Properties of biochar derived from wood and high-nutrient biomasses with the aim of agronomic and environmental benefits
Properties of biochar derived from wood and high-nutrient biomasses with the aim of agronomic and environmental benefits
Rimena R. Domingues 0 1
Paulo F. Trugilho 1
Carlos A. Silva 0 1
Isabel Cristina N. A. de Melo 1
LeoÃ nidas C. A. Melo 0 1
Zuy M. Magriotis 1
Miguel A. SaÂ nchez-Monedero 1
0 Department of Soil Science, Universidade Federal de Lavras, UFLA, Lavras, Minas Gerais, Brazil, 2 Forest Sciences Department, Universidade Federal de Lavras, UFLA, Lavras, Minas Gerais, Brazil, 3 Chemistry Department, Universidade Federal de Lavras, UFLA, Lavras, Minas Gerais, Brazil, 4 Centro de EdafologÂõa y BiologÂõa Aplicada del Segura (CEBAS-CSIC), Departmento de ConservacioÂn de Suelos y Agua y Manejo de Residuos OrgaÂnicos, Campus Universitario de Espinardo , Murcia , Spain
1 Editor: Jorge Paz-Ferreiro, RMIT University , AUSTRALIA
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: XDR analyses were performed at XRD1
beam-line of the Brazilian Synchrotron Light
Laboratory (LNLS), which is supported by the
Brazilian Ministry of Science, Technology,
Innovations and Communications (MCTIC). This
study was funded by the National Council for
Technological and Scientific Development - CNPq,
Biochar production and use are part of the modern agenda to recycle wastes, and to retain
nutrients, pollutants, and heavy metals in the soil and to offset some greenhouse gas
emissions. Biochars from wood (eucalyptus sawdust, pine bark), sugarcane bagasse, and
substances rich in nutrients (coffee husk, chicken manure) produced at 350, 450 and 750ÊC
were characterized to identify agronomic and environmental benefits, which may enhance
soil quality. Biochars derived from wood and sugarcane have greater potential for improving
C storage in tropical soils due to a higher aromatic character, high C concentration, low H/C
ratio, and FTIR spectra features as compared to nutrient-rich biochars. The high ash content
associated with alkaline chemical species such as KHCO3 and CaCO3, verified by XRD
analysis, made chicken manure and coffee husk biochars potential liming agents for
remediating acidic soils. High Ca and K contents in chicken manure and coffee husk biomass can
significantly replace conventional sources of K (mostly imported in Brazil) and Ca,
suggesting a high agronomic value for these biochars. High-ash biochars, such as chicken manure
and coffee husk, produced at low-temperatures (350 and 450ÊC) exhibited high CEC values,
which can be considered as a potential applicable material to increase nutrient retention in
soil. Therefore, the agronomic value of the biochars in this study is predominantly regulated
by the nutrient richness of the biomass, but an increase in pyrolysis temperature to 750ÊC
can strongly decrease the adsorptive capacities of chicken manure and coffee husk
biochars. A diagram of the agronomic potential and environmental benefits is presented, along
with some guidelines to relate biochar properties with potential agronomic and
environmental uses. Based on biochar properties, research needs are identified and directions for future
trials are delineated.
grants 3038592/2011-5 and 303899/2015-8 and
Coordination for the Improvement of Higher Level
Education Personnel (CAPES-PROEX AUXPE 590/
2014). A PhD scholarship for RRD was provided
by CAPES and research scholarships for PFT and
CAS were provided by CNPq. The funders had no
role in study design, data collection and analysis,
decision to publish, or preparation of the
Large amounts of crop residues are generated worldwide and they are not always properly
disposed of or recycled. Wood log production in Brazil generates about 50.8 million m3 of
lignocellulosic residue yearly [
], while nearly 200 million tons/year of sugarcane bagasse is
]. In 2016, 49 million bags of coffee [
] were harvested and almost the same
amount (by weight) of coffee husk was produced. Based on the Brazilian chicken flock and on
the average amount of manure produced per animal, about 12 million t year-1 of manure were
generated in Brazil in 2009 [
]. Chicken manure is characterized by high N, P, Ca, and
micronutrient contents, while coffee husk contains the highest K concentration [
bagasse and wood-derived wastes have low amounts of nutrients and high lignin and cellulose
In humid tropical areas, the application of raw residues on soils is the main management
practice, but this has limited impact on increasing C in soils due to high organic matter
decomposition rates [
]. In natura disposal of coffee husk in crop fields may lead to an
increased population of Stomoxys calcitrans, a pest that may cause damages to dairy cattle
and feedlots [
]. Conversion of wastes into biochar increases the recalcitrance of C due to
increased proportions of condensed aromatic compounds in the biochare, which ensures
higher persistence of C in the soil compared to the C from raw biomass [
]. In addition,
conversion of wastes into biochar reduces residue volume, generates energy, improves the
efficiency of nutrient use by crops, eliminates pathogens, and generates products with high
agronomic value [8±10].
