m 1. Major inorganic solutes in the leachate were K +, NH 4 +, Na +, Mg 2+, Ca 2+, Cl, SO 4 2, and HPO 4 2. Concentrations of DOC, TDN, TDP, and K in the leachate ranged from 30 to 5,430, 30 to 2,330, 7 to 55, and 20 to 6,130 mg L 1, respectively. During 190 d of the simulated weathering process, 21.4 g of DOC, 13.8 g of TDN, 0.6 g of TDP, and 34.6 g of K were released per kilogram of alum-treated poultry litter and subsequently recovered in leachate. The leachate N consisted of 19.3% DON, 75.3% NH 4 -N, and 5.3% NO 3 -N, whereas P was comprised of 62.7% inorganic P and 37.3% of organic P. The nutrient release occurred primarily in the first 5 wk but may last up to 2 yr, especially for P, Ca, and Mg. Cumulative release of DOC, TDP, TDN, inorganic P, NH 4 -N, K +, Na +, Cl, and SO 4 2 followed a first-order ERM model, whereas releases of Ca 2+ and Mg 2+ were best described as a logarithmic process and release of NO 3 -N fitted a first-order sigmoidal logistic equation. The nutrient value of alum-treated poultry litter, based on its nutrient release kinetics and supply capacity during the 190-d weathering, is N, 13.8 g kg 1 ; P, 0.75 g kg 1 ; K, 34.6 g kg 1 ; and S, 24.2 g kg 1. No Al toxicity would be introduced by applying alum-treated poultry litter because Al concentrations in the leachate were consistently below 0.2 mm. In field applications, it is recommended to spread alum-treated poultry litter at 7.3 t ha 1. 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