Potential value of biochar as a soil amendment : A review

This article reviews a range of beneficial impacts of biochar on soil physico-chemical properties and crop yield. Advances in biochar research appeal for identification of beneficial effect of biochar using as a soil amendment before any large-scale field application is recommended. Thus, the purpose of this review are to evaluate the potential beneficial effect of biochar using as a soil amendment. Biochar, a product of biomass pyrolysis, and is usually characterized as rich in recalcitrant C, with a large surface area, and diverse functional groups, although these features largely depend on the feedstock and pyrolysis conditions. Pyrolysis is a thermochemical process that transforms biomass into biochar, bio-oil, and syngas. The use of biochar as a soil amendment has received growing attention due to its ability to enhance crop productivity and improve physicochemical properties of soil. Compared to other soil amendments, the high surface area and porosity of biochar enable it to adsorb or retain nutrients, contaminants and water and also provide a habitat for beneficial micro-organisms. Generally, the ash fraction of biochar consisted of nutrients including N, P, K, S, Ca, Mg, Mn, Fe, and Zn which are required for plant growth. Although biochar has the potential value to use as a soil amendment but still need efficient road-map for biochar production, classification, and its effect in different soil-environment and cost–benefit analysis, must be developed before implementation of field-scale application.


Introduction
Biochar is the product of thermal degradation of organic materials in the absence of air (pyrolysis), and is distinguished from charcoal by its use as a soil amendment.It is a C rich material produced thorough pyrolysis process by heating any biomass like manure, organic wastes, bioenergy crops (grasses, willows) and crop residues.Biochar production is a simple tool that can (1) enhance soil fertility by improving its physico-chemical properties (2) combat global warming (3) reduce organic/ agricultural waste; and (4) produce renewable energy (syngas and bio-oil).The chemical composition of feedstock and pyrolysis temperature has a significant influence on biochar properties because each feedstock has different elemental composition and their thermal degradation at different temperature differs consequently.Characterizing the properties of biochar from different biomass materials under different production conditions will enable a mechanistic understanding of the effects of different biochars on soil properties and crop nutrients.Therefore, characterizing the properties of biochar is important before its use in agriculture and environmental management (Figure 1).[3] reported that cellulose and hemicellulose decompose at 220-400 °C, whereas lignin is resistant to decomposition above 400 °C.Furthermore, biochar originates from the lignin component of the biomass, whereas bio-oil comes from cellulose at a pyrolysis temperature of 500 °C.
The biochar derived from manure is typically rich in soil nutrients such as N, P, Ca, Mg, and K [4-7].The primary reason for the application of wastewater sludge for biochar production and agricultural utilization is higher concentrations of N and P in wastewater sludge biochar, as well as other micro and macronutrients [8].More recently, [9] reported Achnatherum splendens L. biomass for biochar production and found high ash content in biochar obtained at 700 °C indicating that a significant amount of mineral nutrients are present in biochar and offers a greater potential as a soil amendment.The digested and undigested biomass derived biochar have significantly different surface area.The digested biomass derived biochar had significant N2 surface area (336 m 2 /g) and biochar from undigested biomass was very small (2.60 m 2 /g) respectively [10].Keiluweit et al. [11].2010 reported surface area values for wood char (347 m 2 /g) were significantly higher than those of grass char (140 m 2 /g) (Figure 2).found increasing trend in pH, EC and ash content with increasing pyrolysis temperature.So, both feedstock source and pyrolysis temperature decide the final properties of biochar (Figure 3).

Biochar as a soil amendment
A part from being a C source, biochar has been shown to produce changes in the soil chemical, physical properties and enhance plant growth when used as an organic soil amendment.The higher productivity of "Terra Preta" soils that were regularly amended with biochar and other organic materials than un-amendment soil, led to world-wide interest in applying biochar to agricultural soils.

