Biochar Knowledge Gaps


(7 pages)
April, 2011

Helena Paul

This article reviews and discusses biochar with particular reference to the book Biochar for Environmental Management - Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

Examining the claims for biochar

Biochar is biomass burned in the near absence of oxygen and it is basically identical to charcoal, but used for different purposes. It is being widely promoted by various interests as a soil amendment and to sequester carbon, often with little detailed argument or evidence in support of the claims made. The book in question consists of a large collection of articles about biochar by a total of some 50 researchers and specialists from a wide range of universities, government departments and companies. The introduction also cites review by a large number of referees. In view of the potential synergies between biochar and agrofuels, the implications for land-grabbing for biochar production, and the many instruments being proposed whereby users of biochar could be subsidised (carbon trading, public loan guarantees, Payment for Environmental Services (PES), rewards for improving soils, etc), it seemed useful to study the book thoroughly regarding the state of research into biochar. In sum, the book demonstrates clearly that there are major gaps in knowledge about biochar. The issues are complex and the interactions between biochar and soils, especially soil microorganisms are little understood. There are many variables involved, such as different methods of preparation, different raw materials for the biochar and different soils, rainfall and climatic regions to which it may be applied, to mention just a few. However, at the same time, some writers speak of biochar as a means to address climate change and propose it for carbon markets, in spite of these knowledge gaps. This is unfortunate in a book that contains a number of good scientific analyses indicating wide uncertainties with regard to every aspect of biochar.

Current context for biochar

Biochar is proposed both as a soil amendment and a means to sequester carbon. There is a connection between agrofuels and biochar, since the same process of pyrolysis may be used to produce both. Pyrolysis is defined as the decomposition/degradation of organic matter by heating it in the absence of oxygen. At temperatures of between 250-1000ºC, biomass (wood, straw, manure etc) yields different fractions of syngas, bio-oil and biochar. Bio-oil and syngas can be used directly for bioenergy and both can be refined further into agrofuels for transport including aviation. Biochar and related bioenergy including agrofuels for transport could play a major role in the proposed new bioeconomy where biomass is meant to replace products from fossil oil. Thus agrofuels, about which many doubts have been raised since they were first presented as a kind of panacea, can all too easily find a synergy with biochar promotion. Since the market is always searching for new sources of funding, subsidy and profit, especially in the burgeoning climate market, there is a danger that each will help to promote and support the other as co-products from biomass. Governments also hope to benefit from biochar. Several African governments and the United Nations Convention on Combating Desertification (UNCCD)1 have made submissions promoting biochar to address soil degradation and sequester carbon, even though (as will become clear in what follows) a great deal of research has still to be done to test such claims, since there are so many variables and unknowns involved. For example, although applications of the kind proposed by UNCCD would be mostly in drylands, very little is known to date about biochar impacts and durability in drylands. Furthermore, there is no word on where the biomass for such biochar would be sourced, other than so-called waste material, ranging from corn stalks to sewage and manure. However, many commentators agree that waste biomass would only provide a small fraction of what is required, even at a low biochar application rate. Furthermore, such waste biomass would appear to be particularly important for protecting and nourishing soils in dryland regions. Some biochar proponents are actively promoting plantations, for example, Chris Turney, the founder of Carbonscape, has proposed establishing plantations on millions of hectares of “degraded” land for biochar production.2 However, the word “degraded” is ill-defined and its application could easily result in expropriation of local communities that know how to use such land sustainably and the reduction of ecosystem resilience through inappropriate exploitation such as large-scale biomass production for biochar.

