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Research Article

Phosphorus Sorption and Availability from Biochars and Soil/Biochar Mixtures†

Rajesh Chintala1,*, Thomas E Schumacher1, Louis M McDonald2, David E Clay1, Douglas D Malo1,

Sharon K Papiernik3, Sharon A Clay1, and James L Julson4

1 Department of Plant Science, South Dakota State University, Brookings, South Dakota, USA 2 Department of Plant and Soil Science, West Virginia University, Morgantown, West Virginia, USA 3 USDA-ARS, Brookings, South Dakota, USA 4 Department of Agricultural and Biosystems Engineering, South Dakota State University, South Dakota,

USA

Correspondence: R. Chintala, Department of Plant Science, South Dakota State University, Brookings,

South Dakota, USA

e-mail: rajesh.chintala@sdstate.edu

†This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as an ‘Accepted Article’, doi: [10.1002/clen.201300089]. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Received: February 5, 2013 / Revised: April 25, 2013 / Accepted: May 19, 2013

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ABSTRACT

In an energy limited world, biomass may be converted to energy products through pyrolysis. A byproduct

of this process is biochar. A better understanding is needed of the sorption characteristics of biochars which

can influence the availability of plant essential nutrients and potential water contaminants such as

phosphorus (P) in soil. Knowledge of P retention and release mechanisms when applying carbon-rich

amendments such as biochar to soil is needed. The objectives of this study were to quantify the P sorption

and availability from biochars produced from the fast pyrolysis of corn stover (Zea mays L.), Ponderosa

pine (Pinus ponderosa Lawson and C. Lawson) wood residue, and switchgrass (Panicum virgatum L.). We

determined the impact of biochar application to soils with different chemical characteristics on P sorption

and availability. Sorption of P by biochars and soil-biochar mixtures was studied by fitting the equilibrium

solution and sorbed concentrations of P using Freundlich and Langmuir isotherms. Biochar produced from

Ponderosa pine wood residue had very different chemical characteristics than corn stover and switchgrass.

Corn stover biochar had the highest P sorption (in average 79% of the initial solution P concentration)

followed by switchgrass biochar (in average 76%) and Ponderosa pine wood residue biochar (in average

31%). Ponderosa pine wood residue biochar had higher bicarbonate extractable (available) P (in average

43%) followed by switchgrass biochar (33% of sorbed P) and corn stover biochar (25% of sorbed P). The

incorporation of biochars to acidic soil at 40 g kg-1 (4%) increased the equilibrium solution P concentration

(reduced the sorption) and increased available sorbed P. In calcareous soil, application of alkaline biochars

(corn stover and switchgrass biochars) significantly increased the sorption of P and decreased the

availability of sorbed P. Biochar effects on soil P was aligned with their chemical composition and surface

characteristics.

Keywords: P retention, P desorption, acidic soil, calcareous soil

Abbreviations: AEC, anion exchange capacity; ANOVA, analysis of variance; CEC, cation exchange

capacity; ICP-AES, inductively coupled plasma atomic emission spectroscopy; PZNC, point of zero net

charge; VOC, volatile organic compound

INTRODUCTION

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In an energy limited world with concerns about climate change, biomass will increasingly be used

to create energy products that are energy dense and easily transportable. A byproduct of biomass pyrolysis

is biochar, which is recalcitrant, could be used to sequester carbon [1, 2] and exhibits strong sorption

affinity for contaminant organic compounds and heavy metals [3-5]. Biochar also has the potential to

improve soil quality by increasing anion and cation exchange capacities (AEC/CEC), surface area, water

holding capacity, and by affecting the bioavailability of nutrients including phosphorus (P), calcium (Ca),

sulfur (S), and nitrogen (N) [6]. However, biochars are variable in their properties and reactivity in soil due

to different feedstock and processing conditions [7]. If biochar is to be used as a soil amendment, it is

necessary to evaluate its influence on the bioavailability of nutrients such as P in soil.

P undergoes several geo-chemical processes in soil such as solubilization, complexation,

adsorption, and precipitation that determine its mobility and fate. These chemical processes are a complex

function of several soil properties including, Al and Fe oxide form and content, the amount and form of

silicate clays, and CaCO3 content [8]. The impact of these properties on P retention and release depends on

soil acidity or alkalinity. The recovery of P by plants from applied fertilizer is limited in acidic and

calcareous soils due to P fixation. In acidic soils, the P is fixed by high-energy sorption surfaces such as

oxides and hydroxides of Fe and Al by formation of insoluble Fe and Al phosphates by ligand exchange

and precipitation reactions [9, 10]. The high base status and pH of calcareous soils make P sparingly

soluble due to formation of metal complexes such as Ca-P and Mg-P [11, 12].

