Utilization of lignocellulosic plant residues for compost formation and its role in improving soil fertility

2023-10-16 13:27ALOKIKAANUandBijenderSINGH
Pedosphere 2023年5期

ALOKIKA ,ANU and Bijender SINGH,2,*

1Laboratory of Bioprocess Technology,Department of Microbiology,Maharshi Dayan and University,Rohtak,Haryana 124001(India)

2Department of Biotechnology,Central University of Haryana,Jant-Pali,Mahendergarh,Haryana 123031(India)

ABSTRACT Globally,urbanization and a steady increase in population generate a huge amount of wastes,which leads to a series of economic,social,and environmental changes,mainly in developing countries.There is an utmost need for efficient management strategies for the beneficial utilization of these wastes into useful products.Among these strategies,composting is gaining attention due to its benefits of solid waste management,such as proper sterilization,and economical and effective bioconversion of lignocellulosic wastes to valuable products.Composting is an effective and sustainable approach for the management of various lignocellulosic wastes.This process comprises a series of effective waste treatment steps to ensure sustainable agriculture.Different composting methods have been explored for solid waste management.Furthermore,the influence of various factors relevant to composting has been elucidated.Microbes play a significant role in enhancing the degradation of lignocellulosic residues by secreting different hydrolytic enzymes.Compost has been utilized for increasing soil properties and improving plant growth.

Key Words: abiotic factors,composting method,plant growth promotion,remediation,sustainable agriculture,vermicomposting,waste management

INTRODUCTION

Lignocellulosic biomass is an easily available and renewable energy source and an alternative to fossil fuels that can potentially meet the energy requirements of the whole world (Lupoiet al.,2014;Anuet al.,2020a,c).Usually,lignocellulosic crop residues,such as wheat straw,rice straw,corncob,sugarcane bagasse,and many more,are a carbon(C)-rich biomass available in large quantities worldwide.Chemical pretreatment is generally preferred to enhance the nutritive value of these residues(Aquinoet al.,2019),among which sodium hydroxide pretreatment is highly favored by FAO(2008).However,these chemicals are costly and contribute to environmental pollution such as water pollution,air pollution,and others(Khanet al.,2022).These environmental problems can be tackled by developing economically efficient and feasible alternative processes for effective management of these residues.Therefore,composting has been explored as an eco-friendly and sustainable approach for the management of lignocellulosic waste(Heet al.,2021).Composting is a natural bio-process for converting agricultural residues into nutrient-rich effective composts,and thereby promotes sustainable agriculture(Kausaret al.,2016).Furthermore,compost can be extensively utilized as an effective plant growth promoter and soil fertilizer,thus reducing the dependence of farming on chemical fertilizers.Recent studies revealed that compost application alleviates climate and environmental changes,enhances plant growth,stabilizes soil,and minimizes water and air pollution(Limet al.,2016;Guoet al.,2019).However,these positive results are achieved using highly stable and mature compost,whereas immature compost contains phytotoxic compounds that inhibit seed germination and plant growth(Makan,2015).Conventional composting is usually dilatory because of relatively low nitrogen(N)in C-rich lignocellulosic residues,which can be overcome by adding N and cellulose-degrading microbes(Yuet al.,2017).These microbes boost biomass degradation through the enhanced activities of hydrolytic enzymes during composting (Weiet al.,2019).Composting relies on various N-fixing bacteria,includingAzomonas agilis,Bacillussp.,Paenibacillus azotofxans,Stenotrophomonassp.,and others.Composting of lignocellulosic residues is an eco-friendly and cost-effective process that addresses environmental pollution-related issues generated by open burning of the biomass and extensive utilization of chemicals in agriculture(Harindintwaliet al.,2020).Composting has been proven as a promising eco-friendly technique to recycle lignocellulosic wastes.Various byproducts formed during composting play an important role in improving soil properties and promoting plant growth in an eco-friendly manner.In developing countries,the composting process is a crucial technology in recycling of bio-degradable wastes generated during product formation.However,the commonly used conventional composting methods are slow and prevent farmers from implementing succession planting.Therefore,there is a requirement of developing rapid composting techniques for the management and degradation of lignocellulosic residues(Harindintwaliet al.,2020).The compost market is anticipated to reach an estimated 9.2 US dollars billion by 2024,with a compound annual growth rate of 6.8%between 2019 and 2024(Pete,2023).About 800 t of compost is purchased by landscapers annually,with South Korea being the global leader in composting.There was a 65%surge in composting programs globally in the last five years.Therefore,development of an ideal and sustainable process for compost formation from plant-based biomass will help to increase soil fertility and combat environmental pollution.

