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a Center for Animal Health and Productivity, School of Veterinary Medicine, Univ. of Pennsylvania, 382 West Street Road, Kennett Square, PA 19348
b Soil Science Institute, Chinese Academy of Sciences, Nanjing, China
c USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, University Park, PA 16802
* Corresponding author (dou{at}cahp.vet.upenn.edu)
Received for publication July 12, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ANT, anthracite refuse fly ash • FBC, fluidized bed combustion fly ash • FGD, flue gas desulfurization by-product • Pi, inorganic phosphorus • Po, organic phosphorus • Pt, total phosphorus in extract • TP, total phosphorus in acid digest
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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On animal farms under given conditions (soil characteristics, transport factors, etc.), freshly applied manure, especially if surface-spread without incorporation, may function as a major source for P enrichment in runoff waters. Preedy et al. (2001) reported a surge of P export in overland and subsurface drainage waters from grassland receiving dairy slurry or P fertilizer applications. Compared with the control, dairy slurry treatment of 29 kg P ha-1 resulted in a 37-fold increase in P loss within 32 h after the application. In a watershed-scale study, Sharpley et al. (2001) demonstrated that fields receiving manure or fertilizer within 3 wk before rainfall were much more vulnerable to P loss than fields receiving zero P application during the previous 6 mo. Where fertilizer or manure was applied within 3 wk of the simulated rainfall events, dissolved and total P in runoff was not related to Mehlich-3 soil P, but increased with increasing rates of P applications (Sharpley et al., 2001).
Phosphorus concentrations in animal manures typically are many times greater than in soils. For example, on a dry matter basis total P concentrations ranged from 4 to 7 g kg-1 in dairy manures and 7 to 30 g kg-1 in poultry manures (Barnett, 1994), compared with 0.3 to 2.4 g total P kg-1 in 23 surface soils (0–5 cm) receiving long-term manure applications (Sharpley, 1996). More critically, much of the P in manure is water soluble. Dou et al. (2000a)( b; 2002) found that P concentrations in water extracts of manure samples with 1 h of shaking ranged from 2 to 11 g total phosphorus (Pt) kg-1 manure dry matter. In comparison, the 23 surface soils receiving long-term manure applications tested by Sharpley (1996) had labile P (extracted in bicarbonate solution) concentrations less than 0.5 g kg-1. Clearly, it is imperative to carefully manage manures when surface-applied to prevent the large amount of soluble P contained in manures from being dissolved and carried to streams through runoff.
Perhaps, reducing the solubility of manure P before field application can provide an effective source control measure. A number of manure treatments based on chemistry principles have been investigated, one of which is alum (aluminum sulfate). Alum, when added to animal manure, stabilizes manure P through the formation of aluminum phosphate or other Al–P binding mechanisms (Moore and Miller, 1994; Cooke et al., 1986). Substantial reduction in water-soluble P has been reported when alum was added to poultry manure (Moore and Miller, 1994; Moore et al., 2000). Phosphorus in runoff water from field plots receiving alum-treated poultry litter was reduced by 87% compared with the control plots receiving untreated poultry litter (Shreve et al., 1995).
Recent studies also tested coal combustion power plant by-products as manure amendments for reducing soluble P and decreasing its potential loss. Through laboratory incubations, Dao (1999) amended feedlot manure or composted manure with Class C fly ash materials and obtained an 85 to 93% decrease in water-soluble P compared with untreated manure samples. When the fly ash–treated manure was mixed with soils, soil extractable P was reduced substantially. In another study, Elrashidi et al. (1999) packed leaching columns with dairy manure atop fluidized bed combustion fly ash (FBC) material. Water was added and leachate was collected weekly for 10 wk. Phosphorus in the leachate was reduced by more than 80% compared with the control containing dairy manure alone. Presumably, reduction of soluble P in manure treated by FBC or similar fly ash may be attributed to chemical precipitation and surface retention.
Coal-burning power plant by-products have long been introduced into agricultural systems as soil amendments with demonstrated agronomic benefits such as reduced subsoil acidity and improved root growth and crop yields (Stout et al., 1979; Stehouwer et al., 1999). In a more recent study addressing environmental concerns, Stout et al. (1998) amended several high-P soils with FBC or flue gas desulfurization by-product (FGD). The by-products reduced water-soluble P in the amended soils by 45 to 72% compared with untreated soils. The reduction was attributed to the conversion of readily desorbable soil P, measured in water extracts, into less soluble forms.
