Phosphorus bioavailability of biochars produced by thermo-chemical conversion

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com

84 DOI: 10.1002/jpln.201300281 J. Plant Nutr. Soil Sci. 2014, 177, 84–90

Phosphorus bioavailability of biochars produced by thermo-chemicalconversionBernd Weber1*, Ernst A. Stadlbauer2, Elmar Schlich3, Sabrina Eichenauer2, Juergen Kern4, and Diedrich Steffens3

1 Universidad Autónoma del Estado de México–Facultad de Ingeniería, Cerro de Coatepec s/n Col. San Buenaventura , Toluca, Estado deMéxico 50130, Mexico

2 THM-Giessen-Friedberg–Laboratory of Waste Treatment, Giessen, Hessen, Germany3 Justus Liebig University–Department of Process Engineering in Food and Servicing Business, Giessen, Hessen, Germany4 Leibniz-Institute for Agricultural Engineering, Potsdam, Brandenburg, Germany

AbstractRecycling of P is a common strategy in efficient use of P. The aims of our investigation were tostudy the P extractability of biochars produced by low temperature conversion and to determinethe effect of soda application on low-temperature conversion of organic compounds and thebioavailability of P to rye grass (Lolium perenne L., cv. Grazer). In this study canola cake, drieddistillers grains with solubles, and meat-and-bone meal were converted to biochars with thermo-chemical conversion at 400°C. The P availability was measured in terms of solubility in water,2% citric, and 2% formic acid, and in a pot experiment with rye grass (Lolium perenne L.) whichwas cut three times. Application of 8% (w/w) soda to the process of thermo-chemical conversionof canola cake, dried distillers grains with soluble and meat-and-bone meal resulted in anincrease of water-, 2% citric-, and 2% formic-acid-extractable P in the biochars. In contrast tothe application of soda, addition of 12% wood ash (w/w) to the conversion of dried distillersgrains with solubles resulted in a lower increase of water-soluble P in the corresponding biocharcompared to processing biochar without additives. Addition of biochar P (100 mg P [kg soil]–1) toa Luvisol resulted in an increase of CAL-extractable soil P. The P uptake of rye grass from bio-chars produced with the addition of soda was as effective as basic slag and MgNH4 phosphatefertilizers and even better than rock phosphate.

Key words: carbonization / P bioavailability / P recycling / organic residues / soda

Accepted September 30, 2013

1 Introduction

Worldwide many research and implementation projects aimat using biochars for fertilization and C sequestration in soils(Lehmann, 2007; Novak et al., 2009; Graber et al., 2010). Inthe Amazonian region the long-term application of biocharsoriginating from the burning or pyrolysis of organic matter(OM) has resulted in Terra preta soils (Glaser et al., 2001).These soils have a high chemical, microbiological, and physi-cal soil fertility. The highly porous structure of biochars isimportant in encouraging microbial activity in soils (Warnocket al., 2007; Downie et al., 2009). Kammann et al. (2011)have reported that an application of 100 t ha–1 of peanut-shellbiochar increased the water-holding capacity of a sandy soiland the water-use efficiency of Chenopodium quinoa Willd.Furthermore, this biochar reduced the N2O emission of thesoil. Qayyum et al. (2012) have observed a longer half-life ofbiochar-C compared to straw. Application of biochar resultsnot only in an increase of soil organic C (SOC) but also in anincreased concentration of plant nutrients such as P, K, Ca,Mg, B, Cu, Fe, Mn, Mo, Ni, and Zn (Atkinson et al., 2010).Hossain et al. (2011) reported that the P availability of bio-chars was increased due to a higher pyrolysis temperature oforganic materials. Furthermore, the high specific surface

area of biochars (Joseph et al., 2010) and high charge densi-ties (Yao et al., 2012) could improve sorption characteristicsof soil (Chintala et al., 2013). In addition, the alkaline proper-ties of biochar can increase pH of acidic soils with the conse-quence of a better P availability.

