Biochar Carbon Sequestration Research Articles

Exploring resource recovery from diverted organics: Life cycle assessment comparison of options for managing the organic fraction of municipal solid waste

Diversion of the organic fraction of municipal solid waste (OFMSW) from landfills is increasing. Previous life cycle assessment studies have evaluated subsets of OFMSW management options, but conclusions are inconsistent, and none have evaluated diverse applications of material by-products. The primary objective of this work was to identify sustainability-based improvements to the selection, design, implementation, and operation of organics waste diversion management technologies. Process modeling and life cycle assessment were used to compare OFMSW composting, anaerobic digestion, and pyrolysis, with biochar used as a landfill cover, leachate treatment sorbent, and land applicant. Material and energy flows, calculated by newly developed models for the defined functional unit (1 kg MSW over a 20-year timeframe), were translated to environmental performance using ecoinvent and USLCI databases and TRACI method. Additionally, uncertainty, sensitivity, and scenario analyses were conducted to evaluate the implications of model uncertainties, design decisions, and resource recovery tradeoffs. OFMSW pyrolysis usually (65 % of uncertainty assessment simulations) had the best global warming performance mostly due to energy recovery and biochar's carbon sequestration benefit, which was independent of fate. Pyrolyzing the biosolids from OFMSW anaerobic digestion recovered the most energy and had the best performance in 34 % of uncertainty simulations. Material recovery amounts were large (e.g., more biochar was produced than required for novel uses) and warrant feasibility considerations. Global warming performance was more sensitive to uncertainty in carbon sequestration and primary energy production than in fertilizer offset, energy conversion, or heat offset approach. Practical implications include the potential for biochar supply to outweigh demand, and inconsistent revenue from the sale of recovered energy and carbon credits; future research could focus on evaluating the relative social and economic sustainability of the OFMSW management technologies.

Additive effects of basalt enhanced weathering and biochar co-application on carbon sequestration, soil nutrient status and plant performance in a mesocosm experiment

Co-deployment of a portfolio of carbon removal technologies is anticipated in order to remove several gigatons of carbon dioxide from the atmosphere and meet climate targets. However, co-application effects between carbon removal technologies have rarely been examined, despite multiple recent perspectives suggesting potential synergies between basalt enhanced weathering and biochar application. To study the co-application effects of basalt for enhanced weathering and biochar on carbon sequestration, along with related co-benefits and risks, we conducted a fully replicated factorial mesocosm experiment with wheat. Basalt applied alone (74 t ha−1) resulted in an estimated carbon sequestration potential of 1.13 t of equivalent CO2 ha−1 over the course of approximately 6 months. Co-application with biochar (12 t ha−1) did not significantly increase estimated carbon sequestration potential. Total alkalinity fluxes and isotopic evidence indicated nearly exactly additive effects of basalt and biochar co-applied, with no significant interaction effect. Biochar carbon sequestration, approximately 32 t of equivalent CO2 ha−1 in our experiment, was unaffected by basalt addition during our experiment. Co-benefits of basalt and biochar on plant biomass as well as nutrient uptake and availability similarly mostly showed additive tendencies when co-applied. Nonetheless, a few synergistic tendencies were observed when co-applied for plant potassium and magnesium uptake as well as soil calcium availability. Soil calcium availability increased by 126% compared to expected effects based on separate application. Finally, we did not observe a reduction in the increased uptake of potentially harmful trace elements released from basalt when co-applied with biochar. Overall, our results support the co-application of basalt for enhanced weathering and biochar, with additive effects on carbon sequestration and additive, if not synergistic, effects on associated co-benefits.