Sometimes the excessive or poor-timed application of livestock manures has the potential to

mobilize nutrients through runoff and negatively affect the water quality of freshwater systems and causing

eutrophication [13, 14]. Apart from the environmental concerns due to P mobilization, P is an essential

plant nutrient [15] and relatively less available in soil than nitrogen and potassium [16]. An understanding

of P retention and release mechanisms provides crucial information for the effective management of P to

enhance crop production and sustain soil and water quality.

Sorption is one of the commonly used mechanisms to describe P retention in soil [17, 18]. In some

studies, biochar application enhanced the availability and plant uptake of P due to biochar´s high anion

exchange capacity; reduced availability of Al and Fe in soil reduced P fixation [19, 20]. P sorption on pure

ferrihydrite was decreased with the application of rice straw-derived biochar [21]. Understanding the

dynamic interactions of P with biochars will aid in designing biochars as soil amendments to meet various

soil management objectives. In this context, this study was conducted with following objectives, to 1)

quantify the potential of biochars produced from the microwave pyrolysis of corn stover, Ponderosa pine

wood residue, and switchgrass to sorb P from solutions, 2) evaluate the availability of P sorbed to these

biochars, and 3) evaluate P sorption and availability in acidic and calcareous soils amended with each

biochar type.

MATERIALS AND METHODS

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Chemical characterization of biochar materials and soil samples

Biochars derived from corn stover (Zea mays L.), Ponderosa pine wood chips (Pinus ponderosa

Lawson and C. Lawson), and switchgrass (Panicum virgatum L.) were produced as co-products with bio-

oil using microwave pyrolysis at 650 °C and residence time of 18 min. These biochar materials were sieved

to homogenize by using a 2-mm sieve. Acidic and calcareous soils were selected from composite surface

(0-15 cm) samples from two different locations. The acidic soil (Clayey, smectitic, acid, mesic, shallow

Aridic Ustorthents) was collected from a cultivated Grummit soil series (located at 44° 18’ 2.52”N and

103° 20’ 33.39” W) (http:// soils.usda.gov) [22]. The calcareous soil belonged to the Langhei soil series

(fine-loamy, mixed, superactive, frigid, typic, Eutrudepts) and was collected at 44° 13’ 38.85”N and 96°

45’ 02.05” W [22]. Soil was air-dried, crushed and passed through a 2-mm sieve.

Specific surface area of biochars, soils, and soil-biochar mixtures were determined by using the

iodine absorption method [23]. The pH and EC (electrical conductivity) of samples were measured at 1:10

water to solid ratio after shaking for 30 min in deionized water [24]. Soil texture was determined using the

pipette method [25]. The cation exchange capacity (CEC) of biochar and soil/biochar samples were

measured using 1 M ammonium acetate (CH3COONH4) (pH 7) [24]. The point of zero net charge (PZNC)

was measured by determining the cation exchange capacity (CEC) and anion exchange capacity (AEC)

simultaneously at eight pH values (pH 2–10). The PZNC was the pH at which CEC and AEC were equal

on surface of sorbent. Samples (biochars, soils, and soil/biochar mixtures) were initially saturated with 0.01

mol L-1 KCl and the pH was adjusted with 1 mol L-1 HCl and 1 mol L-1 KOH. CEC and AEC were

determined by measuring the concentration of K+ and Cl- from surface of samples using 0.5 M NaNO3 as

an extractant. Potassium (K) was analyzed using flame atomic absorption spectroscopy (GBC Avanta,

USA) and the Cl- ion was determined using flow injection analysis (Quick Chem FIA+, 8000 series,

Latchat Instruments, USA) [26]. The quantitative analysis of volatile organic compounds (VOCs) of

biochar samples were determined using standard ASTM method [27]. Total carbon and total nitrogen were

determined on an Elementar Vario MAX CNS analyzer [28]. The total free Fe, Al, and Mn oxides (Fed,

Ald, and Mnd) were obtained by heating soil samples in a citrate/bicarbonate/dithionate mixture in a water

bath at 85 °C using a citrate/bicarbonate/dithionate mixture [29]. Ammonium oxalate in darkness method

was used to determine amorphous Fe, Al, and Mn oxides in soil samples [30]. Soil exchangeable acidity

(H+ and Al3+) were extracted with 1 M potassium chloride (KCl), and then titrated with 0.1 N sodium

hydroxide (NaOH) and 0.1 N hydrochloric acid (HCl) [31]. The calcium carbonate (CaCO3) content was

determined using the calcimeter method [32]. Bicarbonate extractable P was determined in biochar and

soil/biochar mixtures by shaking samples for 30 min with a 0.5 M sodium bicarbonate (NaHCO3) solution

(1:50 solid/solution ratio), after which P concentration was measured in extracted solutions by the

ammonium molybdate/ascorbic acid method [33]. Soil and biochar samples were digested with

concentrated nitric acid (HNO3) in a pressurized (200 psi) microwave oven (MARS 5, CEM, USA) [34, 35]

equipped with Teflon closed vessels. The wet digested samples were diluted with distilled water and

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nutrient composition (total P, Ca, Mg, Na, and K) measured using inductively coupled plasma atomic

emission spectroscopy (ICP-AES) (Varian, Australia) [36]. Each analysis was conducted using four

separate biochar samples (four replications) from different batches, and mean values are provided in Tables

1 and 2.