Characterization of biochars generated from the main Brazilian organic wastes is the first
step in identifying agronomic and environmental applications and guiding future research
trials. Plant-derived biochars have high aromatic C content due to the greater amount of lignin
and cellulose present, which gives the biochar high stability and resistance to microbial
]. Animal manures have high contents of labile organic and inorganic compounds,
resulting in biochars with high ash content, which is positively related to the nutrient and
chemical composition of the biomass [
]. Higher ash, N, S, Na, and P concentration have
been observed in poultry litter biochar than in peanut hull and pecan shell biochars . High
nutrient concentrations in the biomass can generate biochars with more ash content and
alkalizing capacity [
]. Thus, biochar can be used in soils to correct acidity [
], increase soil
cation exchange capacity (CEC), retain water [15±16, 12], and regulate C and N dynamics [
In addition, researchers have pointed out positive effects of biochar on soil remediation due to
its adsorption of pesticides or metals [18±20].
We characterized biochars derived from wood, sugarcane bagasse, and nutrient-rich
residues (coffee husk, chicken manure) aiming to identify potential agronomic and environmental
benefits for fertilizing soil and enhancing soil quality. Our hypothesis is that nutrient-rich
biochars derived from waste have fertilization potential, while biochars derived from wood and
sugarcane charred at high temperature are potential for increasing C sequestered in soils. We
also hypothesized that the liming value of the biochar is primarily regulated by its ash content,
regardless of its pH; the mineral phase of chicken manure is effective in protecting the organic
compounds from degradation, ensuring production of high CEC biochars even under high
temperature (750ÊC). In this study, we aimed to (i) assess the chemical and physicochemical
properties of biochars derived from wood and nutrient-rich sources in terms their potential
agronomic and environmental benefits, and (ii) identify potential uses and drawbacks in
biochar production from contrasting biomass types and suggest guidelines for future research
trials in biochar-treated soils.
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Materials and methods
Fifteen biochars were produced from five biomass and three pyrolysis temperatures (350, 450,
and 750ÊC). The biomasses selected were those with greatest availability in Brazil: i) chicken
manure (CM); ii) eucalyptus sawdust (ES); iii) coffee husk (CH); iv) sugarcane bagasse (SB);
and v) pine bark (PB). The nutrient concentrations of the biomasses are shown in S1 Table.
The biochars were produced by a slow pyrolysis procedure in an adapted muffle furnace
with a sealed chamber to prevent airflow. Prior to pyrolysis, biomass wasoven dried at 105ÊC.
The amount of material used in each procedure varied according to the density of each
material. A heating rate of 1.67ÊC min-1 was adopted, and the final temperature reached were 350,
450, and 750ÊC. The target temperature was maintained for 30 minutes and the biochar sample
was cooled to room temperature. The yield of the biochar mass was calculated as follows:
% 100 x
105 C dried biomass
Yield and ash content. The volatile material, ash, and fixed carbon concentrations were
determined according to standard procedure D-1762-84, established by the American Society
for Testing and Materials [
]. The biochar samples (< 0.25 mm) were oven dried at 105ÊC
and then heated in a covered crucible inside a muffle furnace at 950ÊC for 6 minutes. The
resulting loss of mass refers to volatile material (VM). The biochar was then returned to the
oven and heated in an open crucible at 750ÊC for 6 hours. The mass of material remaining
after incineration refers to ash. Finally, the fixed carbon (FC) concentration was determined
by the following equation:
Thermogravimetric analysis (TGA) was performed using a Shimadzu DTG-60H device.
Samples of approximately 5 mg were heated from room temperature to 600ÊC at a rate of 10ÊC
min-1 and a nitrogen flow of 50 mL min-1. Then, the first derivative of the TGA curve was
calculated, which establishes loss in mass over the temperature range employed.
Biomass and biochar elemental composition. The elemental composition (C, H, N, S) of
the biochars was determined on 0.5 g of ground and sieved (200 mesh) material by dry
combustion using TOC and CHNS analyzers (Vario TOC cube, Elementar, Germany). Biochar
oxygen concentrations were obtained by difference as follows:
C H N S Ash
The biochar elemental composition was used to calculate the H/C, O/C, and (O + N)/C
Water-soluble organic carbon (WSOC) and water-soluble inorganic carbon (WSIC) was
measured in a 10% (w v-1) biochar-water mixture shaken for 1 h and then filtered through a
0.45 μm membrane filter. In the liquid extracts, WSOC and WSIC were quantified using the
liquid mode of a TOC analyzer (Vario TOC cube, Elementar, Germany). Considering that a
single 1 h extraction is unlikely to solubilize all water-soluble organic and inorganic C from
biochar, it should be take into account that WSOC and WIOC provide an index of part of
water soluble C chemical species rather than 100% of all biochar soluble C; however, they were
considered suitable for comparisons among biochars.
ATR-FTIR analysis. Fourier transform infrared spectroscopy (FTIR) analysis was
performed on a Perkin Elmer Spectrum 1000 device equipped with an attenuated total reflectance
(ATR) accessory, in which the powder of each sample was inserted in a diamond crystal gate.
All biomass and biochars had been dried at 65ÊC and sieved through a 0.150 mm mesh. FTIR
spectra from 32 scans was recorded in the wavenumber range 4000±500 cm-1 with 2 cm-1
resolution. The broad band chemical group assignments described in Jindo et al. [
] were used to
interpret the FTIR-ATR spectra.
