Charcoal and biochar addition to tropical soils has been shown to improve crop yields, sometimes dramatically. In more fertile soils, or in soils that receive high levels of external inputs, and are not liable to water-stress, the corresponding impacts of biochar (and charcoal) on agronomic performance have yet to be demonstrated. Emerging evidence from field trials using charcoal in the UK, and other temperate countries, indicates a modest benefit to the limited number arable systems tested so far. The most important beneficial functions of biochar that have been proposed are given as follows together with an assessment of confidence in the evidence base:

• pH, mineral nutrients and labile carbon: The typically alkaline pH and mineral constituents of biochar (ash content, including N,P,K and trace elements) could provide important agronomic benefit in many soils. Confidence in the evidence base: moderate.
• Water retention: Elsewhere in the world, particularly in poorer-quality sandy or silty soils, the addition of charcoal has been demonstrated to enhance crop yield due in part to the enhanced water retention of a typically porous material. Confidence in the evidence base: low.
• GHG suppression, limiting diffuse water pollution: Other beneficial properties of biochar have been proposed: suppression of soil N₂O and prevention of diffuse water pollution through ammonium. Confidence in the evidence base: very low.
• Long term impacts: A combination of physical and chemical properties may be sufficiently and fundmentally altered that a sustained change in microbial community results, with altered and potentially more efficient patterns of carbon utilisation and nutrient cycling. Confidence in the evidence base: very low.

The possible risks that have been proposed are:

• Contaminants: if the feedstock contains potentially toxic elements they will be retained during thermochemical conversion. During conversion, organic compounds called polycyclic aromatic hydrocarbons (PAHs) can be generated. Confidence in the evidence base: medium (potentially toxic elements) to very low (PAHs).
• Damage to soils: Shifting large amounts of biochar onto fields could cause soil compaction, but the land-owner or tenant will limit this risk. Biochar could have some slow or relatively subtle negative impacts on soils that have not currently been recognised. Confidence in the evidence base: low.
• A possible ‘priming effect’: An accelerated decomposition of soil organic matter from the introduction of biochar has been proposed in the context of charcoal added to forest humus. Confidence in the evidence base: low to very low.
• Feedstock availability and-use impacts: It has been claimed by some environmental organisations that biochar would encourage plantations with deleterious environmental and social impacts. Given the tightly-woven controls over land use in the UK, it is very unlikely that this would occur in the UK and importing large amounts of biochar from overseas is hard to envisage. There is increasing competition for organic feedstocks in the UK and policy incentives need to be designed to promote their sustainable and efficient use. Confidence in the evidence base: high.

Life Cycle Assessment (LCA) of pyrolysis-biochar systems (PBS) suggests abatement of 1.0– .4 t CO₂eq per oven dry tonne (t-1 o.d.) feedstock used in slow pyrolysis. Expressed in terms of delivered energy PBS abates 1.5–2.0 kg of CO₂eq kWh-1,  which compares with average carbon emission factor (CEF) of 0.5 kgCO₂eq kWh-1 for the national electricity grid in 2008, and current CEF for many biomass feedstocks of 0.05–0.30 kgCO₂eq kWh-1. Expressed in terms of land-use, PBS might abate approximately 7–30 t CO₂eq ha-1 yr-1 using dedicated feedstocks compared with typical biofuel abatement of between 1–7 t CO₂eq ha-1 yr-1. By each of these measures PBS appears to offer a more efficient way to abate carbon than alternative uses of biomass feedstock, or land to grow such feedstocks. The largest contribution to PBS carbon abatement (40–50%) is from the feedstock carbon stabilised in biochar. The next largest contribution (25–40%) arises from the more uncertain indirect effects of biochar in the soil (reduced fertiliser needs, reduced N₂O emissions, increased soil organic matter, etc.). Finally, off-setting emissions from fossil fuels accounts for 10–25%. The carbon abatement efficiency of PBS depends, however, on the Carbon Stability Factor (CSF) of biochar, which is the proportion of total carbon in freshly produced biochar that remains fixed as recalcitrant carbon over a defined time period. The LCA in this report suggests that provided the CSF remains above 0.45, PBS will out-perform direct combustion of biomass at 33% efficiency in terms of carbon abatement, even if there is no beneficial indirect impact of biochar on soil greenhouse gas (GHG) fluxes, or accumulation of carbon in soil organic matter. The report notes that unless biochar could be produced at time of harvesting (e.g. using a mobile pyrolysis unit), the scenarios also imply considerable additional truck movements (ca. 220,000 Mt-1) as well as on-farm or dedicated storage requirements. The collection and incorporation of biochar into agricultural soils is a logistical and technological challenge that has not been explored in great detail, and is in need of innovation. It concludes: There are many scientific uncertainties surrounding the role and potential of biochar, in particular concerning its mean residence time, Carbon Stability Factor and how to monitor it in situ. Nevertheless, in principle, biochar has a high carbon abatement efficiency and there are some potentially viable options which may deserve more careful attention, such as pyrolysis of arboricultural arisings and low-cost straw. Integrated, well-design pilot demonstrations of such possible front-runners could, potentially, advance rapidly the science, engineering, regulation and socio-economic evaluation of biochar systems in the UK context. You can find more information here. See here for more about the UK Biochar Research Centre. The report is attached below.