Phosphorus Sorption Isotherms

P sorption isotherms were determined for biochars, soil, and soil/biochar mixtures. The biochar

was mixed with soil at a rate of 40 g kg-1. Soils with and without biochars (control) were incubated at 70%

field capacity (and at room temperature of 28°C) and for 30 days in the dark [37]. After a month of

incubation, the soil (control) and soil/biochar mixtures were air-dried and stored to use for batch

experiments.

Four replicate samples of biochar (0.1 g) and soil/biochar samples (0.5 g) were transferred to 50-

mL centrifuge tubes. To the centrifuge tubes with samples were added 20-mL solutions of 0.01 M calcium

chloride (CaCl2) containing 0, 0.32, 0.64, 1.29, 1.93, 2.58, and 3.22 mmol L-1 P. 0.5 mL chloroform were

added to the centrifuge tubes with samples to inhibit microbial growth. Samples were equilibrated on a

horizontal shaker for 24 h at room temperature and then centrifuged (with fixed-angle rotor) for 5 min to

collect the clear supernatant solution. The supernatant solutions were filtered using Whatman no. 42 filter

paper (with retention: >2.5µm) and then analyzed for P using the ammonium molybdate/ascorbic acid

method [33]. Sorbed P was calculated as the difference between equilibrium solution P concentration (after

shaking) and initial P concentration of solution. Adjustable parameters for Freundlich (Eq. (1)) and

Langmuir isotherms (Eq. (2)) were determined by non-linear regression using Proc NLIN [38]:

q = Kf c1/n (1)

q = KL b c/(1 + KL c) (2)

where q = sorbed P (mg kg-1 of P), c = equilibrium P concentration in supernatant solution after

shaking (mmol L-1), Kf = Freundlich partitioning coefficient, 1/n = sorption intensity, b = sorption

maximum (mmol kg-1 of P), and KL = equilibrium constant that determines sorption energy (L kg-1). The

sorption percentages were calculated by dividing sorbed P mass with initial solution P mass.

Availability of Sorbed Phoshorus

Available P (bicarbonate extractable P) was determined in samples used for the sorption batch

study by using 0.5 M sodium bicarbonate (NaHCO3) solution as an extractant. Samples from sorption batch

study were washed a few times with isobutyl alcohol and then extracted with 50 mL of 0.5 M NaHCO3 at a

1:50 ratio. Centrifuge tubes with sample and extractant were shaken for 30 min on a horizontal shaker [39].

After shaking, the samples were centrifuged for 5 min and the clear supernatant solutions were collected to

analyze bicarbonate extractable P (available P) using colorimetric method as described above. The

available P was determined by dividing the bicarbonate extractable P mass by the sorbed P mass. A one-

way analysis of variance (ANOVA) was performed using the SAS Statistical Package, version 9.2 to

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determine the effect of treatments (biochar type and soil type) on chemical properties, P sorption

(partitioning coefficients) and availability of sorbed P [40, 41].

RESULTS AND DISCUSSION

Characterization of biochar materials, soils, and soil/biochar mixtures

Corn stover and switchgrass biochars were more alkaline with significantly (P < 0.05) higher base

cation concentration when compared to Ponderosa pine wood residue biochar (Table 1). Biochar pH, EC,

CEC, PZNC, VOCs, bicarbonate P, and base cation concentration were highest in corn stover biochar

followed by switchgrass and Ponderosa pine wood residue biochar. Ponderosa pine wood residue biochar

was acidic with highest specific surface area and total carbon content. The high pH values of herbaceous

plant based biochars such as corn stover biochar and switchgrass biochar may be due to hydrolysis

undergone by carbonates and bicarbonates of base cations such as Ca, Mg, Na, and K which were present

in biochar materials [42, 43]. The presence of high calcium carbonate content (CaCO3) in corn stover and

switchgrass biochars determines their utility as liming agents to reduce soil acidity. The liming effect of

these biochars may affect the P sorption and availability. Corn stover and switchgrass biochars had higher

EC values than Ponderosa pine wood residue biochar, indicating the existence of water soluble salts. The

