An overview on biochar production,its implications,and mechanisms of biochar-induced amelioration of soil and plant characteristics

Fasih Ullah HAIDERJeffrey A.COULTERLiqun CAI∗Saddam HUSSAIN∗Sardar Alam CHEEMAJun WU and Renzhi ZHANG

1College of Resources and Environmental Sciences,Gansu Agricultural University,Lanzhou 730070(China)

2Gansu Provincial KeyLab of Arid-land Crop Science,Gansu Agricultural University,Lanzhou 730070(China)

3Department of Agronomyand Plant Genetics,Universityof Minnesota,St.Paul,MN 55108(USA)

4Department of Agronomy,Universityof Agriculture Faisalabad,Faisalabad 38040(Pakistan)

ABSTRACT The degradation of soil fertility and quality due to rapid industrialization and human activities has stimulated interest in the rehabilitation of low-fertility soils to sustainably improve crop yield.In this regard,biochar has emerged as an effective multi-beneficial additive that can be used as a medium for the amelioration of soil properties and plant growth.The current review highlights the methods and conditions for biochar production and the effects of pyrolysis temperature,feedstock type,and retention time on the physicochemical properties of biochar.We also discuss the impact of biochar as a soil amendment with respect to enhancing soil physical(e.g.,surface area,porosity,ion exchange,and water-holding capacity)and chemical(e.g.,pH,nutrient exchange,functional groups,and carbon sequestration)properties,improving the soil microbiome for increased plant nutrient uptake and growth,reducing greenhouse gas emissions,minimizing infectious diseases in plants,and facilitating the remediation of heavy metal-contaminated soils.The possible mechanisms for biochar-induced amelioration of soil and plant characteristics are also described,and we consider the challenges associated with biochar utilization.The findings discussed in this review support the feasibility of expending the application of biochar to improve degraded soils in industrial and saline-alkali regions,thereby increasing the usable amount of cultivated soil.Future research should include long-term field experiments and studies on biochar production and environmental risk management to optimize biochar performance for specific soil remediation purposes.

KeyWords:carbon sequestration,crop yield,pyrolysis,soil amendment,soil fertility,soil remediation

INTRODUCTION

Innovation in agricultural production management is a necessary prerequisite for developing the rural economy and promoting sustainable agricultural systems.Currently,degradation of soil,increasing soil erosion,and dwindling fertility are the major threats to global agriculture(Zhang,1999).Compared with conservation agricultural practices,conventional crop management typically results in the depletion of soil organic matter and increases soil erosion,greenhouse gas emission,and soil degradation(De Meyeret al.,2011).Decreased soil organic matter reduces the stability of soil aggregates(Annabiet al.,2011).Furthermore,the release of methane and ammonia from applied composts and manures can promote global warming and lead to groundwater contamination(Schmidtet al.,2015).Consequently,future research approaches should focus on sustainable and simple methods that can effectively remediate degraded soils(Kammannet al.,2015).

Application of biochar to agricultural soils has attracted considerable attention in recent years,owing to its potential implications with respect to waste management,C sequestration,reduction of greenhouse gas emission,water and soil remediation,and enhancing soil fertility and crop production(Kuppusamyet al.,2016;El-Naggaret al.,2019).Biochar is a C-enriched by-product obtained after the thermal degradation of organic materials in the absence of oxygen(Dinget al.,2016).It is a renewable porous carbonaceous resource loaded with nitrate,ammonium,and phosphate,the application of which can improve soil fertility(Linet al.,2017).From the perspectives of microbial and chemical degradation,the structure of biochar shows a notably high degree of stability(Agegnehuet al.,2017),which makes it superior to other organic materials,because it can provide long-term benefits to soil.Nevertheless,the chemical and physical properties of biochar differ to varying extents,depending on feedstock type,the processing temperature required for pyrolysis(decomposition of raw materials at high temperature)in furnace,and the duration of feedstock pyrolysis(Chenget al.,2008;Sohiet al.,2010).Similar to pyrolysis,other techniques used for the production of biochar,such as hydrothermal carbonization(Meloet al.,2019)and gasification(Yanget al.,2019),also influence the physiochemical properties of the final product(Arthuret al.,2020).An extensive range of feedstock raw materials can be processed to yield biochar,including wood chips,Gallus gallus domesticusL.manure,Sus scrofa domesticusL.manure,Bos taurusL.manure,organic waste,Oryza sativaL.straw,Zea maysL.straw,Triticum aestivumL.straw,and other plant residues(Josephet al.,2010;Dinget al.,2016).Elemental biochar comprises high amounts of C,H,N,K,Ca,Mg,and Na(Zhang Het al.,2015).Generally,the C content in biochar increases,whereas those of H and N decrease with an increase in pyrolysis temperature from 300 to 800°C(Schmidtet al.,2015).Moreover,given that biochar has a high specific surface area and series of non-polar and polar substances,it has a strong affinity for heavy metals,nitrates,and phosphates that exist in nature as inorganic ions(Kammannet al.,2015).Application of biochar not only enhances the chemical and physical features of soil,but also improves soil rhizosphere microbial populations(Grossmanet al.,2010).It has been shown to significantly enhance soil structure,reduce soil bulk density,and improve soil water retention,porosity,and aggregation(Baiamonteet al.,2015).Furthermore,compared with non-biochar-amended soils,the application of biochar to soils has been shown to enhance soil electrical conductivity(EC)by 125%,microbial biomass by 125%,and cation exchange capacity(CEC)by 20%,whilst reducing acidity by 32%(Steineret al.,2008;Lairdet al.,2010).

There is growing interest in using biochar as an amendment for soils with low fertility and to enhance soil C sequestration(Jianget al.,2012;Dinget al.,2016).Numerous mechanisms have been prompted for increased accessibility of plant nutrients in biochar-amended soils,such as a reduction in the leaching of nutrients due to the physicochemical properties of biochar(Lehmannet al.,2009),low unnecessary losses of N2O and N2from denitrification and of NH3viavolatilization(Sohiet al.,2010),addition of soluble nutrients contained in biochar(Uzomaet al.,2011),and mineralization of the labile fraction of biochar containing organically bound nutrients(Lehmannet al.,2009).Additionally,enhancing soil biological activities can promote the preservation and retention of nutrients such as N,S,and P(Zhang Aet al.,2010).

It has been well reported that the application of biochar can enhance crop growth and productivity,along with improved soil quality(Zhang Het al.,2015).For example,the application of 15 and 20 t ha−1ofB.taurusL.manure biochar significantly increase the grain yield ofZ.maysby 150% and 98%,respectively,compared with the control(Uzomaet al.,2011).Nevertheless,in certain cases,the application of biochar to soil can cause negative effects on soil physicochemical properties and the environment(Enniset al.,2012;Xieet al.,2016).Yadavet al.(2018)reported that biochar may reduce the supply of soil nutrients and crop production due to a reduction in plant nutrient uptake and mineralization of soil C,and Asaiet al.(2009)observed that the application of biochar at 4,8,and 16 t ha−1reduced the grain yield ofO.sativaby 23.3%,10.0%,and 26.7%,respectively,compared with the control.Such reductions in crop yield with the application of biochar have been attributed to the presence of toxic substances and highly volatile compounds that inhibit crop growth by reducing plant nutrient uptake(Dinget al.,2016).Accordingly,the effects of biochar application on crop growth and production largely depend on the properties and characteristics of biochar and the associated interactions with soil(Majoret al.,2010).It is important to gain an understanding of the primary mechanisms that contribute to variations in the physiochemical characteristics of biochar during the pyrolysis of different types of feedstock and at different temperatures to assess the capacity of biochar to remediate agricultural soils,which can provide a useful basis for future studies.Moreover,it is crucial to identify the mechanisms that stimulate changes in soil after amendment with biochar(Yuanet al.,2019).

This review focuses on the impact of biochar on soil biological and physicochemical properties and also describe the mechanisms whereby biochar improves soil fertility and plant growth.In order to better understand the effect of biochar on soil,the following aspects are included in this paper:i)biochar as a source of nutrients,ii)the physiochemical attributes of biochar,as affected by different production methods,processing temperatures,and feedstock types,iii)the adsorption and desorption of nutrients on biochar,iv)the influence of biochar on soil properties,and v)the effect of biochar on soil biota.Along with a brief discussion on the challenges associated with the use of biochar,this review provides a foundation for future research on biochar and innovations in the use of this material as an amendment for agricultural soils.

OVERVIEW OFBIOCHAR

Properties of biochar

Biochar is an organic material that is notably enriched with C and other elements,including N,O,and H.The C content of biochar ranges between 380 and 800 g kg−1and is characterized by both aromatic and alkyl structures(Xu and Fang,2015).Biochar also contains inorganic elements,including Si,K,Al,Ca,and P(Wanget al.,2012;Anawaret al.,2015),the contents of which differ depending upon the type of feedstock(Yuanet al.,2019).The type of feedstock can also influence the pH of biochar,which normally varies from neutral to alkaline(Liet al.,2014),although acidic pH has been documented with the use of certain feedstocks and under different pyrolysis conditions(Qiet al.,2017).Generally,the pH value of biochar ranges from 5 to 12(Xu and Fang,2015),and an increase in pyrolysis temperature tends to increase the pH of biochar due to the decomposition of bionic acid(Novaket al.,2009)and increases in the concentrations of mineral alkali elements such as K,Na,Ca,and Mg(Windeattet al.,2014;Konget al.,2015).Moreover,the functional organic groups(i.e.,—OH,—O−,and—COO−)present in biochar are alkaline and consequently contribute to increased pH of biochar(Novaket al.,2009;Yuanet al.,2011).These functional organic groups have a considerable influence on the hydrophobicity,hydrophilicity,and adsorption of biochar,and are also connected to form a buffer between bases and acids(Yuanet al.,2019).Functional organic groups result to hold negative charges on biochar and enhance the CEC(Wanget al.,2012).