X-ray diffraction. The X-ray diffraction (XRD) analysis was carried out at the XRD1
beam-line of the Brazilian Synchrotron Light Laboratory (LNLS), Campinas, SP, Brazil, for
detection of all mineral phases present in the biochars. Powdered biochar samples (< 150
mesh) were inserted in glass capillaries and analyzed in the X-Ray diffractometer through the
range of 4±60Ê 2ɵ in a transmission mode with steps of 0.2Ê 2ɵ and a wavelength of about 1.0
Å. Minerals found in the biochar structure were identified after calculation of the d spacing
according to Bragg's law. The peak areas identified for different minerals were compared with
XRD patterns of standard minerals compiled by the Mineralogy Database available at ªweb
Chemical and physicochemical attributes. Biochar pH was measured in deionized water
and in a 0.01 mol L-1 CaCl2 solution at a 1:10 (w/v) ratio, after shaking the samples for 1h. All
measurements were performed in triplicate. Biochar CEC was determined by the modified
ammonium acetate compulsory displacement method, adapted to biochars [
]. During CEC
determination, a vacuum filtration system was employed, and samples were filtered through a
0.45 um membrane filter. Initially, 0.5 g of biochar sample was leached five times with 20 mL
of deionized water to remove excess salts. After that, the samples were washed three times with
a 1 mol L-1 sodium acetate (pH 8.2) solution, followed by five washes with 20 mL of ethanol to
remove free (non-sorbed) Na+ ions. Samples were then washed four times with 20 mL of 1
mol L-1 ammonium acetate to displace the Na+ from the exchangeable sites of the biochar. The
leachates were collected and stored in a 100 mL volumetric flask, and Na contents in the
leachates were determined by flame photometry. The CEC corresponds to the amount of Na
adsorbed per unit mass of biochar, expressed as cmolc kg-1.
The biochar liming value (LV) was evaluated by the acid-base titration method [
quantity of 0.5 g of biochar (< 0.25-mm) was placed in a 50 mL plastic bottle, and then 20 mL of
distilled water was added. The bottles were stirred for 2 h and then titrated with 0.1 mol L-1 of HCl
solution to a pH 2.0 end point. To ensure that the biochar pH was stabilized at 2.0, after 12 h of
equilibration, the pH was again measured and, if necessary, corrected with the HCl solution
already mentioned. Based on the assumption that alkalinity is the capacity of biochar to accept
protons from a 0.05 M HCl solution (1.3 pH 2) after 72 h of equilibration [
], LV is a
partial measurement of biochar total alkalinity. The volume of acid used and its pH value were
recorded. These results were used to calculate the LV, here defined as the volume of 0.1 mol L-1
HCl necessary to reduce the biochar pH by one unit, according to the following equation:
volume of HCl=pH unit
total volume of HCl to reach the titration end point=pH interval:
Experimental design and statistical analysis
Biochars are hereby referred by the biomass abbreviation and pyrolysis temperature, for
example, CH350 denotes coffee husk pyrolysed at 350ÊC and CH750, coffee husk pyrolysed at
750ÊC. The experimental design used was factorial completely randomized with five biomasses
(CM, ES, CH, SB, PB) combined with three pyrolysis temperature (350, 450, 750ÊC).
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The data were subjected to analysis of variance (ANOVA) for significant differences
between factors as biomasses, pyrolysis temperatures, and their interaction. When significant
F-tests were obtained (0.05 probability level), the factors separation was achieved using Tukey's
honestly significant difference test. Data were statistically analysed employing SISVAR [
Results and discussion
Yield, volatile matter, and ash content
Biochar yields were reduced and ash contents increased with an increase in pyrolysis
temperature (Table 1). The CM biochar at three temperatures (350, 450 and 750ÊC) showed higher
yield and higher ash content than the other biochars (Table 1), due to large amount of
inorganic compounds (K, P, Ca, and Mg) in this biomass (S1 Table), which accumulated after
volatilization of C, O, and H compounds. Coffee husk biochar also showed a high ash content,
which is probably due to the high K (22 g kg-1) content of the biomass. The ES and SB
biochars, regardless of the pyrolysis temperature, showed the lowest ash content (<1.1% and
<2.2%, respectively) (Table 1), explained by their low nutrient content (S1 Table). According
to derivative thermogravimetric (DTG) curves of biomass losses (S1 Fig), ES and SB showed
higher mass loss between 250 and 350ÊC, which is attributed to high cellulose content in the
], which is easily degraded during low-temperature pyrolysis. CM, CH and, PB
biochars showed lower mass loss between 250 and 350ÊC indicating higher thermal stability
Biochar volatile matter values reduced as the pyrolysis temperature was raised from 450ÊC
to 750ÊC (Table 1). This is explained by an the increase in aromatization and greater losses of
gas products, tar oil and low molecular weight hydrocarbons as a result of increasing pyrolysis
]. CM750 and CH750, however, showed the smallest losses of volatiles
(Table 1) in contrast to the other biochars prepared at this same temperature. This was
coincident with higher quantities of ash found in these biomasses, which can protect the organic
fraction and structures of biochars during pyrolysis [29±31]. Chemical activation of KOH
impregnation has a catalytic effect in intensifying hydrolysis reactions, increasing volatile
] and the development of pores in the charcoal structure , suggesting a
role for pores in the adsorption of volatile materials [
]. Fixed C was inversely correlated with
the ash contents and was higher in eucalyptus sawdust and sugarcane bagasse biochar
compared to other biochars produced (Table 1).