CEC of corn stover and switchgrass biochars was higher than Ponderosa pine wood residue biochar which

may be due to low PZNC and high negative charge potential of surface functional groups. VOCs may

provide these oxygenated functional groups with acid-base properties to biochar surfaces [44] which

participate in protonation and de-protonation reactions. Corn stover and switchgrass biochars had

significantly higher VOCs than Ponderosa pine wood residue biochar. This characterization demonstrated

the significant effect of biomass feedstock type on the composition of biochars produced at similar

pyrolytic conditions which was also observed in previous studies [20]. Bicarbonate extractable P and total

P were also higher in corn stover and switchgrass biochars than Ponderosa pine wood residue biochar. The

bicarbonate extractable P of biochars may influence the sorption and availability of P. Total carbon and

nitrogen concentrations were high in these plant based biochars produced from microwave pyrolysis which

was consistent with the results of a study by Gaskin et al. [43].

The texture of both acidic and calcareous soils was clay loam. The acidic soil had higher specific

surface area, bicarbonate extractable P, total P, and oxides of Fe, Al, and Mn. Calcareous soil had

significantly higher clay content, CEC, PZNC, and CaCO3 content (Table 2). Total carbon and nitrogen

content were not significantly different between acidic and calcareous soils. The pH of acidic soil was

significantly increased by 0.88 and 0.59 units after incubation with corn stover biochar and switchgrass

biochar respectively at 40g kg-1. The exchangeable acidity and PZNC were decreased by the incorporation

of corn stover and switchgrass biochars into the acidic soil. Specific surface area, CEC, total C and N,

bicarbonate extractable P, and total P were increased in the acidic soil after incubation with biochars.

Concentration of extractable pedogenic oxides (total free and amorphous) of Fe, Al, and Mn were

decreased in acidic soil which may be due to increased precipitation (as hydroxides) and formation of

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organo-metallic complexes with the organic ligands released from biochars (due to solubility of organic

fraction) as the soil pH increased due to incorporation of biochars. These changes in properties were

generally greater for acidic soil than calcareous soil and for acidic soil, corn stover and switchgrass

biochars had a stronger effect on pedogenic oxides than did Ponderosa pine wood residue biochar. Whereas

the incubation of calcareous soil with biochars increased specific surface area, CEC, CaCO3 content,

bicarbonate extractable P, total C, N, and P. There was decrease in PZNC in calcareous soil with the

application of biochars.

Sorption and availability of P from biochar samples

The Freundlich and Langmuir equations fit the sorption data for all three biochars (Table 3)

although Langmuir isotherms fit better for the alkaline corn stover and switchgrass biochars compared to

the Ponderosa pine wood residue biochar (Fig. 1). P sorption on biochars was significantly (P < 0.0001)

affected by initial P concentration and biochar type. Corn stover biochar had the highest sorption of P

(79%) followed by switchgrass biochar (76%) and Ponderosa pine wood residue biochar (31%). P sorption

by biochars may occur mainly due to the exchange between anions of P in solution with the oxygenated

functional groups on surface of biochars which could be provided by VOCs. FTIR analysis of biochars

produced from corn stover, switchgrass, and Ponderosa pine wood residue showed the presence of surface

functional groups such as C≡N, C=O, C�O, and C�H [45]. The VOCs were significantly higher in corn

stover biochar (15.2%) and switchgrass biochar (12.1%) than Ponderosa pine wood residue biochar

(10.3%). The corn stover and switchgrass biochars had high base cation concentrations which may provide

high positive charge potential to surface and facilitate divalent cation bridging to sorb negatively charged P

ions.

The availability of sorbed P (bicarbonate extractable P) was significantly different among biochars

and increased as the initial P concentration in solution increased from 0 to 3.22 mmol L-1 (Fig. 2). At low P

concentration in solution, the availability of P was low which may be due to formation of bidentate

complexes by anions of P at high energy sorption sites of surface of sorbent [46]. At high solution P

concentrations, the formation of exchangeable monodentate complexes along with bidentate complexes by

P ions on biochar surface may contribute to available pool of P [46]. Extraction of sorbed P was

significantly lower in the alkaline corn stover and switchgrass biochars compared to the Ponderosa pine

wood residue biochar. Available P was larger for Ponderosa pine wood residue biochar (43% of sorbed P)

than for switchgrass biochar (33%) or corn stover biochar (25%), a trend that was consistent with the

Freundlich (Kf) and Langmuir (KL) sorption affinities (Table 3). The sorption percentage was the average of

sorption across all initial solution P concentrations used in this batch study. The higher availability of

sorbed P in Ponderosa pine wood biochar may be due to lower pH, CaCO3 content, and base cation

concentration which can precipitate P on biochar surface. The release of P from sorbed P may also get

some contribution from bicarbonate extractable P of biochars. Biochars can alter the availability of P

directly by influencing the availability of divalent cations (such as Ca and Mg) in solution that interact with

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P. These herbaceous based biochars contain higher concentrations of plant essential nutrients which may

not be easily released. These biochars have a potential to be used as slow release fertilizer and prevent the

mobility of nutrients such as P in soil [47].

