Key Takeaways
- Combining biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More and compost can either speed up or slow down the natural breakdown of carbon in soil, depending on the type of biochar used.
- The study found that mixing poultry biochar with compost increased carbon decomposition by 18% more than expected from their separate effects.
- In contrast, combining rice husk biochar with compost slowed down the decomposition, resulting in 10% less breakdown than predicted.
- This suggests that choosing the right biochar is crucial when combining it with compost to achieve desired outcomes like carbon storage.
- The effects are strongly linked to changes in microbial activity and soil pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More
In an effort to understand how to best manage soils for both fertility and carbon sequestration, researchers Siska Syaranamual, Bahareh Bicharanloo, Shamim Mia, and Feike A. Dijkstra recently published their findings in the journal Discover Soil. Their study investigated the combined influence of biochar and compost on carbon (C) mineralization and the priming effect in low-carbon soil. While modern agriculture often contributes to greenhouse gas emissions, integrating sustainable practices like using biochar and compost presents a promising path toward climate change mitigation. Biochar is generally considered beneficial for sequestering C, but its effect on the breakdown of existing soil C (known as priming) is variable. Compost, containing more labile C and nutrients, typically increases the mineralization of soil C. Given that soils are frequently amended with both, clarifying their combined effect on soil C dynamics is essential.
The researchers conducted a 120-day laboratory incubation study using a low-organic C artificial soil. They applied compost (made from sugarcane residue) and three types of biochar (poultry, rice husk, and Eucalyptus, all produced at 550°C) both individually at 2% and in combination at 1% each. Overall, the compost-only application led to the highest cumulative decomposition after 120 days, which is not surprising since compost has a higher content of easily decomposed carbon compared to the more stable nature of biochar. On average, biochar applied alone resulted in 648 mg C kg−1 soil decomposition, while the co-application treatments averaged 882 mg C kg−1 soil.
The core finding of the study revolves around the non-additive effects when biochar and compost were combined, a phenomenon referred to as the priming effect (PE). The PE is the difference between the actual measured decomposition and the decomposition predicted from the sum of their separate applications. When compost was co-applied with poultry biochar, the actual decomposition was significantly higher than the predicted value, showing a positive priming effect by 18%. This positive effect supports the idea that the readily available C from the compost provided the necessary energy for microbes to decompose the more stable C in the biochar, a process known as co-metabolism. This combination also led to the highest microbial biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More carbon (Cmic) levels at both 60 and 120 days, and the biochar’s liming effect reduced soil acidity. The study showed a significant positive relationship between Cmic and the PE, as well as a strong positive correlation between soil pH and the PE, suggesting that the biochar’s properties, like pH and potential to provide a microbial habitat, modulated the outcome.
Conversely, the co-application of compost with rice husk biochar resulted in a negative priming effect, with actual decomposition being 10% lower than predicted. The negative PE suggests that the rice husk biochar may have stimulated the adsorption of organic matter from the compost or soil onto its surfaces, thereby protecting it from microbial breakdown. Surprisingly, despite rice husk biochar having the highest individual pH (10.7), its combination with compost resulted in only a small, insignificant increase in soil pH compared to the soil-plus-compost treatment, further highlighting the dominance of pH in regulating the PE.
The Eucalyptus biochar co-application, which had a neutral individual pH (6.65), showed no significant difference between actual and predicted decomposition, suggesting an additive effect on decomposition.
In summary, this study clearly demonstrates that the co-application of biochar and compost has non-additive effects on overall carbon decomposition and that the specific properties of the biochar—such as its effect on soil pH, its nutrient content, and its role in microbial habitat—are critical factors in determining whether the interaction promotes C loss (positive PE) or potential C stabilization (negative PE). This means that farmers and land managers need to carefully consider the source and properties of the biochar when combining it with compost for specific soil management goals. The research highlights the need for further studies to assess these interactions in various, more natural soil types and under different environmental conditions, and to look into the long-term effects on C stabilization.
Source: Syaranamual, S., Bicharanloo, B., Mia, S., & Dijkstra, F. A. (2025). Influence of biochar, compost, and their combination on carbon mineralisation and the priming effect in low-carbon soil: an incubation experiment. Discover Soil, 2(1), 110.
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