WORLDWIDE PRODUCTION OF VARIOUS LIGNOCELLULOSIC PLANT RESIDUES

Approximately 1×109t of lignocellulosic crop residues,such as wheat straw,rice straw,sugarcane bagasse,and corn stover,are produced every year,constituting the most abundant and easily available biomass on earth(Zhenget al.,2014).Availability of these residues usually depends on the plant species and differs among regions and countries.Usually,crops such as sugarcane,maize,wheat,and rice constitute the majority of the biomass compared to other crops (Sainiet al.,2015).Among these,sugarcane is the leading agricultural crop worldwide,with 1.9×109t produced annually,followed by maize(1.1×109t),wheat(7.717× 108t),and rice (7.696 × 108t) (FAO,2018).Wheat straw and rice straw are vegetative parts left after harvesting and usually include leaves,stems,panicles,and other parts(Bakkeret al.,2013).China and India,being the leading countries of rice straw production,contribute approximately 50% of the total world’s production of rice straw (Swainet al.,2019).China is the world’s largest producer of wheat straw(6.50×108t)followed by India,the United States,and Russia.About 45%of the world’s total wheat straw is generated by these four countries(FAO,2019).Corn stover mainly comprises cobs,leaves,and stalks left after harvesting corn grains.Approximately 1 kg of corn stover is produced after harvesting 1 kg of crop grain.The United States is the leading country for corn stover production,followed by China and Brazil (Saldivar and Perez-Carrillo,2015).Bagasse is fibrous waste produced after the extraction of juice from sugarcane.Approximately 200—300 kg of bagasse is produced after processing 1 t of sugarcane.Brazil is a leading producer of sugarcane(7.21×108t),followed by India(3.47×108t),China(1.23×108t),and Thailand(9.6×107t)(Melatiet al.,2017).According to data available with Times of India(2022),the civic body produced 1 267 t of raw manure and 231 t of fine manure in 2021,and 1 098 t of raw manure and 91.4 t of fine manure in 2020 in Chennai,India.

COMPOSITIONS OF LIGNOCELLULOSIC PLANT RESIDUES

Lignocellulosic residues,including forestry residues,agricultural residues,plant residues,and grasses,are composed of three main components:cellulose,hemicellulose,and lignin(Table I).Their compositions vary with different residues(Deviet al.,2022).Forestry residues include leaves,branches,bark,and other parts,while agricultural residues consist of wheat straw,rice straw,sugarcane bagasse,rice husk,barley straw,corn stover,and others,which are mainly left on fields after harvest,used as fodder or burnt in the fields(Adhikariet al.,2018).

Cellulose,as the main component of the plant cell wall,is a renewable and abundant resource on earth.It is a linear polymer ofD-glucose subunits linkedvia β-1,4 glycosidic bonds.The long chains are interlinked by van der Waals forces and hydrogen (intra-/intermolecular) bonds (Nijuet al.,2020).Naturally,cellulose mainly exists in bundles which aggregate to form crystalline microfibrils,and a small proportion is amorphous cellulose(Deviet al.,2022).

TABLE ICompositions of various lignocellulosic residues

Hemicellulose is the second most abundant polysaccharide after cellulose in the biosphere and the main component of plant fibrous materials.It is a short,heterogeneous biopolymer of multiple types of sugar units such as xylan,glucan,mannan,galactan,and glucomannan,which are linked byβ-1,4 linkages(Sainiet al.,2015;Harindintwaliet al.,2020;Sharma and Saini,2020).Galactan,arabinan,and xylan are components of straws and grasses,while mannan is present in softwood and hardwood hemicellulose(Anwaret al.,2014).Hemicellulose is an intermediate in the biosynthesis of cellulose and provides linkage between the cellulose microfibrils and the lignin matrix (Aftabet al.,2019).Hemicellulose interacts with lignin and celluloseviacovalent and hydrogen bonds,respectively,resulting in enhanced cell wall strength(Soreket al.,2014).

Lignin is the most complex and smallest fraction of plant residues,contributing nearly 10%—25%of the biomass by weight.It is a heterogeneous polymer composed of phenylpropane units consisting of coniferyl alcohol,ρ-coumaryl alcohol,and sinapyl alcohol linked by ether bonds.These polymers are produced from phenylalanine and tyrosine,the basic building blocks of lignin (Harindintwaliet al.,2020;Sharma and Saini,2020).Lignin behaves like glue,filling gaps around and between cellulose and hemicellulose molecules,producing a multistage fiber structure.Furthermore,lignin hinders the enzymatic degradation of cellulose and hemicellulose(Wertzet al.,2017).

Proteins are minor components of plant cell wall,and they may be covalently cross-linked with polysaccharides and lignin(Sainiet al.,2015).In addition to hydrogen(H),oxygen,and C,lignocellulosic residues contain many other elements,such as sulfur (S),iron (Fe),phosphorus (P),potassium(K),calcium(Ca),copper(Cu),magnesium(Mg),and zinc.When residues are dried at a high temperature,H,C,and S are released in gaseous form,and the remaining ash contains mineral elements in the oxide form.The content of ash differs between different residues;for example,straw ash content is in the range of 10%—25%,and wood ash content is very low(<1%)(Pei,2008).

PRETREATMENT OF LIGNOCELLULOSIC PLANT RESIDUES FOR COMPOSTING

Usually,the complex and recalcitrant structure of lignocellulosic biomass is highly resistant to microbial attack.Therefore,pretreatment of such residues is needed for effective composting to break down the complex packed structure of plant biomass and expose cellulose and hemicellulose to subsequent enzymatic hydrolysis(Anuet al.,2020b).An effective pretreatment also reduces the space and time required for composting.Different pretreatment strategies have been reported by researchers for the preparation of biomass residues/wastes for the composting process.The pretreatment methods for lignocellulosic substrates have been classified into three types:physical,chemical,and biological methods.These methods help to reduce biomass and pathogens,maintain moisture,and separate the lignocellulosic constituents,i.e.,cellulose,hemicellulose,and lignin(Karnchanawonget al.,2017).