Recycling of P is one of the most important strategiesfor achieving efficient use of limited P resources (Cordellet al., 2011). Wang et al. (2012) have shown that P in high-ash biochars is as effective as mineral P fertilizers. Theagronomic efficiency of biochar P can be measured by theextractability of P in 2% formic or citric acid (VDLUFA, 1995).In the thermo-chemical conversion at 400°C under anaer-obic conditions OM matter is catalyzed to conversion oil,reaction water, and a carbonaceous solid product commonlynamed biochar (Bayer and Kutubuddin, 1981; Stadlbaueret al., 2003). The conversion oil characterized by low contendof toxic substances can be used in biofuels (Bayer, 1995).The main characteristic of biochars from organic residues isthe enriched concentration of minerals, whereas the min-eral concentration depends on the composition of the OMfeedstocks. In recent years, sewage sludge, meat-and-bone

* Correspondence: Dr. B. Weber; e-mail: [email protected]

meal (MBM), and the remains of dead animals have beenprocessed by thermo-chemical conversion to oil andcarbonaceous solid product. The P concentration in thesecoals is within the 2.7%–10.3% range. However, little isknown about the P solubility and P bioavailability of varioussuch coals.

In the present investigation, the solubility and bioavailabilityof P in various biochars produced by thermo-chemical con-version was compared with other P-containing residues andfertilizers in order to estimate their suitability to substitute lim-ited P resources. Further modeling experiments applyingsoda to the thermo-chemical conversion were performed toshow that the P availability of the biochars could beenhanced. In addition, we analyzed the bioavailability of P inthe various biochars in comparison to P fertilizers (basic slagP, rock P, Mg NH4 P) with rye grass. In order to restrict thecontribution of the microbial soil biomass and mycorrhizae tothe availability of biochar and fertilizer P, we selected as sub-strate a subsoil with a low concentration of OM and a low soilmicrobial activity.

2 Materials and methods

2.1 Feedstocks

The organic residues selected as feedstock for biochar pro-duction were obtained from the following production facilities:

(1) Canola cake (CC) originated from an oil press was ob-tained from the Bioenergy association (Borken, Germany).

(2) Dried distillers grains with solubles (DDGS) were obtainedas a sample of the product “Protigrain” from the bioethanolproduction facility of the Südzucker Group (Zeitz, Germany).

(3) Meat-and-bone meal (MBM) was supplied as a solid pro-duct from the Saria Group abattoir and knackery plant (Lie-benberg, Germany).

Dry matter, together with the concentrations of C, H, N, S,P, raw-protein, and raw-fat content of the untreatedorganic products are presented in Tab. 1. Phosphorusconcentrations ranged from 0.8% to 5.7% in the dry matter ofthe products.

2.2 Thermo-chemical conversion

The products were converted with and without application ofNa2CO3 by thermo-chemical conversion at the laboratoryscale. An amount of 8% (w/w) of Na2CO3 salt was added tothe organic products. Each substrate and substrate/sodamixture was converted twice. In another treatment, DDGSwas converted twice with 12% of wood ash from a domesticburner belonging to one of the authors. In the thermo-chemi-cal conversion apparatus (Fig. 1), 400 g of organic productwas placed in a horizontal tube furnace (70 mm ∅). Glass-wool packaging fixed the material in the center of the tube.The conversion process started with a temperature of 20°Cand was increased to 400°C with a temperature gradient of1°C to 2°C min–1. The process was continued for a durationof 4.5 h. Further details of this technique are given by Bayerand Kutubuddin (1988) as well as Bojanowski et al. (2007).

Following the thermo-chemical conversion, the chars werehomogenized, ground, and passed through a 1-mm sieve forfurther analysis.

2.3 Plant-pot experiments

The P bioavailability of the various biochars was analyzed insmall Mitscherlich pots in a greenhouse experiment. For thispurpose, rye grass (Lolium perenne L., cv. Grazer) was culti-vated in a treatment without P fertilization (control) and infurther treatments with 100 mg P (kg soil)–1 in the form of


Table 1: Dry matter (DM), organic matter (OM), total element, raw-protein, and raw-fat concentrations in canola cake (CC), dried distillersgrains with solubles (DDGS), meat-and-bone meal (MBM), and wood ash (WA).

Substrate DM OM C H N S P Ca Mg K Na Raw protein Raw fat

/ g (100 g dry weight)–1

CC 95.3 93.4 46.6 7.5 4.7 0.6 1.01 0.80 0.41 1.13 n.d. 29.4 17.1

DDGS 94.4 92.9 44.5 7.1 5.3 0.6 0.76 0.09 0.24 1.47 0.56 33.1 5.5

MBM 97.5 65.4 35.3 5.6 7.6 0.2 5.65 10.0 0.24 0.38 0.55 47.5 12.0

WA nd* nd nd nd nd nd 0.96 25.1 1.90 8.41 0.37 nd nd

* nd = not determined

Figure 1: Laboratory apparatus for biomass thermo-chemicalconversion under a N2 atmosphere (Weber, 2010).