Given its rich sources of C,biochar has an intricate microstructure with numerous pore spaces,which contribute to enhancing its specific surface area(Glaseret al.,2002).Biochar typically has a large surface area,ranging from 100 to 460 m2g−1,and diverse functional groups,including carbonyl,carboxylic,phenolic,and hydroxyl groups(Novaket al.,2009;Yuanet al.,2019).The porosity and specific surface area of biochar can be greatly influenced by pyrolysis temperature(Chenet al.,2018).Within a certain range of temperature,the specific surface area of biochar enhances with an increased pyrolysis temperature(Leeet al.,2010).During biochar pyrolysis,increases in temperature force volatile substances out of the char,thereby resulting in the pore formation and an increase in specific surface area(Pandeyet al.,2020).A high specific surface area and homogenous porous structure contribute to the high waterholding capacity of biochar(Glaseret al.,2002).However,high pyrolysis temperature leads to diminishing of the polar functional groups located on the biochar surface,thereby contributing to an increase in hydrophobicity(Dai and Liu,2013).

The aforementioned physicochemical characteristics of biochar can lead to changes in soil pH,water-holding capacity,base saturation,and CEC following its application to soil(Anawaret al.,2015;Yuanet al.,2019).Generally,the properties of biochar can be modified by altering the preparation conditions(Konget al.,2015),which are described in the following section.

Production of biochar

Various types of waste biomass are extensively used in the production of biochar,including livestock manures,sewage sludge,municipal solid waste,paper mill waste,food processing waste,forestry waste,and crop residues(Cantrellet al.,2012;Ahmadet al.,2014).The production of biochar also yields renewable energy sources,such as gaseous products(CO,H2,CH4,and CO2)and oil(Windeattet al.,2014).During the thermal decay of waste biomass and feedstock into biochar,gaseous products and oil can be obtainedvianumerous techniques,including traditional carbonization,torrefaction,hydrothermal carbonization,gasification,and pyrolysis(Van Der Steltet al.,2011;Yuanet al.,2017).Production of biochar by gasification is quite different compared with biochar production by pyrolysis(Ahmadet al.,2014).For biochar production through gasification,biomass and feedstock are converted into gases that are rich in CO and H2at high temperature(700°C)in a controlled environment(Mohanet al.,2006).However,the yield of biochar obtainedviagasification tends to be low,and consequently this process is not recommended for commercial purposes(Yuanet al.,2019).

Although hydrothermal carbonization produces a final carbonaceous material(hydro-char)from wet feedstock in the absence of the energy-intensive pre-drying step(Kumaret al.,2017),additional thermal energy is typically required for the post-treatment of hydrothermal carbonization material,such as separating the solid and liquid products(Kambo and Dutta,2015).A further pretreatment method that is used extensively in the preparation of biochar is torrefaction(Chen Tet al.,2015),which facilitates the removal of volatile compounds from biomass,thereby enhancing biomass quality(Yuet al.,2017).However,a drawback of the torrefaction process is that high amounts of heavy metals remain within the resulting ash,which represents a major obstacle for the utilization of biochar as a soil amendment(Kambo and Dutta,2015).From the perspective of biochar production,pyrolysis is the thermochemical decomposition of feedstock at 200—900°C under O2-deficient conditions(Sohiet al.,2009;Ahmadet al.,2014).Generally,pyrolysis is classified as slow,intermediate,and fast,depending on temperature and retention time(Mohanet al.,2006).Fast pyrolysis has short retention time,produces large amounts of bio-oil,and yields approximately 75%liquid fuel(Sohiet al.,2010),whereas intermediate and slow pyrolyses have retention times ranging from a few minutes to several hours and generate 25%—35%biochar(Brownet al.,2009).Of these three processes,slow pyrolysis is the most common method of manufacturing biochar(Yuanet al.,2019).

Compared with torrefaction and hydrothermal carbonization,pyrolysis produces a higher C yield(Wilk and Magdziarz,2017).Slow pyrolysis results in a lower yield of liquid fuel and higher yield of biochar than other chemical and thermal processes(Windeattet al.,2014;Yuet al.,2017).Biochar production is a complex physicochemical process that is affected by organic and inorganic substances in feedstock,the mechanisms of pyrolysis,and interactions among components(such as lignin,hemicelluloses,and cellulose)present in feedstock(Windeattet al.,2014;Lian and Xing,2017).Further,the adsorption characteristics and porous structure of biochar are dependent on the precursor methods used for biochar preparation(Parket al.,2013;Higashikawaet al.,2016).Activated C treated with oxygen enhances the surface area and micro-porosity of biochar(Ahmadet al.,2014).The aromatic structure of biochar plays an important role in porous structure formation and the activation of chemical processes in soil(Parket al.,2013).Thus,both the manufacturing procedure and feedstock type can influence the properties of biochar and its effect on soil remediation(Saifullahet al.,2018).

PRINCIPAL DYNAMICS AFFECTING BIOCHAR PRODUCTION

The chemical and physical properties of biochar vary depending on the type of raw material and methods used for biochar preparation,thereby enabling researchers to produce biochar for specific purposes(Wanget al.,2018).The main factors that affect biochar properties(i.e.,feedstock raw material,pyrolysis temperature and time,gas pressure,and additive)are discussed below.

Feedstock

The feasibility of the pre-treatment method is vital in the categorization of feedstock raw material to prepare biochar(Amarasingheet al.,2016).Feedstocks such as agro-industrial waste,livestock and human manure,woody and herbaceous biomass,and crop residues are used to produce biochar(Windeattet al.,2014;Lahoriet al.,2017;Wanget al.,2018)(Table I).For biochar manufacturing,high carbonaceous material is used(Sulimanet al.,2016),with the basic raw material biomass comprising numerous organic constituents,including cellulose,lignin,hemicelluloses,phenolics,phytosterols,sterols,and fats,and various inorganic compounds such as N,S,Si,P,K,Na,and other trace minerals that alter the properties and final structure of the produced biochar(McKendry,2002;Yuanet al.,2019).For a given pyrolysis temperature and duration,the differences in biochar properties are related mainly to variations in feedstock(Windeattet al.,2014;Januset al.,2015)(Table I).Feedstocks rich in nutrients result in biochars with higher nutrient contents.To ensure a suitable,sustainable biotechnological process and maintain a favorable cost/benefit ratio in biochar production,an inexpensive feedstock or raw material is generally preferred(Aparicioet al.,2017).In this regard,over the past decade,algal biomass has been considered as a potential sustainable feedstock for biochar production(Yuanet al.,2019).Due to high ion exchange capacity and nutrient contents,especially N,algal biochar is a suitable option for improving soil fertility(Torriet al.,2011;Yuet al.,2017).

Inexpensive raw waste materials from the paper industry(i.e.,wastewater-treated sludges and deinking sludges)can be utilized for environmental restoration and biochar production,as they are enriched with carbonates and cellulose fibers(Méndezet al.,2014).Material disposed in landfills is also an inexpensive urban organic waste that can be utilized as a feedstock to produce biochar,which is used as a fertilizer for unproductive and low-fertility soils(Cárdenas-Aguiaret al.,2017).The efficacy of biochar produced from waste tea pyrolyzed at 350°C for 30 min has been shown to be similar to that produced from crop residues and woody feedstocks(Amarasingheet al.,2016).In contrast to other types of raw materials,those with lingo-cellulosic compounds and lignin can yield higher amounts of biochar(Siguaet al.,2015;Saifullahet al.,2018).Biochar manufactured from plant residues is generally applied as a soil conditioner rather than as a fertilizer,given that it tend to have low contents of leachable nutrients(Yuanet al.,2019).Conversely,biochar derived from manures are more suitable as fertilizers,owing to their release of plant essential nutrients such as N,P,and K(Uchimiyaet al.,2010).Biochars derived from livestock and human manures are also characterized by high contents of sulfate,phosphate,and carbonate compounds,and accordingly have a superior capacity to adsorb metal ions compared with those derived from plant residues(Higashikawaet al.,2016).However,the yield of biochar may vary to differing degrees depending upon the moisture content of the raw material subjected to pyrolysis(Januset al.,2015).

Feedstock particle size can influence the devolatilization of biomass and conversion to biochar.Zanziet al.(2002)reported that high pyrolysis temperature and feedstock with small particle sizes contribute to a reduction in the yield of biochar.Moreover,the volatilization of latent toxic elements is related to pyrolysis temperature,and the interactions between the organic and inorganic components of feedstocks significantly influence biochar properties and quality(Saifullahet al.,2018).Thus,biochar prepared from different feedstocks shows variations in organic functional groups,porosity,and surface areas,which are vital factors determining the efficacy of biochar for remediation of secondary pollutants in soils(Evangelouet al.,2014).