Effect of biochars on sorption and availability of P in soils

ANOVA was performed to determine the effect of biochar type on sorption and availability of P in

soils. The incorporation of biochars to acidic and calcareous soils resulted in a significant effect on

partitioning (sorption) coefficients (Kf and KL) and availability of sorbed P (Table 4). The acidic soil

exhibited higher P sorption than calcareous soil (Table 5) which may be due to presence of significantly

higher free Fe, Al, and Mn oxides (Table 2).

Acidic soil amended with each biochar significantly decreased the P sorption (partitioning

coefficient), sorption maxima (b), and increased the equilibrium P concentration (Fig. 3). Freundlich

isotherms fit the P sorption data for the acidic soil and acidic soil/biochar mixtures somewhat better (R2 ≥

0.97) than did the Langmuir isotherm (R2 ≤ 0.94) (Table 5). Partitioning coefficients (Kf and KL) were

higher for acidic soil than calcareous soil and decreased with the application of biochars. The average

increase in equilibrium solution P concentrations was larger with the incorporation of corn stover biochar

(22.4%) followed by switchgrass biochar (16.9%) and Ponderosa pine wood residue (7.4%). The

percentage increase in equilibrium solution P for biochar treatments were calculated by considering the

control (acidic soil) as baseline. The availability of sorbed P (bicarbonate extractable P) was significantly

increased with the application of all biochars in the acidic soil (Fig. 4). In addition the availability of sorbed

P was increased in all treatments as the initial solution P concentration increased. The highest availability

of P was observed with the application of corn stover biochar (55% of sorbed P) followed by switchgrass

biochar (53%), and Ponderosa pine wood residue biochar (48%) (P < 0.05). The acidic soil (control)

released less (46%) sorbed P with the remaining being in the non-available pool of P. Overall the

availability (bicarbonate extractable) of sorbed P was increased with the application of all biochars at low

and high levels of initial solution P concentrations.

Sorption of P and its availability is influenced by soil solution pH, metal oxides, and carbonates

[48]. In this batch study, the acidic soil showed higher sorption and lower availability of sorbed P than

calcareous soil due to its low pH (4.52) and larger concentrations of Fe, Al, and Mn oxides (Table 2) which

can fix and reduce P availability in soil solution [49]. Biochar application decreased P sorption and

increased available P in the acidic soil which might be attributed to a decrease in PZNC and increase in the

negative surface charge potential. The soil pH also increased during incubation with biochars which may be

due to proton consumption reactions, such as ligand exchange between functional groups of biochars with

anions of P on aluminol (Al-OH) and ferrol (Fe-OH) surfaces [50, 51] and decarboxylation during

decomposition of partially burnt organic matter present in biochar [52, 53]. Soil pH was increased by 0.99

and 0.74 units when acidic soil was incubated for 165 days with corn stover biochar and switchgrass

biochar respectively and decreased exchangeable acidity (H+ and Al3+) [54]. Apart from changes to soil pH,

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the increased equilibrium solution P may be due to lower concentrations of free Fe, Al and Mn oxides

(Table 2) which may transform to Fe, Al, and Mn hydroxides and reduce the availability of high energy

sorption sites [55]. There was a possibility for P mineralization from biochars into the soil solution during

incubation. In acidic soil, application of biochars seemed to increase the ionic strength of solution due to

release of soluble nutrients/salts (high EC) which may reduce the positive electric potential of soil surface

through a screening effect and ultimately reduced the P sorption. Moreover, some of the increase in

equilibrium solution concentration and availability of P may be attributed to P released from biochars apart

from changes to surface properties of acidic soil during incubation.

P sorption (Kf and KL) and sorption maxima (b) significantly increased in the calcareous soil with

the application of biochars (Tables 4 and 5), especially for the corn stover and switchgrass. Freundlich

isotherms (Kf) best fit for the P sorption data of calcareous soils (R2 ≥ 0.97) when compared to Langmuir

isotherms (KL). The highest sorption of P and sorption maxima were observed with the application corn

stover biochar followed by switchgrass biochar, Ponderosa pine wood residue biochar, and unamended

calcareous soil. Application of corn stover biochar resulted in significantly higher P sorption (28%)

followed by switchgrass biochar (12%) and Ponderosa pine wood residue biochar (3%) (Fig. 5). The

percentage increase in sorption for biochar treatments were calculated using control (calcareous soil) as

baseline. The availability of sorbed P in calcareous soil was significantly decreased with the application of

biochars (Fig. 6). The lowest availability of sorbed P was observed with the application of corn stover

biochar (47% of sorbed P) followed by switchgrass biochar (49%), and Ponderosa pine wood residue

biochar (56%). For the calcareous Langhei soil alone, available P was 62%. Overall the availability of

sorbed P decreased significantly, especially with the application of corn stover and switchgrass biochars.

