Global temperatures have risen by over 1.1°C since the pre-industrial era, with most of this increase occurring in the past five decades, underscoring the urgent need for innovative carbon mitigation strategies. Agriculture and livestock production, particularly through conventional manure management practices such as land application, lagoon storage, and anaerobic digestion, are significant contributors to greenhouse gas (GHG) emissions, especially methane (CH4). Addressing these emissions while enhancing soil carbon storage is critical to mitigating climate change. This study explores the potential of hydrochar and biochar derived from dairy manure, a major waste stream in concentrated animal feeding operations (CAFOs) to reduce GHG emissions and improve soil carbon sequestration. Hydrochar and biochar were produced from dairy manure through hydrothermal carbonization (HTC) at three temperatures (180, 220, 260°C) and pyrolysis at three temperatures (400, 600, and 800°C), respectively. Properties such as carbon content, ultimate and proximate analysis, H/C ratio, O/C ratio, and the R50 recalcitrance index were evaluated to assess the stability and carbon sequestration potential of produced hydrochar and biochar. The yield of hydrochar decreased from 46.34% to 36.75% as the HTC temperature increased from 180°C to 260°C, while biochar yield ranged from 56.88% at 400°C to 45.56% at 800°C. Both hydrochar and biochar exhibited declining H/C and O/C ratios with increasing temperature, indicating higher aromaticity and stability. FTIR analysis confirmed this trend, showing a shift in functional groups toward more aromatic structures. The R50 recalcitrance index, a measure of long-term carbon stability, increased with temperature, reaching 0.51 for biochar at 800°C and 0.43 for hydrochar at 260°C. Biochar demonstrated higher carbon sequestration potential (highest 32.79% at 400°C) compared to hydrochar (highest 22.11% at 180°C). This study also evaluated alternative manure management strategies through HTC, and pyrolysis by analyzing two hypothetical CAFOs: a 1000-head dairy CAFO, where 70% of the manure is stored in lagoons and 30% is land-applied, and a 2000-head dairy CAFO, where 70% of the manure is processed in an anaerobic digester and 30% is used for land application. Integrating HTC and pyrolysis into manure management significantly reduced methane emissions while enhancing soil carbon retention.
