Biochar, also called “black carbon” is a carbon rich material which is produced by heating biomass at around 400°C in the absence of air. The process is called pyrolysis. During pyrolysis (Fig. 1), 50% to 80% of biomass is converted into combustible liquids called biooil which has many industrial applications.

The remaining solid is called biochar, which retains some residual feedstock properties but is essentially composed of stable form of carbon. It is believed that biochar can store carbon in soil for hundreds to thousands of years and thus level of green house gases like CO₂ and methane can be reduced significantly. Along with environmental benefits biochar also improves soil properties and functions. From the Amazonians’ primitive technology called “Terra-Preta” of enhancing soil productivity with charred biomass, biochar has emerged as a viable technique for carbon sequestration in soil (Lehmann et al 2007; Mandal et al., 2015; Chaturvedi et al. 2021).

Effect of biochar on soil

Presence of biochar in the soil mixture influences the texture, structure, porosity and consistency through changing the bulk surface area, pore-size distribution, particle-size distribution, density and packing. Thus biochar’s effect on soil physical properties may then have a direct impact upon plant growth because the penetration depth and availability of air and water within the root zone is determined largely by the physical make-up of soil horizons. Therefore directly and indirectly, many chemical and biological aspects of soil fertility can be affected by application of biochar.

Feedstock for biochar

Any lignocellulosic biomass can be used to produce biochar. Agricultural and forestry residues are suitable feedstock for biochar production. Waste is produced in significant amounts from field crop residues such as paddy straw, wheat straw, maize stalk and many other remnants of agricultural crops in the field. Forest residues include logging residues, dead wood, excess saplings, pole trees etc.

Properties of biochar

The chemical, physical, morphological and spectral properties of biochar are largely influenced by process temperature and duration. Increasing temperature and duration decreases the biochar yield and volatile matter content but increases C, K and P contents as well as mean residence time. It was estimated that mean residence time range from hundreds to thousands of years and generally increasing with charring temperature and duration. pH of biochar falls normally in alkali range. One of the important feature of biochar is the pore structure and pore spaces. With good feedstock the amount of nano and micro pores. increases which is very much helpful for production enriched biochar. Surface area of 1 gm of biochar may even go up to 600 m².

Biochar enrichment

Although biochar has many beneficial impacts on soil and crop growth, its adoption by farming community is limited due to its direct impact on crop productivity. Thus use of biochar for the sake of environmental benefit, can be increased by promoting biochar as a carrier of micro and macro nutrients. Researchers reported increase in oxygenated functional groups, acidity and negative charge at the surface of biochar particles (Chia et al. 2014). These negative charges are suitable to attach cations in and may promote the aggregation of organo-mineral complexes at biochar surfaces.

Research showed that biochar mixed with manure, clay and minerals had higher concentration of plant available P and reduced loss of N. This biochar-mineral complex (BMC) also had higher stability and higher cation retention property. When applied to wheat crop at a rate of 100 kg/ha, higher growth rate and nutrient uptake efficiency was recorded. Higher mycorhizal colonization was also observed at the surface of the BMC (Joseph et al. 2015).

Biochars can absorb up to 5% N when applied in form of urea and reduces the leaching or volatile loss thereby increasing use efficiency (Jassal et al. 2015). Biochar can also be mixed with organic manures to increases soil fertility. Biochar combined with vermicompost significantly increased rice yield by 26.5%–35.3% (Di et al. 2019).  Biochar prepared from rice husk, a byproduct from rice processing industries, at 450°C was used for making bio-urea composite. This composite held urea by >90% when area solution had concentration of 4000 PPM and also released 90% nitrogen within 72 hours (Singh et al. 2020). 

Conclusions

Biochar is most suitable and viable technology for transferring carbon from atmosphere to soil. It is also a sustainable technology for carbon negative energy production. A huge amount of carbon can be stored in soil by converting agricultural and forestry residues, wastes to biochar through simple pyrolysis techniques. While biochar is being applied to soils for the conditioning and its use can be promoted as carrier of nutrients. Studies have shown that biochar is also capable of absorbing N and other macro nutrients and reduce loss of them. Thus biochar enriched with different types of nutrients can effectively used as fertilizer.

References

• Chaturvedi S, Singh SV, Dhyani VC, Govindaraju K, Vinu R, Mandal S (2021) Characterization, bioenergy value, and thermal stability of biochars derived from diverse agriculture and forestry lignocellulosic wastes. Biomass Conversion and Biorefinery. 10.1007/s13399-020-01239-2.
• Chia, C.H., Singh, B.P., Joseph, S., Graber, E.R. and Munroe, P., 2014. Characterization of an enriched biochar. Journal of Analytical and Applied Pyrolysis, 108, pp.26-34.
• Di, W.U., Yanfang, F.E.N.G., Lihong, X.U.E., Manqiang, L.I.U., Bei, Y.A.N.G., Feng, H.U. and Linzhang, Y.A.N.G., 2019. Biochar combined with vermicompost increases crop production while reducing ammonia and nitrous oxide emissions from a paddy soil. Pedosphere, 29(1), pp.82-94.
• Jassal, R.S., Johnson, M.S., Molodovskaya, M., Black, T.A., Jollymore, A. and Sveinson, K., 2015. Nitrogen enrichment potential of biochar in relation to pyrolysis temperature and feedstock quality. Journal of environmental management, 152, pp.140-144.
• Joseph, S., Anawar, H.M., Storer, P., Blackwell, P., Chee, C.H.I.A., Yun, L.I.N., Munroe, P., Donne, S., Horvat, J., Jianli, W.A.N.G. and Solaiman, Z.M., 2015. Effects of enriched biochars containing magnetic iron nanoparticles on mycorrhizal colonisation, plant growth, nutrient uptake and soil quality improvement. Pedosphere, 25(5), pp.749-760.
• Lehmann J (2007) A handful of carbon. Nature 447:143-144
• Mandal, S., Verma, B.C., Ramkrushna, G.I., Singh, R.K. and Rajkhowa, D.J., 2015. Characterization of biochar obtained from weeds and its effect on soil properties of North Eastern Region of India. Journal of environmental biology, 36(2), p.499.
• Singh SV, Chaturvedi S, Dhyani VC, Kasivelu G, (2020) Pyrolysis temperature influences the characteristics of rice straw and husk biochar and sorption/desorption behaviour of their biourea composite. Bioresour Technol  314, p.123674.https://doi.org/10.1016/j.biortech.2020.123674