Eight Facts about Biochar with Professor Colin Snape and Dr Tom Bott

1. Biochar is an umbrella term

Biochar is a catch-all term for carbon-rich material which is produced by heating and carbonising biomass in the absence of oxygen – a process called pyrolysis.

Biochar can be made using a range of different feedstocks including wood and agricultural residues such as straw, under a variety of different pyrolysis conditions. The term ‘biochar’ therefore encompasses a range of charcoal materials which often have very different properties depending on the feedstock from and conditions under which it was made.

When used as a soil amendment, the heterogeneity of biochar can be advantageous. Biochar produced at a higher temperature is very stable in soil and is thus extremely effective when used as a means of storing carbon. Biochars produced at lower temperatures are less stable in soil.  However, biochars from nutrient-rich feedstocks can release nutrients across time to create a fertiliser-based effect.

It is therefore important to consider the type of biochar used as a soil amendment to the potential soil outcome. This has drawn much research into the effects of different biochars on commercial-based farms.

2. Biochar is applied differently in different land uses

Biochar can be spread on arable farms through lime spreaders or muck spreaders. Being an extremely light material, it is difficult to cast and spread evenly and the spreading machines therefore require more passages across the application area. Once the biochar is spread onto the land, it can be left on the surface or incorporated into the topsoil through ploughing.

Similarly, on pasture-based land, biochar can be spread on the soil surface and left to be incorporated into the topsoil by allowing the grass to grow through it.

In forestry, it is common practice to bury biochar. This can be as simple as digging a hole for a tree and inputting some biochar prior to planting. Alternatively, biochar can be placed on the soil surface around the base of the trees and be gradually incorporated in.

3. Soil type, land use and topography are critical considerations

It is important to know the soil type when taking decisions about biochar application. When applied to clay soil, for example, biochar which is very fine will clog the soil pores and waterlog the soil, hence losing many of the potential benefits of biochar application. Ensuring that the physical and chemical characteristics of the biochar chosen for application matches the soil characteristics and land use type is therefore extremely important in achieving the intended outcomes of applying biochar. This will also help farmers to maximise the value of using biochar technologies. In Norfolk, for example, where soils tend to be sandy, the value of biochar application to farmers lies in its ability to increase sandy soils’ water holding capacity, which can then reduce the need for irrigation. Similarly, farmers would be advised, for example, not to apply multiple tons of biochar on top of bare land on a steep slope and not incorporate it, as there is a risk it will be washed or blown away.

Similarly, land use type will dictate ideal biochar application rates to maximise its function. Some initial studies at the Bangor University have indicated that around 40 tonnes of biochar per hectare can be added to grasslands, a concentration which far exceeds the one tonne per hectare per year set by the Environment Agency’s Low Risk Waste Position. Clearly, caution needs to be taken with arable land and the Biochar Demonstrator programme has been aimed at showing that 10 tonnes per hectare is a safe amendment level to increase the potential for carbon sequestration. Better understanding of these practicalities and accounting for them when tailoring a biochar application strategy will help to demonstrate the potential to farmers, which in turn will help to scale up biochar application across the UK.

4. Biochar can be stable for thousands of years if it is designed to be

Some evidence for the long-term stability of biochar derives from ancient charcoals and terra preta soils in South America, cultivated by the Amazonians 7,000 years ago which still contain intact pieces of charcoal. Despite some evidence that charcoal will survive for periods of thousands of years in soil, research has shown that a fraction of biochar will degrade over much shorter timescales, with some degrading over periods of tens of years. It is therefore important to assess the long-term stability of biochar before applying it to soil to ensure that it will sequester carbon over a long-term period.

The higher the temperature at which biochar is prepared, the more stable it is likely to be. This is because higher pyrolysis temperatures build larger clusters of polycyclic aromatic hydrocarbons than lower temperatures. Researchers at the University of Nottingham have developed their own method for using a measure of stable polyaromatic or persistent aromatic carbon to give an indication of the long-term stability of biochar, and this method is gaining a lot of traction globally. Typically, biochar is prepared at high temperature, giving a stability index of 90% or above on the measure developed at the University.

5. Biochar has a positive effect on soil health, biodiversity, function

There is a growing body of evidence which shows a positive relationship between the introduction of biochar to soils and soil microbial and fungal diversity. Biochar has been shown to increase soil biodiversity in both species’ richness and abundance, though the direct effect of biochar on enzyme activity can be difficult to elucidate. The effect of biochar on soil biodiversity is dependent on the existent properties and characteristics of the soil and the biochar, but in general, current research points towards biochar’s positive impact on soil biodiversity, health, and function.

When added to the soil, biochar can act as a surface for bacteria and fungi to grow on. Microbial communities grow on biochar and use nutrients either directly from the biochar or indirectly through nutrients in the soil which are drawn onto the surface of the biochar. This process improves soil organic matter and soil aggregation by providing a habitat for microbial communities and by mobilising soil nutrients. Biochar may also increase the soil temperature slightly by decreasing surface albedo, which promotes greater microbial activity. As well as this, biochar can also alter soil texture and pH and can improve soil porosity and moisture. When taken together, the changes to the soil which result from biochar application can make the soil much more suitable for soil microbial communities to flourish and thrive. Despite the positive association between biochar and soil microbial diversity, the impact of biochar on invertebrate biodiversity is less clear and requires more research.