J. Plant Nutr. Soil Sci. 2014, 177, 84–90 Phosphorus bioavailability of biochars 85

seven different biochars and three P fertilizers. The experi-mental soil was a subsoil (0.6–1.2 m depth) from a Luvisolderived from loess (25% clay, 47% silt, 28% sand, 0.20%total C, 0.04% total N, pH 5.1 in 0.01 M CaCl2, and 7.1 mgCAL-extractable P [kg soil]–1).

Each small Mitscherlich pot contained 6.5 kg of a soil quartzsand (grain size 0.6–1.2 mm) mixture in a ratio of 1:1. Thesoil/sand mixture was fertilized with 1.50 g N (NH4NO3),1.80 g K (50% as KCl + 50% as K2SO4), and 0.16 g Mg(MgSO4) per pot. Micronutrients were added in an aqueoussolution consisting of 13 mg Mn (MnSO4), 2.3 mg Zn(ZnSO4), 1.3 mg Cu (CuSO4), 0.08 mg B (H3BO3), and0.05 mg Mo (NH4 molybdate) per kg of soil. To combatdecreasing a soil pH, 1.95 g CaCO3 was mixed with 6.5 kg ofthe soil/sand mixture. This fertilizer strategy ensured that Palone was the growth-limiting factor in a soil pH 7.0. Theeffects of biochar P on the P bioavailability were comparedwith mineral P fertilizers: rock phosphate (RP, with a total P of13%), basic slag phosphate (BSP, with a total P concentra-tion of 7%), and Mg NH4 phosphate (MAP, with a total P con-centration of 12%). The latter derived from a P-recycling pro-cess, which has been established at a wastewater-treatmentplant in Berlin. This process includes induced MAP precipita-tion in the sewage sludge after addition of MgCl2, as reportedby Kern et al. (2008). In the MAP treatment, the Mg and Ncontent was considered in the fertilization management of ryegrass.

Seven days after mixing the biochars and the mineral saltswith the experimental soil/sand mixture and incubation at50% of the maximum water-holding capacity (169 g waterper kg soil/sand mixture) soil samples were taken. The soilsamples were dried for 24 h at 40°C. Rye grass was sownwith 0.5 g seeds per pot with the seeds placed in two con-centric circles at a soil depth of 1 cm. Then the pots were cul-tivated for 90 d. During this period, the rye grass was cut at30, 50, and 90 d after sowing. After each cut of rye grass,0.5 g N (NH4NO3) and 0.5 g K (50% as KCl + 50% as K2SO4)per pot were applied. Each treatment had four replicationsfrom which means and standard errors (SE) were calculated.After harvesting, rye grass shoots were dried at 105°C for24 h in an oven in order to measure the dry-matter yield. Afterdrying, the rye grass biomass was ground in a rotor mill.

2.4 Analyses

Total concentrations of C, H, N, and S of the organic sub-strates and biochars were analyzed with a Vario EL dry com-bustion analyzer (Elementar Co., Germany). The total con-centrations of P, K, Na, Ca, and Mg in the organic substrates,biochars, and in extracts of biochars were analyzed using anacid digestion method (Rosopulo, 1976). The principle of thismethod was that 0.5 g of a solid sample or 10 mL of anextract were mixed with 2 mL trichloroethylene solution and afollowing wet digestion with a mineral acid mixture at 250°Cfor 9 h. The mineral acid solution contained 10 v/v concen-trated HNO3 (p = 1.40), 1 v/v concentrated HClO4 (p = 1.67),and 0.25 v/v concentrated H2SO4 (p = 1.84).Then the cooleddigestion residue was boiled with 5 mL 5 M HNO3. After-wards, this digestion solution was placed into a 50-mL volu-

metric flask, which was brought to volume with distilled water.This digestion solution was then filtered with “blue-band” fil-ters (Schleicher and Schüll Co, Dassel, Germany). The Pconcentration in the clear digestion extracts was measuredcolorimetrically as an NH4-molybdo-vanado-P complex at450 nm wavelength (Gericke and Kurmies, 1952), and theconcentrations of K, Na, Ca, and Mg were analyzed by AAStechnique.

