The authors have declared that no competing interests exist.
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 750°C 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 450°C) 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 750°C 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.
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 [
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 [
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 decomposition [
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 (750°C). 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.
Fifteen biochars were produced from five biomass and three pyrolysis temperatures (350, 450, and 750°C). 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
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 105°C. The amount of material used in each procedure varied according to the density of each material. A heating rate of 1.67°C min-1 was adopted, and the final temperature reached were 350, 450, and 750°C. 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:
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 [
Thermogravimetric analysis (TGA) was performed using a Shimadzu DTG-60H device. Samples of approximately 5 mg were heated from room temperature to 600°C at a rate of 10°C 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.
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:
The biochar elemental composition was used to calculate the H/C, O/C, and (O + N)/C ratios [
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.
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 65°C 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. [
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 4–60° 2ɵ in a transmission mode with steps of 0.2° 2ɵ and a wavelength of about 1.0 Å. Minerals found in the biochar structure were identified after calculation of the
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 [
The biochar liming value (LV) was evaluated by the acid-base titration method [
Biochars are hereby referred by the biomass abbreviation and pyrolysis temperature, for example, CH350 denotes coffee husk pyrolysed at 350°C and CH750, coffee husk pyrolysed at 750°C. The experimental design used was factorial completely randomized with five biomasses (CM, ES, CH, SB, PB) combined with three pyrolysis temperature (350, 450, 750°C).
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 [
Biochar yields were reduced and ash contents increased with an increase in pyrolysis temperature (
Biomass | Temp. (°C) | Yield (%) | Proximate analysis (wt. %) | ||
---|---|---|---|---|---|
Volatile Matter | Ash | Carbon Fixed | |||
Chicken manure | 350 | 69.7 | 36.9 Ab | 52.0 Ba | 11.1 Cd |
450 | 63.0 | 30.6 Ba | 55.3 Aa | 14.1 Be | |
750 | 55.9 | 26.5 Ca | 56.4 Aa | 17.0 Ae | |
Eucalyptus sawdust | 350 | 42.5 | 36.9 Ab | 0.9 ABe | 62.2 Cb |
450 | 36.0 | 28.5 Bb | 0.7 Be | 70.8 Bb | |
750 | 28.2 | 6.5 Cd | 1.1 Ae | 92.4 Aa | |
Coffee husk | 350 | 43.5 | 34.6 Ac | 12.9 Bb | 52.5 Cc |
450 | 37.7 | 26.2 Bc | 12.9 Bb | 60.9 Bc | |
750 | 31.6 | 17.6 Cb | 19.6 Ab | 62.8 Ad | |
Sugarcane bagasse | 350 | 37.5 | 35.0 Ac | 1.9 Ad | 63.0 Ca |
450 | 33.2 | 24.0 Bd | 2.1 Ad | 73.9 Ba | |
750 | 26.9 | 7.7 Cc | 2.2 Ad | 90.1 Ab | |
Pine bark | 350 | 59.6 | 38.5 Aa | 8.3 Bc | 53.2 Cc |
450 | 49.3 | 29.3 Ba | 7.9 Bc | 62.8 Bc | |
750 | 38.9 | 6.0 Cd | 14.5 Ac | 79.4 Aa |
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
Biochar volatile matter values reduced as the pyrolysis temperature was raised from 450°C to 750°C (
Total C concentrations in plant-derived biochars increased with an increase in pyrolysis temperature (
Biomass | Temp. (°C) | Elemental composition (%) | Atomic ratio | ||||
---|---|---|---|---|---|---|---|
C | H | S | O | H/C | O/C | ||
Chicken manure | 350 | 31.2 Ad | 1.97 Ac | 0.31 Ba | 10.9 Bc | 0.76 Ba | 0.26 Ba |
450 | 27.2 ABd | 1.92 Bc | 0.44 Aa | 11.4 Bc | 0.85 Aa | 0.31 Ba | |
750 | 24.7 Bd | 0.67 Cc | 0.29 Ba | 16.3 Aa | 0.32 Ca | 0.49 Aa | |
