Does size matter? Biochar particle size and nitrogen addition drive biochar-derived carbon mineralisation and soil organic carbon turnover

Biochar represents a promising greenhouse gas removal (GGR) tool due to its recalcitrant nature and resistance to decomposition, with soil application one of the most common storage environments. However, the introduction of organic or mineral substrates can trigger priming effects (PE), leading to microbially-mediated mineralisation of native soil organic carbon (SOC) (positive PE) or preferential degradation of added material (negative PE). The magnitude and direction of PE significantly impact short- and long-term C storage. To examine how biochar characteristics influence C dynamics, we conducted a 30-week mesocosm study combining grassland soil and ammonium-nitrate fertiliser with hazel wood (Corylus avellana L.) biochar produced at 450 °C and 600 °C, separated into three size fractions: small (< 355 μm), medium (355 μm – 2 mm), and large (2 – 9.5 mm). 14C labelling enabled direct measurement of soil and biochar C mineralisation. Biochar mineralisation was significantly higher with small particle sizes, independent of pyrolysis temperature or N addition. SOC turnover was unaffected by biochar particle size or pyrolysis temperature. However, N addition significantly increased the PE of the soil and altered soil microbial community structure. Results indicate that larger biochar particles demonstrate greater soil durability, being less accessible to microorganisms and more resistant to degradation. These findings suggest that biochar particle size critically influences C sequestration potential.