Elemental composition and soluble C fractions
Total C concentrations in plant-derived biochars increased with an increase in pyrolysis
temperature (Table 2), whereas the O and H concentrations diminished (Table 2). Biochars
derived from plant biomass showed the highest C concentration, up to 90% C for ES and SB
pyrolyzed at 750ÊC (Table 2). Increase in C concentrations with a rise in pyrolysis temperature
occurs due to a higher degree of polymerization, leading to a more condensed carbon structure
in the biochar [
]. Similar results were reported for biochars produced from pine straw [
peanut shells [
], sugarcane bagasse [
], and wheat straw [
]. The greater the degree of
formation of aromatic structures is, the higher the resistance of the biochar to microbial
]. The C concentration in CM biochar reduced with an increase in pyrolysis
temperature (Table 2). Such results suggest that the organic compounds found in animal waste
are more labile and are rapidly lost as pyrolysis temperature is increased, before the formation
of biochar with recalcitrant compounds. A 6% reduction in C concentration in poultry litter
biochar was reported when pyrolysis temperature was increased from 350ÊC to 700ÊC , as
well as a decrease in sewage sludge biochar C content [
]. The C concentration in CM biochar
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Uppercase letters compare pyrolysis temperatures within the same biomass and lowercase letters compare biomass at the same temperature. The same
letter do not differ by the Tukey test at p <0.05.
was lower ( 30% C) than wood biochars (Table 2). These results are in agreement with those
of Novak et al. [
The H/C and O/C ratios of biochars derived from plant biomass decreased as the pyrolysis
temperature was increased (Table 2), indicating increasing aromaticity and a lower hydrophilic
Uppercase letters compare pyrolysis temperatures within the same biomass and lowercase letters compare biomass at the same temperature. The same
letter do not differ by the Tukey test at p <0.05.
PLOS ONE | https://doi.org/10.1371/journal.pone.0176884
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Fig 1. Water-soluble organic carbonÐWSOC (A) and water-soluble inorganic carbonÐWSIC (B) of biomasses and biochars at
different pyrolysis temperatures. CM = chicken manure, ES = eucalyptus sawdust, CH = coffee husk, SB = sugarcane bagasse, and
PB = pine bark. Uppercase letters compare pyrolysis temperatures within the same biomass and lowercase letters compare biomass at the
same temperature. Bar followed by the same letter do not differ by the Tukey test at p <0.05.
tendency, respectively [
]. An increase in the aromatic character of biochars is associated
with dehydration reactions and removal of O and H functional groups, as well as the formation
of aromatic structures, as charring is intensified . These features are consistent with the
van Krevelen diagrams generated in this study, which showed a positive relationship between
H/C and the O/C atomic ratios (S2 Fig). Biochars derived from CM did not change H/C and
O/C ratios or the degree of aromaticity as the pyrolysis temperature increased from 350 to
450ÊC (Table 2).
The sugarcane bagasse biomass had the highest WSOC concentration (94.5g kg-1) (Fig 1A).
However, with increasing pyrolysis temperature, WSOC concentration in bagasse were
significantly reduced (< 0.2g kg-1), suggesting that the water-soluble carbon is degraded or
incorporated into the organic compounds of biochar even at a relatively low pyrolysis temperature.
The biochar WSIC concentration increased with pyrolysis temperature (Fig 1A). The
highest WSIC concentration (11.7g kg-1) was verified for CH750. WSIC-coffee biochar was
significantly (p<0.05) different from the other biochars produced at other pyrolysis temperatures.
The WSIC concentrations of CM and SB biochars were also influenced by the pyrolysis
temperature, especially those samples pyrolyzed at 750ÊC, whose WSIC concentration were 2.1 g
kg-1 and 0.8 g kg-1, respectively (Fig 1B). For the other biochar samples, the WSIC
concentration was not significantly (p<0.05) different (Fig 1B). The higher WSIC concentration found
in CH750 in comparison with similar low-temperature biochar is probably due to the presence
of the mineral kalicinite (Fig 2), a K inorganic compound with high solubility in water [
X-ray diffractometry. Mineral components in the crystal form were identified in the CM,
CH and PB biochars (Fig 2). No crystal substances were observed in the X-ray diffraction
spectra for ES and SB biochars. For CM biochars produced at all temperatures, the presence of
calcite (CaCO3) was identified by peaks at 3.85, 3.03, 2.49, 2.28, 2.09, 1.91, and 1.87 Å (Fig 2A).
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Fig 2. X-ray diffraction spectra of biochars pyrolized at different temperatures (350, 450 and 750ÊC). (A) Chicken
manure biochar. (B) Coffee husk biochar. (C) Pine bark biochar.