Biochar – major gaps in knowledge

It is striking that on every aspect of biochar use, the book makes it clear that there are large gaps in knowledge that must be addressed before biochar benefits can be claimed on a scientific basis. For example, critical variables include:

  • type, speed and temperature of pyrolysis
  • type of biomass feedstock used, or mixture of feedstocks
  • types of additive used: eg: compost, synthetic fertiliser, manure, flue-scrub residues
  • size of the biochar particles produced and used and impact on microbes, etc
  • nature of the soil to which it is added including soil microorganisms present
  • previous use of the soil and cultivation patterns
  • impacts on soil nutrients and nutrient leaching
  • different crops to be used: grains, trees, fodder, pasture
  • climatic conditions of the region where it is to be used – temperature, rainfall, natural vegetation
  • method of adding it - scattered on the surface or turned in, and to what depth
  • amount to be added in each case
  • period of time for which biochar will actually sequester carbon in the soil
  • socio-economic contexts for biochar production and use

Fast and slow pyrolysis

There are many issues around the type of pyrolysis used to produce biochar. Slow (below 400ºC for 30 minutes to several hours) yields more biochar (35%) and fast (5-10 seconds for 400-550ºC) yields less (15%). Slow pyrolysis is more energy intensive to carry out than fast pyrolysis. Different temperatures of pyrolysis together with different feedstocks change the characteristics of the biochar, for example the structure, chemistry, composition and action of the material. There are also many issues related to the texture of the biochar produced from different levels of pyrolysis, its qualities, dependent on the different biomass sources, etc, that need to be considered in terms of their effects on the soil and soil biota.

Contamination and fire risks

There are clearly issues around contamination and human health risks, such as volatile organic compounds, carbon monoxide or particulates at different points in the process, plus the risks of biochar black carbon becoming airborne, for example while being applied especially if applied to the surface of the soil. There are also fire hazards including spontaneous combustion of biochar that may be provoked by volatiles in the biochar. Biochar dust must be carefully managed to avoid fire risks. Thus storage and transport of biochar must be safely carried out.

Balance between sequestration versus soil improvement claims

Further research is required into the balance within different biochars between inert carbon for sequestration and activated carbon. The latter may serve useful purposes as it breaks or is broken down, but this means it will not sequester carbon for any length of time, although here too there are widely differing interpretations of how long. Currently, it is impossible to assess or to make assumptions about the carbon sequestration effectiveness of biochar over the short or the long term. Biochar is often compared with terra preta or Amazonian Dark Earths (ADE), which are only mentioned in passing in the book. It is clear that there are still many gaps in knowledge around how precisely ADEs were made, how they have evolved over centuries and what relationship they truly have with biochar, as opposed to blanket claims made by a number of sources that they are identical. Unfortunately terra preta is already being unsustainably exploited by different interests, perhaps just because of claims that it is almost infinitely resilient and fertile. It would be a serious matter if much of it were damaged or eliminated for uses as trivial as packaged garden soil before proper research could be carried out.3

Biochar interactions with soil biota

We need to understand much better how biochar acts on soils and microbial populations. For example mycorrhizae are “common root-fungal mutualisms with key role in terrestrial ecosystems” (Chapter 6, Characteristics of Biochar: Biological Properties: page 99). A summary of current knowledge shows that a great deal of work is required to understand interactions between biochar and micorrhizae, particularly the impacts of biochar on mycorrhizal symbioses. Once again, long-term studies involving all parameters are needed.

Biochar: nutrients, leaching and sorption

Several chapters address biochar and nutrient issues, including effects on soil nutrient transformations. Regarding basic nutrients, we need to understand by what mechanisms biochar affects nitrogen mineralization and immobilisation and the availability of phosphorus and sulphur in different ecosystems. For example, does NH4+ adsorption by biochar reduce nitrogen availability or concentrate it for plant and microbial use? Nutrient leaching is a serious problem in agriculture for which biochar is claimed as a solution, but how precisely does biochar reduce leaching and where? The authors call for laboratory, field and watershed studies using an ecosystem approach. Biochar has different impacts according to the age of the biochar and the local climate regime. Its interaction with soils is complex and requires more work. Biochar adsorbs organic compounds and could decrease bioavailability of organic pollutants, which means that it could potentially act against contamination, but could also reduce the action of pesticides and herbicides. High mineral content biochars from animal manures and industrial byproducts require more research into their sorption capacities. Furthermore, we do not know how long the effect lasts or what happens when it stops working. Partial oxidation of biochar produces carboxyl groups that contribute cation exchange capacity to soils, which could add to the long-term benefits of biochar, but again, we need to know how quickly and under what circumstances such effects are produced.

