Application of biochar to soils changes soil physicochemical properties and stimulates the activities of soil microorganisms that influence soil quality and plant performance. Studying the response of soil microbial communities to biochar amendments is important for better understanding interactions of biochar with soil, as well as plants. However, the effect of biochar on soil microorganisms has received less attention than its influences on soil physicochemical properties. In this review, the following key questions are discussed: (i) how does biochar affect soil microbial activities, in particular soil carbon (C) mineralization, nutrient cycling, and enzyme activities? (ii) how do microorganisms respond to biochar amendment in contaminated soils? and (iii) what is the role of biochar as a growth promoter for soil microorganisms? Many studies have demonstrated that biochar-soil application enhances the soil microbial biomass with substantial changes in microbial community composition. Biochar amendment changes microbial habitats, directly or indirectly affects microbial metabolic activities, and modifies the soil microbial community in terms of their diversity and abundance. However, chemical properties of biochar, (especially pH and nutrient content), and physical properties such as pore size, pore volume, and specific surface area play significant roles in determining the efficacy of biochar on microbial performance as biochar provides suitable habitats for microorganisms. The mode of action of biochar leading to stimulation of microbial activities is complex and is influenced by the nature of biochar as well as soil conditions.
Biochar is the carbon-rich product obtained from the thermochemical conversion of biomass under oxygen-limited conditions. Biochar has attained extensive attention due to its agronomical and environmental benefits in agro-ecosystems. This work adopts the scientometric analysis method to assess the development trends of biochar research based on the literature data retrieved from the Web of Science over the period of 1998–2018. By analysing the basic characteristics of 6934 publications, we found that the number of publications grew rapidly since 2010. Based on a keyword analysis, it is concluded that scholars have had a fundamental recognition of biochar and preliminarily found that biochar application had agronomic and environmental benefits during the period of 1998–2010. The clustering results of keywords in documents published during 2011–2015 showed that the main research hotspots were “biochar production”, “biochar and global climate change”, “soil quality and plant growth”, “organic pollutants removal”, and “heavy metals immobilization”. While in 2016–2018, beside these five main research hotspots, “biochar and composting” topic had also received greater attention, indicating that biochar utilization in organic solid waste composting is the current research hotspot. Moreover, updated reactors (e.g., microwave reactor, fixed-bed reactor, screw-feeding reactor, bubbling fluidized bed reactor, etc.) or technologies (e.g., solar pyrolysis, Thermo-Catalytic Reforming process, liquefaction technology, etc.) applied for efficient energy production and modified biochar for environmental remediation have been extensively studied recently. The findings may help the new researchers to seize the research frontier in the biochar field.
As a class of famous carbon materials, biochars (BCs) and their derivative materials with excellent physicochemical properties and diversified functionalities present great potential in wastewater treatment fields. This review focuses on the latest development in reported biochar-based materials as superior adsorbents or catalysts for removing harmful organic contaminants from wastewater. The construction and properties of biochar-based materials are briefly introduced at the beginning. As one of the major factors affecting the properties of BCs, the wide diversity of feedstocks, such as agricultural and forest residues, industrial by-products as well as municipal wastes, endows BCs different chemical compositions and structures. Woody and herbaceous BCs usually have higher carbon contents, larger surface areas and strong aromaticity, which is in favor of the organic contaminant removal. Driven by the desire of more cost-effective materials, several types of biochar-based hybrid materials, such as magnetic BC composites (MBC), nanometal/nanometallic oxides/hydroxide BC composites and layered nanomaterial-coated BCs, as well as physically/chemically activated BCs, have also been developed. With the help of foreign materials, these types of hybrid BCs have excellent capacities to remove a wide range of organic contaminants, including organic dyestuff, phenols and chemical intermediates, as well as pharmaceutically active compounds, from aquatic solutions. Depending on the different types of biochar-based materials, organic contaminants can be removed by different mechanisms, such as physical adsorption, electrostatic interaction, π–π interaction and Fenton process, as well as photocatalytic degradation. In summary, the low cost, tunable surface chemistry and excellent physical–chemical properties of BCs allow it to be a potential material in organic contaminant removal. The combination of BCs with foreign materials endows BCs more functionalities and broader development opportunities. Considering the urgent demand of practical wastewater treatment, we hope more researches will focus on the applications and commercialization of biochar-based materials.
After entering the twenty-first century, biochar has become a focal point of multidisciplinary research because of its special characteristics, broad application, and promising development prospects. Basic and applied research on the application of biochar in the areas of agriculture, environment, and energy have increased dramatically in the face of food security, environmental pollution, and energy shortage. Although there are some disputes about biochar research, many studies have demonstrated the importance of biochar research from the perspective of scientific advancement and practical application. This paper briefly recalls the history of biochar application; introduces research progress on the basic characteristics of biochar and its associated production technologies; summarizes the research status and existing problems of biochar application in the areas of agriculture, environment, and energy; and analyzes the potential problems and development trends of biochar research in the future.
Converting waste biomass into value-added biochar has been considered as a green and sustainable strategy for resource management and pollution control. In this study, graphitic carbon nitride (g-C3N4) modified biochars (BCs) were produced through one-pot pyrolysis of urea and hickory chips in differential ratios at 520 °C. The resulting BC/g-C3N4 composites were evaluated in laboratory for their physicochemical, adsorptive, and photocatalytic properties. The characterization tests showed the successful synthesis of the BC/g-C3N4 composites that introduced g-C3N4 structure, N-containing surface functional groups, reduced surface area, and better thermal stability to the biochar. After modification, the BC/g-C3N4 composites showed better adsorption ability to reactive red 120 (RR120) than the pristine BC, due to the strong electrostatic attrition between N-containing functional groups of g-C3N4 on biochar surface and anionic RR120. The BC/g-C3N4 composites also inherited g-C3N4’s photocatalytic activity, which is visible light responsive to generate free radicals for RR120 degradation. In addition, the composites with higher urea modification ratios were more effective in the degradation of RR120. Overall, this study demonstrates the feasibility and promising potential of combining biochar and photocatalyst for the removal of aqueous dye. Because of the synergistic adsorption and photodegradation ability, BC/g-C3N4 composites present a novel and cost-effective solution for the removal of aqueous dye and other photodegradable contaminants under natural conditions.