The presence of calcite in CM biochars is consistent with the high Ca content found in the
chicken manure biomass (S1 Table). The presence of calcite in this biochar sample is probably
due to the addition of phosphogypsum in manure, normally used to stabilize N forms during
], as well as the use of calcium carbonate in chicken diets. Similarly, calcite and
dolomite [CaMg(CO3)2] were identified in sewage sludge biochar at 300±800ÊC [
For all CH biochars, the presence of kalicinite (KHCO3) was observed (Fig 2B). The
formation of KHCO3 may have been favored by the reaction of K with CO2 released during thermal
decomposition of hemicellulose and cellulose [
]. An increase in the amounts of KHCO3
may also explain the high WSIC contents found in CH biochars (Fig 1). The peak intensity at
3.67 AÊ increased with increasing pyrolysis temperature, indicating relative accumulation
of kalicinite in CH biochars. The peaks at 3.15, 2.22, 1.82, and 1.41 AÊÂ were found in CH350
and CH450 were attributed to the presence of sylvite (KCl) (Fig 2B). In durian shell biochar,
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kalicinite was also the dominant mineral [
]. The presence of quartz (SiO2) was also
firmed in CH450 and CH750 from peaks at 3.34 and 4.25 AÊ in the X-ray spectra. Identification
of SiO2 was also noted in the biochars produced from PB biochar at the three pyrolysis
temperatures (Fig 2B). Yuan et al. [
] also identified the presence of sylvite and calcite in biochars
from canola straw pyrolyzed at 300, 500, and 700ÊC.
FTIR analysis. The FTIR-ATR biomass and biochar spectra are shown in Fig 3. The
spectra of the all biomass samples showed a broad band at 3200±3400 cm-1, which is attributed to
-OH from H2O or phenolic groups [
22, 40, 11
]. For all biomass sources, absorption in the
region between 2920 and 2885 cm-1 (C-H stretching) was assigned to aliphatic functional
8, 40, 11
], and the strong band at 1030 cm- 1 is due to the C-O stretching and
associated with oxygenated functional groups of cellulose, hemicellulose, and methoxyl groups of
8, 35, 41
] [3±5]. The intense bands at 1270 cm−1 were assigned to phenolicÐOH groups
Changes in biochar organic structure were apparent when biomass was pyrolyzed at 350ÊC,
except for the CM biochars (Fig 3). The intensities of bands of -OH (3200±3400 cm-1),
aliphatic C-H stretching (2920 and 2885 cm-1), -OH phenolic (1270 cm-1), and C-O stretching
region (1030 cm-1) decreased sharply due to degradation and dehydration of cellulosic and
ligneous components, even at low temperatures (350ÊC) [
]. An increase in band intensity
in the 1600 cm-1 region (C = C, C = O of conjugated ketones and quinones) and the
appearance of weak bands between 885 and 750 cm-1 (aromatic CH out-of-plane) were attributed to
an increasing degree of condensation of the biochar organic compounds. An increase in the
degree of biochar condensation as pyrolysis temperature increases is in agreement with the
results reported by Keiluweit et al. , Jindo et al. [
], and Melo et al. [
]. In the FTIR
spectra of ES750, SB750, and PB750 biochars most of the organic functional groups present in the
biochar structure were lost (Fig 3B, 3D and 3E). For CH biochars, weak bands remaining at
the highest pyrolysis temperature were identified, which were assigned to aromatic C = C
stretching (at about 1600 cm-1), -C-H2 bending (1400 cm-1), and aromatic C-H bending (885
cm-1). Losses of chemical groups in CH750 could explain the sharp decrease in CEC of this
biochar in comparison to CH350 and CH450. In the CM biochars, the intensity of all organic
functional bands remained largely unchanged after the biomasses were subjected to the
charring process, regardless of the pyrolysis temperature used (Fig 3A). Protection of organic
groups, even at high pyrolysis temperature, may be associated with the high ash content found
in coffee husk and chicken manure (Fig 3). Ash acts as a heat resistant component, which may
protect organic compounds against degradation and may hinder the formation of aromatic
structures as charring intensity advances [
The pH in water of the biochars ranged from slightly acidic to alkaline (Fig 4A). Overall, the
pH values of biochars were higher than 6.0 units. Compared to the biomass pH, the charring
process increased pH in water and, in some cases, differences were up to 4 pH units for some
of the biomasses pyrolyzed at 750ÊC (Fig 4A). An increase in biochar pH with pyrolysis
temperature has been reported for corn straw [
], sewage sludge [
], pine [
], poultry litter
], and sugarcane straw [
] biochars. With increasing temperature, there is an enrichment
of basic cations in the ashes, which may be associated with alkaline species, such as carbonates,
oxides and hydroxides [
], and a reduction in the concentration of acidic surface
functional groups . Among biochars, the highest pH values were recorded for the CM biochars,
which exhibited a pH of 9.7 (at 350ÊC), 10.2 (at 450ÊC), and 11.7 (at 750ÊC) (Fig 4A). In
general, all biochars pyrolyzed at 750ÊC showed pH values higher than 8.0.
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Fig 3. FTIR-ATR spectra of biomasses and their respective biochars pyrolyzed at 350, 450, and 750ÊC. (A)
Chicken manure. (B) Eucalyptus sawdust. (C) Coffee husk. (D) Sugarcane bagasse. (E) Pine bark.
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Fig 4. Values of pH-H20 (A), liming value (B), ECÐelectrical conductivity (C), and CECÐcation exchange capacity (D) as related to
biomass and biochars. CM = chicken manure, ES = eucalyptus sawdust, CH = coffee husk, SB = sugarcane bagasse, and PB = pine bark.