Physical pretreatment

Physical pretreatment involves mechanical or extrusion methods for disrupting the lignocellulosic biomass.These methods usually decrease the particle size and crystallinity of the biomass,increasing the pore size and accessible surface area of cellulose for enzymatic hydrolysis(Putroet al.,2016).Currently used physical strategies for lignocellulosic feedstocks are classified into mechanical,radiation,pressure,and thermal (Zhenget al.,2014).Among these methods,mechanical pretreatment including milling,chipping,and grinding are widely preferred for the reduction of particle size and enhancing the digestibility of lignocellulosic biomass (Zhuet al.,2010).In this context,the composting process utilizes a novel hyperthermophilic pretreatment(HTPRT)technique for effective lignocellulosic degradation.In this method,the lignocellulosic biomass is preheated in a temperature-adjusted HTPRT reactor,before the traditional procedure of composting is applied(Caoet al.,2017).Although the temperature during HTPRT can be changed or adjusted based on the biomass,high temperature(e.g.,100°C) is usually used (Yamadaet al.,2007;Huanget al.,2019a).As a validated lignocellulosic residue composting method,HTPRT promotes compost maturation and humic substance formation by regulating the formation of microbial communities(Caoet al.,2019;Huanget al.,2019a),it maximizes the retention of N by reducing urease and protease activities(Cuiet al.,2019a,b;Huanget al.,2019b),it regulates the release of ammonia and nitrogenous byproducts during organic waste composting (Yamadaet al.,2007),and it plays an important role in establishing microbial community succession proceeding from the early stage to the final stage of composting(Yamadaet al.,2008).

Another example of physical pretreatment is hydrothermal treatment,which utilizes elevated temperature(e.g.,180°C)and pressure(e.g.,1.0 MPa)to expose the lignocellulosic biomass for composting.Nakhshinievet al.(2014)used hydrothermal treatment to evaluate the maturation and stability of lignocellulosic biomass(i.e.,rice straw)composting and reported effective composting at mild conditions.The main advantage of physical pretreatment is that it is eco-friendly and does not result in the formation of toxic compounds.The main disadvantages of this pretreatment are that it is time-consuming,energy-demanding,and expensive(Zhenget al.,2014).

Chemical pretreatment

This is the most commonly used pretreatment strategy for effective bioconversion of lignocellulosic substrates(Zhenget al.,2018).Chemicals commonly used to enhance composting include hydrochloric acid,sulfuric acid,sodium hydroxide,aqueous ammonia,Fenton’s reagent(solution of hydrogen peroxide and ferrous ion),and others(Karnchanawonget al.,2017;Denget al.,2019).Usually,chemical pretreatment is an effective pretreatment strategy for composting.For example,Fenton’s reagent pretreatment can boost the action of lignocellulosic residue-degrading enzymes and modify the bacterial population during composting(Wuet al.,2019).Wuet al.(2019) observed significant degradation of organic matter during rice straw composting using Fenton’s pretreatment coupled with bacterial inoculation.Fenton’s pretreatment mimics the mechanism of lignin degradation by fungi.Recently,this pretreatment has been proven as an ideal strategy that can enhance the feasibility of the pretreatment process (Shenget al.,2017).However,improper disposal of used chemical residues leads to severe environmental problems such as food web modification,water pollution,corrosion of sewage pipes,soil contamination,and others(Nguyenet al.,2010;Shenget al.,2017).Chemical pretreatment also results in the formation of various lignocellulosic byproducts(phenolic compounds,acetic acid,dicarboxylic acids,etc.)that inhibit microbial catalytic activity(Jönsson and Martín,2016).There is a requirement for the transition from traditional non-sustainable pretreatment to greener sustainable pretreatment methods for various lignocellulosic biomass.Microwave-assisted alkaline pretreatment enhanced the reduction of organic matter by 15% (Zahrimet al.,2017).Similarly,pretreatment ofSamanea samangreen waste with sodium hydroxide(1%—2%)effectively reduced lignin with N loss(18%—50%)as compared to the control,thus improving the composting process(Karnchanawonget al.,2017).

Biological pretreatment

Biological pretreatment involves extracellular enzymes produced by lignocellulolytic microorganisms to modify the lignocellulosic structure of the cell wall(Zhenget al.,2014).This pretreatment includes a microbial consortium,enzymatic hydrolysis,and aeration for the degradation of lignocellulosic biomass.Among these,white rot fungi(Basidiomycetes)are preferred to soft and brown rot fungi for lignocellulosic degradation.Similarly,the microbial consortium used also possesses high cellulase and hemicellulase activities along with lignin-degrading activity(Mielenz,2015).Therefore,bacterial and fungal enzymes (hydrolytic and oxidative)are favored for biological pretreatment of biomass(Zhenget al.,2014).The major advantages of this pretreatment method include cost-effectiveness,eco-friendliness,and low energy consumption.Zainudinet al.(2013)reported a shortened composting process of empty fruit bunch after the incorporation of 27 cellulolytic microbial strains.Similarly,N-rich compost was obtained one month after inoculation of straw waste withAzotobactersp.(Kyawet al.,2018).

We reviewed different types of pretreatments of lignocellulosic biomass,and a comparison between them are presented in Table II.Most conventional physical,chemical,and biological pretreatment methods are not suitable for large-scale composting because they are time,energy,and chemical consuming(Linet al.,2019).Therefore,further research should be focused on developing a novel,green,and cost-effective pretreatment strategy for lignocellulosic biomass composting.

