Geochemistry of carbon sequestration through woody biomass burial

Apparent mechanisms and rates of wood decay under natural geologic burial can be reconciled with general principles of chemical thermodynamics and kinetics, including effects of biotic intermediaries on reaction pathways. A simplified two-step decay model for woody biomass burial (WBB) involves the hydrolysis of wood biopolymers to release monomers, which then decompose into CO2 or CH4. Gibbs free energy values for individual reactions indicate that (a) biopolymer hydrolysis follows a stability sequence of lignin ≫ cellulose > hemicellulose, and (b) monomer decomposition is driven more strongly toward CO2 (compared with CH4) unless biological intervention occurs. Key variables are wood composition, water activity, oxygen activity, and enzymatic activity (from bacteria or fungi) under different burial conditions. Model curves for wood decay under geologic burial indicate that more than 97% of original carbon in tree wood can be preserved (in undecayed form) for 100 y and that 50% (and up to nearly 90%) of original carbon can be preserved for 1,000 y. The model aligns with empirical evidence from ancient tree wood buried thousands of years ago by natural floods, landslides, and volcanic eruptions. It also suggests that WBB can be an effective, nature-based method for carbon sequestration over timescales which are relevant to climatology.