Pyrolysis temperature

Pyrolysis temperature is a vital consideration in the preparation of biochar,as numerous biochar properties,including pore structure,surface properties,and nutrientcomposition,are directly determined by the pyrolysis temperature(Wanget al.,2018).The ash content of biochar generally increases with an increase in the pyrolysis temperature and retention time,whereas the volatile content in biochar decreases as pyrolysis temperature and retention time increase(Yuanet al.,2019)(Table I).At temperatures between 300 and 500°C,relative elementary material showed a rapid loss of O and H(Xieet al.,2016).As the pyrolysis temperature increases,the decreased O and H contents in the char result in the decomposition of organic compounds(i.e.,lignin,carbohydrates,sugars,and cellulose)present in the feedstock,as well as the cracking and cleavage of the fragile bond within the freshly established charred aromatic structure(Tsaiet al.,2012;Tomczyket al.,2020).

TABLE I Physicochemical properties of biochar as affected by feedstock type and pyrolysis temperature

High pyrolysis temperature can also enhance the carbonized fraction and surface area of biochar,thereby improving its capacity for adsorption of inorganic and organic pollutants in contaminated soil(Tanget al.,2013).The properties of biochar pyrolyzed at low temperature are similar to those of the raw material utilized for pyrolysis,whereas the properties of biochar produced at high temperature are clearly distinct from those of the feedstock material(Saifullahet al.,2018).In addition,biochar pyrolyzed at low temperature contains high contents of volatile compounds,although the contents of fixed C and ash are low compared with those in biochar pyrolyzed at high temperature(Rafiqet al.,2016)(Table I).High pyrolysis temperature promotes changes in the carbonization rate and the composition of C,O,H,and N(Chen and Chen,2009;Wanget al.,2018).Biochar pyrolyzed at low temperature comprises mainly cellulose and lignin and has low aromaticity(Keiluweitet al.,2010),while biochar pyrolyzed at high temperature is characterized by rich array of aromatic compounds with low polarity(Sunet al.,2011;Yuanet al.,2019).Parket al.(2013)observed that biochar pyrolyzed at 300—350°C has a specific surface area of＜10 m2g−1,while those pyrolyzed at 500 and 700°C have notably high surface areas of 239 and 321 m2g−1,respectively.

Yuanet al.(2019)reported the Fourier-transform infrared(FT-IR)spectra of biochar samples derived fromO.sativastraw pyrolyzed at different temperatures.The inorganic and organic functional groups of biochar produced at a low pyrolysis temperature(100°C)are dominated by aromatic C=C(1 651 cm−1),−OH(3 405 cm−1),C−O(1 200—1 060 cm−1),and ester C=O(1 732 and 1 162 cm−1),and the amounts of functional groups continue to increase with a decrease in temperature(Qianet al.,2016;Yuanet al.,2019).In contrast,the reduction and elimination of organic functional groups are observed in response to an increase in pyrolysis temperature(Yuanet al.,2019).Although high pyrolysis temperature does not affect the mineral contents in biochar,it enhances the volatilization of organic material(Crombieet al.,2013;Busset al.,2016).

The proportions of certain elements(i.e.,P,Ca,and K),pH,surface area,and C:O and C:N ratios in biochar trend to increase with high pyrolysis temperature(Busset al.,2016;Saifullahet al.,2018).Moreover,biochar pyrolyzed at low temperature(100°C)can improve the biological activity of soil,whereas biochar pyrolyzed at 500°C has a smaller pore volume and surface area,which can reduce soil CO2emission and the bioavailability of Ni and other toxic elements(i.e.,Cr,Pb,Cd,Cu,and Zn)in soil(Méndezet al.,2014;Anyikaet al.,2016;Awadet al.,2018).The toxicity of secondary pollutants in biochar can be minimized by making certain adjustments to pyrolysis conditions that have influences on the equilibrium concentrations and initial adsorption rates of contaminants(Parket al.,2013;Anyikaet al.,2016).Yuanet al.(2019)reported variations in the porosity and surface area of biochar in response to an increase in pyrolysis temperature,which have mainly been attributed to the decomposition of soil organic matter and the development of micropores(Ghaniet al.,2013;Yuanet al.,2019).The degradation of ester and aliphatic alkyls groups and the exposure of the aromatic lignin core at high pyrolysis temperature may also contribute to an increase in the porosity and surface area of the processed char(Rafiqet al.,2016;Tomczyket al.,2020).

The pH value of biochar is closely associated with carbonate formation and inorganic alkaline content(El-Naggaret al.,2019),which are considered as the principal sources contributing to alkaline pH of biochar(Dinget al.,2014).Total carbonates and total base cations have been observed to increase with increasing pyrolysis temperature,resulting in biochar pH value ranging from 6.5 to 11(Saifullahet al.,2018;Zhanget al.,2018).The high pH of biochar produced at high pyrolysis temperature is correlated with increases in O-containing functional groups and ash content that occur during feedstock pyrolysis(Zhaoet al.,2015).Further contributors to enhance the pH of biochar are the absence of acidic functional groups(i.e.,—COOH)and the presence of base functional groups during the pyrolysis of biochar(Zhanget al.,2019).Some authors have reported that changes in the pH of biochar are largely attributed to the separation of alkaline salts from organic compounds at high pyrolysis temperature(Dinget al.,2016;Czekałaet al.,2019).The increase in C and ash contents ranged from 62%to 93%with an increase in pyrolysis temperature from 350 to 750°C,owing to a high degree of polymerization that resulted in the formation of more compact C structure in the biochar(Domingueset al.,2017).Thus,variation in pyrolysis temperature influences the surface structure,pore spaces,functional groups,and other components of biochar,which in turn influence soil physical and chemical properties after biochar application.

Duration of pyrolysis

The retention time for processing biochar also influences the physical and chemical properties of biochar(Januset al.,2015;Yuanet al.,2019)(Table II).Rapid heating rate and short residence duration are necessary for the fast pyrolysisof biochar,which,however,reduces the secondary reactions and yield of biochar.In contrast,slow heating rate and long residence duration lead to higher biochar yield(Mohanet al.,2011;Januset al.,2015)(Table II).Currently,high recovery of biochar is obtained when raw material is pyrolyzed at 450—500°C for 45—60 min(Amarasingheet al.,2016;Yuanet al.,2019).Variation in retention time of pyrolysis significantly influences the pore size of biochar when the pyrolysis temperature is constant(Vitriet al.,2017).

TABLE II Effects of different retention times and temperatures on the characteristics of biochar

Pressure and flow rate of inert gas

Biochar is prepared in the absence of oxygen,and inert gas is used to create a vacuum in reactor chamber(Liet al.,2014).The composition of gas and its flow rate in the reactor chamber are considered essential factors for the pyrolysis of char(Januset al.,2015).For pyrolysis of feedstock,N2is generally used as an inert gas in the reactor chamber to provide an oxygen-deficient condition(Cantrellet al.,2012;Parket al.,2014).The pressure and flow rate of inert gas greatly influence the pyrolysis and biochar formation(Januset al.,2015).Pressure less than 0.5 MPa has been shown to enhance vapor activity in the biochar surface and result in the establishment of secondary biochar(Mašeket al.,2013).The flow rate of gas in the reactor chamber affects the formation of biochar,primary vapors,and generation of secondary biochar(Beiset al.,2002).To improve biochar yield,a low flow rate of inert gas is needed under slow pyrolysis,whereas a high flow rate is needed under fast pyrolysis(Yuanet al.,2019).In order to sustain the inert condition in the reactor chamber and eliminate vapors from the chamber,N2is injected in the chamber from the start of manufacturing biochar until the end of the pyrolysis process(Katyalet al.,2003).

Chemical additives

To enhance the effectiveness of biochars,different chemical compounds(e.g.,FeCl3,AlCl3,KOH,NH4Cl,and ZnCl2)are often added to biochar in pre-or post-pyrolysis(Dinget al.,2016).The main objective of chemical addition is to enhance the adsorption capacity of biochar by increasing its surface area,surface functional groups,and pore volume(Duanet al.,2019).The adsorption capacity of biochar makes this material very efficient with respect to the remediation of heavy metals and organic pollutants(Ahmadet al.,2019b).Feedstocks such as plant residues and wood chips are typically composed of cellulose and hemicellulose.During the manufacturing of biochar,chemical additives can inhibit the decomposition of hemicellulose and enhance the decomposition of cellulose,thereby contributing to dehydration(Yuanet al.,2019).If the phosphoric acid content in feedstock exceeds 30%(weight/weight),cellulose and hemicelluloses will undergo similar rates of decomposition,which could influence the component and yield of biochar(Demirbas,2004;Wanget al.,2018).In recent years,the use of composite feedstock has attracted considerable attention,as it can accommodate low-grade feedstock for the preparation of biochar.Each component in the composite feedstock is found to decompose individually under thermal pyrolysis(Daset al.,2016).Electron microscopy reveals the infiltration of polypropylene into biochar pores and a generally good dispersion in most composites.The chemical and crystal structure of the final bio-composite products shows an additive function of the individual components.However,although offering promising options,employing low-grade waste as a component of feedstocks also presents challenges.Identification of the properties of biochar plays a significant role in the fabrication and application of composite feedstocks(Yuet al.,2017),and therefore future research should focus on assessing the characteristics of bio-composites made from various wastes.