Uppercase letters compare pyrolysis temperatures within the same biomass and lowercase letters compare biomass at the same
temperature. Bar followed by the same letter do not differ by the Tukey test at p <0.05.
Biochars of ES, SB, and PB produced at all pyrolysis temperatures used in this study showed
reduced liming values (capacity to neutralize acidity) (Fig 4B), i.e, the ability to correct soil
acidity should not only be evaluated by the pH value. CM and CH biochars, regardless of the
pyrolysis temperature, showed higher liming values compared to the other biochars (4B),
which were related to the high mineral concentration in chicken manure and coffee biochars,
specifically to the calcium and potassium carbonates found in their respective X-ray diffraction
spectra (Fig 2A and 2B). The presence of carbonates has been previously reported as the main
alkaline components of the biochars [
]. Biochars produced from tomato [
] and paper
] showed high liming value, which was attributed to the presence of calcite and other
carbonate minerals in these biochars. Thus, the biochar liming value is mainly regulated by the
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biochar ash content and chemical composition (especially of basic cations) and, to a much
lesser extent, by the biochar pH. This characteristic should be considered when biochar is
added to soils to correct soil acidity.
Electrical conductivity (EC) was mainly influenced by the biomass used in biochar
production (Fig 4C). At all pyrolysis temperatures, the CH biochar showed the highest EC value,
followed by the CM biochar (Fig 4C). These results, among other factors, may be due to the
presence of soluble minerals, i.e., kalicinite and sylvite, in CH biochar (Fig 2B) and calcite in
CM biochar (Fig 2A), and may be related to the high levels of WSIC in both biochars, as well
Biochar cation exchange capacity (CEC) values varied greatly, and are mainly dependent
on the biomasses and the temperature used in the pyrolysis process (Fig 4D). CH350 and
CH450 stood out from the other biochars due to the high CEC values (means of 69.7 cmolc
kg-1 at 350ÊC and 72.0 cmolc kg-1 at 450ÊC) (Fig 4D). CM biochars produced at low
temperatures (350ÊC and 450ÊC) also showed high CEC values (21.3 cmolc kg-1) (Fig 4D). Negative
charge density on biochar surfaces produced at low temperatures is attributed to the exposure
of functional groups, such as carboxylic acids, ketones, and aldehydes released by
depolymerization of cellulose and lignin [
47, 22, 35
]. CH and CM biomasses also exhibited high K
concentration, which can intercalate and cause the separation of carbon lamellae by the oxidation of
cross-linking carbon atoms, resulting in formation of surface groups at the edge of the carbon
]. ES, SB, and PB biochars shown low CEC, with mean values for biochar
pyrolyzed at 350ÊC of 10.8, 4.6, and 2.4 cmolc kg-1, respectively (Fig 4D). An increase in pyrolysis
temperature from 450ÊC to 750ÊC reduced the biochar CEC values, except for PB biochar (Fig
4D). These results were supported by the FTIR spectra shown in Fig 3, in which most of the
organic group assignments and bands responsible for generating negative charges were lost,
indicating the removal of oxygen-containing functional groups at most of the biochar at high
temperature (750ÊC). Song and Guo [
] also verified that as carboxylic and phenolic group
assignments disappear, the biochar CEC is lower; consequently, depending on the biomass
charred, CEC is inversely correlated with pyrolysis temperature. In conclusion, biochar CEC is
mainly regulated by the biomass rather than by pyrolysis temperature; however, the increase
in temperature from 450ÊC to 750ÊC leads to a drastic reduction in the CEC of some biochars.
Biochar properties related to potential environmental benefits
Carbon concentration, atomic ratios, and biochar FTIR fingerprints can be used as predictors
of C persistence in biochars in soils. High C content, low H/C ratio, and FTIR spectrum
features recorded for biochars derived from high temperatures are key indices of the aromatic
character, stability against degradation in soils, and, consequently, high C residence time in
biochar-treated soils [
34, 6, 48
]. Considering these, it is expected greater aromatic character
for ES750, SB750, and PB750 than nutrient-rich biochars (S1 Table). As pointed out by
Bruun et al. [
], the use of these biochars with a possible high residence time may be an
important strategy to increase C sequestration in Brazilian soils, acting to offset greenhouse
In Brazil, agriculture is the main source of greenhouse gas (GHG) emissions. Most of the
N2O emissions originate from rice fields fertilized with N and from manure deposition by
cattle grazing in low and intensively managed animal production systems. Feedstock type,
production temperature and process, soil properties, biochar rate, and biochar N-source
interactions are the dominant factors that contribute to reductions in N2O emissions from
biochar-treated soils [
]. In fact, Cayuela et al. [
] reported that biochar can still effective at
mitigating N2O emissions even at pyrolysis temperatures of 400±600ÊC (in addition to >600ÊC),
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in application rates of 1±5%, and in coarse-textured soils with water filled pore space of <80%.
In addition to the already mentioned factors, the H:Corg ratio is a suitable factor to infer the
capacity of biochar in reducing N2O emissions [
]. According to Cayuela et al. [
with H:Corg ratio <0.3 (i.e., biochar with high degree of polymerization and aromaticity)
decreased N2O emissions by 73% while biochars with H:Corg ratio >0.5 only diminished
N2O emissions by 40%. Considering only the technical aspects, most of the 750ÊC biochars,
and especially the wood biochars produced in this study, are potential inputs for decreasing
N2O emissions in crop fields, but, due to the high application rates required, biochar use to
offset N2O emissions should be focused on more profitable processes (e.g., composting) instead
of use in soil.
