Composting with Biochar

Compost with Biochar

Now that winter is almost through, gardeners and farmers are planning how to best revive their soil after growing demanding brassica crops. Compost and green manures are often the go-to solution for organically remediating nutrient depleted soil. The application of biochar can complement these methods. Many studies have shown the addition of Biochar to facilitate faster organic matter degradation & enhanced bacterial community composition.

Composting is an excellent low cost method of waste management and nutrient recycling. Adding compost to soil will increase nutrient content and organic carbon while also improving soil structure and composition. Efficient composting methods can destroy weed seeds and pathogenic bacteria, however problems still arise through the composting process, such as; the emission of greenhouse gases, the loss of nitrogen via ammonia volatisation, and safety concerns linked to the absorption of inorganic/organic contaminants from compost substrates (Xiao et al. 2017). Many studies have found Biochar to be a useful additive to aid in combating these issues, and in improving overall compost quality.




Successful composting relies on stabilised high pile temperatures during the thermophilic phase, to ensure the elimination of pathogens and a healthy rate of breakdown (Godlewska 2017). Temperatures above 50°C create ideal conditions for composting processes, largely due the genus Bacillus dominating the microbial community in compost temperatures between 50-55°C (Wei et al. 2014). The addition of Biochar has been found to increase pile temperatures during the thermophilic phase, and to also extend thermophilic periods by up to two weeks (Xiao et al. 2017). This process is attributed to Biochar's large surface area and ability to bulk free spaces between raw material particles, resulting in the reduction of heat losses (Qui et al. 2018). The addition of Biochar also improves water retention in soils and increases aeration, which further creates more favourable environmental conditions for microbial activity. Intensified microorganism activity produces higher compost pile temperatures and a temporary decrease in oxygen concentration, which results in accelerated organic matter transformation into compost (Godlewska 2017).


Physiochemical Changes

Along with improving compost pile temperatures, the highly porous surface area, absorptive capacity, total acid functional groups and cation exchange capacity of Biochar, brings about improved Nitrogen retention and thus an increase in Total Nitrogen concentration in compost. Given the carbon-rich nature of Biochar, the Carbon:Nitrogen ratio of compost substrate becomes stabilised through the amendment of Biochar, as it influences the structure of humic substances and subsequently reduces the loss of Carbon:Nitrogen ratio during the composting cycle (Qui et al. 2018). Biochar can also impact compost matrix mass density and particle size, thus preventing the formation of large clumps in compost heaps. This process improves compost oxygen supply and prevents the development of anaerobic areas (Xiao et al. 2017). Further to this, the addition of Biochar to compost has been found to increase the concentration of mineral elements, stimulate improvements to soil pH and key soil macro-elements, while also decreasing the presence of trace metals in leachates. Unique to other soil amendments, Biochar reduces the likelihood of heavy metal accumulation, for example sewage sludge, linked to re-application of soil amendments (Beesley et al. 2010).



Microbial Activity

The temperature and physiochemical changes promoted by the addition of Biochar to compost piles create ideal conditions for microbial communities and activities to flourish. The most valuable resources for microorganism growth are Carbon and Nitrogen, thus the impact Biochar has on reducing the decrease in Carbon:Nitrogen during composting, is significantly beneficial. Microorganisms need moist conditions to survive, and although Biochar increases compost temperature, its high water-holding capacity ensures greater moisture retention and less evaporation compared to pure compost, hence creating favourable microbial conditions (Godlewska 2017). Further to this, the porous structure of Biochar serves as an ideal habitat for microorganisms, and the improved ventilation achieved with the addition of Biochar establishes an optimum living environment within compost heaps for these communities (Xiao et al. 2017). Microorganisms utilise several macro and micronutrients provided by Biochar, such as labile aliphatic compounds and inorganic nutrients. Studies have found that once microorganisms multiply on the Biochar surface, they are able to extract carbon and various nutrients allowing further proliferation and microbial community diversity. Biochar has been found to be more effective at stimulating microbial activity than other bulking agents such as peat bog or zeolite (Xiao et al. 2017).


Gas Emissions

The emissions of NH3 and other GHG gases through composting, contributes to a decline in compost nutrient quality, while also contributing to climate change and human health problems. Many studies support the addition of Biochar into organic waste composting, as a beneficial method to reduce NH3 and suppress GHG emissions from composting (Xiao et al. 2017). The volatilisation of ammonia typically transpires during the thermophilic stage of composting, due to the decomposition of nitrogenous material. Various aspects such as, pile temperature, pH, and ventilation rates, impact the volatilisation of NH3. Studies have found significant reductions in NH3 emissions in Biochar blended compost, due to Biochar’s propensity to effect NH+4- NH3 equilibrium and N mineralization dynamics (Xiao et al. 2017). Furthermore, the improved aeration qualities of Biochar and compost blends, sustained a more habitable environment for nitrifying bacteria (Sànchez-García et al. 2015). Higher rates of Biochar application to compost piles saw better outcomes in reducing ammonia volatilisation. Biochar can also decrease CH4 emissions in compost through its aerating properties, gas diffusion and ability to improve the condition for methanotrophic populations in the soil (Godlewska 2017). Biochar has also been found to lower total N2O emissions when composting manures, due to a change in the abundance and structure of denitrifying bacteria (SànchezGarcía et al. 2015). The overall reduction in gas emissions will assist in reducing unpleasant odours from compost, and will aid in preventing the release of harmful greenhouse gases into the environment.



Green Man Recommendation

Green Man Char advises a Biochar application rate of between 5 to 20% per volume of compost. When Biochar is added to compost, it will become charged with nutrients, which will bind to its large surface area and free carbon bonds. This process will enable the nutrients in the compost mix to be maintained in the substrate and will allow the Biochar to increase the growth of flowers, trees, fruits and vegetables across your garden and farms.


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Beesley, L., Moreno-Jiménez, E. and Gomez-Eyles, J.L., 2010. Effects of biochar and greenwaste compost amendments on mobility, bioavailability and toxicity of inorganic and organic contaminants in a multi-element polluted soil. Environmental pollution158(6), pp.2282-2287.


Godlewska, P., Schmidt, H.P., Ok, Y.S. and Oleszczuk, P., 2017. Biochar for composting improvement and contaminants reduction. A review. Bioresource technology246, pp.193-202.


Qiu, X., Zhou, G., Zhang, J. and Wang, W., 2019. Microbial community responses to biochar addition when a green waste and manure mix are composted: A molecular ecological network analysis. Bioresource technology273, pp.666-671.


Sánchez-García, M., Alburquerque, J.A., Sánchez-Monedero, M.A., Roig, A. and Cayuela, M.L., 2015. Biochar accelerates organic matter degradation and enhances N mineralisation during composting of poultry manure without a relevant impact on gas emissions. Bioresource Technology192, pp.272-279.


Wei, L., Shutao, W., Jin, Z. and Tong, X., 2014. Biochar influences the microbial community structure during tomato stalk composting with chicken manure. Bioresource technology154, pp.148-154.


Xiao, R., Awasthi, M.K., Li, R., Park, J., Pensky, S.M., Wang, Q., Wang, J.J. and Zhang, Z., 2017. Recent developments in biochar utilization as an additive in organic solid waste composting: a review. Bioresource technology246, pp.203-213.

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