The P solubility of biochars and P fertilizers were measuredby water extraction in a ratio of 1:50 and by 2% formic acidand 2% citric acid extraction in a ratio of 1:100 for 30 min witha rotating shaker (VDLUFA, 1995). The extracts were filteredusing a 614 G ¼ Macherey-Nagel filter paper, and P concen-trations were measured colorimetrically as an NH4-molybdo-vanado-complex at a wavelength of 450 nm.

The P concentration in the grass biomass was determinedafter dry ashing of 0.5 g dry matter in a porcelain crucible at550°C in a furnace overnight. The ash was then moistenedwith 2 mL of demineralized water. After adding 2.5 mL of 5 MHNO3, boiling for 1 min, and filtering, the P concentration wasmeasured as an NH4 -molybdo-vanado-complex at 450 nmwavelength (Gericke and Kurmies, 1952). The P uptake ofrye grass shoots was calculated by multiplying the P concen-tration in the grass shoots with the dry-matter yield per pot.

The effect of the application of biochar and P fertilizer on soilP availability was analyzed with the CAL soil extraction meth-od, using a mixture of 0.1 M Ca lactate, 0.1 M Ca acetate,and 0.3 M acetic acid in a wt/vol ratio of 1:20 (Schüller, 1969).Soil pH was measured in a 0.01 M CaCl2 solution in a ratio of1:2.5.

2.5 Statistical analysis

Experimental data were statistically analyzed at a signifi-cance level of 5% in a LSD test calculated using the programStatgraphics Centurion Version 16.1.

3 Results

3.1 Chemical composition and P extractability ofvarious biochars

The effects of the thermo-chemical conversion on elementconcentrations in the biochars are presented in Tab. 2. Duringthe conversion of canola cake, DDGS, and meat-and-bonemeal, the concentration of K and P increased. It is evident,that N and S were released as gases during the conversionof the organic substances. Mixing of soda to the substratesresulted in higher Na concentrations in corresponding bio-chars.

Addition of soda or wood ash to the conversion process ofcanola cake and DDGS resulted in considerable increase ofwater and organic acid–extractable P in the correspondingbiochars. However, the effect of soda application on the con-version of meat-and-bone meal resulted in a biochar with anincreased water-soluble P but only with a slight increase in


86 Weber, Stadlbauer, Schlich, Eichenauer, Kern, Steffens J. Plant Nutr. Soil Sci. 2014, 177, 84–90

organic acid–soluble P. The organic acid extractability of P inthe chars of meat-and-bone meal is in the range of basic slagphosphate and mono NH4 phosphate. If soda was replacedby wood ash during the conversion process of DDGS thewater solubility of P was strongly reduced. However, thisreduction was not so evident in the organic acids extracts(Fig. 2).

The CAL-extractable soil P was affected by the addition ofvarious biochars and P fertilizers (Tab. 3). Application of sodaor wood ash to the thermo-chemical conversion of canolacake, DDGS, and meat-and-bone meal resulted in anincrease of CAL-extractable soil P. Soil pH increased due tothe mixing of soda or wood ash to the thermo-chemical con-version in biochar synthesis in the corresponding biochartreatments.

The data in Fig. 3 shows the effect of P fertilization in the formof various biochars and P fertilizers on the shoot biomass ofrye grass, both in total and for each cut. Addition of biocharsand P fertilizers resulted in a significant increase of the shootbiomass of rye grass at each cut. The highest dry-matteryield was obtained in the MgNH4 phosphate treatment fol-

lowed by basic slag phosphate. The shoot biomass of ryegrass grown on biochar-fertilized soils was greater in thetreatments where soda was added to canola cake and DDGSprior to the thermo-chemical conversion. Mixing soda to thethermo-chemical conversion of meat-and-bone meal did notinfluence the shoot biomass of rye grass (Fig. 3).

The bioavailability of P from the various biochars is alsoreflected by the P concentration in the shoot biomass of ryegrass. In the treatments with P application, the P concentra-tions were significantly higher than in the control treatment.The P concentrations in rye grass were significantly higher inthe biochar treatment with soda and wood ash applicationprior to the thermo-chemical conversion of canola cake andDDGS than in the treatments without soda application(Tab. 4).