Activation and modification

Biochar is a C-rich material that can be applied to soil to minimize the bioavailability of contaminants present in polluted soil(Choet al.,2009).The porous structure and physicochemical characteristics of biochar are the precursors of activated C,which can be used to effectively remediate contaminated soil at low cost(Brändliet al.,2008).To enhance the adsorptive capacity of biochar,a range of alternative methods have been employed to activate biochar,including microwave activation,CO2activation,and alkali activation(Fig.1)(Yuanet al.,2019).Compared with nonactivated biochar,the biochar activated by excited steam accelerates plant nutrient uptake and retention(Kołtowskiet al.,2017).Therefore,steam activation of biochar is considered an attractive approach for future biochar production(Shimet al.,2015;Bardestani and Kaliaguine,2018).Similarly,biochar activation by alkali(NaOH and KOH)can enhance the porous structure,specific surface area,and adsorption capability of heavy metals(Parket al.,2013;Yuanet al.,2019).The NaOH-activated biochar pyrolyzed at 300°C has a high equilibrium concentration and rapid initial adsorption rate for phenanthrene,whereas the activated biochar pyrolyzed at 700°C shows stronger binding to ascorbate(Yuanet al.,2019).Compared with non-activated biochar,the biochar activatedviamicrowave activation has greater efficiency in reducing the toxicity of soil pollutants,whereas the biochar activated by CO2has a negative effect on soils,owing to heavy metal toxicity(Kołtowskiet al.,2017).Activation of biochar by potassium magnate,H2O2,and O3can contribute to enhancing the physiochemical properties due to oxidation of biochar(El-Hendawy,2003).Biochar oxidized by sulfuric and nitric acid is characterized by a high immobilization of secondary pollutants(i.e.,Cu,Hg,Pb,and Zn)due to having more carboxyl groups(Vithanageet al.,2015).Nevertheless,it is difficult to identify the most effective techniques for the activation of biochar(Subediet al.,2016;Yuanet al.,2019),given that activation can have both positive and negative effects depending on the method of activation,type of raw material,and soil type and properties(Kołtowskiet al.,2017).However,the activation and modification of biochar from various methods provide an opportunity to improve soil fertility,regulate soil chemical and physical properties,and remediate soil pollutants(Zhaoet al.,2013;Windeattet al.,2014).In general,the observed information offers opportunity to future researchers to design experiments based on the desired outcomes and requirements of biochar application.

Fig.1 Methods used for the activation and modification of biochar.

IMPORTANCE OFBIOCHAR FOR AGRICULTURAL SOILS

Physicochemical properties of soil

The application of biochar as a soil amendment helps to enhance the physical characteristics of nutrient-depleted and degraded soils(Agegnehuet al.,2017)(Fig.2).Furthermore,biochar enhances the infiltration rate of soil(Sombroeket al.,2003),which is beneficial with respect to minimizing erosion and overland flow(Glaseret al.,2001).Previous studies have revealed that the application of biochar to barren and infertile soils helps to reduce soil bulk density,enhance the total pore volume,and improve the water-holding capacity of soil(Chanet al.,2007;Abelet al.,2013).Compared with no biochar application,the application of biochar to a Haplic Acrisol soil reduced soil bulk density by 9%(Oguntundeet al.,2008).Moreover,biochar has been shown to improve soil porosity from 45.7%to 50.6%due to its porous structure and high surface area(Oguntundeet al.,2008;Agegnehuet al.,2017).Additionally,the saturated hydraulic conductivity of soil treated with biochar was increased by 88%from 6.1 to 11.4 cm h−1(Oguntundeet al.,2008),and the color of soil treated with biochar was darker,which resulted in higher soil temperatures(Sulimanet al.,2017).

Fig.2 Advantages of the application of biochar to agricultural soils.GHG=greenhouse gas.

The application of biochar to degraded and barren soils also improves the chemical characteristics of soil(Glaseret al.,2002;Agegnehuet al.,2017).Biochar applied to soil at more than 50 t ha−1has been reported to significantly improve soil organic C,pH(Chanet al.,2008),CEC,and N-use efficiency ofRaphanus sativusL.andZ.mays(Lairdet al.,2010;Beraet al.,2016).Application of paper-mill biochar at 10 t ha−1to Ferrosol soils significantly increased the CEC,total C,pH,and exchangeable calcium ions,and reduced the availability of Al,while application to Calcarosol soils increased the contents of exchangeable K+and C(Van Zwietenet al.,2010),and improved nutrient uptake(i.e.,P,Ca,Mg,and N)inVicia fabaL.andMedicago sativaL.(Agegnehuet al.,2017;Zhanget al.,2019).

Application of biochar not only contributes to raising the pH of soil,but has also been shown to enhance Ca level and minimize the toxicity of Al in a red ferralitic loam(Glaseret al.,2002;Steineret al.,2007).Researchers have reported variations in the response of soil pH to the types of biochar(Lehmanet al.,2003).Granatsteinet al.(2009)demonstrated that application of biochar produced from herbaceous feedstock at 39 t ha−1increased the pH of a sandy loam soil from 7.1 to 8.1,whereas Steineret al.(2007)reported that the application of biochar can change soil pH from 6.0 up to 9.6,depending on the type of feedstock and pyrolysis temperature.Biochar derived from woody feedstock had a less prominent effect on soil pH compared with those derived from other feedstocks(Lehmanet al.,2003).Soil CEC plays a vital role in altering soil pH,and biochar applied to Amazonian soils had less influence on soil pH,due to their high buffering and CEC(Granatsteinet al.,2009).In another experiment,application of biochar together with poultry manure compost to saline soil significantly decreased soil salinity by 3.6 g kg−1and increased soil pH by 0.3,soil organic C by 2.6 g kg−1,available P by 27 mg kg−1,and bulk density by 0.1 g cm−3(Lashariet al.,2013).Soils with a high CEC have a greater capacity to bind plant nutrient cations to biochar particles and clay and humus surfaces,thereby retaining these cations in the root zone for uptake by plants and preventing them leaching below the root zone and unavailable to plants(Lairdet al.,2010).

Application of fresh biochar to soil directly exposes this material to water and oxygen,thereby resulting in the oxidation of biochar,which in turn promotes to increase its negative charge and enhance the CEC of soil(Granatsteinet al.,2009).The surface of mature biochar can retain high amounts of negatively charged ions,which thus increases nutrient availability to plants and enhances soil aggregation(Josephet al.,2009).Similarly,biochar derived fromSaccharum officinarumL.bagasse can significantly enhance the cation and anion exchange capacity of soil and improve its nutrient-holding capacity(Inyanget al.,2010).The high reactivity of biochar surface,due to high CEC,is partly determined by the occurrence of reactive functional groups(i.e.,—OH,N,—COOH,Si—O—Si,and—CO)that mostly depends on pH(Cheng and Lehmann,2009).Aged biochar enhances the contents of OH−and R-COOH in soil(Lehmann and Joseph,2015),and the development of various quinine(C20H24N2O2)functional groups occurred due to aging of biochar in soil(Mukomeet al.,2014).Additionally,the presence of O-containing functional groups was observed on the aged biochar surface(Lehmann and Joseph,2015).

The specific surface area of biochar is considered an important factor influencing its adsorption capacity for heavy metals and secondary pollutants,as well as affecting soil microbial population and water-holding capacity(Nget al.,2014;Rajapakshaet al.,2016).Chenget al.(2008)observed that in contrast to younger and fresh biochar,aged biochar had a higher proportion of negative charge.The ion exchange capacity of biochar varies extensively,ranging from an anion exchange capacity of 120 mol kg−1to a cation ion exchange capacity of 250 mol kg−1,depending on pyrolysis conditions and raw material used for pyrolysis and its retention time(Yuanet al.,2011;Chenget al.,2014).

Nutrient retention and soil fertility

Compared with soil treated with other sources of organic matter,biochar-treated soil has higher retention and persistence of cations(Sohiet al.,2010;Loneet al.,2015).However,the cation retention of freshly produced biochar tends to be lower than that of aged biochar,and as yet,the time required for biochar to attain optimal cation-and anion-adsorbing capacities remains undetermined(Cheng and Lehmann,2009;Abivenet al.,2011;Chenget al.,2014).

Oxidation of biochar surface starts within one month following application,but it is limited to the outer surface of biochar particles in the soil,even after several years(Chenget al.,2006;Zimmerman,2010).Plant essential nutrients are retained in soil primarily due to the adsorption of organic matter and minerals.The capacity of soil to retain cations available to plants in the root zone,referred to as CEC,increases with presence of soil organic matter and biochar in soil(Glaseret al.,2002).The improvement of soil CEC is more pronounced in the soil treated with biochar,owing to its high surface area,negative charge,and charge density due to the presence of organic groups(Sombroeket al.,2003;Jaafaret al.,2015).

