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Biochar is the solid product material produced during a process known as pyrolysis from the thermo-conversion of biomass under little or no oxygen for use in soils as an amendment. Biochar is produced from a variety of biomass residues (feed stocks) and under different pyrolytic conditions, and thus has varying nutrient contents. For example, the total nitrogen and phosphorus contents are typically higher in biochar produced from feed stocks of animal origin than those of plant origin.
Traditionally, kilns made from earth, brick or steel were used to produce charcoal. However, these kilns brought about a negative environmental standing because they were related to deforestation and air pollution. In an effort to combat greenhouse gas emissions, modern day pyrolyzers have been designed to capture these volatiles to produce bio-oil and syngas. The biochar solid product resulting from the biomass pyrolysis process is believed to have merit as it may be useful for renewable energy capture.
Today, pyrolysis describes the thermo-chemical process whereby low density biomass (1.5 GJm-3) and other organic materials are transformed into three useful renewable energy products; viz. bio-oil, biochar, and syngas. This transformation occurs by heating the organic materials to temperatures greater than 400 ˚C in the presence of little or no oxygen. During this process, thermal decomposition of the organic materials occurs and concurrently releases a vapour phase, as well as a remnant solid phase which has come to be known as biochar.
Understandings of the chemical changes that occur in biochar-amended soils are keys in managing agricultural soils. This is particularly of important because the application of biochar to soils as an amendment has shown a number of physic-chemical advantages and disadvantages. For example, several studies have provided encouraging evidence that biochar adds basic cations to soils, improves soil water retention, and has liming potential of acid soils. However, although the liming ability of biochar has shown positive responses due to increased biomass production and yields, negative yield responses have also been found because high soil pH values are often associated with micronutrient deficiencies.
The widespread problems of an escalating global human population, diminishing food reserves and climate change (carbon abatement) are a growing concern. It has been predicted that over the next two decades, crop yields of primary foods such as corn (maize), rice and wheat will considerably decrease as a result of warmer and drier climatic conditions particularly in semi-arid areas. In addition to this, agricultural soil degradation and soil infertility are common problems. As a means of addressing these problems, the application of biochar to soils has been brought forward in an effort to sustainably amend low nutrient-holding soils.
Biochar is pyrolyzed (charred) biomass, or also commonly known as charcoal or agrichar, produced by an exothermic process called pyrolysis. Pyrolysis is the combustion of organic materials in the presence of little or no oxygen, leading to the formation of carbon-rich char that is highly resistant to decomposition. As a result thereof, biochar can persist in soils and sediments for many centuries, and has great potential to improve agronomic production.
In previous studies, soils used to investigate the agricultural properties of biochar have mostly been highly weathered soils from humid tropic regions. Only recently has research included the investigation of biochar application on the performance of infertile, acidic soils with kaolinitic clays, low cation exchange capacity (CEC), and deteriorating soil organic carbon contents. Generally, the addition of biochar to soil has been reported to have a multitude of agricultural benefits. These include a high soil sorption capacity, reduced nutrient loss by surface and groundwater runoff, and a gradual release of nutrients to the growing plant.
Furthermore, research on biochar has given evidence that it has potential as a soil conditioner due to its physico-chemical benefits, which include, increased soil water retention and nutrient-use efficiency, improved fertility status and enhanced crop yield. These benefits primarily manifest on account of pyrolyzing dry, fresh biomass to biochar, and thus bring about several gains for nutrient availability. During pyrolysis, labile carbon (C) is converted into a relatively stable aromatised C, while basic cations are transferred from the fresh biomass to biochar. This is advantageous because when biochar is applied to the soil, these basic cations become available to the soil by occupying the soil exchange sites.
On the contrary, a few possible negative implications have been reported to be associated with biochar. i) additional agronomic input costs, ii) the binding and deactivation of synthetic agrochemicals due to an interaction with herbicides and nutrients, iii) the deposit and transport of hazardous contaminants due to the release of toxicants such as heavy metals present in biochar, and iv) an immediate increase in pH and electrical conductivity (EC). Furthermore, although studies have highlighted that contaminants such as organic compounds, heavy metals, and dioxins may be present in biochar, there is limited published research that proves that these contaminants are available.