Biochar Supplementation of Recycled Manure Solids: Impact on Their Characteristics and Greenhouse Gas Emissions During Storage

Recycled manure solids (RMS) are increasingly adopted in dairy farming for their economic advantages and their role in improving nutrient recycling and waste management; however, concerns regarding greenhouse gas (GHG) emissions during storage persist. This study assessed the effects of biochar supplementation at 2.5% (2.5B) and 10% (10B) compared to untreated RMS (C−) and acidified RMS (C+) on GHG emissions (measured both continuously and intermittently) and RMS characteristics during a one-month storage period. The results showed that the addition of biochar increased heavy metals concentration (with the exception of molybdenum) and the electrical conductivity of the RMS. Storage of RMS generally led to an increase in its dry matter content, except in the 10B treatment. The results showed that 10% biochar significantly reduced cumulative CO2 and N2O emissions, resulting in a 32% GWP reduction compared to untreated RMS. In contrast, the 2.5% dose led to higher CO2 emissions, possibly due to microbial stimulation. Adding 10% biochar mitigated GHG emissions similarly to H2SO4 acidification but with fewer environmental and operational risks, making it a preferable farm-scale option. Continuous monitoring captured transient emission peaks, highlighting the importance of high-resolution assessments. Despite the emissions generated during biochar production, its application in RMS bedding systems offsets these environmental costs by mitigating GHG emissions and increasing nutrient content. Biochar’s mitigation potential, especially at higher doses, presents a safer, multifunctional alternative that aligns with EU climate goals. These findings support the integration of biochar into sustainable manure management strategies, though further research is needed to optimize application rates and assess cost-effectiveness in dairy farming. However, continued assessments at a larger scale and with different biochar addition rates are necessary to fully determine the potential of biochar supplementation to RMS.