But the availability of sorbed P was increased as the initial solution P concentration increased which may

be due to exchangeable complexes formed by P ions with surface of sorbent at high solution P

concentrations. Some of the increase in available P may be due to release of P from biochars.

The increased sorption of P and reduction in availability of sorbed P in calcareous soil with the

application of biochars may be due to high base cation concentration in biochars which can provide larger

quantities of positive charge to soil surface and free divalent base cations such as Ca and Mg that

precipitate P ions as Ca or Mg phosphates [56]. This mechanism was observed by Rupa et al. [57] where

exchangeable Ca and Mg concentration in soil precipitated Ca and Mg phosphates and decreased the

availability of P in soil solution. The increased ionic strength of solution due to application of biochars may

reduce the negative electric potential of soil surface due to screening effect which could increase the P

sorption. This calcareous soil had also higher clay mineral content than acidic soil and increased specific

surface area due to application biochars which could result higher sorption and lower availability of P.

This batch experiment demonstrated that biochars produced from pyrolytic process of biomass

feedstock such as corn stover, switchgrass, and Ponderosa pine wood residues have high affinity for P.

Corn stover biochar had the highest P sorption capacity and sorption maximum followed by switchgrass

biochar and Ponderosa pine wood residue biochar. The availability of sorbed P was lesser in the alkaline

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corn stover and switchgrass biochars. The interaction of these biochars with soils may cause the changes in

their ability to sorb and desorb P. Application of these biochars significantly decreased the P sorption and

increased availability of P in acidic soil, with corn stover and switchgrass biochars showing the strongest

effect. The extent of the decrease in P sorption and increase in availability was higher with the application

of corn stover biochar and switchgrass biochar. The opposite effect was observed in calcareous soil:

biochar amendment, increased P sorption and sorption maxima and decreased the availability of sorbed P.

This effect was larger with the application of corn stover biochar followed by switchgrass biochar and

Ponderosa pine wood residue biochar.

Conclusions

This batch study showed that corn stover, switchgrass, and Ponderosa pine wood residue biochars

have high affinity for phosphorus and transform P from readily available P to less available P pools. This

phenomenon was shown by application of alkaline biochars such as corn stover and switchgrass biochars.

Application of biochars may be an effective agronomic remedial tool to reduce the P transport from

vulnerable catchments of a landscape. Field-research is needed to verify these effects of biochars on the

transformations of P in soil. In this study, the incorporation of biochars to acidic soil at 40 g kg-1 increased

the equilibrium solution P concentration, reduced P sorption, and increased bioavailability of P. In

calcareous soil, biochar amendment significantly increased P sorption, especially with alkaline biochars

(corn stover and switchgrass biochars) and decreased the availability of sorbed P. The incorporation of

biochars to soils with low initial P concentration increased the availability of P for plants. To enhance the

bioavailability of P, alkaline biochars can be used on acidic soil and the Ponderosa pine wood residue

biochar should be added to soils with neutral to high pH. Ponderosa pine wood residue biochar had the

smallest effect on P availability but it could still improve soil quality parameters. The effect of these

biochars on soil P was aligned with their characterization, suggesting that similar biochar evaluations might

predict their effect on P availability and inform recommendations on biochar application to soils.

ACKNOWLEDGEMENTS

This project was supported by Agriculture and Food Research Initiative Competitive Grant

no.2011-67009-30076 from the USDA National Institute of Food and Agriculture.

The authors have declared no conflict of interest.

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Fig 1. Phosphorus sorption by corn stover, Ponderosa pine wood residue, and switchgrass biochars. Each

data point is the mean of four replications with standard error.

Fig 2. Availability (0.5 M NaHCO3 extractable) of sorbed P onto corn stover, Ponderosa pine wood residue

and switchgrass biochars. Each bar represents the mean of four replications, with standard error bars.

Fig 3. P sorption by acidic soil and acidic soil (Grummit series) incubated with corn stover, Ponderosa pine

wood residue or switchgrass biochars at 4 g kg-1 soil for 30 days. Each data point is the mean of four

replications with standard error.

Fig 4. Availability of sorbed P (0.5 M NaHCO3 extractable) after 30 day incubation of acidic soil (Grummit

series) with corn stover, Ponderosa pine wood residue, switchgrass biochars at 40 g kg-1 soil. Each bar

represents the mean of four replications, with standard error bars.

Fig 5. P sorption calcareous soil (Langhei series) and calcareous soil incubated with corn stover, Ponderosa

pine wood residue, and switchgrass biochars at 40 g kg-1 soil for 30 days. Each data point is the mean of

four replications with standard error.

