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a Department of Statistics, University of Florida, Gainesville, FL 32611
b Earth and Environmental Science Department, The University of Texas, San Antonio, TX 78249-0663
c Agricultural and Biological Engineering Department, Pennsylvania State University, University Park, PA 16802
* Corresponding author (gao{at}ufl.edu).
Received for publication July 18, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BPR, biological phosphorus removal • TSP, triple superphosphate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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When P considerations dictate biosolids application, the 1995 USEPA design manual (USEPA, 1995) advises consideration of the "relative effectiveness" (50%) of biosolids P compared with fertilizer P. No literature is cited to support the 50% value, but several researchers (e.g., de Haan, 1980; Hall and Williams, 1984; Coker and Carlton-Smith, 1986) report ranges from 10 to 100% effectiveness relative to fertilizer P determined in greenhouse studies. Canadian regulators (Ontario Ministry of Environment and Energy and Ontario Ministry of Agriculture, Food and Rural Affairs, 1996) recommend a value of 40%. The relative effectiveness factor tacitly acknowledges that not all the P in biosolids is phytoavailable. Indeed, biosolids P phytoavailability might be expected to depend on the residuals treatment processes used because they affect the forms (and solubilities) of P in biosolids. This dependency is especially true of biosolids produced through chemical treatment of wastewater for P removal. Pastene (1981) recommended the molar ratio of (Al + Fe) to P as an indicator of the P-supplying power of the biosolids. He suggested that ratio values of <1 were characteristic of biosolids capable of supplying large quantities of soluble P, whereas ratio values of >1 indicated sources of poor P supply. Soon and Bates (1982) found that biosolids treated with Ca salts had greater P phytoavailability than Fe- or Al-treated biosolids. Smith et al. (2002) found that thermal drying significantly reduced release of biosolids P compared with a conventional, dewatered digested cake.
Despite the dominance of inorganic P forms in biosolids (e.g., Pierzynski et al., 1994; Sommers et al., 1976), consideration of organic P mineralization rates may affect phytoavailability. Fine and Mingelgrin (1996) found that mineralization of organic P played only a minor role in P phytoavailability in soils amended with activated sludge biosolids. However, Stratful et al. (1999) found that BPR sludges contained more phytoavailable P than other sludges, and concluded that BPR sludges were excellent P fertilizers. Biological P removal sludges have received limited study, but organic P forms (and their mineralization rates) may be important in determining P phytoavailability from BPR materials.
A further complication to predicting P phytoavailability in biosolids is the P retention characteristics of the soil being amended (Ebeling et al., 2003). Different soils should be expected to convert soluble waste P into soil forms (reaction products) of widely different phytoavailabilities (White, 1981). A given rate of soluble fertilizer P results in different P phytoavailabilities and leaching tendencies in different soils, and reactions of biosolids-borne P should be similarly complex. Further, the added biosolids can alter soil properties [e.g., pH (O'Connor et al., 1986), P adsorption capacity, Fe and Al oxide content (Shrever et al., 1996), organic P mineralization rates (White, 1981)], and add other components, such as soluble salts, soluble organic ligands, and competing ions, that can alter P solubility (Lu and O'Connor, 2001).
The purpose of our study was to assess the relative phytoavailability of P in biosolids representative of residuals produced nationally. Greenhouse studies were conducted with 12 biosolids from around the USA, applied to two P-deficient soils from Florida at levels mimicking P- and N-based application rates. Biosolids P phytoavailability was quantified as aboveground P uptake by a pasture grass, and compared with uptake by grass fertilized with TSP. Statistical analyses of regressions of plant uptake against total P applied in the various P forms were used to estimate relative P phytoavailabilities.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Each P source was analyzed for total P, Fe, Al, Ca, and Mg by inductively coupled plasma spectrophotometry (ICP) following USEPA Method 3050B digestion (USEPA, 1986). Total C and N concentrations were determined by combustion at 1010°C using a Carlo Erba (Milan, Italy) NA-1500 CNS analyzer. Organic matter contents were estimated by the loss on ignition method (Ben-Dor and Banin, 1989) and pH (1:2 solid to solution) and percentage solids were determined by standard methods (Sparks, 1996).
