Long-term influence of biochar on native organic carbon mineralisation in a low-carbon clayey soil

Biochar can influence native soil organic carbon (SOC) mineralisation through "priming effects". However, the long-term direction, persistence and extent of SOC priming by biochar remain uncertain. Using natural ¹³C abundance and under controlled laboratory conditions, we show that biochar-stimulated SOC mineralisation ("positive priming") caused a loss of 4 to 44 mg C g⁻¹ SOC over 2.3 years in a clayey, unplanted soil (0.42% OC). Positive priming was greater for manure-based or 400°C biochars, cf. plant-based or 550°C biochars, but was trivial relative to recalcitrant C in biochar. From 2.3 to 5.0 years, the amount of positively-primed soil CO₂-C in the biochar treatments decreased by 4 to 7 mg C g⁻¹ SOC. We conclude that biochar stimulates native SOC mineralisation in the low-C clayey soil but that this effect decreases with time, possibly due to depletion of labile SOC from initial positive priming, and/or stabilisation of SOC caused by biochar-induced organo-mineral interactions.