Recovery of Essential Plant Nutrients from Biofuel Residual

Essential plant nutrients contained in residues and wastes generated during biofuel processing can be recovered for further production of bioenergy biomass. The objective of this study was to determine the relative agronomic efficiency of " processed " biofuel residual (PBR). Liquid biofuel residual was " processed " by precipitating phosphate and ammonium in the residual with magnesium into a struvite-like material. Then, in a series of greenhouse experiments, we evaluated the fertility potential of PBR, using sweet sorghum (Sorghum bicolor (L.) Moench), as a test bioenergy crop. We compared the agronomic effectiveness of PBR to inorganic commercial fertilizers, biosolids, and poultry manure as nutrient sources. The sources were either applied alone or in combination with supplemental essential plant nutrients (S, K, Mg, and micronutrients). In each of the greenhouse experiments, the crop was grown for 12 wk on soil of minimal native fertility. After each harvest, sufficient water was applied to the soil in each pot over a 6-wk period to yield ~2 L (~one pore volume) of leachate to assess potential total N and soluble reactive phosphorus (SRP) losses. Dry matter yields from the PBR treatment applied alone were significantly greater than yields from inorganic fertilizers, biosolids, and poultry manure treatments applied alone, and similar to yields obtained when the supplemental essential plant nutrients were added to the inorganic fertilizer, biosolids, and manure treatments. Leachate N and SRP concentrations from the PBR treatment were significantly lower than in the treatments with inorganic fertilizers, poultry manure, and biosolids. We conclude that PBR can substitute for inorganic fertilizers and other organic sources of plant nutrients to produce bioenergy biomass cheaply, without causing offsite N and P losses in vulnerable soils.


Introduction
Growing environmental and economic concerns over use of fossil fuels have prompted the search for alternative fuels, including biofuels such as ethanol.Ethanol is a renewable, environmental-friendly energy source produced through fermentation of the constituent sugars of biomass, and can either be used as mixtures with conventional gasoline fuels or as a sole fuel source [1].In addition to being nontoxic, ethanol is biodegradable and essentially sulfur-free.Switching from fossil fuel to biofuel to power vehicular engines will reduce carcinogenic air toxics, carbon monoxide, and unburned hydrocarbons that contribute to smog and ozone formation [1].Despite the ecological gains attributed to biofuel use, a major obstacle to its widespread use is the increased expense over conven-tional vehicular fuel [2].Recycling the residues and wastes generated during biofuel processing for subsequent biomass production can potentially reduce the increased expense.
The biochemical platform for production of ethanol from cellulosic biomass typically involves thermal and/or chemical processing of the feedstock to convert the macromolecules to constituent sugars, followed by fermentation of the sugars to ethanol [3].Both processes employ dilute or concentrated acids, particularly phosphoric acid, and later, ammonia at elevated temperatures to enhance the fermentation process [3].The added phosphoric acids and/or ammonia, combined with N and P from the cellulosic biomass, accumulates in the biofuel residual [4].The residual, thus, represents a potentially valuable nutrient source as well as a hazard if the nutrients escape to the environment, particularly to surface water bodies.The ever-increasing demand for biofuel in recent times, particularly in the developed countries, could lead to accumulation of large volumes of liquid biofuel wastes/ residuals in limited areas, which may eventually be discharged to landfill or surface water bodies.Biofuel residual discharges to natural waters could promote eutrophication problems due to the high N and P contents and consequences for aquatic life and water supplies for domestic and industrial uses.
One of the proposed solutions to the potential environmental problem associated with biofuel residual discharge is to recover nutrients from the residuals and utilize it to fertilize subsequent biomass production.The key feature of this recovery technique is the combined removal of ammonium  4  NH  , phosphate  3 4 PO   and magnesium from supersaturated biofuel residual.The resulting product is magnesium ammonium phosphate hexahydrate (MgNH 4 PO 4 •6H 2 O), commonly known as struvite, which precipitates according to following reaction [5]:

Several studies have explored recovery of ammonium an
this study was to assess the fe
The overall objective of asibility of using PBR as a cost-effective and environmentally benign nutrient source for bioenergy biomass production.Specific objectives were to 1) characterize PBR to determine its suitability as plant nutrient source, 2) evaluate agronomic effectiveness of PBR relative to inorganic fertilizers and other organic sources of plant nutrients (biosolids and manure), and 3) determine the potential offsite N and P losses when PBR is land applied.