Effect of various potassium agents on product distributions and biochar carbon sequestration of biomass pyrolysis

Biomass to biochar through pyrolysis is promising for carbon-negative biomass energy. Potassium could enhance biochar formation whereas the difference among potassium agents has not been understood. Herein, 13 potassium agents were adopted, and the product compositions and carbon distributions of slow and intermediate pyrolysis were investigated. Results show that neutral agents have a weak influence on the products, while high-basicity agents enhance biochar formation, decrease organics yields, and promote H2 generation (9–144 cm3/g) and phenols selectivity (64–79 area%). Especially, due to the anion properties, borate and phosphates deviate from the basicity tendency; acetate contributes to increasing acetic acid selectivity (up to 43 area%) while oxidizing agents promote CO2 and H2O yields. Potassium performs obvious enhancement of biochar carbon sequestration, increasing by 2–49 % for intermediate pyrolysis and -8–51 % for slow pyrolysis with biochar carbon retention reaching 55 % and 77 %, respectively. Meanwhile, the thermally unstable phases of biochar were also generated. General correlation analysis indicates that CO yields and phenols selectivity highly relate to biochar carbon retention. This study sheds light on the key role of acid-base properties and anion properties, and provides insight into AAEM-catalytic biomass pyrolysis.

Potential for biochar carbon sequestration from crop residues: A global spatially explicit assessment

AbstractGlobal warming necessitates urgent action to reduce carbon dioxide (CO2) emissions and remove CO2 from the atmosphere. Biochar, a type of carbonized biomass which can be produced from crop residues (CRs), offers a promising solution for carbon dioxide removal (CDR) when it is used to sequester photosynthetically fixed carbon that would otherwise have been returned to atmospheric CO2 through respiration or combustion. However, high‐resolution spatially explicit maps of CR resources and their capacity for climate change mitigation through biochar production are currently lacking, with previous global studies relying on coarse (mostly country scale) aggregated statistics. By developing a comprehensive high spatial resolution global dataset of CR production, we show that, globally, CRs generate around 2.4 Pg C annually. If 100% of these residues were utilized, the maximum theoretical technical potential for biochar production from CRs amounts to 1.0 Pg C year−1 (3.7 Pg CO2e year−1). The permanence of biochar differs across regions, with the fraction of initial carbon that remains after 100 years ranging from 60% in warm climates to nearly 100% in cryosols. Assuming that biochar is sequestered in soils close to point of production, approximately 0.72 Pg C year−1 (2.6 Pg CO2e year−1) of the technical potential would remain sequestered after 100 years. However, when considering limitations on sustainable residue harvesting and competing livestock usage, the global biochar production potential decreases to 0.51 Pg C year−1 (1.9 Pg CO2e year−1), with 0.36 Pg C year−1 (1.3 Pg CO2e year−1) remaining sequestered after a century. Twelve countries have the technical potential to sequester over one fifth of their current emissions as biochar from CRs, with Bhutan (68%) and India (53%) having the largest ratios. The high‐resolution maps of CR production and biochar sequestration potential provided here will provide valuable insights and support decision‐making related to biochar production and investment in biochar production capacity.

Biochar carbon sequestration potential rectification in soils: Synthesis effects of biochar on soil CO2, CH4 and N2O emissions

Biochar production and its soil sequestration are promising ways to mitigate global warming. Effects of biochar on soil CO2, CH4 and N2O release have been studied extensively. In contrast, few studies have comprehensively quantified and synthesized the effect of biochar on soil greenhouse gas (GHG) emission and coupled it to the calculation of carbon sequestration potential. This study obtained the influence coefficient of biochar on soil GHG release relative to biochar carbon storage potential in soils under different environmental conditions, by literature statistics and data transformations. Our results showed that the overall average effect of biochar on soil CO2, CH4, N2O and CO2e release observed in our databases would compensate the potential of biochar soil carbon storage by −2.1 ± 3.3 %, 13.1 ± 9.8 %, −1.6 ± 8.6 % and 5.3 ± 11.4 %, respectively. By combining biochar induced soil GHG emission reduction mechanism and results from our literature statistics, some specific application environmental scenarios (such as biochar with high pyrolysis temperature of 500–600 °C, application in flooded soils, application in straw-return scenarios, etc.) were recommended, which could increase the actual carbon sequestration potential of biochar by an average of about 43.3 ± 30.2 % relative the amount of carbon buried. Our findings provide a scientific basis for developing a precise application strategy towards large scale adoption of biochar as a soil amendment for climate change mitigation.

