Forestry: Conifer brash as biochar brings big benefits

THE UK is home to over 2,000 abandoned mine sites, posing a contamination risk to land, waterways and surrounding watersheds. In some cases, these abandoned mine lands (AMLs) aren’t contaminated, but still provide no/minimal ecological function due to soil degradation and loss. Mining has had adverse ecological impacts on soils, most notably with acidification, topsoil macronutrient depletion and the reduction of soil water-holding capacity.

Biochar can be used for AML restoration, due to its capacity to increase soil’s water-holding capacity, improve its nutrient retention and elevate its pH, countering mining-induced soil acidification. 

Biochar’s biological, chemical and physical qualities have been shown to improve soil health by decreasing particle density by up to 64 per cent, improving the soil’s water-holding capacity. Biochar particle pores have also been shown to act as microscopic structural habitats for fungi, plant roots and bacteria. There are some concerns surrounding biochar use for land restoration, which are owed largely to their own pollutant potential. This is most notable in the forms of polycyclic aromatic hydrocarbons (PAHs), which are acutely toxic and can potentially enter groundwater systems and the food chain – although the presence of PAHs is typically below existing environmental quality standards.

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The characteristics of biochar can vary drastically, depending on the pyrolysis temperature and duration. Longer pyrolysis periods can facilitate higher biochar yields, due to lower temperatures. Conversely, higher temperatures can improve certain characteristics of the biochar, such as an improved surface area, increased porosity, and an elevated pH – all of which are favourable characteristics for AML restoration.

Another big determiner of a biochar’s characteristics is its feedstock, and this can inform feedstock decisions as a part of a proper mine site risk assessment. This is essential as certain (but few) feedstocks run the risk of worsening the soil’s health, such as Arbutus menziesii, which produces a biochar with a lower pH that can worsen acidification.

Conifer brash, however, is an optimal feedstock for biochar, as it has a high pH after pyrolysis, despite its low pH in its raw form. In a trial, it had the highest pH out of several other wood-based biochars, while also recording the second-highest carbon content (89 per cent, behind the highest at 94 per cent) – indicating its potential for carbon. However, the suitability of biochar for carbon sequestration is contested, as the carbon can only be ‘locked away’ with proper land management.

Biochar made from coniferous wood is also preferable when considering the concerns surrounding biochar application, as they have a lower PAH concentration compared to other biomass types like straw. For example, biochar made from Pinus radiata (a coniferous wood stock) reduced the soil’s phenanthrene concentration by over nine per cent. Conifer brash biochar also serves other industrial land-remediation functions across the UK, such as peatland ‘forest-to-bog’ restoration, due to its capacity to remove phosphate (up to 9.9 per cent) and ammonia (up to 65.2 per cent). Conifer biochar can also be useful at the smaller commercial level with potential uses for water treatment and as a potential peat/substrate replacement in potting soil.

Economically, utilising conifer brash as a biochar feedstock is pragmatic, as it arises as a byproduct of forestry. Additionally, it is a common waste product of commercial gardening, as it’s challenging to burn without extensive drying and it is often unsuitable for mulch due to high acidity. The toxic resin in conifer brash poses ecological risks, taking up to five years to decompose even when chopped into small pieces.

Current harvesting practices create a ‘brash mat’ on forest floors, offering soil coverage during forestry operations, which is left to either decompose or be burnt – leading to potential pollution and/or greenhouse gas emissions. Redirecting biomass to biochar production enables the storing of 50 per cent of emitted carbon as CO2, with the remainder contributing to biofuel production, offering a more efficient and environmentally friendly alternative to current conifer brash management practices.

Brash is often left to decompose on forestry sites. (Image: Supplied)

I further propose the implementation of a coniferous wood disposal system for commercial gardeners to efficiently dispose of organic waste. By leveraging existing infrastructure for industrial conifer brash collection, this system could provide a cost-effective feedstock source for biochar. Temporary Christmas tree recycling points have demonstrated this is feasible. Currently, gardeners face high disposal fees, encouraging inefficient practices like multiple trips in smaller vehicles to avoid charges. This not only increases GHG emissions but also risks illegal dumping, posing threats like wildfires, water pollution, and harm to biodiversity.

This, coupled with the removal of forestry byproducts from felling sites, would align the industry with the Forestry Commission’s advocation for the value of conifers, benefitting various stakeholders, and contributing to a more circular economy.

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