COMPOSTING OF LIGNOCELLULOSIC PLANT RESIDUES

Composting is an aerobic or anaerobic process of biological degradation of organic matter into humus-like material called compost(Kuhadet al.,2011,Lim L Yet al.,2015).Mesophilic and thermophilic microorganisms consume organic matter and convert it into mineralized products (Qianet al.,2014).Various factors such as pH,temperature,moisture content,aeration,porosity,and initial C/N ratio affect the composting process(Shafawati and Siddiquee,2013).These parameters are used under controlled conditions to provide an optimum environment for degradation of organic matter by microorganisms during the composting process(López-Gonzálezet al.,2015).Composting of lignocellulosic residues is a sustainable and cost-effective way for reducing environmental pollution resulting from the utilization of chemical fertilizers in agriculture and open burning of the biomass.

TABLE IIAdvantages and disadvantages of the physical,chemical,and biological methods used to pretreat lignocellulosic plant residues for composting(Tan et al.,2021;Khan et al.,2022)

Various organic agricultural wastes,including sugarcane trash,rice straw,and sugarcane bagasse,have been used for composting(Singh and Nain,2014).Various organisms,such as bacteria,fungi,actinomycetes,and earthworms,play an important role in the composting process(Maccreadyet al.,2013)(Table III).Owing to their higher thermal tolerance,bacteria have been at the center of attention(Bonitoet al.,2010).CytophagaandCellulomonasare aerobic mesophilic bacteria that degrade cellulose,whereasBacillus polymyxa,Bacillus pumilus,Bacillus subtilis,Bacillus megaterium,andBacillus circulansdegrade both cellulose and hemicellulose.Fungi(Pleurotus ostreatus,Phanerochaete chrysosporium,Trichoderma harzianum,etc.)also play an important role in composting through production of extracellular enzymes that catalyze the decomposition of various polymeric compounds(De Ganneset al.,2013).Various actinomycetes such asStreptomyces,Micromonospora,andThermoactinomycesare capable of degrading crystalline cellulose(Sarithaet al.,2013).

Lignin composition in lignocellulosic residues affects the length of the composting process.Composting of paddy straw takes 180 d due to high lignin content,but the combination of cellulose-and lignin-degrading microorganisms hastens the composting of lignocellulosic residues such as trash,straw,and leaves(Singh and Nain,2014).The bulking agents and organic substrates,which are used in composting,are mostly obtained from plant materials.The composite polymer of vascular plant biomass is lignocellulose,which is composed of polysaccharides such as hemicellulose,cellulose,and lignin.Microorganisms can degrade these components by producing enzymes that are needed for degradation.A more comprehensive and extensive enzyme system is required for degradation of more complex substrates into smaller molecules that can be then utilized by microbial cells(Singh and Nain,2014).

There are different methods of composting,such as in-vessel composting,windrow composting,aerated-pile composting,continuous-feed composting,and vermicomposting.In-vessel composting is conducted in different types of vessels(e.g.,agitated bin,horizontal plug-flow,vertical plug-flow),whereas in windrow composting,solid waste is arranged in long rows.In aerated-pile composting,waste is arranged in piles and air is forced through the piles for faster composting.Continuous-feed composting is an extensive,fast composting protocol used for municipal waste and conducted through the control of environmental factors.Vermicomposting is a simple process of composting,in which both microorganisms and earthworms are utilized(Kuhadet al.,2011).

The composting process can be divided into three phases:the mesophilic phase,the thermophilic phase,and the cooling and maturation phase(Chowdhuryet al.,2013).Most of the decomposition occurs in the thermophilic phase in which microorganisms degrade the lignocellulosic residues.During microbial catabolism,heat is released,causing temperature to rise,which is crucial for sanitization and pathogen reduction(Singh and Kamaldhad,2014).At the end of the thermophilic stage,the temperature decreases and the maturation phase starts.At the end of the composting process,the temperature is decreased to ambient one(Sánchez-Monederoet al.,2010).

FACTORS AFFECTING COMPOSTING PROCESS

Composting is considered a type of microbial farming,where the microbes in the organic pile demand C and N as energy source to develop and maintain cells and enzymes.Composting process is highly affected by various chemical and physical factors,such as temperature,pH,moisture content,aeration,and properties of organic waste(particle size,C/N ratio,nutrients,etc.),which are discussed below(Silvaet al.,2014;Khater,2015;Khainget al.,2019).

C/N ratio

An efficient composting process requires an optimum C/N ratio of the organic biomass.The most effective composting occurs at the C/N ratio of approximately 25:1.However,some previous studies reported good results with C/N ratio in the range of 20—50 (Petricet al.,2015;Yanget al.,2015).As this ratio increases to 50:1 or decreases to 10:1,the overall conversion of biomass declines.For example,at a high C/N ratio,excessive accumulation of substrates leads to nutrient deficiency to microbes,slowing down the composting process.Similarly,at a low C/N ratio,ammonia and volatile fatty acids are generated,an offensive smell is developed,and the process of composting is inhibited(Kutsanedzieet al.,2015;Linet al.,2019).Bulking agents are usually mixed with the biomass to enhance its porosity during composting (Wanget al.,2016;Zhang and Sun,2016).These agents are effective in attaining appropriate composting by maintaining optimum aeration conditions throughout the process.Bulking agents not only absorb the excess moisture but also maintain a suitable C/N ratio of the wastes (Shaoet al.,2014;Jainet al.,2019).They also maintain the porosity of feedstock,which enhances the aeration rate and microbial activity(Uçaroğlu and Alkan,2016;Jainet al.,2019).Similarly,Silvaet al.(2016)proved the feasibility of an effective waste degradation process using wood chips as bulking waste and developed a compost with a validated standard value.