The capacity of biochar to improve soil CEC makes it a unique C compound that helps to enhance the uptake of nutrients by plants,minimize environmental pollution,and improve crop production(Lehmann,2007).Biochar contributes to reducing the depletion of nutrients,soil acidification,and fertilizer costs and enhancing crop yield(Chenget al.,2014).Biochar derived fromUmbellularia californicaL.significantly reduced the leaching of phosphate,ammonium,and nitrate by 20.6%,34.7%,and 34.0%,respectively(Agegnehuet al.,2017).Yaoet al.(2012)reported that the nitrate and ammonium leaching were reduced by 34%and 14%,respectively,with the application ofArachis hypogaeaL.hull biochar.In short,the addition of biochar can sustain soil fertility and reduce leaching of nitrate,N,K,Ca,P,Mg,and Na(Lairdet al.,2010;Majoret al.,2012).

Biochar is considered a source of plant-available nutrients(Sohiet al.,2010).Once biochar is added to soil,biotic and abiotic surface oxidation increases carboxyl groups,enhances negative charge,and improves its capability to adsorb cations(Yadavet al.,2018).Biochar also has ability to adsorb polar compounds,including both organic and inorganic environmental pollutants(Yuet al.,2006).The nutrients and C retained in biochar promote the process of mineralization in nutrient-depleted soil(Wardeet al.,2008).In sandy loam soil,addition of biochar enhances plant growth(Lehmanet al.,2003),increases soil organic matter,CEC,available P,and exchangeable Mg,Ca,and K,and improves nutrient uptake inZ.mays(Sukartonoet al.,2011).Similarly,the recovery of N in sandy loam soil is improved by the addition of biochar,whereas in silty loam soil,the addition of biochar does not facilitate the recovery of N(Yeboahet al.,2009).The addition of biochar has also been shown to enhance the availability of Mg and Ca in the soil by 77% and 320%,respectively(Majoret al.,2010).Biochar has a notably higher capacity to absorb phosphate than other crop residues and organic manures(Agegnehuet al.,2017).When applied in combination with fertilizers,biochar has been shown to increase plant available nutrients in soil,compared with the addition of fertilizer alone(Blackwellet al.,2009).

Although the addition of biochar can reduce nutrient leaching and greenhouse gas emission and improve C sequestration and fertilizer-use efficiency(Lehmannet al.,2003;Chan and Xu,2009),biochar addition without fertilizers causes the immobilization of N due to high C/N ratio in biochar(Lehmannet al.,2003).The efficacy of biochar and availability of nutrients in biochar are highly dependent on the type of feedstock,with that derived from livestock manures generally being more nutrient-rich than those derived from plant residues(Yuanet al.,2019).

Carbon sequestration and stabilization

Soil C sequestration refers the removal of CO2from the atmosphere,either artificially or naturally,and storage in liquid or solid form in soil for a long duration(Yadavet al.,2018).Conversion of organic matter to a highly stabilized form such as biochar can minimize CO2emission from the soil by reducing the decomposition rate(Sohiet al.,2009).The immediate proportion of CO2emission is released due to pyrolysis and is affected by the temperature used for pyrolysis(Gaunt and Lehmann,2008;Lehmannet al.,2011).

The production of biochar and its deposition in soil seem to be a promising and viable method for permanent storage of C in soil(Shaabanet al.,2018).Owing to the relative inertness of biochar,its deposition in soil augments the refractory organic C reservoir of soil(Bruunet al.,2011;Matovic,2011).Biochar application is therefore an alternative method to sequester more C,compared with other conventional agricultural practices that require direct biomass absorption and result in rapid and immediate mineralization and CO2release(Crombie and Mašek,2015).

When added to soil,biochar can persist in soil for decades,which efficiently enables C to store for long term sequestration(Bashiret al.,2018).Incubation analysis at the end of a year of C mineralization revealed that total soil C increased from 410 to 650 g kg−1in biochar-amended Inceptisol soils in India(Purakayasthaet al.,2015).The ability of biochar application for sequestration of soil organic C may reach 1 Pg C year−1(annual mean of C budget)or even higher(Lehmannet al.,2006;Sohiet al.,2010).The C content of biochar may be as high as≥600 to 800 g kg−1,which is equivalent to≥2.20 to 2.94 t CO2sequestered per ton of biochar(Vermaet al.,2014).If 10% of the global net primary production is converted to charcoal,at 50%yield and 30%energy from volatiles,it would sequester 4.8 Gt C year−1,which is approximately 20% more than the current annual increase of atmospheric C(4.1 Gt C year−1)(Matovic,2011).Biochar deposition in soil at a rate of 13.5 t ha−1offers storage space that would last two centuries(Sohiet al.,2010).Biochar is a viable C sequestration option for the whole world.The overall biomass reserves seem to be sufficient to fulfill the sequestration need and still provide other substitutions for fossil fuel use(Dinget al.,2016;Yuanet al.,2019).

Greenhouse gas emissions

The emissions of greenhouse gases and the consequent increase in the absorption of solar radiation in the atmosphere enhance global warming potential(Wahidet al.,2007).The application of biochar to soil helps to minimize the emissions of CO2and other greenhouse gases such as NO and CH4(Sohiet al.,2009)(Fig.2).Emission of NO from the soil is influenced by tillage practices and soil moisture content(Pekrunet al.,2003),and the application of biochar has been shown to minimize NO emission under no-tillage conditionsviaenhancement of soil water-holding capacity(Gaunt and Lehmann,2008).

Biochar has high surface area and therefore confines N to small pores that are inaccessible by denitrifying bacteria.Due to biochar’s high surface area,it enhances the waterholding capacity of soil and has been shown to suppress 90%of NO emission in wet soils containing 73%—78% waterfilled pore space(Yuanet al.,2019).With more than 83%of pore spaces filled with water,biochar shad a negative effect and enhanced the emission of NO from soil(Sohiet al.,2010).Biochar addition in arable soil reduced NO emission by 15%,where C content was low(i.e.,22 g kg−1)(Sohi,2012).In Columbia,addition of biochar to soil suppressed NO emission by 80%(Renner,2007).

The global warming potential of CH4is 21%,and the CH4concentration in the atmosphere is 6 times higher than that of NO.Industrial emissions,exploitation of natural gas,and paddy soils are the main sources of CH4emission into the atmosphere(Wahidet al.,2007).Several studies reported that biochar addition to rice paddy soil reduced CH4production(Inyanget al.,2014;Xu and Fang,2015).Biochar amendment can enhance the populations of methanotrophic proteobacteria that oxidizes methane production in soil and can reduce the populations of methanogenic archaea and methanotrophic archaea,which minimize the CH4emission from soil(Chen W Het al.,2015;Lahoriet al.,2017).

Furthermore,the addition of biochar to soil alters the abundance and microbial activity of denitrifying genes(nirKandnosZ)that regulate the emission of N2O(Shaabanet al.,2018).Biochar application at the rate of 20 g kg−1in paddy soil triggered N transformation and effectively minimized the N2O emission due to abundance ofnirKandnosZgenes in the soil(Aameret al.,2020).Borchardet al.(2019)conducted a meta-analysis and concluded that addition of biochar to paddy soil significantly reduced N2O emission and NO−3leaching by 38%and 13%,respectively,whereas in another meta-analysis,Cayuelaet al.(2014)reported a 54%reduction in N2O emission from soil in response to the application of biochar.A further meta-analysis of field study revealed that the application of biochar could contribute to a reduction of 12%—32% in N2O emission from soil(Verhoevenet al.,2017;Liuet al.,2018).

Biochar has also been shown to increase the dissolved organic C oxidation and reduction potential in soil(Agegnehuet al.,2015;Wanget al.,2018).Biochar amendment can reduce the macroporosity and saturated hydraulic conductivity of soil,thereby limiting the emission of CH4from soil to the atmosphere(Nguyenet al.,2004;Subediet al.,2016).Methanogenic gene abundance is positively correlated with nitrate and CH4production,and biochar has been shown to reduce the abundance of methanogenic genes in soil(Wanget al.,2016).Addition of 20 and 40 t ha−1ofO.sativastraw biochar to paddy soil reduced the emission of CH4by 29.7%and 15.6%,respectively,compared to no biochar addition(Shaabanet al.,2018).In some studies,the incorporation of biochar into soil provides potential substrates for methanogens and thereby enhances the production of CH4(Wanget al.,2012).These contrasting results are mainly ascribable to differences in biochar feedstock,application rate,pyrolysis temperature,retention time for pyrolysis,soil texture,and agronomic practices(Busset al.,2016;De Boecket al.,2016;Dinget al.,2016).

Remediation of soil pollutants

Agricultural soils are polluted with a diverse range of organic and inorganic pollutants(e.g.,nitrates,phosphates,antibiotics,pesticides,polyaromatic hydrocarbons,and metalloids)(Ahmadet al.,2018b,2019b;Zulfiqaret al.,2019).Environmental contamination with potentially toxic elements has increased dramatically since the beginning of industrialization and anthropogenic activities such as the smelting of ores and mining(Petruzzelli,2012).These pollutants can migrate off-site as a consequence of leaching into groundwater(Pugaet al.,2015).Addition of biochar tends to reduce the availability of metalloids and their toxicity to plants(Sohi,2012;Lyuet al.,2016).Moreover,biochar has a great ability to absorb organic pesticides,polycyclic aromatic hydrocarbons,antibiotics,dyes,and heavy metals(e.g.,Cd,Pb,Hg,and Cr)(Tanget al.,2013;Ahmadet al.,2014;Tranet al.,2015)(Fig.2).Nevertheless,physiochemical properties of biochar mainly control its adsorption ability for organic and inorganic pollutants(Zhuet al.,2005;Caoet al.,2011)(Table III).

