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Biochar: Coming From the Past to Improve the Future

Q. The term ‘biochar’ is a relatively recent development, and there is a range of terms when discussing biochar, such as charcoal, agrichar, biocoal… What is biochar, and how does it differ from other forms of chars?

Biochar is a black, charcoal-like material. Scientifically, it is defined as a highly recalcitrant form of carbon resulting from a process known as pyrolysis. This involves heating the feedstock to about 450-700°C under limited oxygen supply. The feedstock for biochar production may come from plant residues or remnants from agriculture. The main purpose of producing biochar is adding it to soil to improve soil fertility, whereas charcoal and biocoal are produced to provide energy benefit. Another important point is that biochar can be pyrolysed in a much more sustainable way than charcoal because it results mainly in inert materials, so the release of gas contaminants is minimal. 

Q. In scientific literature, biochar is usually used in conjunction with the term ‘Terra Preta’, meaning ‘Dark Earths’, in the Amazonian rainforest. Tell us about this phenomenon.

The Amazonian Indians in the pre-Columbian period are thought to have mixed charcoal from their cooking fire with organic matter, for example, remainders of the meal, to create a dark, highly fertile organic material to use as a soil amendment. This type of artificially created soils was found by scientists thousands of years later and received the name ‘Terra Preta’. We don’t know the reason why the Indians first started using charcoal in agriculture. Maybe, initially it happened by accident, but it became a purposeful soil management strategy.

Q. So, the char used by the Indians about 6000 years ago is still present in the soil?

Yes, the reason for it is that biochar contains a recalcitrant carbon, which is highly resistant to bacterial activity and organic decomposition. That is why scientists are considering applying biochar as a means of sequestering atmospheric carbon. If the plant is allowed to grow naturally, it will release carbon dioxide (CO2) back to the atmosphere at the end of its life. However, when you pyrolise this decomposable biomass, it will obtain a higher stability than the organic carbon from which it was made. As a result, the whole decomposition process will occur at a much slower rate, meaning less CO2 is released from soil.

Q. Most of the research that has been going on worldwide demonstrates that biochar is generally positive when added to soil. What properties help it to provide such a benefit?

There are several factors which allow biochar to enhance soil fertility. The main physical factor is the porous structure of biochar. This is primarily due to the structure of the plant material, which was used as a feedstock. If you look at biochar, you will see that it is formed from plant transport vessels, which are maintained in the pyrolised char. Porosity helps with storing water and soluble nutrients. In addition, biochar helps to increase the cation exchange capacity of the soil because it has a charged surface, which allows different mineral ions to attach to it. Some studies show that biochar is also capable of reducing the bulk density of the soil itself, which allows better aeration and water infiltration. Another biochar attribute is its high adsorptive capacity, meaning that it can adsorb toxic chemicals present in the soil and help with remediation. However, it may also adsorb substances, which were intentionally put into the soil, such as pesticides. So, in the meantime it is important to understand how biochar would interact with different compounds in the soil environment.

Q. Does biochar also affect microbial life?

Yes, recent studies showed that biochar increases the diversity of different communities of soil bacteria and fungi. This may be especially beneficial for symbiotic bacteria and rhizo-fungi, which enhance plant growth. But we don’t know the mechanism of this interaction. Some scientists relate it to the property of biochar to provide micro-habitats that allow bacteria to live inside the biochar pores and use it as escape refugia from predators. Another possibility is that a small labile fraction of biochar can be used as a potential energy source for microorganisms. So there are different hypotheses, but they all need further clarification.

Q. You have mentioned that biochar can be produced from different types of organic feedstock. Would all these biochars act in the same way?

No, there can be a variety of biochars available for different types of soils so it is important to understand which biochar to add to which climatic region and soil type. For example, if you have a highly alkaline soil, you may want to add wood-based biochar with pH around 5-6 to reduce alkalinity. In contrast, manure-based biochar with pH about 8-10 could be added to combat soil acidity.

Q. If biochar is really beneficial to soil and helps to increase crop yield, does it have the potential to resolve the world’s food security problems?

Yes, it certainly has the potential to play a part. There are different non-governmental organization (NGO) projects such as Biochar Fund, Agrichar, and Re:char that are helping to improve food yield in developing countries with low fertility soils. But it is important to understand which feedstock to use, because there may be competition with other uses of biomass. So, there should be a reliable and environmentally sustainable biochar production, which wouldn’t aggravate problems with deforestation, for example. However, the main limitation to use biochar in developing countries is a lack of pyrolysis units required for biochar production. The majority of pyrolysis machines is based only in scientific labs and cannot be used commercially. But some NGOs are developing other options such as char kilns or cook-stoves, which could produce some biochar to local communities.  

Q. Is there going to be competition between the production of biochar and fertilisers? 

Biochar may increase nutrient content, which is already present in the soil. So, it can actually be used alongside fertilisers. Some studies have been looking at mixing biochar with soil organic inputs such as manure and compost. And it seems that biochar helps to hold these organic nutrients and prevent them from being lost from the soil.

Q. Are there any other implications for biochar?

One of the obvious implications is to produce energy by capturing syngas and bio-oils, which are generated as by-products in the biochar pyrolysis reaction. Another option which is currently emerging is to use biochar to mitigate climate change. As I have mentioned before, biochar is capable of controlling soil pH, water content, aeration, and microbial life. These factors are also important in nitrous oxide and methane generation, which are harmful greenhouse gases with global warming potential higher than that of carbon dioxide. So, adding biochar to agricultural soil could help to suppress the release of these gases and therefore reduce climate change. However, the majority of the research projects are lab-based and it is unclear whether biochar would have a temporary or a long-term effect on gas emissions. At the moment, there are three PhD students at the UK BRC who are researching this topic.

Q. How did the UK BRC get started?

The UK BRC was established in April 2009 and led by Saran Sohi (Soil Science), Ondrej Masek (Pyrolysis), and Simon Shackley (Systems and Social Science), who then expanded the centre by welcoming postdoctoral fellows, PhD, and MSc students to research biochar. Now there are about twenty people based at the UK BRC.

Q. Is the Centre limited only to the UK?

Initially, it was an alliance with the University of Newcastle and the Rothamsted Research Institute. Then, the centre became involved in several international projects– with Interreg (EU), Soil Research Institute in Ghana, and two biochar groups in China and Japan. There are also some links with EMBRAPA (Brazilian Enterprise for Agricultural Research) and Cornell University (USA). Our future plan is to establish an international network to build partnerships with other biochar centres around the world by conducting shared research and running visiting exchange programmes for scientists and students. Another aim is to develop the ‘Charchive’, a database containing information on all sorts of biochar samples to facilitate an exchange of knowledge between different research groups worldwide.

Q. Biochar seems to be highly versatile. Don’t you think that it sometimes appears too good to be true?

Yes, sometimes it seems like a win-win situation. Biochar is indeed very versatile. It can be used both for carbon storage and as a soil amendment. But the scale of its use is currently very limited, primarily due to all the uncertainties that we have. And because there are so many different feedstocks that we can use, there can be an even wider range of biochar types. So, there should be integrated research to cover all the aspects and different properties of biochar. It is a new and emerging area of research, which cannot be considered a silver bullet to solve all environmental problems. We need many options, and biochar could be part of our toolbox.

Alua Suleimenova is an undergraduate in Environmental Sciences

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