Biochar permanence : a policy commentary

Abstract

The application of biochar to soil is a highly durable nature-based carbon dioxide removal (CDR) pathway. It provides certifiable climate-change mitigation, with mean carbon residence times exceeding 1,000 years, and additional co-benefits for soil health and fertility. 2. Biochar persistence in soil depends on both intrinsic material properties and environmental factors. Its longevity is determined not only by the polyaromatic structure of the biochar itself but also by soil mineralogy, biological activity, and climatic conditions. 3. Biochar aging involves both decomposition and stabilization processes. The complementary mechanisms of decomposition and stabilization include interactions of biochar with minerals and native organic matter, as well as aggregations with soil particles that maintain its long-term persistence. 4. Biochars and inertinite-ranked fossil coals cannot be equated. Inertinite has been protected from biotic and abiotic oxidation for millions of years through burial in sediments and inclusion in minerals under high pressure and temperature. Biochar produced today in modern pyrolysis facilities is a fundamentally different material. 5. No carbonaceous material is completely inert. Field and laboratory studies consistently show measurable, though small, mineralization across a wide range of biochar types. Declaring that soil-applied biochar carbon persists at 100% over millennia is inconsistent with current scientific understanding. 6. Analytical proxies indicate relative, but not absolute, biochar persistence. 7. Policy definitions of biochar CDR should reflect climate-relevant timescales. The degree of persistence should be estimated on the order of centuries rather than millennia, supported by registered material properties, traceable application data, conservative modeling, and continued long-term field experiments for model validation.