pH

pH is an important environmental factor that affects theactivity of microorganisms involved in composting.Usually,a pH value in the range of 5.5—8.0 is recommended as the optimum pH for composting (Zhang and Sun,2016).Low pH during the process leads to the volatilization of ammonia,resulting in the production of high amounts of acids and carbon dioxide(Wanget al.,2016).In contrast,the produced ammonia elevates the pH (>8) during the subsequent composting stage(Guoet al.,2012).Elevation in pH leads to the alkalization of composting biomass,thereby inhibiting the growth rate of pH-sensitive microbes and resulting in compost sanitation(Hachichaet al.,2009).Although bacteria can tolerate pH in the range of 5.0—8.0,acidic pH (4.0—5.0) supports an effective breakdown of cellulose and lignin by fungi(Torres-Climentet al.,2015).

Zhang and Sun (2016) reported a reduction in microbial activity and elevated pH (9.0) due to the production of nitrogenous compounds.

TABLE IIIEffects of various microorganisms on the composting of lignocellulosic plant residues

Oxygen

Among the composting factors,the aeration rate is one of the most important factors responsible for successful composting.Proper oxygen supply during the composting process is an essential requirement to reduce anoxic conditions(Cayuelaet al.,2012).Composting is usually conducted in aerobic conditions as aeration provides favorable environment for aerobic microbes,resulting in high-quality compost(Jusohet al.,2013).Latifahet al.(2015)recommended that the optimum oxygen considered for efficient composting should be between 15%—20%.Proper aeration during the initial composting stages shortens the processing time of waste stabilization and is thus responsible for the complete conversion of C into carbon dioxide and lower methane emission,whereas excessive aeration results in drastic outcomes related to decomposition rate of the wastes(Awasthiet al.,2014).Similarly,low aeration rate results in an anaerobic process that produces methane and hydrogen sulfide gas,whereas excessive aeration somewhat decelerates the composting due to loss of water,ammonia,and other compounds(Linet al.,2019).Various composting studies reported that the optimum aeration rate is highly affected by the raw material composition and the aeration method used during composting.Cabezaet al.(2013)reported an increment in biomass degradation and compost stability at an aeration rate of 0.05—0.175 L min-1kg-1during the composting of municipal solid waste(MSW)with legume trimmings.Considering that aeration is an important factor for effective composting,its optimization is necessary to achieve better results.A turning regime is a prerequisite for optimum aeration of the compost as it strongly affects the C/N ratio,temperature,pH,moisture content,and N content of the composting pile(Moheeet al.,2015).Awasthiet al.(2014) compared the bacterial succession in MSW composting under different turning regimes and observed a significant progress in composting under the three days per week turning regime in comparison to the daily turning regime.Furthermore,proper mixing of the composting waste for 30 min on a daily basis improved the composting of poultry manure and MSW(Petricet al.,2015).

Based on oxygen supply,conventional composting is classified into three types: windrow composting,aerated static pile composting,and in-vessel composting(Fig.1).Windrow composting,also known as on-farm composting,involves mechanical aeration(Fig.1a).Lignocellulosic residues are mixed in long narrow piles(windrows)followed by periodic agitation of composting materialviacompost turners.In the aerated static pile composting,the waste material remains static,that is,no mechanical aeration is conducted(Fig.1b).Air is pulled or pushed through the composting pile to maintain proper aeration.The in-vessel composting occurs in a vessel in which aeration is maintained by forced type aeration or agitation(Fig.1c).

Fig.1 Schematic diagrams showing three types of conventional composting based on oxygen supply:windrow composting(a),aerated static pile composting(b),and in-vessel composting(c).

Temperature

Composting is a natural self-heating bioprocess comprised of three phases:mesophilic(25—40°C),thermophilic(40—80°C),and cooling and maturation(10—40°C).Different types of microbial communities are dominant in each phase for degrading organic materials,resulting in heat production and temperature elevation.Temperature thus acts as a reliable factor in determining composting efficiency(Linet al.,2019).Temperature stimulates microbial growth and activity in composting piles,thus affecting the decomposition of organic matter during the composting process(Waszkieliset al.,2013).The optimum temperature stimulates the biodegradability of organic wastes and enhances the degradation rate of biomass during composting(Rastogiet al.,2020).Salamaet al.,(2016)reported 50—55°C as the optimum temperature for biomass degradation and enhanced sanitization of the overall composting process.Furthermore,temperature with appropriate processing time collectively stimulates the eradication of pathogens in composting biomass(Pandeyet al.,2016).Degradation of organic materials occurs at a faster rate in the temperature range of 32—60°C.Low temperature(<32°C)slows down the composting process,whereas high temperature (>60°C) kills many microbes,thus limiting the decomposition rate(Gerba and Pepper,2015).Excessive heat can be regulated by modifying the shape and size of composting massviathe turning process (Chowdhuryet al.,2013).Usually,the optimum composting temperature depends on the structure,shape,and properties(e.g.,C/N ratio,porosity,and humidity)of the composting pile,amount of oxygen,and other factors(Proiettiet al.,2016).Enhancement in temperature(50°C)was also reported by Jara-Samaniegoet al.(2017)during MSW composting.

