Biochar refers to fine grained, carbon rich, porous product produced by thermal decomposition of biomass under low oxygen supply and is used for agriculture or environmental purposes. Biochar is a type of charcoal that differs from conventionally produced charcoal being generally created at high temperature than the conventional charcoal produced at low temperature. It can be obtained from any organic material including wood, crop residues, municipal solid waste, manure, bones, pine needles, maize stalk, weed biomass and dried leaves of banana, chickpea stover, outer shell of jatropha pods, millet cones, paper mill waste, saw mill waste, piggery waste, poultry waste, shells of palm fruit and sugar cane trash by pyrolysis. Pyrolysis involves heating organic material in the absence of air at a temperature above 300-700°C. Initially some volatiles are lost followed by conversion of unreacted residues to volatiles, gases and biochar. Pyrolysis breaks down the complex chemical substances into simple ones. Higher the temperature during pyrolysis, better the quality of biochar produced. Best quality biochar is produced at a temperature above 650°C. Raw material and temperature of pyrolysis affect the porosity and chemical properties of biochar produced, affecting its impact on soil. The yield of biochar varies with the initial moisture content of substrate. Once produced, the biochar can be mixed with soil to improve fertility. Biochar is more useful in degraded soil as well as in soil with low nutritive levels.

Characteristics of biochar

Physical and chemical constituents of feed stock used for its preparation determine its properties
• It has large number of microscopic pores that provides it with large surface area to hold water and serve as a niche for beneficial microorganisms.
• Increase in pyrolysis temperature from 400-600°C decrease the volatile and N component of biochar and increase the content of ash and fixed carbon.
• Total carbon content of biochar produced from different feedstock varies between 33-82.0 %. Its C:N ratio varies between 19-221.
• Biochar produced at high temperature has low N and S content.
• High temperature biochars (> 650°C) have high pH, electrical conductivity and extractable nitrate
• Low temperature biochars (350°C) have greater content of extractable P, NH4 and phenols
• Biochar contains appreciable amount of Ca, Mg, K and P.

Effects on soil properties
• Biochar has high pH and act as liming material in acidic soils
• Reduction in soil acidity improves the ability of plant to absorb most nutrients
• It increases the water retention capacity of soil and reduce the cost of irrigation
• Higher water retention capacity of biochar gives more time for nutrient ions to move from the mineral to water where plants absorb them
• Biochar removes CO2 from atmosphere and stores it in soil where it can remain for longer duration
• Application of biochar to soil increases its carbon content
• Its incorporation in soil increases plant biomass production
• It also helps in increasing plant growth and extra plant growth removes CO2 from air and contains it well within the plant and soil.
• Decreases soil tensile strength
• Improves soil conditions for earthworm population
• Biochar also serves as a source of carbon and energy for microorganisms that take part in nutrient cycling
• Improve fertilizer use efficiency
• Prevent soil disease
• Biochar act as an efficient sorbent of various organic and inorganic contaminants because of its huge surface area and special structure and decreases Al toxicity
• Due to the presence of carboxylic, alcohol and hydroxyl group etc in the surface of biochar, complexes between heavy metals and these functional groups are easily formed during biochar and heavy metal interactions.

Method of application
• It can be applied to soil by broadcasting, band application, spot placement and deep banding etc. Deep banding is a low impact method of application and puts biochar directly into rhizosphere.
• Biochar can also be mixed with manure or compost that may improve nutrient performance over time due to slow nutrient leaching rates.
• The combined application of biochar and inorganic fertilizer has the potential to increase crop productivity, thereby increasing the income of farmers and improving the quality of soil.

Economic viability
• The cost of feedstock is negative in the case of biomass that need effective waste disposal
• On application to soil there is increased production and reduced fertiliser requirements.
• Benefit from some form of carbon credit under an emissions trading scheme
• The producer could receive credit for stabilising organic carbon, avoiding emissions from organic matter decomposition
• Landholder may receive credit for increasing the soil carbon stock in his field where biochar is applied.
• The growing cost of waste disposal, and implementation of renewable energy targets, is likely to make the production and application of biochar for electricity and waste management economically viable.

Agriculture wastes do not have the ability to slow down climate change. Converting them into biochar is a meaningful low cost technology for waste management. This technology has promising potential for development of sustainable agriculture system and also for global climate change mitigation.

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