What are the likely benefits of biochar?
In some soil types and with certain crop species, some biochars have been shown to:
- increase water holding capacity of the soil
- increase biomass (crop) production
- increase soil carbon levels
- increase soil pH
- decrease Aluminium toxicity
- decrease tensile strength
- change microbiology of the soil
- decrease emissions from soil of the greenhouse gases CO2, N2O and CH4
- improve soil conditions for earthworm populations
- increase CEC, especially over the long-term
- improve fertiliser use efficiency
It should be noted that the wide variety of biochar feedstock
materials, process conditions and applications leads to a huge and
diverse range of responses that are often contradictory. Some
biochars have been shown to have no influence on some of the factors
noted above; some biochars have been shown to have adverse effects on
crop productivity. More research is required to verify the observed
effects and to distinguish beneficial from detrimental biochar products.
Studies thus far have shown that the greatest positive effects of biochar applications have been in highly degraded, acidic or nutrient-depleted soils. Thus, biochar research is of particular relevance in the Australian context, as many Australian soils exhibit very low nutrient and carbon levels, and are at risk of acidification.
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What is the carbon sequestration and greenhouse gas mitigation potential of biochar?
The potential for biochar technology to achieve carbon sequestration
and greenhouse gas mitigation is very large. The realisation of its
potential is determined by several key factors, including:
- available renewable biomass resource which can be harvested in a sustainable manner
- the efficiency of the production technology
- long-term stability of biochar in the soil
- production and utilisation of the co-produced bioenergy to displace fossil energy sources
- implementation of biochar investment schemes and adoption of biochar use
How is ‘biochar’ made?
Sensu strictu biochar is made by heating biomass under oxygen-limited conditions (e.g. slow pyrolysis). Biomass feedstocks can include forestry and agricultural waste products, municipal greenwaste, biosolids, animal manures, some industrial wastes such as papermill wastes etc. The thermo-chemical conversion drives off the volatile components of the biomass and stabilises the remaining carbon into a black, highly aromatic solid.
Is all biochar the same?
Key chemical and physical properties of biochar are greatly affected both by choice of feedstock and process conditions (mainly temperature, residence time, heating rate and feedstock preparation). These properties affect the interactions of biochar with the environment of its application as well as its fate. Several analysis techniques can be employed to differentiate biochar types. Research on matching the unique properties of biochars to different applications is ongoing. There is no rapid screening technique currently available that provides the means for biochar products to be compared or matched to a particular use.
What techniques are currently used to characterise biochar?
Currently research scientist are using a large range of analytical techniques to understand the structure, composition and interactions of biochar materials. The chemical structural aspects of biochar can be characterised spectroscopically (e.g. 13C-NMR, ESR, Raman), chemical/thermal analysis (TGA-MS, Py-GCMS) or microscopically (SEM, TEM). Chemical characteristics of biochar can be assessed using standard chemical and agricultural soil testing, although somemethods require modification. Ecotoxicological testing such as earthworm avoidance assays and plant germination inhibition assays can be used to test the ecological safety of the biochars.
How stable is it?
Studies of charcoal from natural fire and ancient anthropogenic activity indicate millennial-scale stability. However, the stability of modern biochar products is uncertain: it is difficult to establish the half life of newly produced biochar through short term experiments, and aging processes are expected to affect turnover in the longer term. The limited data available suggest that turnover time of newly produced biochar ranges from decades to centuries, depending on feedstock and process conditions. At the moment there is no established method to artificially age biochar and assess likely long term stability.
Is it safe to use?
Prior to the large-scale endorsement of biochar usage, its safe use with regard to human and environmental health needs to be assured. Pyrolysis systems that meet first world regulatory standards on emissions, health and safety have been demonstrated. Some biochars have been tested for toxicity, and found to meet quideline levels of dioxins, PAHs and heavy metals, The OECD ecotoxicological test involving response of earthworms has demonstrated lack of toxicity of biochar made from paper sludge. Air emissions from biochar production, and composition of the biochar product, are highly dependent on the production systems and the biomass feedstock. Therefore it is critical that the safety of all proposed facilities is assessed Where biochar is used as a soil conditioner it must conform to relevant Australian and international standards and legislation (e.g . the NSW Protection of the Environment Operations (Waste) Regulation). The “earthworm avoidance test”, an ecotoxicological test method prescribed by the OECD, can be used as an initial environmental test. It is a cheap test that is suitable for developing countries.
What are the agronomic benefits?
A number of studies have been conducted where biochar
application has shown significant agronomic benefits. However, these
results are not universal as other studies have shown no difference, or
even some decline, in productivity. The reason lies in the wide range
of properties between different biochars, and variation in impact due
to interaction with different soil types. Our incomplete understanding
of the processes that occur when biochar is applied to soil limits our
ability to predict agronomic impacts of biochars in different
situations. There is a need for models to allow extrapolation
of location-specific findings by accounting for mechanistic effects of
variations in soil type, climate, crop species and pyrolysis feedstock.
See also “What are the likely benefits of biochar?”
Is it economically viable?
The economic viability of biochar is dependent on the price of the product and the benefits to the user. The price will be affected by the cost of feedstock (which may be negative in the case of biomass that would incur a waste disposal fee), and returns from renewable energy generated in the pyrolysis process. Financial benefits to the user may include increased production and reduced fertiliser requirements. Furthermore, the biochar producer or user may 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; alternatively, the landholder may receive credit for increasing the soil carbon stock in his field where biochar is applied. Thus the economic viability of biochar is influenced by policy; uncertainty over future policy may risk investment in biochar production facilities. The growing cost of waste disposal, and implementation of renewable energy targets, are likely to make the production and application of biochar for electricity and waste management economically viable. Potential returns from carbon trading will be enhanced if biochar is accepted under the Clean Development Mechanism (CDM) of the Kyoto Protocol.
What are the environmental and societal benefits?
Models exist for viable agronomic use of biochar in subsistence agriculture. However, appropriate technology and policy needs to be implemented to deal with environmental issues such as methane and particulate emissions, that could contribute to climate change and human health risks. Socio-economic constraints and benefits are areas for ongoing research. Higher crop yields resulting from biochar applications would be expected to mitigate pressures on land and would also have relevance to land restoration and remediation. Other environmental benefits may include waste re-use and avoided landfill, offset of fossil fuels through renewable energy production, carbon sequestration, potentially reduced soil emissions of non-CO2 GHGs, improved crop performance and biomass production.