Biochar application significantly increases soil organic carbon under conservation tillage: an 11-year field experiment

Biochar application and conservation tillage are significant for long-term organic carbon (OC) sequestration in soil and enhancing crop yields, however, their effects on native soil organic carbon (native SOC) without biochar carbon sequestration in situ remain largely unknown. Here, an 11-year field experiment was carried out to examine different biochar application rates (0, 30, 60, and 90 Mg ha−1) on native SOC pools (native labile SOC pool I and II, and native recalcitrant SOC) and microbial activities in calcareous soil across an entire winter wheat–maize rotation. The proportions of C3 and C4-derived native SOC mineralization were quantified using soil basal respiration (SBR) combined with 13C natural isotope abundance measurements. The results showed that 39–51% of the biochar remained in the top 30 cm after 11 years. Biochar application rates significantly increased native SOC and native recalcitrant SOC contents but decreased the proportion of native labile SOC [native labile SOC pool I and II, dissolved organic carbon (DOC), and microbial biomass carbon (MBC)]. Biochar application tended to increase the indicators of microbial activities associated with SOC degradation, such as SBR, fluorescein diacetate hydrolysis activity, and metabolic quotient (qCO2). Meanwhile, higher biochar application rates (B60 and B90) significantly increased the C4-derived CO2 proportion of the SBR and enhanced C4-derived native SOC mineralization. The effect of the biochar application rate on the content and proportion of native SOC fractions occurred in the 0–15 cm layer, however, there were no significant differences at 15–30 cm. Soil depth also significantly increased native labile SOC pool I and II contents and decreased qCO2. In conclusion, the biochar application rate significantly increased native SOC accumulation in calcareous soil by enhancing the proportion of native recalcitrant SOC, and biochar application and soil depth collectively influenced the seasonal turnover of native SOC fractions, which has important implications for long-term agricultural soil organic carbon sequestration.Graphical

Biochar produced from wood waste for soil remediation in Sweden: Carbon sequestration and other environmental impacts.

The use of biochar to stabilize soil contaminants is emerging as a technique for remediation of contaminated soils. In this study, an environmental assessment of systems where biochar produced from wood waste with energy recovery is used for remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAH) and metal(loid)s was performed. Two soil remediation options with biochar (on- and off-site) are considered and compared to landfilling. The assessment combined material and energy flow analysis (MEFA), life cycle assessment (LCA), and substance flow analysis (SFA). The MEFA indicated that on-site remediation can save fuel and backfill material compared to off-site remediation and landfilling. However, the net energy production by pyrolysis of wood waste for biochar production is 38% lower than incineration. The LCA showed that both on-site and off-site remediation with biochar performed better than landfilling in 10 of the 12 environmental impact categories, with on-site remediation performing best. Remediation with biochar provided substantial reductions in climate change impact in the studied context, owing to biochar carbon sequestration being up to 4.5 times larger than direct greenhouse gas emissions from the systems. The two biochar systems showed increased impacts only in ionizing radiation and fossils because of increased electricity consumption for biochar production. They also resulted in increased biomass demand to maintain energy production. The SFA indicated that leaching of PAH from the remediated soil was lower than from landfilled soil. For metal(loid)s, no straightforward conclusion could be made, as biochar had different effects on their leaching and for some elements the results were sensitive to water infiltration assumptions. Hence, the reuse of biocharremediated soils requires further evaluation, with site-specific information. Overall, in Sweden's current context, the biochar remediation technique is an environmentally promising alternative to landfilling worth investigating further.

Country-level potential of carbon sequestration and environmental benefits by utilizing crop residues for biochar implementation