Tensions between lack of knowledge and the push for markets

Determining the quantity of biochar in soils is complex, time consuming and expensive because biochar is heterogeneous and complex, and it changes over time. According to the authors of chapter 18 (Biochar, Greenhouse Gas Accounting and Emissions Trading), there is simply not enough data to make verifiable measurements regarding biochar for carbon trading, especially in agriculture. While acknowledging the limited amount of data and the fact that credible default values are not available, the authors propose a number of assumptions as a possible basis for emissions trading. They also acknowledge that projects may not be economically viable without aggregating and integrating them across sites and producers. There is considerable pressure for assumption-based, aggregated processes in order to assist the development of markets. However, before any of this should even be considered, we need serious long-term research into the impacts and longevity of biochar in soil. Issues of permanence, uncertainty, additionality and leakage must all be addressed and tested over time. Yet the authors advocate monetising the GHG offsets from biochar and pyrolosis, in spite of the difficulties that they themselves identify. This push towards market mechanisms when the science is still so imperfectly understood is regrettable.

Potential gains, costs and negative impacts from biochar

In spite of all the uncertainties, many different streams of potential benefits are mentioned in the book, which, according to some writers, could then generate returns from the market or from CDM and other carbon trading – or from both. However, these all need further research, some over several years, before such claims can safely be made:

  • GHG sequestration
  • Avoided fertiliser application (double gain, from market and credits)
  • Soil “improvement”
  • Dedicated biochar (and agrofuel) plantations
  • Improved yields
  • Avoided irrigation
  • Co-products: agrofuel and other forms of non-fossil energy
  • Other more valuable fractions, equivalent to those from fossil fuel
  • Application to so-called degraded land for regeneration

Yet it is also clear that the costs, both financial and environmental, must be taken into consideration, including the funding of the essential research:

  • Land use change, both direct and indirect
  • Removal of residues – impact on soils, water, etc
  • Biomass production costs
  • Biomass harvesting costs
  • Biomass hauling and storage infrastructure
  • Pyrolysis – size and nature of installations
  • Preparation of biochar for application, eg: pelleting, compression
  • Biochar application methods and machinery
  • Researching amounts of biochar to be applied and its actions in short and long term

Impacts on communities

There is one chapter on social impacts: Socio-economic Assessment and Implementation of Small-scale Biochar Projects (chapter 20), but this is mainly theoretical and proposes a model scenario and a large number of assumptions. It is therefore not an appropriate basis for tackling the practical implementation of biochar technologies. All the claims should be properly researched and tested with communities before there is any value in trying to examine possible gains and costs.

Large and smallscale use

Biochar promoters often speak of the likely benefits to smallscale operators and communities, yet those who wish to propose biochar as a method of securing "large-scale carbon removals from the atmosphere and stocking it somewhere safer" (chapter 22: Policy to address the Threat of Dangerous Climate Change: A leading Role for Biochar, page 395) want to see biochar being applied on a massive scale, and speak of between half a billion and over a billion hectares. This implies industrial monocultures and installations for economies of scale. Previous experience suggests that involving smallholders in industrial scale production is likely to lead to them being aggregated and subordinated to large companies and involved in contract schemes that will not necessarily benefit them. Then of course there are the issues of property rights, over land, biomass, machinery, installations and infrastructure, and the carbon itself, which again are likely to favour large international operators.