Moisture content

Optimum moisture content is considered an essential factor to optimize the composting system as decomposing microorganisms require sufficient water for growth and various functions.Usually,a composting process requires 50%—60%moisture depending on the composition of lignocellulosic materials(Bernalet al.,2009).Wet composting matter limits the oxygen supply and favors anaerobic activity,whereas dry composting declines the microbial degradation of biomass (Lianget al.,2003).Moisture content essentially affects microbial activity,temperature,oxygen uptake,and porosity level of composting(Petricet al.,2015).Moisture content and temperature show an inverse relation:as temperature increases,moisture content decreases andvice versa(Varma and Kalamdhad,2015).Elevated temperature promotes evaporation,thereby reducing the decomposition of organic matter.Thus,wetting of composting piles is requisite to maintain suitable moisture conditions for proper functioning of the compost microbes.Inversely,higher moisture content results in water logs and anaerobic conditions that halt the composting process(Makanet al.,2013).Usually,high moisture content is needed to soften the hard fibrous component of the compost mass(Kádáret al.,2012).Kimet al.(2016)reported 57%as the optimum moisture content for the composting of rice hull and beet cattle manure mixture.

Particle size

The particle size of composting biomass is an important factor to ensure porosity level,proper aeration,and water or gas exchange during composting(Zhang and Sun,2016).Geet al.(2015)reported sieving as a convenient and fundamental process in determining the optimal distribution of differently sized particles in composting biomass.Chipping and shredding are also effective methods for achieving an appropriate particle size.Optimal particle size provides greater surface area and thereby enhances the microbial activity in composting piles.Usually,small particles promote compaction of biomass,leading to anaerobic conditions,whereas large particles have less surface area available for microbial action,resulting in more air pockets and subsequently lower matrix temperature and slower decomposition(Verma and Marschner,2013).Zhaoet al.(2017)reported that 25 mm is the optimum particle size for a tobacco composting process.

Inoculation

Inoculation is the addition of living microorganisms(bacteria,fungi,etc.)to a mixture of composting materials.Inoculation somewhat enhances the microbial metabolism indicated by the consumption of a high amount of organic acids as the nutrient source by microbes (Lim P Net al.,2015).Generally,composting does not require specific inoculum because various types of ubiquitously present microbes can recycle a variety of nutrients from dead organic matter and other waste products (Haug,2018).However,some composting studies reported that mature compost is usually produced in a short time after inoculation with specific microorganisms.Inoculation enhances the rate of changes in composting piles (Parveen and Padmaja,2011;Wanget al.,2011).Sahaet al.(2012)used an innovative strategy of paddy straw composting by inoculating the biomass with white rot fungus followed by the addition of poultry dropping in successive stage.This strategy resulted in 13.9%degradation of lignin,while effectively maintaining the C/N ratio.Awasthiet al.(2014)observed an improvement in the humification of municipal waste composting after inoculation with spore suspension of a thermophilic fungal consortium ofAspergillus flavus,Aspergillus niger,andTrichoderma viride.Therefore,inoculation of lignocellulosic residues with specific microorganisms is a feasible approach for enhancing the composting process and compost quality.Weiet al.(2019) used a consortium of thermophilic actinobacteria and reported enhanced lignocellulosic biomass(rice,wheat,soybean,and corn straws)degradation(34.3%).

VERMICOMPOSTING

Vermicomposting is a biological decomposition process that involves the interaction of earthworms and microorganisms for the conversion of different types of organic wastes into nutrient-rich manure.The resulting vermicompost has high porosity,low C/N ratio,high water-holding capacity,high enzyme activities,and high hormone,nutrient,and humic acid contents(Lim L Yet al.,2015;Amoueiet al.,2017;Sharma and Garg,2018).It is odor-free,homogeneous,and stabilized and contains significant quantities of nutrients but low levels of toxicants (Sharma and Garg,2018).Vermicomposting is an eco-friendly,viable,rapid,and cost-effective method as compared to other remediation processes used for the management of solid wastes.The method is highly affected by various factors such as pH,moisture content,C/N ratio,and the nature of organic waste.During the process,earthworms aerate the composting pileviatheir burrowing activity,which creates a more suitable environment for the biochemical breakdown of organic wastes by microbes (Arora and Kaur,2019).Chenet al.(2015)reported that various lignocellulosic residues,such as bagasse,leaf litter waste,olive cake,and agro-residues,have been successfully converted into vermicompost byEisenia fetida.Earthworms are classified into three categories named anecic,endogeic,and epigeic.Epigeic species(e.g.,E.fetida,Eisenia andrei,andEudrilus eugeniae)are most effective for the vermicomposting process (Suthar,2014;Lim P Net al.,2015).Both anecic and endogeic species are detritivores.Epigeic earthworm species feed directly on microorganisms and litter,and fungi compose a major component of earthworm’s diet (Brownet al.,2004).Fracchiaet al.(2006) found bacterial communities such asAcidobacteria,Gemmatimonadetes,andBacteroidetesin the vermicompost produced from animal manure mixed with crop residues and straw.Various lignocellulosic residues can be converted into vermicompost with the help of earthworm and microbes,which is used as manure in the fields.Composting also helps to reduce pollution and waste in the environment.The activity of earthworms during vermicomposting works in two phases: the active phase in which earthworms process the waste by modifying its physical state and microbial community,and the maturation phase in which microbes take over earthworms’ decomposed waste and earthworms are displaced toward the fresh layer of undigested waste(Domínguez,2004;Loreset al.,2006).Microbes are the best decomposers,and earthworms indirectly stimulate microbial population by comminuting organic waste and providing extra space for decomposition(Domínguezet al.,2010).