Byproducts FBC and FGD are generated at coal-burning power plants where limestone (CaCO3) is used to remove SO2 from the flue gas. These by-products are often mixtures of coal ash, anhydrite, and limestone residues with alkaline components in the form of carbonates, hydroxides, and oxides (Stehouwer et al., 1999). Adoption of SO2 removing methods is required by the USEPA to conform to the Clean Air Act and would generate more than 50 Tg of by-products on an annual basis (USEPA, 1988). This vast amount of by-products, if tested successfully, may be used as manure amendments for agronomic and environmental benefits.
The primary purpose of this study was to investigate the efficacy of alum and three by-products of coal combustion power plants for reducing P solubility when amended to dairy, swine, or broiler litter manures through laboratory incubation. Changes in P fraction distributions in manure resulting from the various treatments were also studied.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Chemical amendments included technical-grade alum [Al2(SO4)3·14H2O] and coal-combustion power plant by-products FBC, FGD, and anthracite refuse fly ash (ANT). Chemical and physical characteristics of the amendments are presented in Table 1 . Preliminary trials were conducted using dairy manure to determine effective amendment rates and incubation time. A basic consideration for selecting the range of treatment rate was to achieve a sizable reduction in water-soluble P while avoiding excessive use of amendment. The final rates were set as following: alum at 25, 50, 100, and 250 g kg-1 manure (dry matter basis); FBC at 50, 100, 200, and 400 g kg-1; and FGD at 50, 100, 150, and 250 g kg-1. The ANT fly ash material was obtained at a later stage of the study, and its rates were arbitrarily set at 25, 50, 100, and 250 g kg-1 (same rates as alum). In terms of incubation time, Dao (1999) reported little difference in soluble P reduction when manure was amended with Class C fly ash and incubated for 1, 3, or 6 wk. We tested 3, 5, and 7 d of incubation and found little time difference in water-soluble P reduction. The full-scale investigation was performed with 3 d of incubation.
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Laboratory Analyses
At the completion of incubation, subsamples of treated and untreated moist samples were processed immediately for the determination of pH and readily soluble P. Moist samples of 2 g each were weighed into plastic tubes, capped, and shaken in 98 mL deionized H2O for 1 h on a reciprocal shaker and filtered through Whatman (Maidstone, UK) 42 paper. Aliquots of the filtrates were analyzed for pH using a glass electrode, inorganic phosphorus (Pi) by the phosphomolybdate blue method (Murphy and Riley, 1962), and Pt by inductively coupled plasma (ICP) spectrometry. Concentrations of organic phosphorus (Po) in filtrates were calculated as the differences between Pt and Pi. Additionally, aliquots of filtrates were used to determine ammoniacal N concentrations using the indophenol blue method (Keeney and Nelson, 1982).
The remaining treated and untreated samples were oven-dried at 60°C and ground to pass a 2-mm sieve. Samples of selected treatments, including control, alum at 100 g kg-1, FBC at 400 g kg-1, and FGD at 250 g kg-1, were tested for P fraction distributions. These samples were extracted through a rigorous sequential extraction procedure described by Dou et al. (2000a), which was adapted from a soil P fractionation scheme (Hedley et al., 1982). Briefly, this procedure involves extracting 0.30-g, dried, ground samples with deionized H2O, 0.5 M NaHCO3, 0.1 M NaOH, and 1 M HCl solutions, in that order. Repeated 1-h shaking and filtering is performed with each extractant (30 mL) until the P in the last filtrate is negligible before proceeding to the next extractant. Individual filtrates were analyzed for concentrations of Pi and Pt. The sums of P in the relevant extracting solutions are referred to as H2O-P, NaHCO3–P, NaOH-P, and HCl-P fractions, respectively. Phosphorus in residues at the completion of the sequential extractions was estimated by subtracting the sum of P in the four fractions from acid digest total P of the manure samples. It is apparent that P extracted in H2O, NaHCO3, NaOH, and HCl in that order is increasingly stable, or decreasingly susceptible to potential environmental loss.
Analysis of variance and mean separation by Duncan's New Multiple Range Test (P = 0.05) were performed using the General Linear Models procedure (SAS Institute, 1999). All results were reported as treatment means and based on manure dry matter after adjusting for the amount of amendment added. Where appropriate, standard errors were reported to illustrate variability about the means.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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At the completion of incubation, the untreated control samples had readily soluble P concentrations averaging 3.06, 5.65, and 2.81 g Pt kg-1 for the dairy, swine, and broiler samples, respectively. These accounted for 39, 21, and 15% of the acid digest total P of the manures. Also, 85 to 91% of the readily soluble P was inorganic P. That water extracts of manure samples contain much higher Pi than Po is consistent with our previous findings using manures from various sources (Dou et al., 2000a; 2002).