Eucalyptus sawdust | 350 | 70.4 Ca | 3.81 Ab | 0.02 Ac | 24.0 Aab | 0.65 Aa | 0.26 Ab |
450 | 78.6 Ba | 3.42 Ba | 0.01 Ac | 16.6 Bab | 0.52 Bb | 0.16 Bc | |
750 | 90.9 Aa | 1.52 Ca | 0.04 Ac | 5.6 Cc | 0.20 Cc | 0.05 Cc | |
Coffee husk | 350 | 60.5 Bc | 3.92 Ab | 0.09 Bb | 19.5 Aab | 0.78 Aa | 0.24 Aa |
450 | 61.3 Bc | 3.65 Ba | 0.10 Bb | 19.0 Aa | 0.71 Aa | 0.23 Ab | |
750 | 66.0 Ac | 1.57 Ca | 0.23 Ab | 9.8 Bb | 0.29 Bb | 0.11 Bb | |
Sugarcane bagasse | 350 | 74.7 Ca | 4.26 Aa | 0.03 Ac | 17.9 Ab | 0.68 Aa | 0.18 Ab |
450 | 81.6 Ba | 3.66 Ba | 0.05 Ac | 11.3 Bbc | 0.54 Bb | 0.10 Bc | |
750 | 90.5 Aa | 1.64 Ca | 0.06 Ac | 4.3 Cc | 0.22 Cc | 0.04 Cc | |
Pine bark | 350 | 67.6 Cb | 3.73 Ab | 0.01 Ac | 28.7 Aa | 0.66 Aa | 0.32 Aa |
450 | 75.2 Ba | 2.74 Bb | 0.02 Ac | 24.7 Ba | 0.44 Bb | 0.25 Bc | |
750 | 86.3 Aab | 1.16 Cb | 0.04 Ac | 19.1 Ca | 0.16 Cc | 0.17 Cc |
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
The H/C and O/C ratios of biochars derived from plant biomass decreased as the pyrolysis temperature was increased (
The sugarcane bagasse biomass had the highest WSOC concentration (94.5g kg-1) (
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
The biochar WSIC concentration increased with pyrolysis temperature (
(A) Chicken manure biochar. (B) Coffee husk biochar. (C) Pine bark biochar.
Mineral components in the crystal form were identified in the CM, CH and PB biochars (
For all CH biochars, the presence of kalicinite (KHCO3) was observed (
The FTIR-ATR biomass and biochar spectra are shown in
(A) Chicken manure. (B) Eucalyptus sawdust. (C) Coffee husk. (D) Sugarcane bagasse. (E) Pine bark.
Changes in biochar organic structure were apparent when biomass was pyrolyzed at 350°C, except for the CM biochars (
The pH in water of the biochars ranged from slightly acidic to alkaline (
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
Biochars of ES, SB, and PB produced at all pyrolysis temperatures used in this study showed reduced liming values (capacity to neutralize acidity) (
Electrical conductivity (EC) was mainly influenced by the biomass used in biochar production (
Biochar cation exchange capacity (CEC) values varied greatly, and are mainly dependent on the biomasses and the temperature used in the pyrolysis process (
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 [
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 [
For the purpose of reducing CO2 emissions, the use of low labile C biomass pyrolyzed at >550°C is recommended [
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 [
Differentiation of biochars was established by the parameters evaluated, which allowed the identification and discussion of agronomic benefits. Characterization by proximate analysis (
Low—temperature biochars provided the largest CEC (chicken manure and coffee husk pyrolyzed at 350–450°C), which can make them possible to adsorb N-NH4+ up to 2.3 mg g-1 and to reduce N leaching rates [
Wood- and sugarcane-derived biochars, regardless of the charring conditions, can potentially improve C storage in tropical soils (
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 [
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 450°C) 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 750°C. 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.
1The contents of P, K, Ca, Mg, Cu, Fe, Mn, and Zn were determined in extracts from the nitric-perchloric digestion procedure. 2Total content of B extracted with hot water.
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CM = chicken manure, SE = eucalyptus sawdust, CH = coffee husk, SB = sugarcane bagasse, and PB = pine bark.
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SE = eucalyptus sawdust, CH = coffee husk, SB = sugarcane bagasse, and PB = pine bark.
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The authors are grateful to Claudinéia 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 Development—CNPq, 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 manuscript.