Collection and Processing Residual
biofuel resi essing plant at the Agricultural and Biological Engineering Department of the University of Florida.Ethanol is produced in the facility through fermentation of various organic materials, including waste grass clippings, cellulosic biomass, and municipal waste.The biofuel residual (~12% solids) contains high amount of suspended organic materials.Prior to processing, the m organic residuals, e.g.poultry manure, into struvitelike materials is presented in Yetilmezsoy and Sapci-Zengin [13].Briefly, the biofuel residual processing plant utilized for the present study consists of a precipitation reactor with stainless steel pipes for influent and reactants and effluent discharges, and a peristaltic pump (Figure 1).The reactor is composed of two parts: the bottom part is the reaction zone, and the top part is a settling zone that prevents fine particles from being lost in the effluent [13].The settling zone is located above the reaction zone, is cone-shaped with an angle of 45˚ between the two zones, and is equipped with a baffle to guide the flow.The effluent flows out at the top of the settling zone over a weir (Figure 1).
The success of struvite precipitati in factors: the Mg 2+ : PO  activities exceeds the equilibrium ion-activity product [11] or the thermodynamic solubility product [13].Although H + concentration does not directly enter the ion activity product equation, MgNH PO  are pH dependent [11].Nelson et al. [11] showed that struvite solubility decreases with increasing pH.Gadekar and Pullammanapallil [4] suggested that an optimum pH of PO  ions.The pH was adjusted to ~9.0 [4 h 0.1 M 4 OH through a membrane pump, and the suspension was agitated for 6 h.All the runs were carried out at room temperature of ~25˚C.The processed material was allowed to settle for ~2 h to separate the precipitate from the bulk liquid.
Conventional struvite production from liq ] wit NH aterials involves centrifugation to settle suspended materials, and the supernatant is used for the struvite production.The resultant struvite is generally white stable orthorhombic crystals [13].In the present study, however, the supernatant was not separated from the suspended solids, and the resulting precipitated material includes organic matter and other constituents contained in the raw biofuel residual.Therefore, the precipitated material does not qualify as a pure struvite, but rather as a struvite-like material (referred throughout this paper as "processed" biofuel residual and abbreviated as PBR).

"Processed" Biofuel Residual
ve chemical analyses were conducte to determine its agronomic potential, and possible environmental impact if the material is land-applied.Selected chemical characteristics of the raw biofuel residual and PBR are presented in Table 1.For total elemental analysis, subsamples of PBR were digested [14], and the digest was analyzed for the major plant nutrients including P, K, Ca, Mg and S; minor plant nutrients including B, Cu, Fe, Mn, Mo, and Zn.Digest was also analyzed for elements of environmental concerns including Al, As, Cd, Cr, Pb, Hg, and Se.Mehlich-3 extractable P, K, Ca, and Mg were determined using the procedure described in Mehlich [15] for the extraction, and the extracts were analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (PerkinElmer Plasma 3200; PerkinElmer, Wellesley, MA).Percent solids were determined by drying the material at 105˚C [16], electrical conductivity (EC) and pH measurements were performed on the material as described by Thomas [17].Total N concentration of the material was determined using the Kjeldahl procedure as described in Bremmer [18].Inorganic N (ammonium and nitrate) was determined by the potassium chloride (2 M KCl) procedure [19], and Organic N content was calculated as a difference between total N and inorganic N content [20].Sulfate (SO 4 -S) content was determined based on the method outlined in B termined following the procedure described in Brandt et al. [22].

Biofuel Residual
. Soil and Nutrient S Immokalee fine sand (sandy, siliceous Arenic Alaquods) was used for the study.Native Immo-kalee fine sand, not previously contaminated by manure depositions and having "very low" native fertility, was collected from uncultivated site at the University of Florida Research and Education Center in Immokalee, FL.Multiple random bulk samples were collected from the surface horizon (0 -20 cm) and thoroughly mixed to yield a composite sample.
Five nutrient sources (PBR, two biosolids with contra