In addition, all materials (following air-drying) were extensively characterized in terms of their P chemistries. Analyses included organic P (by difference; Saunders and Williams, 1955), oxalate-extractable Fe, Al, and P (McKeague et al., 1971), Mehlich 1–extractable P (Hanlon et al., 1990), and sequentially extracted inorganic P forms. The sequential extraction (modified from Chang et al., 1983) begins by shaking material with 30 mL KCl (rather than NH4Cl) for 2 h at 250 rpm on an orbital shaker, centrifuging (4000 rpm, 15 min), filtering (0.45 µm), and measuring soluble reactive phosphorus (SRP) by the Murphy and Riley (1962) colorimetric procedure. The KCl-extractable P is defined as "soluble/exchangeable P" and is regarded as readily available to plants. The second sequential step is extraction of the residual from Step 1 with 30 mL 0.1 M NaOH overnight (17 h), followed by filtering and SRP determination. This fraction represents "Fe- and Al-bound P" and can be indicative of labile P that buffers soluble P levels. The fraction is a commonly observed pool of biosolids P in many materials and in amended soils. The third sequential extractant, 30 mL of 0.5 M HCl, was shaken with residual from Step 2 for 24 h, filtered, and analyzed for SRP, and represents "Ca- and Mg-bound P." The fraction can be an important component of some biosolids, but tends to be a minor soil reaction product pool, even in biosolids-amended calcareous soils (Chang et al., 1983). The sum of KCl-, NaOH-, and HCl-extractable P is usually identified as inorganic P in a material, although some organic P can be extracted in the NaOH extraction (Sui et al., 1999).
Biosolids were also subjected to a 10–1 M citric acid (2% citric acid) extraction procedure (Charleston, 1984), recommended by Häni et al. (1981) for estimating plant-available P in biosolids. One gram dry equivalent weight of material was combined with 100 mL of 2% citric acid and shaken at room temperature for 30 min. Following centrifugation and 0.45-µm filtration, the filtrate was analyzed for P by ICP. Water-extractable P was determined following a slightly modified method described by Sharpley and Moyer (2000). Here, 1 g dry equivalent weight of material was combined with 250 mL of deionized water and shaken end-over-end for 1 h. Following centrifugation and 0.45-µm filtration, the samples were acidified and analyzed for P via ICP.
Amendments were mixed with surface-horizon samples (0–15 cm) of two acid, low organic matter Florida sands (Table 1): Immokalee sand (sandy, siliceous, hyperthermic Arenic Alaquods) and Candler sand (hyperthermic, uncoated Lamellic Quartzipsamments). The base sand for the columns (discussed later) was obtained from the E2 horizon of Myakka soil (sandy, siliceous, hyperthermic Aeric Alaquods). The surface horizon samples differ primarily in their oxalate-extractable Fe + Al contents and corresponding P retention capacities (Table 1), as indicated by the relative phosphorus adsorption (RPA) values (Harris et al., 1996). Both soils adsorb P poorly (RPA < 15), but the Candler soil sorbs about three times as much P as the Immokalee soil. Both soils test "very low" (i.e., <10 mg kg–1) as Mehlich-1 P (Hanlon et al., 1990) and are P deficient in their native state, a desirable condition for use in studies of plant response to applied P (de Haan, 1980).
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In Year 1, all amendments were thoroughly mixed with 3.5 kg of each soil (Immokalee and Candler) in a V-shaped blender for 5 to 10 min. The soil–amendment mixtures were then deposited on top of 28 cm of a sand (Myakka E horizon) base of negligible P retention capacity (Table 1). The base sand in each column was packed to a bulk density of 1.7 Mg m–3 and weighed 8.5 kg. Treated surface soils were packed to a bulk density of 1.3 Mg m–3 and required 3.5 kg of soil and amendments. Soil samples were packed in 15-cm-diameter, 45-cm-long columns made of PVC tubing, and were supported on nylon netting secured to the column bottom. Each column was fitted with a reservoir (PVC cap) equipped with a drainage hole and tubing (with clamps) to control drainage. Wooden racks supported the columns (six columns per rack).
In Year 1, each column was wetted with tap water by a siphon from a reservoir to the bottom access port until the soil surface was visibly wet. The columns were allowed to drain overnight and then weighed to establish initial "pot holding capacity" water content estimates. While soluble P in leachate was minimal (Elliott et al., 2002), the wetting procedure may have moved some soluble nutrients out of the treated soil depth (0–15 cm). Thus in Year 2, both the treated soils and base sand were prewetted to near field capacity initially, and only soil surface moistening occurred until grass seed had fully germinated. In columns utilizing Immokalee soil, one pore volume (water held at pot holding capacity) was about 2200 mL; for the Candler columns, it was about 2000 mL.