Conversion of biomass into biofuel and biochar with a subsequent soil storage is assumed as a prospective strategy of reducing atmospheric CO2 concentrations. However, substantial uncertainties exist in this field regarding the country-level potential of biochar carbon sequestration, indirect effects of biochar implementation on overall environment, and dominating factors. This study conducted a life cycle assessment of country-wide incorporation of biochar in agriculture, and associated potential benefits. Results showed that over 920 kg CO2e (CO2-equivalent) could be sequestrated via converting 1 t of crop residues into biochar. As an example, based on crop residues availability statistics for China in 2014, the estimated annual carbon sequestration potential could be as high as 0.50 Pg CO2e (1 Pg = 1 × 109 t). The most significant potential for biochar carbon sequestration was identified in the central south, east and northeast of China, which contributed 65% of the national biochar carbon sequestration potential. The biochar system could also contribute to mitigation of the following environmental problems: marine aquatic biodiversity destruction, surface soil and water acidification, etc. Sensitivity analysis demonstrated that biochar yield, carbon content in biochar, electricity conversion efficiencies of bio-oil and pyrolysis gas were the critical parameters determining the biochar system’s overall carbon sequestration potential and environmental effects. This study provides guidance on evaluating biochar’s potential carbon sequestration capacity and comprehensive environmental impacts, as well as research and development needs.

Small-scale biochar production on Swedish farms: A model for estimating potential, variability, and environmental performance

Several small-scale pyrolysis plants have been installed on Swedish farms and uptake is increasing in the Nordic countries. Pyrolysis plants convert biomass to biochar for agricultural applications and syngas for heating applications. These projects are driven by ambitions of achieving carbon dioxide removal, reducing environmental impacts, and improving farm finances and resilience. Before policy support for on-farm pyrolysis projects is implemented, a comprehensive environmental evaluation of these systems is needed. Here, a model was developed to jointly: (i) simulate operation of on-farm energy systems equipped with pyrolysis units; (ii) estimate biochar production potential and its variability under different energy demand situations and designs; and (iii) calculate life cycle environmental impacts. The model was applied to a case study farm in Sweden. The farm’s heating system achieved net carbon dioxide removal through biochar carbon sequestration, but increased its impact in several other environmental categories, mainly due to increased biomass throughput. Proper dimensioning of heat-constrained systems is key to ensure optimal biochar production, as biochar production potential of the case farm was reduced under expected climate change in Sweden. To improve the environmental footprint of future biochar systems, it is crucial that expected co-benefits from biochar use in agriculture are realised. The model developed here is available for application to other cases.

The financial trade‐off between the production of biochar and biofuel via pyrolysis under uncertainty

AbstractThere is a lack of widespread interest in slow pyrolysis biochar pathways relative to other bioenergy pathways because of the perceived absence of biochar's market value when produced at a commercial scale. Thus, most refereed techno‐economic analyses focus on fast pyrolysis. This study quantifies the carbon price point at which the economic feasibility of the slow‐pyrolysis pathway for biochar production is equal to or greater than a fast‐pyrolysis pathway for biochar and biofuel production using baseline minimum carbon prices (MCP). These factors are then modeled under uncertainty to generate stochastic cash flows for the calculation of a 20‐year net present value and carbon abatement cost probability distributions. This article examines whether a slow‐pyrolysis pathway is ever more financially attractive than a fast‐pyrolysis pathway at realistic carbon prices. The results show fast pyrolysis to fuels and biochar achieving the lowest baseline MCP of $61.38/Mg, while the slow pyrolysis to biochar and methanol at 450 °C scenario yields the highest baseline MCP of $642.40/Mg. A notable result is the baseline MCP for the slow pyrolysis to biochar scenario achieving $123.48/Mg, when compared with the fast pyrolysis to fuels and electricity scenario, resulting in a baseline MCP of $182.03/Mg. The results suggest that carbon prices, when high enough, can incentivize biochar carbon sequestration produced from slow‐pyrolysis pathways rather than carbon abatement with biofuels, and that slow pyrolysis to biochar and methanol scenarios require a higher baseline MCP and are less financially competitive than the other pathways. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd

Different alkaline minerals interacted with biomass carbon during pyrolysis: Which one improved biochar carbon sequestration?