Biochar derived from municipal sewage sludge has been shown to enhance the removal of Cd,with the removal efficiency increasing with an increase in the application rate of biochar in contaminated soil(Chen Tet al.,2015).The removal capacity of Cu for biochar derived fromArachis hypogaeaL.,Glycine maxL.,andBrassica napusL.straw was 89.0,53.0,and 37.0 mg g−1,respectively(Tonget al.,2011),which indicates that removal capacity of legumederived biochar is higher than that of biochar derived from non-legumes.Furthermore,biochar derived fromSaccharum officinarumL.straw was found to reduce soil Zn,Pb,and Cd concentrations by 54%,50%,and 56%,respectively(Pugaet al.,2015),and limited the uptake of heavy metal pollutants by plants(Dinget al.,2016).A combination of biochar and green waste compost reduce the concentrations of Pb and Cu in the shoots ofLoliumspecies(Karamiet al.,2011).The adsorption mechanisms are described as(Tanet al.,2015):1)increase in pH,CaO+H2O=CaOH2→Ca2++2OH−,2)precipitation,Cd2++2OH−=Cd(OH)2,3)cation exchange,M-Ca2++Cd2+→M-Cd2++Ca2+,and 4)competition and inhibition,M-Ca2++2H+→M-2H++Ca2+and M-Cd2++2H+→M-2H++Cd2+,where M represents the biochar matrix.

With respect to the interaction between biochar and heavy metal pollutants,several adsorption mechanisms have been described,including ion exchange,cation-π interactions,electrostatic attraction,precipitation of surface functional groups,and surface mineral adsorption(Inyanget al.,2012;Yanget al.,2014).The electrostatic attraction between biochar and heavy metal ions is ascribed to the negatively charged biochar surfaces(Inyanget al.,2012).With theaddition of biochar to soil,the electrostatic attraction between positive charges of heavy metals and the soil is enhanced,thereby increasing the adsorption of heavy metals(Uchimiyaet al.,2011).The main mechanisms involved in the adsorption of heavy metals are surface complexation and ion exchange(Gaskinet al.,2008).Mineral components(i.e.,Mn,Fe,Mg,Ca,and Si)present in biochar can increase the affinity and adsorption of heavy metals compared with other organic C compounds(Gaskinet al.,2008).The chemical reaction equations for adsorption of heavy metal pollutants are presented as:Cπ+2H2O→Cπ—H3O++OH−,Cπ—H3O++Cd2+→Cπ—Cd2++H3O+,or Cπ+Cd2+→Cπ—Cd2+,where Cπ is the cation-π complex(Lyuet al.,2016).

TABLE III Adsorption capacity of heavy metal pollutants in aqueous solution as affected by biochar type

Atmospheric N decomposition,agricultural wastes(farm manures and crop residues),and N fertilizers are the main sources of N that are becoming the part of food chain by dietary crops and contaminating the groundwater through surface and subsurface soil leaching(Xueet al.,2015;Ahmadet al.,2018a).High concentrations of NO−3cause eutrophication that contributes to harmful algal blooms that have potential paralyzing,diarrheal,and neurotoxic effects on terrestrial animals,humans,and marine life(Ahmadet al.,2018a;Aameret al.,2020).Given its highly porous structure,biochar is noted for the retention of nitrate within its pores(Haideret al.,2016).Addition of biochar to agricultural soils may influence the processes that regulate denitrification,nitrification,and other N transformation and loss pathways(Hagemannet al.,2017;Liuet al.,2018).

Numerous pharmaceutical antibiotics,i.e.,quinolones,macrolides,sulfonamides,tetracyclines(TCs),and chlortetracycline(CTC),have been widely used in animal and human medicine for the prevention against infection(Yuet al.,2016;Tanet al.,2019).Mainly,70%—90%of the ingested TCs cannot be metabolized by animals and poultry and the non-digested antibiotics will be excreted with excreta and manures(Acostaet al.,2016;Chenet al.,2017).Application of manures enriched with antibiotics to the soil can cause deterioration of the environment(Tanet al.,2019).Thus remediation of TCs and CTC from wastewater has gained much intention in recent years.Biochar modified by H2O2improves the sorption of TCs from aqueous solution on biochar,which indicated that the dominant sorption mechanisms in this process are H-bonding and π—π electron donor-acceptor interactions(Tanet al.,2019).Similarly,in another study,the modification ofPhoenixdactyliferaL.-derived biochar with zeolite,silica,and nano-zerovalent iron was found to significantly enhance the sorption capacity of biochar used for the remediation of CTC from aqueous solution,with the mechanisms being identified as inter-particle diffusion,H-bonding,and chemisorption(Ahmadet al.,2019b).

Addition of biochar in the soils polluted with pesticides increases soil water-holding capacity and aeration,and provides habitats for microbes to facilitate their growth in soil(Guptaet al.,2018).These phenomena help microbes to enhance their metabolic activities and degrade pesticide residues in soil(Raniet al.,2017).

Disease control

Addition of biochar to soil can minimize the vulnerability of plants to diseases because of the changes in metabolic pathways of plants(Gaoet al.,2016).Biochar addition to soil reduces fungal diseases ofSolanum lycopersicumL.andCapsicum annuumL.caused byBotrytis cinereaL.in Switzerland(Mehariet al.,2015).Similarly,the addition of biochar enhances the resistance ofFragaria ananassaL.againstPsdosphaera apahanisL.,Botrytis cinereaL.,andColletotrichum acutatumL.(Harelet al.,2012).Moreover,biochar application to soil reduces Fusarium root rot infection by 50%inAsparagus officinalisL.,compared with the soil without biochar addition(Głuszeket al.,2017).Biochar addition can also enhance the resistance ofC.annuumL.against tetranychidae species(Eladet al.,2010).Furthermore,biochar can absorb phenolic and xenobiotics from soil(Wanget al.,2014).Absorption of allelochemicals by biochar can minimize soil pathogens and improve the plant resistance against different diseases(Gaoet al.,2016).Biochar is composed of 2 phenoxyethanol,quinines,o-cresol,benzoic acid,butyric acid,hydroxyl-propionic,propylene glycol,and ethylene glycol,which can affect the growth of soil microbiota.With the addition of biochar in low concentrations,the above-stated compounds can suppress susceptible microbial species,enhance plant resistance,and improve crop production(Graberet al.,2010).

Improvement of soil microbiomes

The physiochemical properties of biochar can alter the biomass and activity of microbes,soil enzymatic activity,the ratio between bacteria and fungi,and microbial community structure and diversity(Ahmadet al.,2016).In certain cases,addition of biochar does not alter microbial activity and biomass,but significantly changes the microbial community structure in soil(Ajema,2018).In order to clearly identify the response of soil microbes to biochar amendment,analysis of gene copy number is more informative than determination of microbial biomass(Chenet al.,2013).Variations in the abundance and occurrence of Verucomicrobia,Gemmatimonadetes,Actinobacteria,and Acidobacteria in soil have been reported in response to biochar application(Nielsenet al.,2014;Mackieet al.,2015).The pore spaces and structure of biochar provide shelter for microbes(Quilliamet al.,2013),while the nutrients present in and adsorbed by biochar are available to support microbial growth(Josephet al.,2013).The addition of biochar has been shown to promote microbial population and modify microbial habitats by altering the water content,pH,and aeration of soil(Quilliamet al.,2013;Fanget al.,2014).Biochar stimulates the activities of microbial enzymes to differing extents,thus affecting the elemental cycles in soil associated with the microbial community(Lehmanet al.,2011).The interruption in inter-and intraspecific communication among microbial cells through hydrolysis and adsorption have been related to biochar in soil(Gaoet al.,2016;Yanget al.,2016),which is ascribed to the presence of certain molecules in biochar that serve as signals for the interaction and communication between microbes(Masielloet al.,2013).Biochar can remediate heavy metal pollutant toxicity and bioavailability to microbes in soil(Gaoet al.,2016;Stefaniuk and Oleszczuk,2016).The addition of 2%(weight/weight)of biochar in a pot experiment enhanced the growth of root-promoting bacteria in soil(Joneset al.,2011),and De Tenderet al.(2016)reported that the application of biochar increased the bacterial population in the rhizosphere by 72%compared with no biochar application.Enhanced growth ofFragaria ananassaL.and increased bacterial population were observed in white peat soil treated with 3%(weight/weight)Quercuswood biochar(De Tenderet al.,2016).The impact of biochar on the microbial biomass and colonization in the rhizosphere is linked to soil type.Increased microbial population in soil due to the addition of biochar increases soil respiration activity(Kolbet al.,2009).

Biochar type also influences soil microbial activity(Chintalaet al.,2014).Biochars derived from wood residue,Pinus ponderosaL.,Panicum virgatumL.,andZ.maysL.stover have been found to reduce the population of microbes and their activities(Ajema,2018).Foxet al.(2016)reported that biochar addition to soil enhanced the abundance of genera of S-and P-mobilizing bacteria,such asRhodanobacter,Acidothermus,Pseudorabies,Rhodobium,Bradyhizobium,Planctomyces,Isophaera,andBacillus.Moreover,Wanget al.(2016)reported an increase in the population of a newRhizobiumstrain T1-17 in soil in response to the addition of biochar derived fromO.sativastraw.The microbial oxidation of biochar is more efficient in soil with higher organic matter,and biochar oxidation does not occur in the absence of organic matter(Chenget al.,2006).