In Year 1, 2 g bahiagrass seed was sown on each column and covered with about 3 mm of base sand. In Year 2, the seeding rate was increased to 5 g to compensate for the poor germination and uneven grass coverage experienced in some treatments of Year 1 (see below). The sand surface was then covered with a filter paper, which was wetted two or three times daily with a hand-mister, until seed germination. After about a week, the filter papers were removed, and the seedlings began receiving irrigation water twice daily with 25 to 50 mL of tap water to maintain favorable soil moisture (approximately 80% pot holding capacity). Daily watering volumes increased to 100 to 150 mL as the grass grew, but did not provoke drainage. The columns were also weighed periodically to identify significant soil-moisture decreases (plant water stress). If soils were drying appreciably due to excessive water deficits, the watering scheme was adjusted to maintain the columns at moisture contents close to the target of 80% of pot holding capacity. In Year 1 the tap water used for irrigation had a pH of 8.3 due to lime softening at the main water treatment plant. In Year 2, all water used in the greenhouse was first adjusted to pH 5 (average pH of rainwater in Florida) with small additions of HCl. Tap water contained undetectable quantities of P (<0.05 mg L–1) and minimal N (<10 mg total Kjeldahl N L–1).
Despite careful nurturing, bahiagrass seeds failed to germinate in the Year 1 columns receiving the Tarpon Springs N-Viro product (at both rates). We reseeded the columns after a week, but germination failed again. The pH of the N-Viro material (N-Viro International, Toledo, OH) was 11.9, and we hypothesized that high pH and salinity of the amended soils restricted seed germination. To resolve the problem, the columns were leached with 500 mL tap water to reduce pH and salinity, and the columns were reseeded. A majority of the new seeds germinated, but seedling growth was extremely slow and grass quality was visibly poor.
The first crops of grass grew for 30 to 35 d after the filter papers were removed. The subsequent three harvests occurred approximately monthly thereafter. Grass was harvested 5.0 cm above the soil surface and placed in paper bags for drying (65°C), and dry weight yields were determined. After each harvest, columns were weighed and weights compared with previously determined pot holding capacities. Sufficient water was added to return the columns to pot holding capacity, plus additional water to obtain about 500 mL (approximately 0.25 pore volume) of leachate from each column. Following the leaching irrigation, the columns were allowed to drain overnight. Harvesting and leaching procedures were repeated monthly for the next three months. The total volume of water applied to each column equaled evapotranspiration plus approximately 11 cm of "leaching rains," resulting in the collection of one pore volume of leachate. Leaching data have been previously reported (Elliott et al., 2002).
Plant Tissue Analysis
Bahiagrass was harvested approximately monthly and placed in cloth bags for drying at 65°C for several days. Dried material was weighed for yield determinations, ground in a Wiley mill with stainless steel blades to pass a 20-mesh sieve, and stored in airtight polyethylene containers. Ground plant material was ashed, treated with strong acid, and brought to final volume with distilled water following procedures described by Plank (1992). Phosphorus in the diluted digests was determined colorimetrically (Murphy and Riley, 1962).
Statistics
Cumulative yield and P uptake were statistically analyzed using PROC GLM (SAS Institute, 1989). Mean separation of treatment differences was by LSD using a significance level of 5%. The regression relationship between P applied and P uptake was estimated under the common intercept model assuming a linear relationship. Under this model, comparisons of different P sources begin with the same value of the control. As a consequence, differences in response are proportional to rate. All regression parameters were estimated simultaneously. Comparisons among the P source regression coefficients were means of the Student's t test. Again, a 5% significance level was used.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Total P concentrations of the various biosolids (Table 2) were similar (about 20–40 g kg–1; 2–4%) and typical of biosolids produced nationwide (USEPA, 1995). The N-Viro product had a total P concentration of 3.2 g kg–1, reflecting significant dilution of the biosolids cake with non-P-containing solids. Inorganic P forms dominated most biosolids (73–90%; Table 3), again, representative of biosolids nationwide. Even the Tarpon Springs, Largo, and Wisconsin materials, generated via biological P removal, were dominated by inorganic P (organic P, as a percentage of total = 4–24%). Analysis of the Utah aerobic cake material suggested only 48% inorganic P, but oxalate and sequential extraction data (discussed below) suggest that this organic P determination is erroneously high.
