Every organic carbon pool has an associated oxidative ratio (OR) value, where OR is defined as the moles of O₂ emitted during photosynthesis per moles of CO₂ fixed (O₂/CO₂), or the moles of O₂ consumed per mole of CO₂ emitted for reverse processes (e.g. respiration, fire, etc.). This exchange ratio between O₂ and CO₂ in the biosphere is an Earth system tracer, with a natural span of values ranging from 0 (CO₂) to 2.0 (CH₄). OR values for the atmosphere, terrestrial biomass, and soil samples typically fall in the range of 0.8 – 1.2 [Severinghaus, 1995; Masiello et al., 2008; Hockaday et al., 2009; Worrall et al., 2013; Clay and Worrall, 2015a; Gallagher et al., 2014; van der Laan et al., 2014].

Like many other organic matter tracers, OR can be both a property of a pool or a flux. This is analogous to δ ¹³ C, which can be measured in biomass, litter, soils, sediments, or atmospheric samples. Like the δ ¹³ C signature of the photosynthetic flux, the OR associated with photosynthesis can be constrained through gas measurements, or it can be estimated after CO ₂ has been fixed by measuring the signature retained in biomass, soils, or sediments.

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