Fig 6. Availability of sorbed P (0.5 M NaHCO3 extractable) after 30 day incubation of calcareous soil

(Langhei series) with corn stover, Ponderosa pine wood residue or switchgrass biochars at 40 g kg-1 soil.

Each bar represents the mean of four replications, with standard error bars.

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Table 1. Physical and Chemical properties of biochars produced from fast pyrolysis of corn stover,

switchgrass, and Ponderosa pine wood residue at 650 °C with standard errors.

Properties Corn stover biochar Ponderosa pine wood

residue biochar

Switchgrass biochar

Specific surface area (m2 g-1) 43.4 ± 2.5 52.1 ± 4.1 48.0 ± 2.6

pH 11.4 ± 0.10 5.82 ± 0.05 10.4 ± 0.12

EC(μS cm-1) 3000 ± 60.6 200 ± 22.1 890 ± 20.5

CaCO3 content (g kg-1) 2.51 ± 0.05 0.30 ± 0.02 2.00 ± 0.05

CEC (Cmolc kg-1) 60.1 ± 4.6 34.2 ± 2.3 50.6 ± 2.1

PZNC 2.35 ± 0.64 1.92 ± 0.05 2.03 ± 0.88

VOC (%) 15.2 ± 1.1 10.33 ± 0.83 12.16 ± 1.82

Ca (g kg-1) 7.51 ± 0.31 2.57 ± 0.03 7.12 ± 0.22

Mg (g kg-1) 5.34 ± 0.72 0.62 ± 0.31 5.25 ± 0.11

K (g kg-1) 21.4 ± 1.3 1.96 ± 0.04 2.70 ± 0.10

Na (g kg-1) 0.69 ± 0.01 0.62 ± 0.03 0.67 ± 0.01

Bicarbonate extractable P (mg kg-1) 10.2 ± 0.96 4.15 ± 0.11 6.41 ± 0.55

Total P (g kg-1) 2.0 ± 0.14 0.36 ± 0.03 1.89 ± 0.05

Total N (g kg-1) 12.3 ± 1.0 3.53 ± 0.63 16.4 ± 1.7

Total C (g kg-1) 740 ± 23.4 833 ± 29.7 780 ± 19.5

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Table 2. Selected physical and chemical properties of acidic soil, calcareous soil, and soils mixed with

biochars at 40 g kg-1 with standard errors.

Property Acidic soil Calcareous soil

Control +CSB +PWRB +SGB Control +CSB +PWRB +SGB

Texture Clay loam Clay loam

Clay (g kg-1) 290 ± 17 350 ± 23

Specific surface area

(m2g-1)

141 ± 16 216 ± 14 277 ± 22 230 ± 12 165 ± 12 252 ± 20 298.0 ± 19 265 ± 20

pH 4.52 ± 0.19 5.40 ± 0.14 4.78 ± 0.11 5.11 ± 0.03 7.90 ± 0.25 8.36 ± 0.34 7.62 ± 0.21 8.20 ± 0.44

CEC

(Cmolc kg-1)

14.6 ± 1.4 19.3 ± 1.2 16.4 ± 2.1 18.9 ± 2.0 37.8 ± 3.1 46.14 ± 5.8 40.42 ± 2.3 43.35 ± 4.1

PZNC 6.93 ± 0.33 5.51 ± 0.17 5.79 ± 0.42 5.70 ± 0.22 7.41 ± 0.81 7.26 ± 0.93 7.40 ± 0.44 7.31 ± 0.51

CaCO3content

(g kg-1)

0.26 ± 0.02 0.06 ± 0.01 0.19 ± 0.01 4.05 ± 0.44 4.41 ± 0.52 4.03 ± 0.46 4.33 ± 0.62

Exchangeable acidity

(Cmolc kg-1)

3.50 ± 1.0 1.51 ± 0.66 3.26 ± 0.82 1.83 ± 0.17 nd nd nd nd

Total Nitrogen

(g kg-1)

0.02 ± 0.01 0.47 ± 0.11 0.15 ± 0.08 0.64 ± 0.41 0.02 ± 0.01 0.42 ± 0.04 0.14 ± 0.01 0.61 ± 0.23

Total carbon (g kg-1) 0.24 ± 0.06 29.6 ± 2.0 33.3 ± 4.1 31.2 ± 2.6 0.17 ± 0.12 30.5 ± 3.0 36.4 ± 2.9 33.3 ± 1.8

Bicarbonate extractable

P (mg kg-1)

12.1 ± 2.7 28.4 ± 4.1 17.1 ± 2.5 22.2 ± 3.2 8.4 ± 0.73 22.1 ± 2.9 24.5 ± 3.0 20.2 ± 3.3