Figure 4 shows the P uptake of rye grass related to P fertiliza-tion of soils in the form of biochars and P fertilizers. Mixingsoda or wood ash to the thermo-chemical conversion of theorganic products improved the solubility of P in the corre-sponding biochars so that the P uptake of rye grass signifi-cantly increased.


Table 2: Ash and total element concentrations in biochars produced from various substrates with and without an addition of soda and woodash prior to thermo-chemical conversion. DDGS: dried distillers grains with solubles.

Biochar Ash C H N S P Ca K Mg Na

/ g (100 g dry weight)–1

Canola cake 17.0 67.0 3.9 7.2 0.2 2.89 2.08 3.07 1.01 n.d.

Canola cake + 8% soda 24.2 59.3 3.9 5.6 0.2 2.44 1.72 2.73 0.94 5.92

DDGS 16.9 64.6 3.5 7.6 1.6 2.11 0.19 4.17 0.67 1.56

DDGS + 8% soda 27.7 55.9 3.8 5.2 1.4 1.90 0.22 3.88 0.66 8.42

DDGS + 12% wood ash 36.9 46.0 2.6 4.8 1.2 1.75 7.67 4.85 0.95 1.19

Meat-and-bone meal 62.5 25.4 1.9 4.5 0.0 11.2 18.30 0.82 0.43 0.93

Meat-and-bone meal + 8% soda 64.6 25.2 1.8 3.4 0.1 10.3 17.40 0.79 0.40 4.65

0102030405060708090

100

Water

Formic acid

Citric acidP so

lubi

lity

/ % o

f tot

al P

Figure 2: Effect of soda (So) and wood-ash (WA) application to thermo-chemical conversion of canola cake (CC), dried distillers grains withsolubles (DDGS), and meat-and-bone meal (MBM) on water, 2% formic acid, and 2% citric acid solubility of total P in various biochars and Pfertilizers (RP: rock phosphate, BSP: basic slag phosphate, MAP: MgNH4 phosphate).


4 Discussion

Shoot dry-matter yield, P concentration, and P uptake of ryegrass was higher in those treatments in which soda wasadded prior to the carbonization of canola cake and DDGS bythe thermo-chemical conversion (Fig. 3 and 4; Tab. 4). Thus,we can confirm our hypothesis that soda application to thethermo-chemical conversion of various organic materialsimproves the P bioavailability of the chars. In contrast tocanola cake and DDGS, mixing soda to the thermo-chemicalconversion of meat-and-bone meal only slightly improved thesolubility of P. The question therefore arises as to why sodaaddition to the thermo-chemical conversion of canola cake

and DDGS promoted P solubility. We assume that during thethermo-chemical conversion, soda digests organic P toCaNaPO4 or Na3PO4. This process did not develop as muchduring the carbonization of DDGS plus wood ash, since woodash contained a relatively high concentration of Ca (Tab. 1).During conversion of the DDGS and wood ash mixture thishigh Ca concentration appeared to enhance the synthesis ofwater-insoluble Ca phosphates (Fig. 2). Carbonization ofDDGS or canola cake without soda resulted in much moreinsoluble P. The addition of soda to the carbonization ofcanola cake and DDGS improved the chemical and biologicalavailability of P.

However, soda application to the conversion of meat-and-bone meal had only a limited effect on P availability. The rea-son for this is that historically soda digestion is processed at1200°C–1300°C. This process was used in the thermodiges-tion of rock phosphate to CaNaPO4 (Rhenania phosphate orSinter phosphate) in which rock phosphate is mixed withsoda and quartz sand (Werner, 1967).

In this study, we did not analyze the heavy metal transfer intothe food chain since canola cake, DDGS, and meat-and-bone meal are relatively uncontaminated with heavy metals.A thermo-chemical conversion of sewage sludge is a promis-ing tool from an energetic point of view. From a food safetypoint of view, biochars from sewage sludge should be inte-grated as energy carriers into process schemes—in order toreduce the contamination of heavy metals. One of these pro-cesses could be a thermo-chemical treatment with KCl orMgCl2 to form volatile heavy metal chlorides that are releasedfrom solid material at temperatures > 1000°C (Adam et al.,2009).