Biochars have the potential to reclaim mine-impacted soils; however, their variable physico-chemical properties incite speculation about their successful remediation performance. This investigation examined the capability of biochars produced from three different feedstocks along with a compost blend to improve switchgrass growth conditions in a mine-impacted soil by examining influences on soil pH, grass metal contents, and soil-extractable metal concentrations. Cadmium (Cd)- and zinc (Zn)-contaminated mine soil was collected from a site near Webb City, Missouri, USA—a location within the Tri-State Mining District. In a full factorial design, soil was treated with a 0%, 2.5%, and 5% (w/w) compost mixture (wood chips + beef cattle manure), and 0%, 2.5% and 5% of each biochar pyrolyzed from beef cattle manure, poultry litter, and lodgepole pine feedstocks. Switchgrass (Panicum virgatum, ‘Cave-In-Rock’ variety) was grown in a greenhouse for 50 days and the mass of shoots (above-ground biomass) and roots was assessed, while soil pH, deionized H2O- and 0.01 M CaCl2-extractable Cd and Zn concentrations were measured. Poultry litter biochar and compost had the greatest ability to raise soil pH (from 4.40 to 6.61), beef cattle manure biochar and compost moderately raised pH (from 4.4 to 5.92), and lodgepole pine biochar and compost weakly raised pH (from 4.40 to 5.05). Soils treated with beef cattle manure biochar, poultry litter biochar significantly reduced deionized H2O- and 0.01 M CaCl2-extractable Cd and Zn concentrations, while lodgepole pine biochar-treated soils showed mixed results. Switchgrass shoot and root masses were greatest in soil treated with compost in combination with either beef cattle manure biochar or poultry litter biochar. Soils treated with 5% beef cattle manure biochar + 5% compost had greater reductions in total Cd and Zn concentrations measured in switchgrass shoots and roots compared to the other two treatments. The three biochars and compost mixtures applied to heavy metal, mine-impacted soil had considerable performance dissimilarities for improving switchgrass productivity. Switchgrass growth was noticeably improved after treatment with the compost in combination with biochar from beef cattle manure or poultry litter. This may be explained by the increased soil pH that promoted Zn and Cd precipitation and organic functional groups that reduced soil-available heavy metal concentrations. Our results imply that creating designer biochars is an important management component in developing successful mine-site phytostabilization programs.
Dairy pastures can be a major source of soil nitrous oxide (N2O) emissions due to the combination of intensive nitrogen (N) fertiliser use and high soil water content, from either rainfall and/or irrigation. Biochar application is a promising approach to lower soil greenhouse gas emissions, particularly under high soil moisture conditions where denitrification is the primary N-transformation pathway. In a replicated field trial, we evaluated the effects of two contrasting biochars derived from poultry litter and from hardwood on soil N2O emissions, soil ammonium (NH4+) and nitrate (NO3−) status, pasture productivity and herbage nutrient content. A liming treatment to mimic the liming equivalence of the poultry litter biochar was used to separate any effects observed from changes in soil pH. To further separate the effects of biochars on soil N status, N2O emissions and pasture N uptake, high and low N fertiliser doses (annual application of 672 kg N ha−1, 336 kg N ha−1) were superimposed across all of the treatments. The N fertiliser dose had no significant impact on pasture yield. Application of poultry litter biochar resulted in significant increases in pasture productivity under both high and low N inputs. This was achieved by alleviating soil P, and possibly K nutritional constraints that are typical in Australian Ferralsols. Under the high N fertiliser dose, emissions of N2O from the treatments and control were not significantly different (p > 0.05) and ranged between 1.14 and 1.78 kg N2O-N ha−1 across the 11-month study. The low N dose resulted in significantly lower emissions of N2O of between 0.80 and 0.84 kg N2O-N ha−1, but biochar had no significant effect on net emissions across the season. The lack of impact of biochar on N2O emissions was attributed to the relatively dry conditions over the trial period resulting in nitrification being the most likely N-transformation pathway. During brief episodes of high soil moisture, peak emissions from the biochar plots were lower than from the control or lime treatment, but these differences did not impact on the emission budget over the 11-month sampling campaign.
With the aim to comparatively investigate the effect of the addition of additives such as biochar (BC), microbial inoculation (MI) and biochar/microbial inoculation (BCMI) on composting enhancement, nitrogen conservation, greenhouse gas emissions, the quality improvement of pig manure compost were comparatively investigated in a 42-day aerobic pig manure composting experiment. The results showed that the duration of the thermophilic stage, the degradation of organic matter and the detoxification of the compost were enhanced in the BC, MI and BCMI treatments compared with those in the control (without additive). Moreover, the content of total Kjeldahl nitrogen in the BC, MI and BCMI treatments was increased by 38.1, 48.9 and 59.0%, respectively, through the reduction of NH3 volatilization and N2O production, which were higher than those of the control (31.8%). A reduction in CH4 release during composting was not observed with the MI and BCMI treatments. Following 42 days of composting, the final product of the three treatments exhibited acceptable potential for use as a fertilizer in agriculture; the BCMI treatment showed an especially synergistic effect on pig manure composting enhancement.