Biochar from slaughtered bone waste: circular economy and sustainable agriculture

Abstract This thesis set out to explore the potential of bone char as a sustainable solution to two interconnected challenges: waste management in the meat industry and the need for alternative phosphorus (P) sources in agriculture. By transforming slaughterhouse bone waste into biochar, this research aligned with the principles of the circular economy, contributing to both environmental sustainability and agricultural productivity. Through a comprehensive review of existing literature, a series of controlled experiments, and a comparative analysis of biochar and bone char, this thesis has provided new insights into the role that bone char can play in improving soil health, enhancing crop yields, and mitigating climate change. • Chapter 1: State-of-the-Art The introductory chapter laid the groundwork by providing an in-depth review of bone char, examining its respective roles in agriculture and environmental sustainability. Biochar has long been recognized for its ability to improve soil structure, enhance water retention, and sequester carbon (C), making it a valuable tool in addressing the twin challenges of soil degradation and climate change. However, the use of biochar derived from plant biomass has limitations, particularly in P-deficient soils, where it does not supply sufficient nutrients. Bone char, in contrast, is rich in P and Ca, offering a viable solution to the depletion of P reserves—a critical issue in global food security. The literature reviewed demonstrated that bone char can act as a slow-release P fertilizer, providing a more sustainable alternative to conventional fertilizers derived from non-renewable phosphate rock. Moreover, the ability of bone char to improve soil fertility while reducing the environmental impact of waste disposal presents a strong case for its adoption in agriculture. However, gaps in the literature, particularly regarding the long-term effects of bone char application on soil health and crop productivity, highlighted the need for further research, which this thesis aimed to address. • Chapter 2: Experimental Findings The experimental portion of this thesis built upon the knowledge gained from the literature review, focusing on the practical application of bone char in agricultural soils. Through a series of controlled pot experiments, the effects of bone char and biochar on soil properties, plant growth, and nutrient uptake were compared across different soil types, particularly soils with low and high P content. The results demonstrated that bone char had a significant positive effect on plant growth, particularly in P-deficient soils. The slow-release nature of P in bone char allowed for more consistent nutrient availability throughout the growing period, leading to improved shoot and root biomass, enhanced root morphology, and higher P uptake in comparison to both biochar and control treatments. In soils with low P content, bone char increased shoot biomass by ten times compared to the control and five times compared to biochar, illustrating its effectiveness as a P-rich soil amendment. Furthermore, the experiments revealed that bone char improved soil structure and water retention, contributing to better overall plant health and growth. In soils with higher P content, the benefits of bone char were less pronounced, but still evident. While biochar outperformed bone char in terms of enhancing soil physical properties and moisture retention in high-P soils, its ability to provide long-term P availability makes it a valuable addition to soil management strategies in nutrient-deficient environments. These findings highlight the importance of selecting soil amendments based on specific soil conditions, reinforcing the role of bone char as an effective tool for sustainable P management. • Chapter 3: Environmental Impacts In addition to its role as a soil amendment, this thesis also explored the environmental implications of bone char application, particularly in relation to greenhouse gas emissions. Biochar has been widely recognized for its C sequestration potential, as it stabilizes C in soils for extended periods, thus reducing the amount of CO2 released into the atmosphere. The same is true for bone char, which, due to its stable C structure, contributes to long-term C storage in soils. The research presented in Chapter 3 evaluated the effects of biochar and bone char on greenhouse gas emissions, particularly focusing on nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions from treated soils. The results showed that both biochar and bone char reduced the daily and cumulative flux of greenhouse gases compared to the control, with bone char demonstrating a slightly higher capacity for reducing N2O emissions in P-deficient soils. This reduction in greenhouse gas emissions, combined with the ability of bone char to sequester C, positions it as a key tool in climate change mitigation strategies within agricultural systems. Furthermore, the cumulative global warming potential (GWP) and greenhouse gas intensity (GHGI) of the treated soils were significantly lower in biochar and bone char treatments compared to control soils. This suggests that integrating bone char into agricultural practices not only improves soil health and crop productivity but also contributes to reducing the environmental footprint of agriculture. • Chapter 4: Phosphorus Efficiency While Chapters 1 through 3 of thesis provided a solid foundation for understanding the potential of bone char in sustainable agriculture, Chapter 4 further investigated its P efficiency of bone char in different particle sizes including macro and micro. The aim was to assess how particle size influences P uptake, distribution across soil fractions, and nutrient use efficiency. Results of this chapter demonstrated that bone char in micro size outperformed macro size in promoting maize growth and P accumulation, highlighting its efficiency in supporting root P uptake. On the other hand, the Hedley P fractionation results indicated that bone char enhanced all extracted P fractions of the soil, suggesting that bone char provides both immediate and long-term P availability. This was particularly notable in the stable fraction, which formed a major portion of the total P pool in bone char-treated soils, indicating its potential as a slow-release P source that could reduce the need for frequent fertilization. The 33P isotope tracing showed relatively similar results. Finally, micro bone char emerged as a more effective P source than macro for the plant, improving both P uptake and phosphorus use efficiency in maize. These findings support the potential of bone char, whether in macro or micro size, as a sustainable P source, offering a slow-release mechanism that could reduce dependence on conventional fertilizers, minimize phosphorus runoff, and contribute to sustainable nutrient management practices in agriculture. Concluding remarks highlighted bone char as a nutrient provider and environmental protection agent, suggesting its inclusion in agricultural systems. It recommended innovative production approaches, site-specific application strategies, and expand field applications to facilitate adoption. This project contributed to the European Green Deal's goals of resource efficiency and climate neutrality while offering practical solutions for sustainable farming practices worldwide.