Improved crop yield

As a soil amendment,biochar can improve soil health,fertility,and microbial activity,reduce heavy metal toxicity,and subsequently enhance biomass accumulation and yield of crop plants(Table IV)(Yadavet al.,2018).Biochars derived from poultry litter,wood chips,paper pulp,and forest woods have been shown to positively enhance crop grain yield and biomass(Table IV).Application of biochar at a rate of 30 t ha−1increased the productivity of legume crops,vegetables,T.aestivumL.,Z.maysL.,andO.sativaL.by 30%,29%,11%,8%,and 7%,respectively,compared to no biochar application(Liuet al.,2014).The addition of biochar to soil can enhance soil microbial biomass,Rhizobiumnodulation,and nutrient availability to improve plant aboveground productivity and dry biomass(Biederman and Harpole,2013).Improved soil conditions and availability of P,K,and N from biochar can improve the yields of cereal and legume crops(Bashiret al.,2018;Yadavet al.,2018).Shaabanet al.(2018)found that the use of biochar significantly enhanced the grain yield ofZ.maysand reduced N2O emissions.Given its high N and low P contents,biochar is well suited for utilization as a fertilizer(Beesleyet al.,2011),and its application to soil can positively influence fertilizer-use efficiency in soil and thereby improve crop yield(Steineret al.,2008).

TABLE IV Increases of biomass yield of agronomic and horticultural crops as affected by biochar application,compared with the no-biochar control

MECHANISMS OFBIOCHAR-INDUCED AMELIORATION OFSOIL AND PLANT CHARACTERISTICS

Biochar has numerous attractive properties(e.g.,high CEC,surface area,pH,water-holding capacity,biodegradation,H/C and O/C ratios,and microporosity)for agricultural and environmental applications.These features of biochar can alter the physicochemical characteristics of soil.Biochar application in soil can enhance the absorption of nutrients by plants(Thies and Rillig,2009).The ash content of biochar plays a major role in the regulation of its pH and provides a rich source of inorganic minerals,particularly K,for plant growth(Chenet al.,2013;Chen W Het al.,2015).When applied to acidic soils,alkaline biochar can enhance crop production and soil quality by raising soil pH(Amelootet al.,2013;Curtin and Trolove,2013).Moreover,applying biochar in combination with other soil amendments,such as lime,compost,manure,crop residues,and organic fertilizers,can reduce the leaching of nutrients,compared with application of these amendments alone(Lairdet al.,2010;Iqbalet al.,2015).The pH of biochar also contributes to the distribution of charge within the soil,which improves soil CECviathe binding of soil organic matter with cation groups(Bassoet al.,2013).Biochar surface charge density can improve soil CEC and facilitate the retention of cations and maintenance of pH in soil(Lianget al.,2006;Czekałaet al.,2019).

The unique ability of biochar to absorb nutrients can be attributed to its porous surface and the interactions with surface functional groups(i.e.,hydroxyl,carboxyl,and alkyl groups).The interactions of these functional groups with nutrients prevent nutrient leaching and enhance the slow diffusion of nutrients within soil(Novaket al.,2009).The functional groups facilitate the adsorption of associated nutrients(i.e.,S,Ca,Mg,K,P,and N)and organic molecules on biochar,whereas microbial mineralization promotes the solubilization of complex-bound organic compounds present on the surface of biochar,thereby making them available for plant uptake.A probable mechanism for the improvement of crop production in biochar-amended soil is an increase in the water-holding capacity of soil(Jefferyet al.,2011).The high porosity of biochar can hold water in small pores and enhance the water-holding capacity of soil(Keiluweitet al.,2012;Peakeet al.,2014).Biochar can also enhance the infiltration of water derived from precipitation(Lianget al.,2006;Asaiet al.,2009).Soil amended with biochar interacts with several inorganic and organic substances;therefore,the interruption magnitude of microbial signaling depends on the free space on biochar surface where these substances can be adsorbed.In modern agriculture,the depletion of soil minerals,primarily as a consequence of soil degradation and nutrient leaching,is a cause of some concern(Dinget al.,2016).Since biochar contains organic matter and nutrients,its addition increases soil pH,EC,organic C,total N,available P,and CEC(Ahmadet al.,2014;Shaabanet al.,2018).It has also been reported to increase the availability of C,N,Ca,Mg,K,and P to plants,viathe absorption and slow release of applied fertilizers(Pandeyet al.,2020),which can improve plant growth and crop production(Zhanget al.,2019).

Abiotic stresses(e.g.,salinity and heavy metal toxicity)can reduce crop production by increasing the generation of reactive oxygen species and reducing antioxidant activities(Gallegoet al.,2012;Abbaset al.,2018).In addition,these abiotic stresses minimize gas exchange parameters in plants(i.e.,mesophyll conductance to CO2,stomata opening and closing,and activity of Rubisco),thereby reducing photosynthetic activity(Ehsanet al.,2014;Rizwanet al.,2016).Such changes restrict crop growth by causing a reduction in C fixation,membrane damage,loss of organelle function,ultrastructural alternation,and reduced plant nutrient uptake(Arshadet al.,2016).Addition of biochar to soil helps to remediate the toxicity of organic and inorganic pollutants and improves the antioxidant activities in plants(Zhanget al.,2019).Biochar application to heavy metal-polluted soil can enhance the activities of catalase and superoxide dismutase and reduce electron leakage and the activity of hydrogen peroxide,which can improve plant growth and productivity(Youniset al.,2016;Tomczyket al.,2020).

The high pH,active surface functional groups,and aromatic and porous structure of biochar(Ahmadet al.,2014,2019a)play vital roles in the remediation of organic and inorganic pollutantsviadiverse mechanism,including precipitation,complexation,electrostatic interaction,ion exchange,and physical adsorption(Wanget al.,2019).

Biochar can co-precipitate heavy metal ions to form insoluble carbonates and phosphates,thereby immobilizing these toxic ions in soil(Yuanet al.,2019).Biochar obtained at high temperature has high contents of C and ash(containing S,Si,Ca,Mg,P,K,and Na),which can react with toxic metal ions and convert these ions to insoluble minerals in soil(Shenet al.,2017).In addition,biochar derived from cow manure feedstock contains a high concentration of phosphate,which can immobilize the Pb in aqueous solutionviaprecipitation,forming insoluble pyromorphite,Pb5(PO4)3(OH),complexes(Caoet al.,2011;Wanget al.,2019).The surface of biochar is enriched with O-containing functional groups(—OH,—COH,and—COOH)that complex with metalloid ions to form stable complexes(Ahmadet al.,2014).Biochar produced by low-temperature pyrolysis comprises high concentrations of these O-containing functional groups that can efficiently immobilize metal ions through metal complexation(Matuštíket al.,2020).The O-containing functional groups of biochar also significantly increase ligands on the soil surface,which in turn facilitates to immobilize more heavy metal ionsviathe formation of ligand-heavy metal complexes(Wanget al.,2019).The mechanism of functional groups in biochar to form complexes with metal ions(Mn+)viacomplexation is described as:(Chenet al.,2012).

Due to having high negative charges,biochar can significantly enhance the electrostatic interaction between metal ions and soil particles to immobilize metalloid ionsviaelectrostatic attraction(Bashiret al.,2018).Application of biochar to soil enhances the pH and CEC of soil and therefore facilitates electrostatic interaction between soil particles and toxic metal ions(Penget al.,2011;Shaabanet al.,2018).The elective exchange of ionizable protons/exchangeable metal ions(i.e.,Mg2+,Ca2+,Na+,and K+)on the biochar surface with heavy metal ions is recognized as an ion exchange mechanism for metalloid remediation(Kaluset al.,2019).The high CEC of biochar can enhance the ion exchange effect between metal ions and biochar particles(Tomczyket al.,2020).

Biochars derived from feedstock pyrolyzed at temperature ranging from 250 to 300°C are characterized by a high CEC(El-Shafey,2010),whereas higher pyrolysis temperatures result in a reduction in the number of acidic O-containing functional groups and the O/C ratio,thereby reducing the CEC of biochar(Arthuret al.,2020).The ion exchange capacity of biochar is closely related to the pH of soil solution(Zhanget al.,2018;Ahmadet al.,2019a).When pH value of soil solution is below the pH of biochar at the point of zero charge,a larger number of heavy metal ions are attracted to the biochar surfaceviaion exchange process(Wanget al.,2019).Physical adsorption,also referred to as van der Waals adsorption,occurs during the interaction among adsorbent molecules and adsorbates(Beesleyet al.,2014).This physical adsorption is generally caused by the intermolecular forces of metal ions adsorbed on biochar and is typically dependent on the pore volume,surface area,and specific surface energy of biochar(Zhanget al.,2009;Wanget al.,2019).The physical adsorption of biochar tends to be enhanced with the increase in pyrolysis temperature of biochar,which enhances the pore volume and specific surface area of biochar,thereby improving the contact area between toxic metal ions and biochar(Dinget al.,2016;Yuanet al.,2019).