5 Conclusions

The bioavailability of P in biochars from canola cake, DDGS,and meat-and-bone meal produced by the thermo-chemicalconversion was acceptable compared to P fertilizers such as


Table 3: Effect of P fertilization (100 mg P [kg soil]–1) in the form ofvarious biochars with and without soda and wood-ash application tothe thermo-chemical conversion of canola cake, DDGS (drieddistillers grains with solubles), meat-and-bone meal, and in theform of rock phosphate, basic slag phosphate, and MgNH4phosphate on CAL-extractable soil P and soil pH. The standard errorrefers to a total of four replicates. Different letters indicate significantdifferences at the 5% probability level between the treatments.

Treatment CAL-extractable P Soil pH (CaCl2)

/ mg P (kg soil)–1

Control 4.6 ± 0.47a 7.0 ± 0.11

Canola-cake char 11.3 ± 2.17a 7.1 ± 0.05

Canola-cake / soda char 75.9 ± 16.58d,e 7.7 ± 0.05

DDGS char 13.5 ± 1.54a 7.1 ± 0.09

DDGS / soda char 58.7 ± 3.79c 7.7 ± 0.09

DDGS / wood-ash char 40.2 ± 4.61b 7.6 ± 0.03

Meat-and-bone-meal char 67.4 ± 11.54c,d 7.0 ± 0.2

Meat-and-bone-meal / soda char 70.3 ± 10.69c,d,e 7.1 ± 0.09

Rock phosphate 36.8 ± 1.01b 7.1 ± 0.1

Basic slag phosphate 85.2 ± 9.67e 7.6 ± 0.04

MgNH4 phosphate 84.3 ± 16.67e 7.2 ± 0.07

3. Cut

2. Cut

1. Cut

a

b b

c

fg

dc, dc, dc, d

e

Dry

-mat

ter y

ield

/ g

DM

pot–1

Figure 3: Effect of P fertilization (100 mg P [kg soil]–1) in form of various biochars with and without soda (So) and wood-ash (WA) application tothe thermo-chemical conversion process of canola cake (CC), dried distillers grains with solubles (DDGS), meat-and-bone meal (MBM), and inform of P fertilizers (RP: rock phosphate, BSP: basic slag phosphate, and MAP: MgNH4 phosphate) on shoot dry-matter yields of rye grass atthree cuts. (Each data point is the mean ± standard error of four replications per treatment. Different letters indicates significant differences atthe 5% probability level between the treatments).


basic slag phosphate and MgNH4 phosphate from sewagesludge. Application of soda to the thermo-chemical conver-sion of the organic compounds significantly improves the Pbioavailability of the corresponding biochars. The thermo-chemical conversion is a practicable technique in biochar pro-duction for P recycling of organic waste.

Acknowledgments

This work was supported by the Ministry of Industry andTrade of Federal State Hesse, Germany by grant LOEWE(Landes-Offensive zur Entwicklung wissenschaftlich-ökono-mischer Exzellenz); Project No. 146/08-08.

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Table 4: Effect of P fertilization (100 mg P [kg soil]–1) in the form of various biochars with and without soda and wood-ash application to thethermo-chemical conversion of canola cake, DDGS (dried distillers grains with solubles), meat-and-bone meal, and in the form of rockphosphate, basic slag phosphate and MgNH4 phosphate on P concentration in the shoot biomass of rye grass during three cuts. The standarderror refers to a total of four replicates. Different letters indicate significant differences at the 5% probability level between the treatments.

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Basic slag phosphate 2.8 ± 0.05e 2.3 ± 0.16e 2.4 ± 0.12c,d

MgNH4 phosphate 3.4 ± 0.06f 2.8 ± 0.19f 2.7 ± 0.19d,e

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100120140160180200

3. Cut

2. Cut

1. Cut

a

bc

d eef

g

hi i

P up

take

/ m

g P

pot–1

Figure 4: Effect of P fertilization (100 mg P [kg soil]–1) in form of various biochars with and without soda (So) and wood-ash (WA) application tothe thermo-chemical conversion process of canola cake (CC), dried distillers dried grains with solubles (DDGS), meat-and-bone meal (MBM),and in form of P fertilizers (RP: rock phosphate, BSP: basic slag phosphate, and MAP: MgNH4 phosphate) on the P uptake of rye grass at threecuts. (Each data point is the mean ± standard error of four replications per treatment. Different letters indicates significant differences at the 5%probability level between the treatments).


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Phosphorus bioavailability of biochars produced by thermo-chemical conversion