During the pyrolytic production of biochar, only 50% of carbon (C) could be retained in biochar, and about 10% C would be mineralized after biochar is amended into soil. In this study, we aim to search a material to pretreat the biomass and improve carbon sequestration along biochar production and application. Alkaline minerals are widely reported to have catalytic effect on biomass pyrolysis and alter the liquid and gas products, while few studies have focused on their influence on biochar C sequestration. This study selected KCl, NaCl, MgCl2, and CaCl2 to dope in a biomass, cow manure, to explore their influence on the carbon sequestration based on carbon retention before and after pyrolysis and the carbon stability assessed by K2Cr2O7 oxidation and biological mineralization in real soil. Results showed that dosage of these minerals with a mineral/biomass ratio of 1: 5 (w/w) increased C retention in biochar from 49.8% (without minerals) to 52.5% (KCl), 53.7% (NaCl), 64.1% (MgCl2), and 60.7% (CaCl2), respectively. However, only MgCl2-fixed C in the produced composite biochar (Mg-BC) exhibited significant C stability both for bulk C (about 60.0%) and surface C (5.0 mg CO2·kg−1 soil). Addition of CaCl2 also could retain more C in biochar during pyrolysis, while the C in this composite biochar was not stable indicated by K2Cr2O7 oxidation and biological mineralization. Crystals of MgO and MgO3(CO3)2 were detected on the surface of Mg-BC by X-ray diffractometer (XRD), which could sorb small organic molecules such as C3–C7 compounds from the thermal decomposition verified by thermogravimetry-mass spectrum analysis (TG-MS). Moreover, a physical barrier was formed covering on biochar surface and enhance its capacity against oxidation and biological mineralization. This study offers a potential strategy to strengthen biochar-carbon sequestration by Mg-doping from the production of biochar to the application in soil environment.

Biophysical Potential of Crop Residues for Biochar Carbon Sequestration, and Co‐benefits, in Uganda

To what extent greenhouse gas emissions could be offset by soil biochar amendment is hotly debated. Residues from staple food crops are an obvious and promising choice, for gasifier energy appliances and biochar production in farming system of tropical Africa, for example, Uganda. Yet until now, the quantities of biomass residues from staple crops available for these technologies and their potential carbon sequestration have not been comprehensively investigated. Our interdisciplinary research demonstrates that biochar from unused biomass waste on farms in Uganda presents substantial opportunities for mitigation of climate change, generation of low-carbon energy, and intensification of agriculture. Photo credit: D. Roobroeck. Photo credit: D. Roobroeck. These photographs illustrate the article “Biophysical potential of crop residues for biochar carbon sequestration, and co-benefits, in Uganda” by Dries Roobroeck, Rebecca Hood-Nowotny, Dianah Nakubulwa, John-Baptist Tumuhairwe, Jackson Majaliwa, Isaac Ndawula, and Bernard Vanlauwe published in Ecological Applications. https://doi.org/10.1002/eap.1984

Biophysical potential of crop residues for biochar carbon sequestration, and co-benefits, in Uganda.

Increasing organic matter/carbon contents of soils is one option proposed to offset climate change inducing greenhouse gas (GHG) emissions, under the auspices of the UNFCC Paris Agreement. One of the complementary practices to sequester carbon in soils on decadal time scales is amending it with biochar, a carbon rich byproduct of biomass gasification. In sub‐Saharan Africa (SSA), there is a widespread and close interplay of agrarian‐based economies and the use of biomass for fuel, which makes the co‐benefits of biochar production for agriculture and energy supply explicitly different from the rest of the world. To date, the quantities of residues available from staple crops for biochar production, and their potential for carbon sequestration in farming systems of SSA have not been comprehensively investigated. We assessed the productivity and usage of biomass waste from maize, sorghum, rice, millet, and groundnut crops; specifically quantifying straw, shanks, chaff, and shells, based on measurements from multiple farmer fields and household surveys in eastern Uganda. Moreover, allometric models were tested, using grain productivity, plant height, and density as predictors. These models enable rapid and low‐cost assessment of the potential availability of feedstocks at various spatial scales: individual cropland, farm enterprise, region, and country. Ultimately, we modeled the carbon balance in soils of major cropping systems when amended with biochar from biomass residues, and up‐scaled this for basic scenario analysis. This interdisciplinary approach showcases that there is significant biophysical potential for soil carbon sequestration in farming systems of Uganda through amendment of biochar derived from unused residues of cereals and legume crops. Furthermore, information about these biomass waste flows is used for estimating the rates of biochar input that could be made to farmlands, as well as the amounts of energy that could be produced with gasifier appliances.