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Biosolids water-extractable P values generally followed KCl-extractable P values, varying from below method detection (<0.01 g kg–1) for the N-Viro material to 8.83 g kg–1 for the Largo cake. Inexplicably, the moderately high KCl-extractable P value of 3.48 g kg–1 for the Utah anaerobic cake was not reflected in water-extractable P (0.29 g kg–1). The BPR materials (Largo, Tarpon Springs, and Wisconsin) all had high water-extractable P values (2.63–8.83 g kg–1).
The Mehlich-1 soil test was also applied to biosolids. Previous work (O'Connor and Sarkar, 1999) suggested that Mehlich-extractable P in biosolids was at least qualitatively related to phytoavailable P, although not as indicative as KCl-extractable P. Mehlich 1–extractable P was poorly correlated (r = 0.17) with total P for the biosolids examined here. Extractable P values were very low for N-Viro materials (Table 3) because extract pH values were alkaline rather than acid. The KCl-extractable P was also poorly correlated with total biosolids P (r = 0.24).
Citric acid–extractable P correlated reasonably well (r = 0.77) with total P in the biosolids. Citric acid is used to characterize available P in fertilizers in most states, and has been suggested as a measure of phytoavailable P in biosolids. Our data, however, suggest that citric acid is an aggressive extractant that mirrors total P rather than available P (Table 3). Data collected here and previously (O'Connor and Sarkar, 1999) suggest poor correlations of citric acid–extractable P with other availability indices (e.g., KCl-extractable P and Mehlich-extractable P) and with P taken up by plants or leached. Correlations of the various measures of labile P in biosolids with plant uptake (including both greenhouse and field data) will be the subject of a subsequent paper.
Oxalate-extractable P correlated well (r = 0.84) with total P, indirectly confirming the sequential extraction dominance of Fe and Al-P "forms" in most materials. Oxalate extracts poorly crystalline (amorphous) Fe and Al oxides and, thus, is expected to release P associated with amorphous Fe and Al solids. Oxalate-extractable P also tracked total P well for the Utah and Wisconsin materials, where Ca and Mg forms were prevalent. Oxalate P in the Utah aerobic cake was nearly 87% of total P, again suggesting much more inorganic P than estimated and reported in Table 3 as organic P.
We also calculated the phosphorus saturation index (PSI = [Pox]/[Alox + Feox]) of the biosolids (Schoumans, 2000; Elliott et al., 2002) from the moles of oxalate-extractable P, Fe, and Al data (Table 3). The index is used here as an indication of the degree to which biosolids P is potentially bound (unavailable?) with Fe and Al in the biosolids. Thus, PSI values of <1 suggest excessive Fe and Al for binding of P (little available P), whereas values of >1 suggest abundant P not associated with Fe and Al precipitates (abundant available P). Obviously, soil oxalate-extractable Fe and Al will affect P phytoavailability once biosolids are land-applied, but the PSI value of biosolids may prove useful as an a priori index of biosolids P lability (Elliott et al., 2002). Since only oxalate-extractable P, Fe, and Al are considered, the index is not as useful for Ca-dominated materials (the N-Viro product and the Utah and Wisconsin biosolids). Nevertheless, the PSI qualitatively identifies the Largo and Wisconsin materials as possibly good "labile" P sources. The Baltimore (high Fe) and the Tarpon Springs and Utah anaerobic (high Al) materials are qualitatively identified as poor labile P sources.
Biosolids P was dominantly in Fe- and Al-associated forms (NaOH extract) in the Year 1 materials, except for the N-Viro product where Ca and Mg forms (HCl extract) dominated (Table 3). In the Year 2 materials, however, Ca and Mg forms were much more prevalent, or dominated biosolids P. The Utah materials have alkaline and high Ca source streams and, thus, would be expected to contain an abundance of Ca-P forms. Unexpectedly, Ca and Mg-P forms also dominated the Wisconsin material, possibly reflecting significant whey additions in the source water (D. Taylor, personal communication, 2002).
The sum of sequentially extracted P forms is usually taken to approximate total inorganic P. Thus, given the dominance of biosolids total P by inorganic forms, we expected the good correlation (r = 0.94) between the sum of sequential extracts and total P. The sum of P in the sequential extracts for the Utah aerobic cake was 76% of total P, much more than expected for a material with the estimated 48% organic P mentioned previously.
