Forest soil carbon oxidation state and oxidative ratio responses to elevated CO₂

The oxidative ratio (OR) of the biosphere is the stoichiometric ratio (O₂/CO₂) of gas exchange by photosynthesis and respiration - a key parameter in budgeting calculations of the land and ocean carbon sinks. Carbon cycle-climate feedbacks could alter the OR of the biosphere by affecting the quantity and quality of organic matter in plant biomass and soil carbon pools. This study considers the effect of elevated atmospheric carbon dioxide concentrations ([CO₂]) on the OR of a hardwood forest after nine growing seasons of Free-Air CO₂ Enrichment. We measured changes in the carbon oxidation state (C_ox) of biomass and soil carbon pools as a proxy for the ecosystem OR. The OR of net primary production, 1.039, was not affected by elevated [CO₂]. However, the C_ox of the soil carbon pool was 40% higher at elevated [CO₂], and the estimated OR values for soil respiration increased from 1.006 at ambient [CO₂] to 1.054 at elevated [CO₂]. A biochemical inventory of the soil organic matter ascribed the increases in C_ox and OR to faster turnover of reduced substrates, lignin and lipids, at elevated [CO₂]. This implicates the heterotrophic soil community response to elevated [CO₂] as a driver of disequilibrium in the ecosystem OR. The oxidation of soil carbon pool constitutes an unexpected terrestrial O₂ sink. Carbon budgets constructed under the assumption of OR equilibrium would equate such a terrestrial O₂ sink to CO₂ uptake by the ocean. The potential for climate-driven disequilibriua in the cycling of O₂ and CO₂ warrants further investigation.