Sequential extraction results for the untreated control samples revealed some differences in P fraction distribution between manure types (Fig. 1) . The H2O-P fraction dominated the dairy samples with a distribution pattern of 70% H2O-P, 16% NaHCO3–P, 4% NaOH-P, and 3% HCl-P, plus 6% in residues. This pattern is consistent with our previous findings using various manures (Dou et al., 2000a; 2002). However, for the swine and broiler samples, although the amount of cumulative water-extractable P was considerable (7.61 g Pt kg-1 for swine and 5.37 g Pt kg-1 for the broiler), H2O-P as a fraction was no longer dominating. The fraction distribution was 28% H2O-P, 21% NaHCO3–P, 7% NaOH-P, and 30% HCl-P, plus 14% in residues for the swine, and 30, 13, 13, 41, and 3% for the broiler litter, respectively. Also, the NaOH-P and HCl-P fractions of the swine and broiler samples consisted of more Po than Pi, which is contrary to the dairy samples (detailed data not presented).
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Limited pH change may help explain why there seems to be a lack of, or inconsistent, treatment effects on the ammoniacal N concentrations measured in water extracts of moist samples with 1 h of shaking (Table 2). Moore et al. (2000) reported significant decreases in ammonia volatilization loss (i.e., increases in ammoniacal N in manure) in alum-treated poultry litter; their findings were coupled with substantial decreases in pH with alum treatments. On the other hand, Dao (1999) recorded no treatment effect on total Kjeldahl nitrogen (TKN) when beef feedlot manure or manure compost was treated with alum or fly ash, despite considerable changes in pH (up to 3 units). Dao explained that the manure samples possibly contained little ammoniacal N to begin with, due to volatilization losses before sample collection, while manure organic N is relatively stable and would not be affected by the amendments or pH change. In the present study, the ammoniacal N of the untreated control samples appeared to be considerably lower than published book values (American Society of Agricultural Engineers, 1993). We do not know if substantial volatilization loss occurred during the time period from farm sample collection to post-incubation analyses. But for the dairy samples (control or treated) with very low ammoniacal N concentrations, substantial volatilization loss is unlikely given the low pH. It is possible that the dairy sample, collected fresh from the floor of a free stall barn, consisted of mainly fecal material with little urine. Nitrogen in dairy feces is primarily organic and relatively stable.
Reduction in Readily Soluble Phosphorus
There was a general trend of decreasing readily soluble P with increasing treatment rates (Fig. 2)
. Also illustrated in Fig. 2 is that decrease in readily soluble P is primarily associated with Pi, whereas changes in Po are small and inconsistent. The latter might be attributed to the low Po concentrations in water extracts, coupled with a relatively large variation since Po values were calculated (Pt - Pi) instead of direct measurements.
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At the accentuated rates, estimated molar ratios of Al to P were 1.33:1, 0.96:1, and 0.57:1 for dairy, swine, and broiler samples, respectively, based on the amount of Al added through alum and the acid digest total P concentration of the manure. If we use readily soluble Pi instead of acid digest total P for the calculation, the ratios would be 3.7:1, 4.3:1, and 5.4:1, a range considerably narrower among the three manures. We argue that readily soluble Pi is a better measurable parameter than acid digest total P for reflecting the relative magnitude of manure P that is most responsive to the treatments of this study.
Cooke et al. (1986) proposed two different mechanisms responsible for P removal when alum is used for wastewater treatment: formation of AlPO4 under acidic conditions (pH < 6) and sorption and entrapment of inorganic and organic P on Al(OH)3 flocs at higher pH. Accordingly, the sorption–entrapment mechanism seems to play a major role in the reduction of readily soluble P in the current study because of favorable pH. Sorption–entrapment being the major mechanism also helps explain the high Al to P mole ratios estimated in this study (from 3.7:1 to 5.4:1 based on readily soluble Pi). Obviously, formation of AlPO4 would have a 1:1 Al to P mole ratio, whereas the sorption–entrapment mechanism may be associated with a much higher ratio because Al–P bonding on the surface of Al(OH)3 flocs can be space-restricted and thus less efficient.
The FBC treatments reduced readily soluble P (mainly Pi) at rates of 200 and 400 g kg-1 for the dairy, 400 g kg-1 for the swine, and at all rates (50, 100, 200, and 400 g kg-1) for the broiler sample (Fig. 2b). At a rate of 400 g kg-1, where the most reduction occurred for all three manures, readily soluble Pi decreased by 50 to 60% compared with the untreated control samples. The FGD treatments were ineffective for the dairy but effective for the swine and broiler manures with steady decreases in readily soluble Pi as the rate increased (Fig. 2c). At 150 g kg-1 for the swine and 250 g kg-1 for the broiler litter manure, decrease in readily soluble Pi was accentuated with approximately 80% reduction compared with the untreated control samples. The ANT treatments were generally ineffective for all three manures with less than 20% reduction in readily soluble P even at the highest rate (data not presented).