Soil s
ere air-dried and passed through a 2 mm

Experimental Setup in the Greenhouse
re con-sting phosphorus (P) phytoavailabilities [Gainesville Regional Utility (GRU)-high P phytoavailability, and Milorganite biosolids-low P phytoavailability], poultry manure, and commercial fertilizers (urea + triple superphosphate) were used in the study.The GRU biosolids was produced through aerobic digestion, and was obtained from the water reclamation facilities of the Gainesville Regional Utilities (Gainesville, FL).Milorganite biosolids, obtained from Milwaukee Metropolitan Sewerage District, Milwaukee, WI, was generated from anaerobically digested material that was heat-dried and pelletized.The Milorganite biosolids is stabilized with iron salts to decrease P solubility of the biosolids.The poultry manure was obtained from an egg producing farm in Indiantown, FL.Previous work [23] characterized the P phytoavailability of each nutrient source other than PBR.Used amples w sieve before analyses.Particle size distribution of the samples was determined using the pipette method [24].Soil pH was determined in a 1:2 soil:water ratio using a glass electrode [25].Total C and N of soil samples were determined by combustion at 1010˚C using a Carlo Erba NA-1500 CNS analyzer (NA-1500 CNS, Carlo Erba, Milan, Italy) as outlined in Nelson and Sommers [26].Soil test P concentration was determined using the Mehlich-3 extraction protocol [15], and WEP was determined following the procedure of Kuo [27].The biosolids and manure were analyzed for solids content, total N, SO 4 -S, Mehlich-3 extractable P, Ca, Mg, and K; total P, and WEP concentrations as described above in Section 2.2.Selected properties of the soil and nutrient sources used are presented in Tables 2 and 3, respectively.
Three consecutive greenhouse experiments we ducted, using "fresh" soil + treatments at each cropping, to verify and confirm results for the agronomic evaluations of PBR.The studies were conducted in a greenhouse at University of Florida campus in Gainesville, FL.Greenhouse temperature was maintained at 27˚C (day) and 17˚C (night).The five nutrient sources were each mixed with 8 kg of the soil at a recommended N applica- tion rate of 150 kg•PAN•ha −1 for sweet sorghum [28].
The plant available N (PAN) was calculated based on the inorganic N content of the nutrient sources, and an assumed mineralization of the organic N content.Previous studies [23,29] suggested ~40% annual organic N mineralization from the manure and biosolids used for the study; therefore PAN for the nutrient sources was adjusted based on 40% organic N mineralization rate.The PBR-, manure-, and biosolids-amended soils were equilibrated (~80% water holding capacity) in zip-lock plastic bags at room temperature for 2 wk in the laboratory prior to use in the greenhouse.Chemical analyses of the PBR showed that the material contained adequate concentrations of Mg, K, S, and micronutrients for crop growth so as a component of the study, we introduced a Sul-Po- Mag [a blend of sulfate, potassium and magnesium (18% S, 22% K, 11% Mg)] and micronutrient treatments where the plants were either supplied with or without Sul-Po- Mag and micronutrients.A total of 24 treatment combinations were obtained [6 (5 nutrient sources + control) × 2 (with and without Sul-Po-Mag) × 2 (with and without micronutrients)] and each treatment was randomly assigned to a pot.The pots were arranged in a randomized complete block design to minimize greenhouse positioning effects.Four replications for each treatment were used, yielding a total of 96 pots in each greenhouse experiment.The soil in each pot was wetted to, and maintained at ~80% of the water holding capacity, and allowed to equilibrate for additional 1 wk in the greenhouse before planting.Sweet sorghum (Sorghum bicolor (L.) Moench), variety CSH-5 was selected as the test bioenergy crop and used in each greenhouse experiment.Studies have shown that sweet sorghum is a promising bioenergy crop, well adapted to several agro-ecosystems, and produces high bioenergy yields [30,31].Six seeds of sweet sorghum were sown in each pot and misted daily to facilitate uniform germination.The inorganic N + P fertilizer treatment was applied at the recommended rates of 150 kg•N•ha −1 and 55 kg•P•ha −1 , respectively, at planting [28].Five days after emergence, the seedlings were thinned to two plants in each pot, and the Sul-Po-Mag and micronutrient treatments were applied.Sul-Po-Mag solution was applied at 0.91 g•pot −1 to supply adequate and uniform S, K, and Mg.Micronutrients were supplied using soluble trace element mix (The Scotts Company, Marysville, OH) [a blend of boron, copper, iron, manganese, molybdenum and zinc (1.35% B, 2.3% Cu, 7.5% Fe, 8% water soluble Mn, 0.04% Mo, 4.5% Zn)] at 13.9 mg pot −1 .The pots were weighed daily and the loss in weight was made up by adding distilled water to maintain ~80% of the water holding capacity of the soil.The above-ground plant material of each pot was harvested at anthesis stage (12 wk after emergence) for yield determination and plant tissue analyses.