Total phosphorus

(mg kg-1)

18.1 ± 1.5 38.4 ± 3.3 21.1 ± 3.1 32.2 ± 2.7 12.4 ± 2.0 35.1 ± 4.1 24.5 ± 2.4 30.2 ± 2.2

Fed (g kg-1) 10.3 ± 1.2 4.66 ± 0.35 7.22 ± 0.77 5.13 ± 0.42 6.15 ± 2.4 6.01 ± 0.81 6.00 ± 1.0 6.10 ± 0.24

Ald (g kg-1) 0.94 ± 0.32 0.16 ± 0.05 0.55 ± 0.02 0.21 ± 0.02 0.40 ± 0.03 0.31 ± 0.03 0.40 ± 0.03 0.38 ± 0.03

Mnd (g kg-1) 0.62 ± 0.26 0.22 ± 0.01 0.29 ± 0.06 0.22 ± 0.01 0.31 ± 0.44 0.29 ± 0.01 0.31 ± 0.03 0.31 ± 0.01

Feox (g kg-1) 5.92 ± 1.1 2.66 ± 0.56 3.12 ± 0.44 2.83 ± 0.17 2.66 ± 0.61 2.42 ± 0.26 2.68 ± 0.11 2.68 ± 0.34

Alox (g kg-1) 2.36 ± 0.14 0.83 ± 0.03 1.03 ± 0.52 0.84 ± 0.22 1.02 ± 0.13 0.88 ± 0.01 0.91 ± 0.02 0.88 ± 0.08

Mnox (g kg-1) 1.41 ± 0.62 0.03 ± 0.02 0.09 ± 0.01 0.03 ± 0.01 0.45 ± 0.11 0.04 ± 0.01 0.04 ± 0.01 0.03 ± 0.01

*CSB - corn stover biochar, PWRB – Ponderosa pine wood residue biochar, SGB – switchgrass biochar

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Table 3. Freundlich and Langmuir P sorption parameters with standard errors for corn stover, Ponderosa

pine wood residue, and switchgrass biochars. Different letters in the same column represent significant

differences at α = 0.05.

Freundlich Langmuir

Biochar Kf 1/n R2 KL b R2

Corn stover biochar 1400 ± 71a 0.28 ± 0.05b 0.97 0.86 ± 0.01a 111 ± 6a 0.99

Ponderosa pine wood residue

biochar

55 ± 9c 0.66 ± 0.05a 0.97 0.014 ± 0.04c 58 ± 10b 0.89

Switchgrass biochar 1161 ± 57b 0.31 ± 0.06b 0.97 0.47 ± 0.01b 109 ± 3a 0.98

Kf and KL are expressed in mmol1-1/n kg-1 L1/n and b is in mmol L-1

Table 4. Analysis of variance for the partitioning coefficients and available (bicarbonate extractable) P

Source of variation Degrees of freedom Freundlich (Kf) Langmuir (KL) Available P

P > F

Soil 1 <0.001 <0.001 0.013

Biochar 2 <0.001 0.006 <0.001

Soil/biochar 3 <0.001 <0.001 0.005

Available P – 0.5 M NaHCO3 extractable P

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Table 5. Freundlich and Langmuir P sorption parameters with standard errors for the acidic and calcareous

soils and soil biochar mixtures at 40 g kg-1.

Freundlich Langmuir

Treatment Kf 1/n R2 KL b R2

Acidic soil 287 ± 5.3 0.47 ± 0.06 0.97 0.31 ± 0.02 35 ± 8.1 0.94

Acidic soil + corn stover biochar 195 ± 13 0.54 ± 0.04 0.98 0.23 ± 0.05 32 ± 4.6 0.91

Acidic soil + Ponderosa pine wood

residue biochar

246 ± 6.1 0.49 ± 0.05 0.97 0.29 ± 0.05 33 ± 2.2 0.93

Acidic soil + switchgrass biochar 218 ± 10 0.52 ± 0.03 0.98 0.23 ± 0.01 34 ± 3.6 0.94

Calcareous soil 54 ± 9.1 0.43 ± 0.04 0.98 0.05 ± 0.01 12 ± 2.34 0.87

Calcareous soil + corn stover

biochar

119 ± 5.8 0.48 ± 0.04 0.98 0.11 ± 0.01 24 ± 5.9 0.99

Calcareous soil + Ponderosa pine

wood residue biochar

48 ± 5.4 0.49 ± 0.04 0.98 0.06 ± 0.01 13 ± 2.16 0.84

Calcareous soil + switchgrass

biochar

63 ±5.6 0.53 ± 0.01 0.99 0.06 ± 0.01 20 ± 4.6 0.94

Kf and KL are expressed in mmol1-1/n kg-1 L1/n and b is in mmol L-1

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.