Prospective Life Cycle Assessment of Large-Scale Biochar Production and Use for Negative Emissions in Stockholm.

Several cities in Sweden are aiming for climate neutrality within a few decades and for negative emissions thereafter. Combined biochar, heat, and power production is an option to achieve carbon sequestration for cities relying on biomass-fuelled district heating, while biochar use could mitigate environmental pollution and greenhouse gas emissions from the agricultural sector. By using prospective life cycle assessment, the climate impact of the pyrolysis of woodchips in Stockholm is compared with two reference scenarios based on woodchip combustion. The pyrolysis of woodchips produces heat and power for the city of Stockholm, and biochar whose potential use as a feed and manure additive on Swedish dairy farms is explored. The climate change mitigation trade-off between bioenergy production and biochar carbon sequestration in Stockholm's context is dominated by the fate of marginal power. If decarbonisation of power is achieved, building a new pyrolysis plant becomes a better climate option than conventional combustion. Effects of cascading biochar use in animal husbandry are uncertain but could provide 10-20% more mitigation than direct biochar soil incorporation. These results help design regional biochar systems that combine negative carbon dioxide emissions with increased methane and nitrous oxide mitigation efforts and can also guide the development of minimum performance criteria for biochar products.

Potassium doping increases biochar carbon sequestration potential by 45%, facilitating decoupling of carbon sequestration from soil improvement

Negative emissions technologies offer an important tool to limit the global warming to <2 °C. Biochar is one of only a few such technologies, and the one at highest technology readiness level. Here we show that potassium as a low-concentration additive in biochar production can increase biochar’s carbon sequestration potential; by up to 45% in this study. This translates to an increase in the estimated global biochar carbon sequestration potential to over 2.6 Gt CO2-C(eq) yr−1, thus boosting the efficiency of utilisation of limited biomass and land resources, and considerably improving the economics of biochar production and atmospheric carbon sequestration. In addition, potassium doping also increases plant nutrient content of resulting biochar, making it better suited for agricultural applications. Yet, more importantly, due to its much higher carbon sequestration potential, AM-enriched biochar facilitates viable biochar deployment for carbon sequestration purposes with reduced need to rely on biochar’s abilities to improve soil properties and crop yields, hence opening new potential areas and scenarios for biochar applications.

Interaction of Inherent Minerals with Carbon during Biomass Pyrolysis Weakens Biochar Carbon Sequestration Potential

Biomass carbon could be sequestrated in the form of biochar, an aromatized carbon structure produced by pyrolysis. Inherent minerals are reactive constituents that interact with organic contents du...

Environment Friendly Agricultural Brand “Cool Vege” Through Carbon Sequestration by Biochar for Sustainable Management of Food and Water

&lt;p&gt;The reduction of of greenhouse gas to mitigate or adapt to drastic climate change are one of the most important issues for human beings. On the other hand, rural development is also important issue for sustainable rural natural resources to secure food and water. Then, we propose the new socio-economic scheme to solve these issues at the same time through biochar carbon capture and sequestration. This scheme contains 4 measure factors that 1) Carbon Capture &amp;amp; Storage(CCS) via biochar, 2) Biochar CCS should be carried out at agricultural lands for rural development, 3) Biochar CCS should be monitored and measured to generate carbon credits and social creditability, 4) The ECO-brand “Cool Vege” for agricultural products derived from biochar CCS. And, it consists of many stake holders and actors that local community, compost center, farmers, CCS local committee consisted by local governments and universities as scientific authority, companies, retailers and normal citizen as consumers. Therefore, when proceeding this scheme, it is needed to have holistic aspect like bird view.&lt;/p&gt;

Biochar carbon sequestration in soil - A myth or reality?