Biochar effect on soil biota
There is some evidence from scientific literature that soil microbial communities are responsive to biochar amendment.Biochar incorporation to soil has been suggested as a strategy to improve crop productivity and soil quality, which may also affect microbial activity.It is well documented that biochar physico-chemical properties as well as induced changes by biochar in soil physicochemical can change the activities of soil microorganisms.Biochar is porous body where pores serve as habitat for soil microorganisms [42,43].It can be concluded from the study of [43] that biochar macropores (>200 nm) are the right size to accommodate bacteria which probably represent most of the protected microbial habitats.The biochar production temperature determine the size of these pores, where at elevated temperature organic matter volatilized creating larger pores.Moreover, the pore size and abundance is also determined by the biochar feedstock.The biochar greater surface area leads to more chances for microbial colonization.The biochar chemical properties like (1) its surface charge, which binds microbial cells, chemical compounds and ions, and (2) the concentration of nutrients and DOC that are desorbed or solubilized from the biochar, can account for microbial growth on biochar surfaces and within its pores [44].According to [44] that biochar pH as well as the nature of the DOC and other metabolisable C compounds are likely to be important controllers of microorganisms growing on biochars.Moreover, biochar surface, porosity and the size can exemplifies a suitable niche for microbial colonization, where biochar does not provide microorganisms with as much mineralizable C and nutrient sources as the bulk soil.Biochar for remediation of contaminated soil Biochar has recently been recommended to remediate contaminated soil by sorption of both heavy metal and organic pollutants and reduce their mobility.Cao et al. [5] tested the capability of biochar produced from dairy manure at 500 °C, in removing Pb and atrazine from aqueous solution.The biochar exhibited significant ability of adsorption for Pb and atrazine, with Pb and atrazine removal by as high as 100% and 77%, respectively.The author concluded that dairy manure can be converted into biochar as an effective adsorbent for application in environmental remediation.More recently, [45] suggested the long-term effectiveness and potential of biochar application in immobilising heavy metals in contaminated soil.It was investigated that the freely dissolved concentration of PAH in sewage sludge can significantly decrease in the presence of biochar, with the 0-57% reduction depending on the added amount of biochar [46].Oleszczuk et al. [47] inspected the sorption of the terbuthylazine in biochar-amended soils and found that the adsorption coefficient increased by 63 and 2.7 times in the BC700 and BC350 biochar amendment soil respectively.Based on many published research reports, two important mechanism such as surface adsorption and partition is responsible for organic pollutants removal.Adsorption refers to the surface interactions leading to adhesion of pollutant molecules to biochar surfaces, whereas sorption includes both surface adsorption as well as partition of pollutant molecules in the micropores of biochar (without differentiating the two processes).

Biochar as a nutrients source
Biochar can serve as a direct nutrient supply or indirectly increase soil nutrients availability.Smider et al. [40] reported agronomic performance of a high ash biochar produced from tomato green waste by incorporation into soil resulted increased in the shoot dry matter of corn.The increased in shoot dry matter of corn was attributed to release of nutrients from the biochar and biochar liming effect and associated increased availability of nutrients.Generally, the ash fraction of biochar consisted of nutrients including N, P, K, S, Ca, Mg, Mn, Fe, and Zn which are required for plant growth.In a greenhouse experiment, radish yields (up to 96%) increases from application of biochar produced from poultry litter and suggested that this increased yield was mainly due to the biochar's ability to increase N availability [36].During the two rice/wheat rotations, seasonal application of 4.5 t ha −1 and 9.0 t ha −1 biochar increased the total rice/wheat crop biomass 24.3% and 34.3%, respectively [41].However, not all soils demonstrate broader improvements, and not all crops behave in the same way with biochar amendment [48].Güereña et al. [49] reported that biochar addition did not improve maize yield and plant N uptake.In fact, some biochars may have adverse effects on plant growth, and not all soils respond to biochar additions in the same way.

Conclusions
Biochar is seen to be beneficial an improving soil physico-chemical properties, soil biota, crop productivity and remediating contaminated soil and recycling agricultural wastes.Biochar, using as a soil amendment have multiple benefits, interlinked and include both direct and indirect effect.Therefore, biochar can be potentially an attractive soil amendment in modern agriculture to solve food and environmental problems.Obviously further research is needed to evaluate full potential of biochar as a soil amendment with possibly various benefits to the agriculture and environment.

Figure 2 .
Figure 2. Different feedstocks and their biochars.Ondrej Masek, UK Biochar Research Centre (UKBRC) Increasing data show that amending soil with biochar has remarkable effects on the physical, chemical, and microbiological properties of soil [19-21].The most notable effects may be the increase in the stable soil C pool and reduced greenhouse gas emissions [22,23].Thus, amending soil with biochar is a way to sequester C due to the persistence of biochar-C in soil .

Figure 3 .
Figure 3. Multi-dimensional benefits of biochar an agriculture and environment