Plant Tissue Analyses
The fresh plant materials were placed in pre-weighed stant weight to represent bags and dried at 60˚C to a con dry matter yield (DMY).The dried samples were ground in a Wiley mill (Model 4 Thomas-Wiley Laboratory Mill, Thomas Scientific, Swedeboro, NJ) to pass a 1-mm screen for tissue analyses.For tissue P determination, the ground samples were digested as described by Andersen [14], and analyzed for P via the molybdenum blue method [32].Tissue N concentration was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. [33], followed by automated colorimetry with a Technicon Auto Analyzer.Phosphorus and N uptake were calculated as the product of DMY and tissue P or N concentration.The indices of relative agronomic effectiveness (RAE) were estimated based on DMY [34] as follows: where Y 1 = DMY from the nutrient sources app or in combination with Sul-Po-Mag and micronutrients, hing and Leachate Analyses After harvest in each greenhouse experiment, sufficient lied alone, Y 2 = DMY from treatment receiving the inorganic fertilizer supplied with Sul-Po-Mag and micronutrients, Y 0 = DMY from the control.

Post-Harvest Leac
deionized water (adjusted to pH 5.0) was initially applied to each pot to yield ~500 mL (~0.25 pore volume) of leachate.The soil was covered and allowed to equilibrate for 2 wk and the leaching was repeated; followed by a third and fourth leaching each at 2 wk intervals.Thus, a total of ~2000 mL (~1 pore volume) leachate was collected over a period of 6 wk after harvest to assess nutrient (N and P) losses.After each leaching event, the leachate collected was analyzed for SRP using the molybdenum blue method [32].Leachate samples were also analyzed for N concentration using the USEPA Method 353.2 [35].The product of the leachate volume and leachate N or P concentration yielded mass of N or P leached.Although leachates from the pots were not highly colored, total P was determined on the leachate to confirm that organic P loss from the pots was not significant.Since the P content of the various nutrient sources varied (Tables 1 and 3), P applied in the sources were not equal because application rate was based on N requirement of the crop (N-based rate), which also resulted in the applied P in all organic sources exceeding the recommended P rate.Therefore P leached was expressed as a percentage of P applied.

Statistical Analyses
Differences in DMY, RAE, and N and P uptake among lly analyzed as a factorial omplete block design, f the "Processed" PBR typical range of soil pH ade-e able sorghum production.The Mehlich-3 P and (Al, Fe the treatments were statistica experiment with a randomized c using the general linear model procedure (PROC GLM) of the SAS software [36].Means of the various treatments were separated using a single degree of freedom orthogonal contrast procedure.The leachate N and P concentration data were not normally distributed (based on Kolmogorov-Smimov procedure and the normal probability plots [37]), and were normalized with a squareroot transformation before statistical analysis, based on the result of the Box-Cox transformation procedure [38].The transformed data were then analyzed using the PROC GLM procedure.Means of the leachate N and P content resulting from the treatment were separated using a single degree of freedom orthogonal contrast on the transformed data.Data were back transformed for all discussions in the manuscript.Treatment differences were considered significant at P ≤ 0.05.