The soil carbon sequestration is the long-term storage of carbon in soil which could well be accomplished by the application of biochar as a soil amendment. Biochar (BC) is a fine grained, highly carbonaceous, pyrolysed (low temperature) product of biomass. The pyrolysis temperature strongly influences the stability of biochar in soil; the higher the pyrolysis temperature higher would be the stability. Biochar being highly stable in soil due to its aromaticity, presence of amorphous structure and turbostatic crystallites, rounded structures and reduced accessibility to decomposers has lot of potential for long-term carbon sequestration. The higher stability of biochar in soil is also due to strong interactions with mineral surfaces. Biochar interacts with native soil organic matter (SOM) in a complex way; sometimes biochar showed positive priming effect or negative priming effect or no effect on native SOM. This depends upon the feedstock type, pyrolysis temperature and organic matter level of soil. The soils richer in organic matter status provide positive priming effect of native SOM due to biochar addition and vice-versa. Biochar has high carbon sequestration potential and long-term influence on native SOM. Biochar has huge potential for reduction of greenhouse gas emission form paddy field soils. Therefore, optimisation of feedstock, pyrolysis temperature for preparation biochar and its application in a specific soil is extremely essential for stability of biochar and protection of native SOM and greenhouse gas reduction for long-term carbon sequestration. Thus biochar carbon sequestration is not a myth rather it would be a reality in near future.

Characteristics of biochars from crop residues: Potential for carbon sequestration and soil amendment

Biochar has potential to sequester carbon in soils and simultaneously improve soil quality and plant growth. More understanding of biochar variation is needed to optimise these potential benefits. Slow pyrolysis at 600 °C was undertaken to determine how yields and characteristics of biochars differ when produced from eight different agricultural residues. Biochar properties such as carbon content, surface area, pH, ultimate and proximate analysis, nutrient and metal content and the R50 recalcitrance index were determined. Significant variations seen in biochar characteristics were attributed to feedstock variation since pyrolysis conditions were constant. Biochar yields varied from 28% to 39%. Average carbon content was 51%. Ash content of both feedstocks and biochars were correlated with biochar carbon content. Macronutrients were concentrated during pyrolysis, but biochar macronutrient content was low in comparison to biochars produced from more nutrient rich feedstocks. Most biochars were slightly alkaline, ranging from pH 6.1 to pH 11.6. pH was correlated with biochar K content. Aromaticity was increased with pyrolysis, shown by a reduction in biochar H/C and O/C ratios relative to feedstock values. The R50 recalcitrance index showed biochars to be either class 2 or class 3. Biochar carbon sequestration potential was 21.3%–32.5%. The R50 recalcitrance index is influenced by the presence of alkali metals in the biochar which may lead to an under-estimation of biochar stability. The residues assessed here, at current global availability, could produce 373 Mt of biochar. This quantity of biochar has the potential to sequester 0.55 Pg CO2 yr−1 in soils over long time periods.

Insights on the molecular mechanism for the recalcitrance of biochars: interactive effects of carbon and silicon components.

Few studies have investigated the effects of structural heterogeneity (particularly the interactions of silicon and carbon) on the mechanisms for the recalcitrance of biochar. In this study, the molecular mechanisms for the recalcitrance of biochars derived from rice straw at 300, 500, and 700 °C (named RS300, RS500, and RS700, respectively) were elucidated. Short-term (24 h) and long-term (240 h) oxidation kinetics experiments were conducted under different concentrations of H2O2 to distinguish the stable carbon pools in the biochars. We discovered that the stabilities of the biochars were influenced not only by their aromaticity but also through possible protection by silicon encapsulation, which is regulated by pyrolysis temperatures. The aromatic components and recalcitrance of the biochars increased with increasing pyrolysis temperatures. The morphologies of the carbon forms in all of the biochars were also greatly associated with those of silica. Silica-encapsulation protection only occurred for RS500, not for RS300 and RS700. In RS300, carbon and silica were both amorphous, and they were easily decomposed by H2O2. The separation of crystalline silica from condensed aromatic carbon in RS700 eliminated the protective role of silicon on carbon. The effect of the biochar particle size on the stability of the biochar was greatly influenced by C-Si interactions and by the oxidation intensities. A novel silicon-and-carbon-coupled framework model was proposed to guide biochar carbon sequestration.