For the purpose of reducing CO2 emissions, the use of low labile C biomass pyrolyzed at
>550ÊC is recommended [
]. Based on these assumptions, sugarcane bagasse, pine bark,
and eucalyptus biochars pyrolyzed at 750ÊC are suitable for reducing CO2 emissions.
Nevertheless, it has been suggested that the application of biochar can increase CH4 emissions [52,
53]. However, these studies were carried out in paddy soil, where species of methanogenic
bacteria predominate and, thus, the addition of some biochars to the substrate creates a favorable
environment for methanogenic microbial activity . Therefore, it is very difficult to
anticipate the role that may be played by the biochars characterized in this study in decreasing CH4
fluxes from soil to air, but wood and high-surface area biochars are potential inputs for use in
soil to reduce CH4 emissions.
The labile C fraction in biochars can be easily decomposed and, in some cases, can stimulate
the mineralization of native soil organic matter, through a positive priming effect [
54, 33, 55,
]. In general, these events occurred in soils treated with biochar produced at low
temperature, but this condition may not be generalized. An increase in the biochar mineralization rate
can be explained by the volatile material contained in the biochar, which may also be present
in high concentrations in biochars produced at high temperatures . Under these
assumptions, chicken manure and coffee husk biochars both pyrolyzed at 750ÊC are not expected to
increase C storage in soils due to their possible rapid decomposition in treated soils. The
magnitude of volatile matter content in biochar is an important attribute to evaluate in C
bioavailability and N cycling in biochar in the soil ecosystem. High aliphatic character (high O/C
ratios and more intense FTIR peak) observed at low temperature (350 and 450ÊC) can be
considered an index of biochar susceptibility to degradation by soil microorganisms, causing
short-term immobilization of inorganic N in soil [
]. This N immobilization may hamper
the supply of N to plants in biochar-treated soils [
]. Nevertheless, N immobilization can
be seen as a beneficial mechanism for mitigating N2O emissions and for reducing inorganic-N
leaching from soils [
Biochar properties related to potential agronomic benefits
Differentiation of biochars was established by the parameters evaluated, which allowed the
identification and discussion of agronomic benefits. Characterization by proximate analysis
(Table 1) showed clear differentiation in ash contents among the biochar samples. In many
cases, high ash content ensures biochars rich in nutrients with high alkalizing capacity [
]. The high ash content was associated with alkaline chemical species, such as KHCO3 and
CaCO3, as verified by XRD analysis (Fig 2). Such characteristics make chicken manure and
coffee husk biochars potential materials to increase soil acidity buffering capacity and to
neutralize soil acidity, which may partially replace the large amounts of limestone used to correct
soil acidity in crop fields in Brazil (Fig 5). The solubilization of these alkaline chemical species
can increase soil pH, decrease Al3+ toxicity, reduce Fe and Mn availability, and increase soil
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Fig 5. Simplified schematic representation in which wood, sugarcane, coffee husk, and chicken manure
biochars are typified according to chemical and physiochemical properties and potential for carrying
out trials on weathered soils in regard to their potential agronomic or environmental services.
59, 25, 60
], which may decrease the precipitation and adsorption of P [
], as well as
enhance the supply of Ca and K to plants. The high Ca and K contents in chicken manure and
coffee husk biomass (S1 Table) can significantly replace conventional sources of K (mostly
imported in Brazil) and Ca, which suggests the high agronomic value of these biochars (Fig 5).
However, despite the high total concentration of these chemical elements, the availability of
nutrient forms in biochars should not be neglected, since an increase in pyrolysis temperature
can drastically reduce the labile P forms in biochars according to . Other uses of these
biochars could be for remediation of some cationic trace element found in contaminated soils due
to their alkalinity and high CEC (Fig 5) [
63, 45, 49
LowÐtemperature biochars provided the largest CEC (chicken manure and coffee husk
pyrolyzed at 350±450ÊC), which can make them possible to adsorb N-NH4+ up to 2.3 mg g-1
and to reduce N leaching rates [
]. Although high-surface-area biochars generated at high
temperature (>600ÊC) usually generate low CEC biochars, the aging effect may come into
play, oxidizing the organic biochar, increasing the negative charge density and increasing the
formation of biochar-mineral complexes [
Recommendations and suggestions for future trials
Wood- and sugarcane-derived biochars, regardless of the charring conditions, can potentially
improve C storage in tropical soils (Fig 5). The agronomic value of biochars from wastes poor
in nutrients is questionable since they have low CEC, and low ash contents. Charring intensity
improved the potential capacity of wood and sugarcane biochars to offset GHG emissions due
to their C-fixing and aromatic character. The potential of these aromatic biochars for
increasing C sequestration is probably mediated by soil texture and organic matter contents. It is more
plausible to use low nutrient and high C content biochars to decrease emissions of CO2 rather
than N2O, due to the high biochar rates required to offset N gas emissions from soil. The
potential of biochars from wood and sugarcane bagasse for remediating contaminated soils and/or
14 / 19
increasing water retention capacity should not be overlooked. In this case, supplementary
fertilization, especially with N, should be used to avoid immobilization and maintain soil fertility
]. In Brazil, the cost associated with the use of biochars to sequester C in soils may be offset
by governmental incentives such as that offered by the Brazilian government through the
LowCarbon Agriculture (Agricultura de Baixa Emissão de CarbonoÐABC) Program.