It appears that Ca and pH are the main factors affecting P solubility and its reduction in the by-product treatments. The dairy manure had a relatively low pH (6.01) and thus the effectiveness of the by-products depended on the increase of pH in addition to the increase in Ca. The FBC was effective only at the two highest rates (200 and 400 g kg-1) when the pH was raised to above 7.0 (Table 2) coupled with considerable addition of Ca. The FGD was ineffective at all rates for the dairy manure because of limited addition of Ca at the lower rates and insufficient pH increase at the higher rates. In contrast, the swine and broiler manures had initial pH above 8.0, and thus the relative effectiveness of the by-products was largely driven by the amount of Ca added in the treatments. As illustrated in Fig. 2b and 2c, both swine and broiler samples exhibited a trend of steady decrease in readily soluble Pi with increasing rates of FBC or FGD. The ANT fly ash contained the smallest amount of Ca as compared with FBC and FGD (Table 1) and thus was ineffective for all three manures regardless of pH.
By rough estimates, pH above 7.0 and a Ca to P mole ratio (readily soluble Pi based) greater than 10:1 appeared to be the prime conditions for the coal combustion by-products to be effective with a 50% or more reduction in readily soluble Pi. Elrashidi et al. (1999) discussed possible mechanisms involved in reduction of soluble P for FBC treated dairy manure, including (i) precipitation of insoluble Ca–phosphate compounds, (ii) retention of soluble organic P on FBC mineral surfaces, (iii) sorption of soluble inorganic P by Al- and Fe-hydroxides, and (iv) coprecipitation and/or entrapment of P by newly formed carbonates due to carbonization. It is likely that most of these mechanisms would involve Ca–P binding with relatively less strength than the Al–P binding in alum treatments. Results from the sequential extractions, discussed next, seem to support this supposition.
Shift in Phosphorus Fractions
Sequential extraction results clearly demonstrate the shift of P from water-extractable to more stable fractions. Mostly, the alum treatment shifted P into NaOH-P while the by-product treatments shifted P into NaHCO3–P (Fig. 3)
. The shifts involved primarily Pi, whereas concentrations of Po in the relevant fractions remained unchanged except in the alum treatments. All three manures with the alum treatment had much higher Po concentrations associated with the NaOH-P fraction, as compared with the untreated control samples (detailed data not presented). This perhaps reflects the sorption–entrapment of organic P on Al(OH)3 flocs discussed earlier.
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Implications
Surface-spreading manure onto no-till or pasture fields is a common practice on many animal operations. Also, manure applications occur both in and out of the growing season because of limited storage capacity as well as seasonal labor constraints. Phosphorus contained in manures surface-applied during the nongrowing season is particularly prone to runoff losses. Treating manure with alum or coal-burning power plant by-products before field application to stabilize soluble P can be a management option to address the special needs. This would be an addition to the traditionally recommended best management practices emphasizing in-the-field tactics such as application rate, time, and method.
However, addition of Al to soils through the use of alum-treated manures might be a concern. A cursory estimate was made to calculate the relative magnitude of Al addition. Assuming the swine manure in this study is to be applied at 30 kg total P ha-1 yr-1, the equivalent manure dry matter applied would be 1106 kg ha-1 yr-1. If the manure is treated with 250 g alum kg-1 dry matter before field application, the amount of Al added would be 25 kg ha-1 yr-1. The consequent Al loading would be about 0.0125 g kg-1 yr-1 assuming the treated manure is mixed to a 15-cm depth with a soil bulk density of 1.33 g cm-3. The estimated Al loading is negligibly small compared with Al contained in typical agricultural soils (14–40 g Al kg-1 soil; Stout et al., 1988). Periodic tillage is desirable to prevent Al and P from accumulating near the soil surface.
For the by-products, we estimated heavy metal loading rates assuming the swine manure treated with FBC at 400 g kg-1 in the current study is to be applied at 30 kg total P ha-1 yr-1. The results suggest that the FBC-treated manure may be applied for many years without exceeding the USEPA's heavy metal loading limits (Table 3) . Also, levels of heavy metals contained in nine samples of FBC tested by Stout et al. (1988) are within ranges usually found in soils; these elements are mostly within the maximum tolerable dietary levels for dairy cattle (Table 3).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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