Biochar is obtained by pyrolysis of feedstock at high temperature for the burning of organic content such as lignin and cellulose(Pariset al.,2005;Purakayasthaet al.,2015).As the major proportion(70%—80%)of biochar is C,it can potentially provide the soil with more C compared with plant residue(approximately 40%)of comparable density(Matovic,2011;Smith,2016).Enhancing the concentration of stable C in biochar is one of the ways to increase the C sequestration potential in soil(El-Naggaret al.,2020).An alternative strategy to improve C sequestration in soil is to increase the yield of biochar,which can be achieved by increasing the pyrolysis temperature in conjunction with long retention time,thereby reducing solid yield and enhancing C release in a gaseous form,primarilyviaCO2emission(Yanget al.,2019;Yuanet al.,2019).However,from the perspectives of C capture and sequestration,it is generally more important to maximize stable C production,that is,to enhance the volume of stable C that can be produced from the same volume of biomass(Xieet al.,2016).Increasing and improving stable C yield is in turn equivalent to increasing the volume of biomass required to sequester a unit of C(El-Naggaret al.,2018).

The C stability of biochar is associated with its concentrated aromatic compounds that are influenced by the composition of feedstock and pyrolysis temperature,particularly high temperatures(Luet al.,2020).In addition to organic compounds(lignin,hemi-cellulose,and cellulose),feedstock also comprises inorganic compounds,with their concentrations and compositions depending on the feedstock type used for pyrolysis(Matuštíket al.,2020).Moreover,with respect to biochar yield,the composition of stable C and consequently the yield of stable C are the most important factors influencing C sequestration ability of biochar(Kaluset al.,2019).Addition of alkali metals during pyrolysis can enhance the yield of biochar,through catalysis,and the lignocellulose materials in feedstock(El-Naggaret al.,2019).Due to the presence of alkali metals,in the main decomposition of the holo-cellulosic fraction,the reaction cycle favors charring and dehydrating reactions over de-polymerization and fragmentation pathways(Tomczyket al.,2020).Addition of alkali metals enhances decarboxylation,demethoxylation,dehydration,and formation of biochar during lignin pyrolysis,thereby enhancing the stability of biochar(Xieet al.,2016;Yuanet al.,2019).

Another way to improve C stability is the addition of Si to feedstock during pyrolysis(Zimmermanet al.,2011;Ahmadet al.,2018b).Addition of SiO2particles with feedstock during pyrolysis may alter the C structural composition by creating Si-C coupling compounds,and consequently leads to form biochar-Si matrix(Xiaoet al.,2014).The encapsulation of Si-C bonding increases biochar density,which helps to prevent the thermal degradation of C by preserving C particles in the inner core and thereby enhances biochar stability(Guo and Chen,2014).Ahmadet al.(2018b)also found that SiO2can protect C particles from degradation,thereby enhancing C stability due to formation of Si-C complexes.After addition to soil,biochar becomes stabilized by interacting with soil particles(Kaluset al.,2019).Clay particles have a larger surface area for interacting with biochar and are therefore more effective in stabilizing biochar than other soil types(Shaabanet al.,2018).

NEGATIVE EFFECTS OFBIOCHAR

The effects of biochar on soil properties and soil microbial populations depend on the types of biochar and soil(Dinget al.,2016).The organic compounds,such as polyphenolics and phenolics,that may be present in charcoal can harm soil microbes(Gellet al.,2011).Warnocket al.(2007)noted that the addition of biochar to soil reduced microbial biomass and mycorrhizae.Other organic compounds present in biochar,such as bio-oil,toxic substances,pesticides,and polycyclic aromatic and non-aromatic hydrocarbons,can reduce the activities and abundance of microbes in soil(Enniset al.,2012).Application of biochar derived from residues ofO.sativaL.had a negative impact on the population of earthworm,including decreases in abundance and biomass and increases in genotoxicity and mortality(Haefeleet al.,2011;Weyers and Spokas,2011).The negative impact on earthworm population was possibly due to an increase in soil pH and high heavy metal concentrations in soil following biochar application(Mukherjee and Lal,2014).

It should not be concluded that if a particular biochar has positive effects on one type of soil,it might also have comparable effects on another soil type(Dinget al.,2016).Furthermore,biochar might enhance the growth of arbuscular mycorrhizae,but reduce plant growth(Rilliget al.,2010).Salts(Na or Cl),biochar property,volatile and non-volatile compounds,and soil biota are the factors that are responsible for negative response of biochar on plant growth(Gellet al.,2011).In an incubation experiment,incorporation of 4%(weight:weight)biochar derived fromT.aestivumL.residues in soil significantly increased the emission of N2O by 291%,compared with no biochar application(Linet al.,2017).The possible mechanism was due to the abundance of ammoniaoxidizing bacteriaamoAgenes(Shaabanet al.,2018).The addition ofPinuswood chip andArachis hypogaeaL.hull biochar to an acidic soil in USA reduced the yield ofZ.mays,compared with the control without biochar,under recommended fertilizer management(Gaskinet al.,2010).Similarly,the application of biochar to an Alfisol soil reduced the growth ofZ.mays(Rajkovichet al.,2012).In another study(Nzanzaet al.,2012),application of biochar derived from eucalyptus wood significantly reduced leaf P content and root dry weight ofSolanum lycopersicumL.by 26%and 13%,respectively,compared with the control.Such findings indicate that biochar may have a range of effects on crops and crop production,which is plausibly associated with the application of a specific type of biochar to a specific type of soil(Shaabanet al.,2018).

FUTURE CHALLENGES AND PERSPECTIVE

Currently,there is a rapid growth in the application of biochar to soil(Alburquerqueet al.,2014).The addition of biochar as a slow-release fertilizer can enhance the transfer of single and multiple nutrients from soil to plant roots(Dinget al.,2016).However,with regards to alkaline soils,the effects of biochar application on crop yield and soil quality remain unclear.Research,modification,and standardization are needed regarding pyrolysis conditions and feedstock type to produce biochar for specific applications,such as the adsorption of contaminants and mineralization desorption of nutrients(Agegnehuet al.,2017).In addition,much emphasis is required on studying the chemical interactions between biochar and soil surface,particularly with regards to micronutrients and ion exchange mechanisms.To date,biochar studies have focused on limited parameters,and accordingly,future research needs to examine a broader range of different parameters and involve collaborative efforts so as to fill knowledge gaps.Knowledge must be based on complete information(i.e.,biochar characteristics related to targeted application),which is reliable and replicated in the field.It is critical to determine the amounts and methods of biochar application to attain the maximum benefits.Consequently,more advanced research should focus on the effective rates,methods,and application times based on cropping system and crop growth stage.

The role of microbes in plant growth through the transformation of nutrients in biochar-amended soil is not well characterized.The correlations of microbes with biochar properties such as pH,nutrient content,microporosity,and particle size have not been examined so far.Soil nutrient deficiency is an important issue that can be offset by the application of biochar produced from waste residues,which thus provides new opportunities for the application of micronutrient-enriched biochar to nutrient-depleted soils.Apart from agricultural application to enhance crop nutrient uptake and reduce environmental pollution,biochar also has application potential in the industrial sector for waste management and thus the betterment of society,although a future challenge is to determine the relationships among the qualities and properties of biochar and the biochar manufacturing process for optimal utilization of biochar in soil.Moreover,there is a need to gain a thorough understanding of how biochar can minimize adverse effects on the atmosphere and soil.Models can be beneficially applied to evaluate the impact of alternative bioenergy systems on C footprints,at global,national,and regional scales,to support decisions regarding incentives and policies.

CONCLUSIONS

This paper provides an overview of the effects of different feedstocks and pyrolysis temperatures on biochar physiochemical properties and its practical utilization as a conditioner in agricultural soils.The physiochemical properties of biochar(i.e.,volatile matter,CEC,pore size,specific surface area,pH,and C and ash contents)can differ according to the pyrolysis temperature,retention time,feedstock type,and activation and modification of biochar during its production.These findings highlight that biochar should be carefully produced by selecting feedstock type and modifying preparation conditions according to the particular purpose of biochar utilization.Biochar shows the potential to contribute to resolving public health,economic,and environmental problems that are spreading worldwide and need to be addressed.Application of biochar improves soil physiochemical properties,soil fertility,soil microbiomes,and C sequestration,reduces greenhouse emissions,facilitates the remediation of soil organic and inorganic pollutants,and ultimately enhances crop productivity.Biochar may have a range of effects on crops and crop production,which can plausibly be attributed to the soil type-specific effects of biochar.The study of the effects of biochar on soil structures for long duration needs to be established under field conditions.Moreover,in the future,there is a need to further enhance the absorption capacity of biochar,either by activation or modification,and to examine the underlying mechanisms and practical implementation.Likewise,the potential risks associated with the application of biochar need to be addressed.In short,biochar still requires rigorous research or policy analyses to evaluate optimal biochar processing and popularization methods,and increasing attention should be paid to the suitability and sustainability of biochar as an effective soil remediation tool.

ACKNOWLEDGEMENTS

The authors wish to express their thanks to the Provincial Government of Gansu,China,for supporting the research.We are also grateful to the anonymous reviewers who helped us to improve the quality of the manuscript.