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Review of the Potential Benefits, Costs and Issues Surrounding the addition of Biochar to Soil: An Expert Elicitation Approach - SP0576

Background: Biochar is a fine-grained, highly porous material similar to charcoal, that is produced from the decomposition of plant-derived organic matter (biomass) in a low- or zero-oxygen environment (a process known as pyrolysis). Biochar is already found in soils around the world as a consequence of naturally-occurring fires and, in the Amazon, as a deliberate result of its addition by past human populations (the so-called terra preta soils). These terra preta soils are famous for enhancing the year-on-year fertility of soils and are, nowadays, highly valued as composts. Contemporary interest in biochar is, first and foremost, driven by its potential role as a response to the problem of climate change. This is through the long-term storage of carbon in soils in a stable form. If untreated (i.e. non-pyrolysed) organic matter is added to soils, many of the nutrients contained therein are released. The carbon in the material is rapidly converted into carbon dioxide (‘mineralised’) and released to the atmosphere. Usually, all the carbon has disappeared as CO2 within 1 to 5 years, which is why adding organic matter to soils doesn’t help very much in efforts to limit climate change. By contrast, the carbon atoms in biochar molecules are strongly bound to one another, and this makes biochar resistant to attack and decomposition by micro-organisms.

Potential Role of Biochar: In addition to the carbon contained in a stable form within biochar, there are some further ways in which biochar assists in limiting atmospheric CO2 concentration. These are: 1. bioenergy released during the production of biochar, so avoiding the use of fossil fuels (gas or coal) with their attendant CO2 emissions; 2. reduced use of fertilisers due to the soil fertility benefits of biochar (fertilisers being produced from, and using, fossil fuels); 3. reduced emissions, in some situations, of nitrous oxide and methane from soils into which biochar is added (these being powerful greenhouse gases); 4. reduced diesel requirements during field operations; and 5. increased net primary productivity (NPP), so enhancing the quantity of carbon retained in vegetation and soils. The combined effect of these processes in terms of avoided and sequestered carbon emissions is considerable. One estimate for bioenergy crops used for generating energy and biochar, is that about 15 tonnes of carbon per hectare per year could be prevented from entering / removed from the atmosphere. At the global scale, early estimates by the International Biochar Initiative suggest that it is feasible to remove one gigatonne of CO2 (10 to the power of 9 tonnes) by about 2030, which makes it as potentially important as other major carbon mitigation activities (CO2 Capture and Geological Storage (CCS), renewables, efficient vehicles, etc.).

Key Uncertainties: There are, however, some major uncertainties surrounding the role of biochar, its impacts upon soils and crops, its overall performance and its costs compared to other carbon mitigation options. A) The longevity of carbon in soils. Whilst it is known that most of the carbon remains in a stable form, more knowledge is needed of what happens to the carbon, what other factors have an influence and over what timescales. B) The influence of biochar on nitrous oxide and methane emissions. How much is really known about the effects of biochar on such emissions? C) Reduction in greenhouse gas emissions arising from reduced fertiliser use and modified field operations. To what extent are the effects upon crop productivity really understood, and on what timescales? Over how many years does an increase in crop productivity occur? D) Whilst biochar might improve productivity, is this effect really understood well enough that we can factor-in a long-term enhancement of the carbon sink in vegetation and soils?

Aims and Objectives: The aim of this project is to provide a detailed state-of-the-art and critical review of what we really know about biochar, identification of the key gaps, uncertainties and risks, and suggestions on key research questions that need to be addressed before biochar can be widely deployed. We will also provide some initial estimates of the overall carbon emissions and economic costs associated with producing, transporting and applying biochar to soils. This will allow a comparison to be made between biochar and other low-carbon options and technologies.

Methodology: The project will involve an extensive literature review, both of published and still-to-be-published research (where we can realistically access it). To supplement this, we will hold expert interviews with a range of key specialists from around the world and, following this, a web-based survey will be developed and implemented. An Expert Advisory Group of 9 experts will be convened in order to provide an independent, multi-disciplinary assessment of the framing of the key issues as well as a screening of the project findings as they emerge. The project team will respond to the critical evaluation of the Group and go back to Group members where necessary to ensure that the Group’s points have been taken sufficiently into account.

Outputs: A report consisting of an Executive Summary, a Technical Summary and a Main Report.
Project Documents
• Final Report : An assessment of the benefits and issues associated with the application of biochar to soil   (2081k)
• Final Report - Annex : Analysis of scientific studies published on the function of char, its quantification, and its stability in soil   (37k)
Time-Scale and Cost
From: 2009

To: 2009

Cost: £42,458
Contractor / Funded Organisations
University - Edinburgh
Environmental Protection