Department of Earth System Science

Degree of oxidation of organic carbon (C_ox) is a fundamental property of the carbon cycle, reflecting the synthesis and decomposition of natural organic matter. C_ox is also related to ecosystem oxidative ratio (OR), the molar ratio of O₂ to CO₂ fluxes associated with net ecosystem exchange (NEE). Here we compare two methods for measuring C_ox and OR: (1) %C, %H, %N, and %O elemental analysis, and (2) heat of combustion (ΔH_c) measured by means of bomb calorimetry coupled with %C elemental analysis (hereafter referred to as calorimetry). Compared with %C, %N, %H, and %O elemental analysis, calorimetry generates C_ox and OR data more rapidly and cheaply. However, calorimetric measurements yield less accurate C_ox and OR data. We additionally report C_ox and OR data for a pair of biomass standards and a suite of biomass samples. The OR values we measured in these samples were less variable than OR data reported in the literature (generated by simultaneous measurement of ecosystem O₂ and CO₂ gas mixing ratios). Our biomass OR values had a mean of 1.03 and range of 0.99–1.06. These estimates are lower than the OR value of 1.10 that is often used to partition uptake of fossil fuel CO₂ between the ocean and the terrestrial biosphere.