Chemical Characteristics o Biofuel Residual
Selected chemical characteristics of the raw biofuel residual and PBR are presented in Table 1.The pH of the was higher than the quate for sorghum growth [28], but based on N-based application rates, quantities required for land application should not alter native soil pH.Electrical conductivity of PBR was well below the reported critical EC value of 4 dS•m −1 associated with reduced plant growth caused by soil salinity [39].Organic C content was much greater than typical soil organic C levels, and considerably greater than organic C levels of most biosolids produced and/or marketed in Florida [29].The PBR contained high levels of plant available N (NH 4 -N and NO 3 -N), which portends that application of PBR will immediately supply adequate quantities of N to the plant.Phosphorus concentrations of the raw biofuel residual and PBR were similar, and ranged from 32 to 36 g•kg −1 , however WEP values of the raw biofuel residual (~6.4 g•kg −1 ) was significantly greater than that of the PBR (0.3 g•kg −1 ).This suggests that >95% of P precipitated out of the raw biofuel residual during processing into the struvite-like material.Several studies have demonstrated that WEP is a good measure of the environmentally relevant portion of P in soils and residuals, including manure and biosolids [23,29,40].WEP is used to calculate percent water-extractable P (PWEP = WEP/TP × 100), which is a measure of the potential of P sources to cause offsite runoff and leaching losses.Chinault and O'Connor [29] demonstrated that biosolids with PWEP values ≥ 10% has the potential to cause significant leaching losses in sandy soils with limited P sorption capacity.The PBR had PWEP values ~0.16%, below values observed from other studies [29] for most biosolids stabilized with Fe-and Al (PWEP ~0.5%) to reduce off-site P losses, and suggests that offsite P losses will be minimal when PBR is land-applied.The Mehlich-3 extractable K and Mg, and sulfate (SO 4 -S) values suggest that the PBR could be a source of the three plant nutrients, in addition to N and P. Furthermore, PBR also contained micronutrients (Fe, Zn, B, Mo, etc.) essential for plant growth (Table 1).Total arsenic (As) concentrations for the PBR were below the residential direct exposure limit for As in Florida (0.8 mg•kg −1 [41]), suggesting no threat for As contamination when PBR is land applied.Total Cd and Cr concentrations were well below the respective residential direct exposure limits, and also below the ranges of concentrations commonly found for soils in Florida and other states [41].Total Pb and Hg concentrations were below the detection limits of the ICP-AES (0.1 µg L −1 ).The chemical characteristics of the PBR, thus, suggest that it could be an excellent nutrient source for plant growth, without causing adverse environmental impact if land applied.

Properties of Soil, Manure, and Biosolids Used
The soil was acidic (Table 2), but the pH was within th reported range of pH values (5 -7.5 [28]) deemed suitfor sweet total N contents of the soil identify the soil as having "very low" native fertility [28], and requiring the maximum recommended rates of N and P to support normal growth and development of crops.The soil contained low total carbon and clay contents, which implies only a small capacity to retain cations.The coarse soil texture, with small clay and organic matter (Table 2) suggests that the soil could be highly susceptible to nutrient leaching losses.Intense rainfall or excessive irrigation combined with N and P application in such soil could enhance the potential risk of N and P leaching losses.
The two biosolids had total N and P concentrations (Table 3) typical of biosolids produced nationally in the USA [42].Total concentrations of major elements , and Ca) were also representative of biosolids produced nationally, and reflected individual wastewater and sludge treatment processes.O'Connor and Elliott [43] observed that Fe or Al concentrations of biosolids were generally ≤10 g•kg −1 , unless chemicals were added to the waste stream for P removal (e.g., Milorganite biosolids).The manure and biosolids varied widely in the amount of labile P estimated as WEP.The Milorganite biosolids, stabilized with Fe salts, had low WEP values.The critical role of Fe and Al in determining P solubility and release from biosolids-amended soils is documented [44].The GRU biosolids contained high PWEP values that exceeded the critical PWEP value of 10% [29], and suggests that land application of the material could result in extensive P leaching in vulnerable soils.As expected, the poultry manure had high Ca content (Table 3) because Ca is a basic ingredient of poultry diet, and a representative PWEP value of 9.78%.