Tension between generalised claims and the need for detailed, iterative research

Most of the authors are clear that much research needs to be carried out in order to assess the values of different kinds of biochar under different conditions, as noted above. This contrasts with broad claims made elsewhere, in the media and on blogs, about a generalised material, called biochar. The realities are complex, particular and context specific. Many general advocates appear to treat soil as an undifferentiated, passive recipient medium, whereas in fact soils are highly complex and variable and we still know very little about them. This is especially true of the interactions between the different parts of the soil food web, including soil microbes, insects and other detrivores, substances added to the soil, and the crops planted in it. As the book itself notes: “Thus in the strictest sense, each biochar made of a particular feedstock and process combination presents a unique mixture of phases and microenvironments that gives rise to a unique set of chemical properties. In some respects the chemical complexity of biochars rivals that of incipient soils” (Chapter 3: Characteristics of Biochar: Microchemical Properties, page 34). This highlights the tensions between the pressure to market biochar and the lack of knowledge plus the need for long-term research to assess the claims being made for it.

Important Omissions

Some key issues are barely addressed in the book. Most noticeable are matters relating to charcoal dust and emissions throughout the life-cycle of biochar processes. The climate and health impacts of charcoal dust are vulnerable points for biochar advocates, yet there is little discussion of these hazards, except to note that rice husk biochar made at higher temperatures can contain toxic crystalline materials such as silica that may be harmful to human health and that biochar can also pose a fire risk, including spontaneous combustion (page 216). There is no discussion of the climate risks from black carbon if it becomes airborne, although it is noted in chapter 12: Biochar Application to Soil, regarding top-dressing with biochar, that care should be taken to prevent erosion by wind and water and to manage health risks from biochar dust. The emissions involved in biochar production and use, whether during pyrolysis, storage, transport or during and after addition to the soil are not directly addressed anywhere in the book. In chapter 8, Biochar Production Technology, there are passing references to emission control, which led this reader to wonder about the safety implications of proposals for biochar production in remote rural areas, and issues of training, plus maintenance, repair and safety of pyrolysers in the field.

Biochar and carbon trading

Emissions trading issues are mainly addressed in chapter 18: Biochar, Greenhouse Gas Accounting and Emissions Trading. Here, as already noted, the writers admit that they are using assumptions, mass balance methods, and default figures in order to calculate emissions for the purposes of constructing their tables. It may be that the sheer complexity and interactivity of the issues involved renders exact measurements nearly impossible. However, this is certainly no reason to rely on premature assumptions instead. It is vital to carry out the research, in order to find out whether the claimed carbon sequestration/climate and soil benefits of biochar are genuine or not. Until such research is carried out, there is no justification in promoting biochar or proposing it for carbon markets on the basis of assumptions and generalisations. This is especially the case when many interests would like to propose biochar as a “quick fix” for addressing climate change, while avoiding sharp and immediate emission reductions from transport, industry and power generation.


Biochar is often presented as if it were a simple uniform substance, whereas in fact each type of biochar has different impacts, depending on the source material, the preparation, production and application processes, as well as the soils and geographical regions in which it is applied. As noted above, its chemical complexity rivals that of soils. Something as complex and variable as this cannot be presented as a generalised panacea. Far more research is needed in order to find precisely what its value may be as a soil additive, and under what circumstances. We certainly do not know enough to make assumptions about its potential for soil sequestration or use in carbon trading. Accurate measuring, reporting and verifying, a condition for any financial reward under the Climate Convention, is not currently feasible.While it is clear that some charcoal has remained in soils for thousands of years, it may also be lost within decades or less, and all of it will eventually be released as CO2. This means it is vital to understand what causes this variability before applying it to soils and certainly before considering it as a means of sequestering carbon. Biochar also brings its own hazards in the form of dust which is dangerous to human health, while airborne black carbon has an impact some 5-800 times as great as CO2 over a century because of its ability to reduce albedo and absorb solar radiation. Finally, the social impacts of biochar use have not been investigated and models and assumptions are no substitute for research on the ground with communities and farmers.