As reported in Carbon Research, Jinfeng He and colleagues investigated a novel way to improve the recycling of livestock waste. The researchers created a specialized additive by combining coconut shells with oyster shells and heating them at high temperatures without oxygen. This process resulted in a calcium-modified biochar that was then mixed into a composting pile of pig manure and rice straw. The study sought to address the common problem of slow and inefficient organic matter breakdown in traditional composting, which often results in lower-quality fertilizer. By using waste products like oyster shells as a calcium source, the team developed a sustainable method to enhance the chemical and biological transformations that occur during the composting cycle.

The results showed that the modified biochar significantly altered the physical and chemical environment of the compost pile. During the active phase of composting, the groups treated with the modified biochar reached higher peak temperatures, hitting nearly 70 degrees Celsius. This intense heat is vital for killing pathogens and speeding up the breakdown of organic materials. Furthermore, the modified biochar acted as a stabilizing agent for the acidity levels of the mixture. Because the oyster shells slowly released calcium, they helped neutralize organic acids produced during the early stages of decomposition. This stabilization created a more favorable environment for the microorganisms responsible for turning waste into nutrient-rich humus.

The study found a major improvement in the humification process, which is the conversion of raw organic matter into stable substances called humic acids. In the piles containing the modified biochar, the final amount of humic acid was significantly higher than in the control groups. Specifically, the researchers observed a 4.92% increase in overall humification efficiency. This change is important because humic acids are the “gold standard” for soil health, helping to retain moisture and keep nutrients available for crops. The spectroscopy analysis confirmed that the surface of the modified biochar provided active sites that helped small molecules join together into these larger, more complex humic structures.

Safety for agricultural use also saw a measurable boost through this treatment. The seed germination index, which measures how well plants can grow in the finished compost, was nearly 23% higher in the modified biochar group than in the control. This indicates that the additive effectively reduced the presence of harmful chemicals that can sometimes linger in immature compost. The compost treated with the modified biochar reached the safety threshold for plant growth a full week earlier than the standard compost. This faster maturation could allow commercial composting facilities to process more waste in less time, improving the efficiency of waste management systems.

From a biological perspective, the addition of the oyster shell-modified biochar dramatically shifted the types of bacteria present in the compost. The researchers discovered that the prevalence of two major groups of bacteria, Proteobacteria and Bacteroidetes, was 107% higher in the modified biochar treatment compared to the control. These specific microbes are known for their ability to break down tough plant materials like lignin and help stabilize nitrogen. The porous structure of the biochar provided a protected home for these bacteria to thrive. By fostering a more active and specialized microbial community, the modified biochar ensured that the composting process was not only faster but also produced a more stable and nutrient-dense final product.

Source: He, J., Li, L., Shi, Y., Wang, K., He, J., Ruan, Y., Bao, H., Khan, M. U., Li, D., Chen, S., & Fan, P. (2026). Oyster shell-functionalized biochar enhanced compost humification during the co-composting of pig manure with rice straw. Carbon Research, 5(7).