COMPOST AND SOIL PHYSICAL PROPERTIES

United States Environmental Protection Agency(2003)articulates soil erosion and runoffstorm water as major nonpoint sources of pollution.The rate of soil erosion or soil loss is 10—20 times higher in urban areas.Urbanization results in the removal of vegetation,stripping of upper soil,and compaction due to equipment,ultimately leading to the degradation of soil functions.This development usually results in increase of bulk density,loss of soil structure,and loss of organic matter.Various investigations confirm the important role of composting in improving the physical,chemical,and biological characteristics,nutrient content,organic matter,and vegetation establishment of the soil(Adugna,2016;Beck-Broichsitteret al.,2018).Usually,mature compost contains highly stable C,thus effectively enhancing soil organic matter in comparison to immature and fresh compost (Adugna,2016).The organic matter maintains soil fertility and prevents nutrient loss.It also promotes soil water-holding capacity and provides better aeration for seed generation and plant growth(Edwards and Hailu,2011).Compost has numerous beneficial effects on soil properties as it stabilizes and promotes crop productivity and quality.Compost incorporation usually decreases bulk density,enhances infiltration rate,and improves available water to plants(Kranzet al.,2020).Composting can also be a better option for developing efficient management strategies related to plant nutrients under various conditions(Adugna,2016).

Compost and bulk density

Soils in urban areas or construction sites are usually compact,with increased bulk density(Layman,2010).This increase in bulk density promotes excessive strength in dry soils and inappropriate aeration in wet soils,limiting root growth.Compost decreases soil density and affects soil structure by providing a mixture of low-density organic matter to soil.This reduction in bulk density somewhat enhances the porosity due to various interactions of organic and inorganic constituents(Amlingeret al.,2007;Adugna,2016).An increment in the amount of compost in soil decreases the bulk density of soil as reported by Brown and Cotton(2011).Reduction in bulk density enhances the pore space and increases macro-and meso-pores in soil,thus improving the soil tilth as well as soil stabilization and aggregation(Liuet al.,2007;Adugna,2016).The organic fraction is lighter than the mineral fraction of soil.Therefore,an increment in the organic fractionviacomposting reduces the bulk density and weight of soil(Brown and Cotton,2011).Somervilleet al.(2018)compared the effect of composting on bulk density in different soils (coarse sandy loam and loamy coarse sands)and reported a reduction in bulk density in these soils after 3(15%—26%)and 15(14%—25%)months of compost incorporation.They found a continuous reduction in bulk density in the case of deep tilling as compared to shallow tilling.Shiraziet al.(2017)also observed a reduction in bulk density of soil incorporated with yard waste compost compared to the control without compost application.

Compost and aggregate stability

Soil structure is determined by the size,spatial arrangement,and aggregation of soil particles.Generally,the more the soil is compacted,the more adverse is the soil for plant growth(Adugna,2016).The addition of compost somewhat increases the aggregation stability.Usually,humified and fresh aggregates are expected to produce positive results.The effect of compost application depends on compost quantity and type,soil type,and application period.Bouajila and Sanaa(2011)reported that the application of organicrich compost(household waste and manure)resulted in a remarkable increase in soil structural stability.

Compost and infltration rate

The amount of available water to a plant depends on the amount of water that can infiltrate into the soil.Reduction in infiltration rate increases runoff,water ponding,and soil erosion,which inhibits the establishment of plants.Compost application increases water infiltration rate(Logsdonet al.,2017).Bouajila and Sanaa(2011)reported enhanced water infiltration with household waste and manure composts(596.46 and 549.25 cm,respectively) as compared to the control(332.16 cm).Enhanced infiltration improves water efficiency as soil with a high infiltration rate is capable to absorb the excessive fraction of rainfall or irrigation water(Fischer and Glaser,2012).Soil texture also shows a prominent effect on infiltration rate.Fine-textured soil has a lower infiltration rate as compared to coarse soil(Brown and Cotton,2011).Compost addition reduces soil erosion and improves soil structure,infiltration rate,aggregation stability,and pore volume(Adugna,2016).Many studies reported an increase in infiltration rate with compost incorporation due to an increase in porosity and organic matter,a reduction in soil bulk density,and other factors(Chen,2015).Similarly,Logsdonet al.(2017) reported increased infiltration rate with yard waste compost application.

Compost and water-holding capacity

The water-holding capacity of a soil is the amount of water it can hold.Water-holding capacity and field capacity are usually affected by particle size,the content and structure of organic matter,and other parameters (Adugna,2016).Compost has a high water-holding capacity to provide sufficient water to plants(Cogger,2005;Kranzet al.,2020).Various studies reported increased soil water retention with the addition of compost(Logsdonet al.,2017;Schmidet al.,2017).Logsdonet al.(2017)examined the water content in lawn soil with or without yard waste compost and observed high water content in the soil incorporated with compost as compared to the unamended soil.Schmidet al.(2017)investigated the effect of yard waste compost incorporation and reported increased soil water content(6%—9%)as compared to the control.However,the improved soil water content was only observed in the first 12 months,and no significant difference in soil water content between the compost-amended soil and control was observed thereafter.Brown and Cotton(2011)reported that texture and organic matter content are important factors affecting soil water-holding capacity.They also documented that compost showed a stronger positive effect on the water-holding capacity of coarsely textured soil in comparison to finely textured soil.