All biosolids tested represent low-grade fertilizer materials suitable for agronomic use. The array of materials studied includes wastewater treatment processes, stabilization methods, and elemental compositions representative of biosolids produced nationwide. All biosolids used (except the Wisconsin material) had been previously thickened and/or dewatered (Table 2). "Liquid biosolids" (very low solids percentages) were, thus, underrepresented in our evaluation. The array of chemical and physical properties of biosolids examined suggests that the selected materials were otherwise appropriate to study biosolids P–plant relationships.
Fertilizer
Triple superphosphate was obtained from a local fertilizer distributor and identified as being 46% citric acid–soluble P2O5 (200 g kg–1 P). Our analysis confirmed the total (Table 2) and citric acid–soluble (Table 3) P concentrations. Much of the P was KCl-extractable (Table 3) as expected for a commercial fertilizer, and Ca was the dominant (total) cation (137 g kg–1). Impurities in the rock phosphate treated with H3PO4 to produce the fertilizer, however, contributed measurable amounts of Fe (15.7 g kg–1), Al (10.0 g kg–1), and Mg (6.15 g kg–1) as shown in Table 2.
Bahiagrass Yields
Year 1
Analysis of variance of the yield data revealed that all treatments and interaction terms were significant. Thus, cumulative bahiagrass yield data for three harvests in Year 1 are separated in Table 4 for Immokalee and Candler soils. Four harvests were conducted, but total yields for all treatments were dominated by material collected in Harvests 1 to 3. Cool weather and shorter day lengths in October resulted in very poor yields of the warm-season grass in Harvest 4. Analysis of tissues for N (data not presented) revealed progressively lower N concentrations in tissues from later harvests, suggesting the development of N limitations, which may have also contributed to the limited fourth harvest yields. The maximum dry matter yield for any treatment in Harvest 4 was 1 g, and many treatment yields were <0.5 g. Accordingly, tissue from Harvest 4 was not analyzed for P. The effect of Harvest 4 on cumulative yield was minimal, so the effects on P uptake could be safely ignored.
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Yield results in the Candler soil (Table 4) reflected treatment effects similar to those observed in the Immokalee soil. Cake materials generally produced the greatest yields (similar to TSP) and pelletized and compost materials tended to reduce yields. High soil pH values associated with both rates of the N-Viro product severely reduced bahiagrass growth. The Candler soil contained more soil test P than the Immokalee soil (Table 1), and the control treatment yields were greater in the Candler than the Immokalee soil. However, symptoms of P deficiencies were observed in control treatment plants in both soils.
Year 2
Bahiagrass yields in Year 2 (Table 5) were generally greater than in Year 1, because four harvests were possible. Also, warmer temperatures and more careful management of N nutrition occurred in Year 2. Nitrogen fertilizer supplements were split among harvests (rather than being added all at once before cropping in Year 1), which promoted excellent grass growth throughout the growing season.
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Year 1 Phosphorus Uptake
Phosphorus uptake values from Harvests 1 to 3 for Year 1 are presented in Table 4 for treatments applied to the Immokalee and Candler soil. Recall that Harvest 4 yields were so small in Year 1 that no attempt was made to quantify P uptake. Analysis of variance of the uptake data revealed that all treatments and interaction terms were significant.
The high P rate of most P sources resulted in greater P uptake than the low P rates on both soils, but the effects were greater for grass grown in the Immokalee soil (Table 4). The exceptions were both N-Viro rates, probably a result of poor growth (of biomass) in these treatments.
Year 2 Phosphorus Uptake
Phosphorus uptake values from Harvests 1 to 4 for Year 2 are presented in Table 5 for treatments applied to Immokalee and Candler soils. As in Year 1, the high P rates of most P sources resulted in greater P uptake than in the low P rates on both soils, but the effects were more obvious in the low P sorbing Immokalee soil (Table 5).
Relative Phytoavailability
Uptake data were combined from both years for each soil and regressed against P applied to assess the bioavailability of P in each organic source relative to fertilizer P. The linear regressions were based on data from the two rates of applied P and a control. While more rates would have undoubtedly improved the mathematical description of P uptake as a function of applied P, a practical and useful protocol was employed to estimate biosolids P bioavailability. More accurate estimates could be obtained by conducting field trials with more P application rates.