The agronomic value of the biochars generated in this study is predominantly regulated by
the nutrient richness of the biomass. CM and CH biochars have high agronomic value and
they should be tested in crop fields in order to identify their potential for supplying K (CH and
CM) and Ca (CM) to plants and for correcting soil acidity. Several experiments have been
performed trying to enrich biochars with clays and minerals to modify the final characteristics of
the biochars [
]. With the use of chicken manure or other nutrient-rich biomasses like
coffee husk, it may be possible to create biochars to reach similar results in a natural way.
Among the potential uses of biochars discussed in this study, the K content in coffee husk
biochars enables them to act as a slow-release K fertilizer. Considering the average coffee husk
biochar yield of 63% and a mean K2O content of 16% in the final coffee husk biochars, each
ton of the potential organo-mineral K biochar fertilizer produced may be sold for < US$100
per ton, considering the current cost of K2O in Brazil (US$ 0.625/kg). In short, all the aspects
and possible functions of biochars in soil emphasize the fact that the ªone biochar fits all
approachº  is not an option for the main organic wastes available in Brazil and for the
biochars produced in the charring conditions of this study. Following Yargicoglu et al. [
whatever the potential agronomic or environmental use, screening of biochars is highly
recommended, given the range of variability that biomass and the extent of thermal degradation
may cause in the chemical and physicochemical properties of the chars produced.
In this study, the biomass source, rather than pyrolysis temperature, is the primary factor
conditioning the biochar characteristics and the agronomic and environmental value of the
biochar. However, pyrolysis temperature acts as a modify, changing the chemical nature and
increasing the aromatic character of the organic compounds of most of the biochars
investigated. In this study, characterization of the biochars was used to identify the main differences
and similarities between them, offering guidelines for selecting a biomass and charring
conditions to biochar end-users according to their specific soil and environmental requeriments.
Biochars manufactured from ES, PB, and SB, regardless of the pyrolysis temperature
employed, have potential for increasing C storage in soils, as the biochar aromatic character
increases along with pyrolysis temperature. Both CH and CM biochars were also characterized
by their high liming value, which make them potential materials for correcting soil acidity in
crop fields. Both CH and CM biochars have a role as P and K sources for plants. High-ash
biochars, such as CM and CH, produced at low-temperatures (350 and 450ÊC) exhibited high
CEC values, which can be considered as a potential applicable material to retain nutrients.
Inorganic components found in CM biochar can protect its organic compounds from
degradation or hinder the charring process at 750ÊC. A diagram with the potential agronomic and
environmental benefits of biochars is presented, and some guidelines are shown to relate the
properties of biochars with their possible use. Research needs are identified and suggestions
for future trials are also made.
S1 Table. Total nutrient contents in the biomasses investigated. 1The contents of P, K, Ca,
Mg, Cu, Fe, Mn, and Zn were determined in extracts from the nitric-perchloric digestion
15 / 19
procedure. 2Total content of B extracted with hot water.
S1 Fig. dTG curves of biomass. CM = chicken manure, SE = eucalyptus sawdust, CH = coffee
husk, SB = sugarcane bagasse, and PB = pine bark.
S2 Fig. van Krevelen diagram. SE = eucalyptus sawdust, CH = coffee husk, SB = sugarcane
bagasse, and PB = pine bark.
The authors are grateful to ClaudineÂia Olimpia de Assis, PhD, for producing some of the
biochar samples. XDR analyses were performed at XRD1 beam-line of the Brazilian Synchrotron
Light Laboratory (LNLS), which is supported by the Brazilian Ministry of Science, Technology,
Innovations and Communications (MCTIC). This study was funded by the National Council
for Technological and Scientific DevelopmentÐCNPq, grants 3038592/2011-5 and 303899/
2015-8 and Coordination for the Improvement of Higher Level Education Personnel
(CAPESPROEX AUXPE 590/2014). A PhD scholarship for RRD was provided by CAPES and research
scholarships for PFT and CAS were provided by CNPq. The funders had no role in study
design, data collection and analysis, decision to publish, or preparation of the manuscript.
Conceptualization: RRD CAS LCAM MASM.
Data curation: RRD PFT CAS ICNAM LCAM ZMM MASM.
Formal analysis: RRD CAS.
Funding acquisition: RRD PFT CAS ICNAM LCAM ZMM.
Investigation: RRD PFT CAS ICNAM ZMM.
Methodology: RRD PFT CAS ICNAM ZMM.
Project administration: RRD PFT CAS ICNAM LCAM MASM.
Resources: RRD PFT CAS ICNAM LCAM ZMM.
Supervision: RRD CAS LCAM MASM.
Validation: RRD CAS LCAM ZMM MASM.
Visualization: RRD CAS LCAM ZMM MASM.
Writing ± original draft: RRD CAS LCAM ZMM MASM.
Writing ± review & editing: RRD CAS LCAM ZMM MASM.
16 / 19
17 / 19
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