Sweet Sorghum Dry Matter Yield
Nutrient application, in any form, enhanced biomass pros obtained e 2).Sor-ci duction of sweet sorghum, relative to yield from the control treatment (~25 g•pot −1 ; Figur ghum yields were similar for respective treatments for the three growing seasons.Thus, there were no seasonal effects on DMY and yields were averaged across the three growing seasons and presented in Figure 2. The PBR was as effective as the other nutrient sources (inorganic fertilizer, manure, and biosolids) in enhancing biomass production of sweet sorghum, but was superior to the other nutrient sources when applied alone without addition of S, K, Mg, and micronutrients (Figure 2).Thus, PBR applied alone is sufficient to supply the crop's nutrient needs and to completely substitute for fertilizer inputs.When the nutrient sources were applied alone, without addition of S, K, Mg, and micronutrients, treatments with PBR produced the greatest DMY of ~110 g•pot −1 , whereas yields from the nutrient source treatments occurring in the commercial fertilizer treatments were much lower (~70 g•pot −1 ).Addition of S, K, and Mg (Sul-Po-Mag) to the nutrient sources significantly increased yields for the treatments with commercial fertilizers by ~45%, ≤40% for the two biosolids, and ~30% for manure treatments.However, Sul-Po-Mag addition to the PBR did not significantly increase DMY, suggesting that the S, K, and Mg content of the PBR (Table 1) was adequate to supply those nutrients to meet the crop requirement when PBR was applied at N-based rates.Whereas addition of micronutrients resulted in further increases in DMY for the inorganic fertilizer treatment, DMY from the treatments with manure, biosolids, and PBR did not increase significantly over respective treatments having Sul-Po-Mag additions.This suggests that the manure, biosolids, and PBR could be applied without micronutrient addition when applied at N-based rates.
Relative agronomic effectiveness (RAE), as calculated with Equation ( 2), shows that PBR applied alone was ~85% as effective as the complete package of commer al fertilizers (N, P, K, S, Mg, and micronutrients) (Table 4).Thus, in low-input production systems where supplemental plant nutrients and low quantities of N and P fertilizers are used, PBR could be the most cost-effective nutrient source for bioenergy biomass production.The RAE data show that with the addition of S, K, Mg  ul-Po-Mag) a d micronut ients, the or nic sources nutrients, except manu c Several studies have shown that struvite and struvitelike materials can function as slow release fertilizers for environmentally sound crop production [45,46].In greenuse studies, Yetilmezsoy et al. [46] showed an increase in DMY of Perennial Ryegrass (Lolium perenne) by >400%, when the plants grown in a "garden soil" received nutrients from struvite-like material obtained by precipitating N and P from poultry manure wastewater with Mg.When the material was applied to sandy soils, a DMY increase of 60% was observed, relative to control treatments, where no nutrients were applied.Other studies have reported an increase in DMY of garden cress (Lepidum sativum) and purslane (Portulaca oleracea) up to 207% when the grasses were grown on sands, and supplied with nutrients solely from precipitated struvite-hat of P uptake nt sources that were applied in ent.Offsite losses of excess P through runof from soil treated with the nutrient sources either applied alone r, potash and magnesium).
like material [46].Plaza et al. [45] reported that struvite from an anaerobic digester supernatant was as effective as inorganic fertilizers in increasing DMY of ryegrass.
The DMY data in the present study suggest that in lowinput bioenergy biomass production systems, PBR could be a better nutrient source option than commercial fertilizers, manure, and biosolids.With the current prices of commercial N fertilizers at $ 0.37 -0.78 kg −1 (depending on the N content of the fertilizer type), P fertilizers at $ 0.65 -0.73 kg −1 [47], and Sul-Po-Mag at 0.32 kg −1 , application of PBR as a sole nutrient source to grow sweet sorghum could result in a significant reduction in the input cost, if it replaces the required application rates of 150 kg•N•ha −1 , 55 kg•P•ha −1 , and 225 kg•Sul-Po-Mag•ha −1 .