Statistical comparisons of the control and TSP treatments (for each soil) for Years 1 and 2 were first conducted to evaluate the possibility of a year effect. Analysis revealed that although uptake in Year 2 was greater (four harvests) than in Year 1 (three harvests), the regression lines were parallel (year x P applied interaction not significant). Thus, data for both years could be combined and an average regression equation estimated using a common intercept model. Regressions for all treatments were significant (slopes significantly different from zero) with the exception of the Baltimore pellets and Philadelphia compost treatments. Regression equations for other P sources that were originally compared with TSP for each year could then be compared with TSP and other P sources used in both years. For each soil, regression coefficients (slopes) based on the common intercept model were compared. Using Student's t test, significant differences among the relative phytoavailabilities of the P sources were determined. The corresponding regression coefficients are arrayed in Table 6 in absolute and relative order, with TSP representing 100% relative phytoavailability. The critical differences for comparing regression coefficients for each soil were calculated in a manner similar to LSD values for individual means comparisons.
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Phosphorus sources tended to segregate into three groups of similar relative phytoavailabilities qualitatively described as high, moderate, and low. Biosolids produced via BPR tended to be as phytoavailable as fertilizer P, and constituted the members of the high phytoavailability group. The Wisconsin material, in fact, was significantly more phytoavailable than TSP in the Candler soil (Table 6). The Largo cake (BPR) was slightly less phytoavailable than TSP, but not significantly so in the Candler soil. The other BPR material tested (Tarpon Springs cake) is produced in a treatment process that adds Al to reduce final effluent P concentrations. The material's elevated total Al + Fe concentration (39.5 g kg–1) reduced P phytoavailability to values commensurate with the moderate bioavailability group.
Biological P removal products not normally high in Fe and Al (e.g., Wisconsin slurry and Largo cake) can be expected to mimic fertilizer P with regard to fertilizer value. Stratful et al. (1999) found that BPR sludges contained more plant-available P than other sludges and were excellent P fertilizers. Ebeling et al. (2003) recently reported that the same Wisconsin material as used here was as phytoavailable as TSP using wheat (Triticum aestivum L.).
The moderate bioavailable group included most of the biosolids tested, and displayed relative bioavailabilities of 53 to 26% (average = 39%) in Immokalee soil and 72 to 42% (average = 57%) in Candler soil. Averaged across both soils, the relative bioavailability for the moderate group was 48%. This value compares with the 50% suggested by current USEPA guidelines (USEPA, 1995) and 40% suggested by Canadian regulators (Ontario Ministry of Environment and Energy and Ontario Ministry of Agriculture, Food and Rural Affairs, 1996). De Haan (1980) evaluated 15 biosolids in greenhouse pot studies and found most (12) materials to be 36 to 90% as phytoavailable as fertilizer P. The exceptions he noted were materials with greater than normal total Fe and Al concentrations; those materials had phytoavailabilities ranging from 17 to 54%.
Within the moderate group identified here are biosolids produced by common wastewater treatment processes (aerobic and anaerobic digestion) and additional treatments ranging from composting to Fe and Al addition. Also included is the Largo BPR material that had been pelletized (discussed below). All Utah materials were produced from the same waste stream and offered a unique opportunity to compare treatment processes on relative P phytoavailability. Surprisingly, all three products had greater relative phytoavailabilities in the Candler soil (average = 63%) than in the Immokalee soil (average = 40%), but the average across soils was 52% (Table 6). Thus, there was no clear effect of digestion process, or additional treatment (composting), on relative P phytoavailability in the Utah materials.
Data collected in Year 1 suggested lower P phytoavailability in materials containing more than the 10 to 30 g kg–1 of total Fe and Al combined concentration normal for biosolids produced nationally (USEPA, 1995). This was exemplified by data presented for the Baltimore and Philadelphia cake materials (total Fe and Al = 70–80 g kg–1; Table 2). Both materials had lower absolute and relative phytoavailabilities than many of the other moderate group materials (Table 6), but the differences were frequently not significant, and phytoavailabilities were only slightly below the averages for the moderate group on either soil. Thus, total Fe and Al concentration in biosolids is not the quantitative indicator of P availability suggested from Year 1 results.
