Estimating the oxidative ratio of the global terrestrial biosphere carbon

The oxidative ratio (OR) is the amount of CO₂ sequestered in the terrestrial biosphere for each mol of O₂ produced. The OR governs the efficiency of a terrestrial biome’s O₂ production and it has been used to calculate the balance of terrestrial and oceanic carbon sinks across the globe. However, the value used in carbon cycle calculations comes from only one study of one environment. Here we perform a meta-analysis of studies of soil organic matter and vegetation composition to calculate the first global ecosystem OR value. We use data from 138 samples across 31 studies covering 9 USDA global soil orders, 7 global biomes and 5 continents and combine this information as a weighted average based upon biome land area or organic carbon content of the soil order. Organic matter fractions could not be shown to be reliable proxies for whole soil or vegetation OR. The resulting analysis suggests that although the presently used value of 1.1 is within the range of natural occurrence, it is not the most accurate choice, representing between the 97th and 99th percentile value. Our study yields a global terrestrial OR = 1.04 ± 0.03. This value of OR means that the sink of anthropogenic carbon fluxes to land has been underestimated (and the sink to the ocean overestimated) by up to 14 %. Recalculating with our OR value, the fossil fuel carbon flux to land is 1.48 ± 0.04 Gt C/year and flux to oceans is 2.02 ± 0.03 Gt C/year.