Nitrogen and Phosphorus Uptake and Leaching Losses
Nitrogen and P uptake followed similar pattern as t DMY.Across nutrient sources, the greatest N and occurred in the nutrie combination with S, K, Mg, and micronutrients (Table 5).However, for the PBR treatments, there were no significant differences among respective N and P uptake of the treatments receiving PBR applied alone, or in combination with micronutrients and/or Sul-Po-Mag (Table 5).This explains the similarity in yield quantities observed for the PBR in the different application strategies (either applied alone or with micronutrient and/or Sul-Po-Mag) (Figure 2).
Applying the organic sources of nutrients based on crop N needs simultaneously supplied P in excess of the crop requirem f and leaching can cause undesirable environmental effects, threatening surface waters bodies with eutrophication when the receiving waters are P-limited [40].This prompted us to determine potential P and N leaching looses following each harvest of the sorghum plants.As pointed out, application of the organic sources of nutri-Table 5. Nitrogen and phosphorus uptake of sweet sorghum or in combination of with micronutrients and Sul-Po-Mag (sulfu ents (biosolids, manure, and PBR) at N-based rates resulted in differential total P application rates (416 g•P•pot −1 for PBR; 354 g•P•pot −1 for GRU biosolids; 300 g•P•pot −1 for Milorganite biosolids; and 240 g•P•pot −1 for manure) due to differences in P content of the organic nutrient sources.Therefore, P measured in leachate was expressed as a percentage of P supplied in the nutrient sources.The greatest percent P leached (~51%), occurred within the treatment having the inorganic fertilizers applied alone, with the least occurring in the treatments having Milorganite biosolids and PBR (~18%) (Figure 3(a)).Low leachate P concentrations resulting from the Milorganite biosolids was expected because the biosolids is stabilized with Fe to reduce offsite losses of excess P [29].However, without any stabilization with Fe, the PBR material performed as well as the Milorganite biosolids in having reduced P leaching (Figure 3(a)).
The P leaching data were consistent with the calculated PWEP values of the nutrient sources (Tables 1 and  3), and also consistent with the observations of Brandt et al. [22] and Agyin-Birikorang et al. [23] that PWEP values of P sources were strongly related to off-site P losses emanating from the P sources.The PBR material in the present study had PWEP value of ~0.2% (Table 1) and showed the least amount of P leached, followed by Milorganite biosolids (Figure 3(a)), which had a PWEP value of ~0.6% (Table 3).Thus, as suggested by Chinault and O'Connor [13], PWEP could be a priori measure of the offsite P losses potential of P sources in vulnerable soils.
Improved N uptake, through the use of PBR, significantly reduced masses of N lost to leaching, compared with the other organic sources of nutrients and the inorganic fertilizers (Figure 3(b)) irrespective of whether the nutrient sources were applied alone or in combination with micronutrients and/or Sul-Po-Mag.Supplemental micronutrients and Sul-Po-Mag addition to the nutrient sources (except PBR) improved biomass yields (Figure   2) and consequently, N and P uptake (Table 5), resulting in reduced residual soil N and P content subject to leaching (Figure 3).The N and P leaching losses data s gests that land application of PBR will not only improve biomass yields at reduced cost (no additional S, K, Mg, and micronutrients fertilizers application), but could be environmentally friendly, resulting in reduced offsite nutrient losses.

Conclusion
The chemical plant growth, with ental impact when land-applied.This was confirmed in the greenhouse experiments where PBR applied alone was at least 85% as effective as inorganic N and P fertilizers applied together with S, K, Mg, and micronutrient, but with minimal N and P leaching losses.Thus, the combined data suggest that PBR can be substituted for commercial fertilizers to supply nutrients for biomass pro-surface water bodies.Utilization of biofuel residuals for biomass production could minimize the indiscriminate disposal of the residuals to landfills.If biofuel residual processing can be exploited in a practical engineering process, there is potential to extract struvite from raw biofuel residual in commercial quantities for slow release N and P fertilizer production.The present results were obtained in greenhouse pot experiments, and more research is needed to evaluate the agronomic effectiveness of the PBR in field trials, especially for their long-term effects on soil health, and for other test crops.

Acknowledgements
This study was funded by State of Florida Federal Stimulus Funds.We wish to express appreciation to Dr. J. E. Erickson of the Agronomy Dept., University of Florida for his collaboration, and to Mr. Richard Fethiere of the Forage Evaluation Support Lab., University of duct reduce pr plem nutrients r uired) and inimize po

4 NH  and 3 4 PO
 contents of the raw biofuel residual w easo determine the quantity of reactants required to produce struvite.
4 PO 4 •6H 2 O precipitation is highly pH dependent because the activities of both 4 NH  and 3 4

Figure 1 .
Figure 1.A schematic diagram of the reactor utilized for processing the biofuel residual (a modification of Yetilmezsoy and Sapci-Zengin [13]).

Figure 2 .
Figure 2. Sweet sorghum dry matter yield as a function of nutrient sources either applied alone or co-applied with micronutrients and/or Sul-Po-Mag (sulfur, potash, and

Figure 3 .
Figure 3. Percent of applied phosphorus (a) and total nitrogen (b) measured in leachate collected as a function of nutrient sources either applied e or co-applied with mic ronutrients and/or Sul-Po-M, potash, and magne- . Bars with same letters within a particular (a) or (b) are not significantly different (P > 0.05).

Selected chemical properties of the biosolids and
T manure

used for the study. Numbers are mean values of six replicates ± one standard deviation.
b percent water extractable phosphorus =