COMPOST APPLICATIONS

Due to the biological decomposition of lignocellulosic residues such as manure from cattle farms,crop residues,and unsold agricultural residues,production of compost has become economically beneficial (Fig.2).These residues are easily degradable and improve soil nutrient and organic matter contents,C sequestration,resistance to erosion,groundwater retention capacity,plant disease resistance,and plant growth (Pergolaet al.,2018).They reduce the use of costly synthetic fertilizers,which cause environmental pollution(Zhaoet al.,2017).

Fig.2 Flow diagram showing the formation of compost from lignocellulosic residues and its application in sustainable agriculture.

Compost in plant growth promotion

Nowadays,there is a focus on alternatives to inorganic fertilizers and chemical pesticides.In this case,there is the use of compost,which is rich in microbes and promotes plant growth(Hameedaet al.,2007)(Table IV).Compost promotes plant growth by providing primary and secondary nutrients such as S,K,Mg,Fe,Cu,Ca,N,and P at low levels (Risse and Faucette,2017).Lazcanoet al.(2009)reported a significant impact on root and shoot growth of tomato plants after the application of compost in comparison to peat moss.Some studies have reported a reduction in soil-borne plant disease after compost application(Mehtaet al.,2014).Nitrogen is a major plant-growth limiting nutrient.Arifet al.(2017) reported improved seed quality and higher fatty acid and seed oil contents in sunflower with the combined application of N-enriched compost and plant growth-promoting rhizobacteria.Compost usage reduces the use of fertilizers in the fields and increases crop yield.Premalathaet al.(2017) found that the combined use of crop residue compost with bacterial consortium reduces fertilizer usage without compromising the yield and reduces the production cost of black gram as well.Ilayaraja and Dhanarajan (2011) reported that the use of compost with a microbial consortium(Azospirillum,Rhizobium,andPseudomonas)improves the growth and yield of plants.Soil fertility is also increased with compost application in organic farming practices.Radyet al.(2016)reported that the use of organo-mineral fertilizer compost showed a positive effect on soil physio-biochemical characteristics and the growth ofPhaseolus vulgarisas compared to the control.According to Hameedaet al.(2006),compost combined with a bacterial consortium (Serratia marcescens,Pseudomonassp.,andBacillus circulans)enhanced the growth of pearl millet.

Compost for pollution prevention and remediation

A major problem in the world is air pollution,which causes millions of deaths worldwide every year (Lovettet al.,2009).Air pollution is caused by the use of pesticides,fertilizers,and insecticides and open burning of crop residues,which releases a huge amount of toxic gases into the atmosphere (Wuet al.,2018).An alternative to chemical fertilizers that can reduce air pollution in the environment is compost.Compost releases nutrients slowly to plants.Composting also helps to reduce greenhouse gas emissions and stabilizes the nutrients(Kausaret al.,2016).

The application of compost for bioremediation of organic pollution is a new topic of research.Compost has the capability to remove recalcitrant organic pollutants such as total petroleum hydrocarbons,polychlorinated dibenzo-pdioxins and furans,diesel,polychlorinated biphenyls,and organochlorine pesticides(Huanget al.,2019;Tranet al.,2021).Compost is helpful in treatments of toxic pollutants at lower concentrations.Composting as a biological process assures enhanced bioremediation due to suitable oxygen content,particle size,and moisture content.Compost application for bioremediation is beneficial over traditional approaches due to its ease of operation and diverse microbial community(Linet al.,2022).

Compost for sedimentation and soil erosion control

Compost is also used in sedimentation and soil erosion control.Risse and Faucette (2017) reported that the application of compost significantly improved the quality of water by boosting plant establishment,enhancing flood management,and reducing nutrient loads.One study has reported reduction of soil erosion and runoffin erosionsensitive areas along with faster establishment of vegetation in those areas after compost application(Persynet al.,2004).

CONCLUSIONS AND FUTURE PERSPECTIVES

Excessive and intensive utilization of chemicals adversely affects the environment as well as food quality and safety.Compost as an organic amendment acts as an ecofriendly and economical alternative rich in organic matter and important nutrients that can improve soil fertility.Compost releases nutrients slowly for a longer period,thus effectively increasing soil organic matter content and maintaining soil water-holding capacity and structure.From an environmental point of view,biological composting is an appropriate method for maximal degradation of biomass.Further incorporation of cost-effective pretreatment strategies,such as genetically modified organisms,can improve the bioconversion of organic materials and composting efficiency.Government can also play an important role by addressing the issues related to waste collection,management,segregation,and other processes.Additionally,compost quality should be determined to assure its non-toxicity to plant growth.Vermicomposting in combination with suitable microbial consortia can be a suitable and ideal strategy for compost formation from lignocellulosic biomass that is generated after crop harvesting.

ACKNOWLEDGEMENTS

Mrs.Alokika acknowledges the financial assistance as Senior Research Fellowship (No.09/382(0179)/2016-EMR1) from the Council of Scientific and Industrial Research(CSIR),New Delhi,India during the tenure of this research work.Ms.Anu acknowledges the Haryana State Council for Science and Technology,Panchkula,India(Nos.1743 and HSCST/R&D/2017/62) for providing financial support during the tenure of this research work.

CONTRIBUTION OF AUTHORS

Alokika and Anu contributed equally as first authors.