Pelletizing (heat drying) either the Largo or Baltimore cakes reduced their relative phytoavailabilities (Table 6). Pelletization lowered Largo BPR (cake) from the high to the moderate category. Relative phytoavailability decreased from 74 to 48% in Immokalee soil and from 85 to 62% in Candler soil. Pelletizing the Baltimore material (high total Fe and Al) changed its category from moderate to low, and relative phytoavailability to approximately 10% in both soils. Smith et al. (2002) reported that high-temperature drying significantly reduced P release from biosolids due to the formation of insoluble Ca-P minerals, and that P availability (extractability) was further reduced by thermal drying of Fe-enriched biosolids. The pelletizing effect may also be at least partially physical. Preliminary lab studies and field observations (O'Connor and Sarkar, 1999) confirmed that pellets formed by heat drying resist degradation, and this probably slows soluble P release. Shaking the pellets for extended times, or grinding, increased KCl-extractable P (data not presented). While pelletization reduced P phytoavailability for the 5 mo of our greenhouse studies, soluble P input to a system in the long term may eventually be the same, regardless of whether the material is pelletized. Long-term field studies are necessary to test the hypothesis.
Physical and/or chemical effects may have affected the Philadelphia compost results (relative phytoavailability of <7% in either soil; Table 6). Composting alone had no effect on the phytoavailability of Utah aerobic cake. The Philadelphia compost, however, contained even more total Fe and Al (96 g kg–1) than the Philadelphia cake (80 g kg–1), and the additional Fe and Al may have reduced P phytoavailability chemically.
Data for the N-Viro product of the Tarpon Springs cake are confounded by exceptionally poor growth of the "acid-tolerant" bahiagrass in soils, with pH values well above 8. The regression coefficients were, in fact, negative, indicating that P uptake decreased with P applied in the N-Viro material. As noted earlier, the application rates of N-Viro used here are unreasonable for such a liming agent and would not be recommended.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Within the moderate group are biosolids produced by conventional wastewater and solids digestion and additional treatments like composting. Also included is the Largo BPR material that had been pelletized and Tarpon Springs BPR where supplemental Al is added. Averaged across both soils, the relative bioavailability for the moderate group was 48%. This value compares with the 50% suggested by current USEPA guidelines (USEPA, 1995) and 40% used by Canadian regulators (Ontario Ministry of Environment and Energy and Ontario Ministry of Agriculture, Food and Rural Affairs, 1996).
Significantly lower P availability seems to characterize biosolids containing very high (>50 g kg–1) total Fe and Al concentrations and which have been processed by methods that result in very dry materials (>60% solids). This low category includes the Baltimore pellets (total Fe + Al = 83 g kg–1, 87% solids) and the Philadelphia compost (total Fe + Al = 96 g kg–1, 63% solids).
A biosolids produced by advanced alkaline stabilization had P phytoavailability in the low category. However, greenhouse results were confounded by exceptionally poor growth by the "acid-tolerant" bahiagrass in soils with pH of >8 as a consequence of applying large amounts of this highly alkaline material. The application rates required in the research study were unreasonable for such a liming agent, and would not be recommended. This serves to underscore an important principle: P phytoavailability is a complex function of many properties of the soil, climate, and vegetation, as well as the nature of the P source. This research used sandy P-deficient soils where the biosolids properties are primary determinants of the P chemistry and behavior. Application of biosolids and other organic sources of P to soils already containing adequate P for crop growth may have little effect on P uptake and crop yields, thereby masking inherent differences in the P phytoavailability of various materials.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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V. Antoniadis, C. D. Tsadilas, and S. Stamatiadis Effect of Dissolved Organic Carbon on Zinc Solubility in Incubated Biosolids-Amended Soils J. Environ. Qual., January 25, 2007; 36(2): 379 - 385. [Abstract] [Full Text] [PDF]
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M. L. Silveira, M. K. Miyittah, and G. A. O'Connor Phosphorus Release from a Manure-Impacted Spodosol: Effects of a Water Treatment Residual J. Environ. Qual., February 2, 2006; 35(2): 529 - 541. [Abstract] [Full Text] [PDF]
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G. M. Pierzynski and K. A. Gehl Plant Nutrient Issues for Sustainable Land Application J. Environ. Qual., January 1, 2005; 34(1): 18 - 28. [Abstract] [Full Text] [PDF]
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R. K. Bastian Interpreting Science in the Real World for Sustainable Land Application J. Environ. Qual., January 1, 2005; 34(1): 174 - 183. [Abstract] [Full Text] [PDF]
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