The improvements to soil carbon content related to biochar are caveated by some research which shows some positive priming effects of biochar as a soil amendment. Sometimes, biochar application causes disturbance of the soil which results in a flush of CO₂ and an increase in soil CO₂ emission. However, these effects are often transient and tend to disappear after three to six months.

6. Biochar can help to reduce soil erosion and improve downstream water quality

Evidence shows that adding biochar to soil can help reduce soil erosion and loss. By supporting aggregate formation and improving soil infiltration rates, biochar can help to increase soil drainage, which in turn reduces the effects of, and vulnerability to, erosion. Again, the degree to which biochar improves soil drainage depends on the soil type. Adding biochar to soils can also reduce soil compaction and promote root growth, which will help to stabilise soils through a more resilient root network and will support cover crop growth. This in turn reduces both soil erodibility and soil damage from direct precipitation.

It is worth noting, however, that there is a soil-biochar saturation point, after which more biochar addition will damage the soil structure, which will result in more soil erosion. Although this saturation point will vary by soil type and biochar function, research from the University of Nottingham indicates that biochar application rates which fall within the UK regulations will not cause harm to soils.

By increasing soil infiltration and reducing soil runoff from agricultural land, biochar reduces the levels pesticides and inorganic fertilisers which reach water bodies downstream and therefore has a positive impact on water quality. Moreover, biochar which is applied to soil binds pesticides and some of the nitrogen and phosphorus from the fertilisers applied to agricultural land, potentially reducing the amount entering the water system, again improving water quality. Current research exploring the use of biochar as a filtration technique has indicated that it may have use in wastewater and stormwater treatment, or in ditches and buffer strips as a filter for agricultural runoff.

Preliminary evidence from contemporary biochar research has also indicated that pyrolysing agricultural waste which is spread onto land as fertiliser may reduce the nutrient load in agricultural runoff. Biochars therefore yield as many benefits in water quality regulation, improving soil health, and encouraging plant growth when applied to field margins as when applied to crop land.

7. Biochar must be safe and stable before application

Biochar material is sorted into three principal grades: the top grade is permissible for animal feed, and this is attracting attention, particularly for cows. The next grade down is agricultural grade, which is still very stringent. Finally, the lowest grade is biochar which will be used in aggregates for construction or roads.

Biochar is liable to be broken up by the farm machinery used in its application and therefore the material needs to have reasonable mechanical strength. Prior to its application, biochar must be kept damp while being stored to avoid spontaneous combustion. This also helps in avoiding creating a dust cloud when applying biochar to the soil. Once applied to the soil, biochar cannot be extracted.

Voluntary standards, such as the European and World Biochar Certificates limit pollutant levels, including heavy metals and toxic organic species, including polycyclic aromatic hydrocarbons and dioxins, and exist to ensure that biochar materials will not damage the soils to which they are applied or the wider environment. Typically, pollutants are limited in biochar to levels lower than found in soils.

Various carbon trading platforms such as Pure Earth have their own methodologies and voluntary standards for measuring and assessing the long-term stability of biochar, but most biochar which is prepared at high temperature will meet the required stability criteria. Making biochar using high pyrolysis temperatures may lose some of the biochar’s surface functionality and so limit the extent to which metals in contaminated land might bind to biochar.

Despite these quality regulations, ensuring the safe application of biochar can be limited by a lack of consistency in monitoring, reporting, and verification (MRV) in the regulation process. Carbon platforms are rarely closely monitored, and, for agricultural use, there are few checks to verify that the quantity of biochar that’s being reported is being spread.

This presents an opportunity for the Government to set statutory regulations on top of the voluntary standards and rules set by the carbon trading markets. This will help the government to calculate the amount of carbon sequestered through biochar application and to incorporate these data into the national inventory.

In all, the trials being conducted by researchers at the University of Nottingham wherein ten tonnes of biochar are applied per hectare of land, no negative environmental impact has yet been observed. This is building a strong evidence base which supports a potential shift in the biochar regulatory framework which from the Environment Agency’s Low Risk Waste Position currently prevents the application of more than one ton per hectare per year to allowing ten tonnes per hectare.

8. Making biochar application pay for farmers

Increasing the uptake of biochar application by farmers will likely be conditional on providing the maximum value possible from the technology to the farmer. Understandably, unless farmers are rewarded for using biochar, it is unlikely to be widely applied. The ideal feedstock of the biochar used will therefore be one which can extend – rather than curtail – its lifecycle through pyrolysis. These include feedstock from low-cost bio residues, such as digestate from food waste that would rapidly be converted to greenhouse gases, as opposed to using virgin wood where the cost may be prohibitive.