Biochar suppresses subsoil carbon decomposition

Biochar is widely promoted as a climate-mitigation strategy, yet its effects on carbon persistence in subsoil remain poorly constrained. Here we show that long-term surface biochar application substantially increases soil organic carbon (SOC) storage in subsoil by suppressing SOC decomposition. In a 14-year field experiment with compound-specific δ¹³C and Δ¹⁴C analyses, SOC increased by 40 ± 3.7 Mg C ha^–1 across a one-meter profile, with 17% of this increase occurring below 20 cm. Biochar stabilized plant-derived carbon in topsoil via enhanced microbial transformation and mineral association, while the reduced vertical carbon transport constrained carbon supply to subsoil, suppressing native SOC decomposition and extending its radiocarbon age by 690 ± 57 years. These patterns were confirmed by five additional biochar experiments and a global meta-analysis, indicating broad applicability. Globally, this mechanism could offset 2.2–6.9% of annual agricultural greenhouse-gas emissions, highlighting subsoil as a resilient, underappreciated carbon sink under long-term biochar management.