Making and interpreting high precision ecosystem oxidative ratio measurements

Background/Question/Methods

One source of uncertainty in the size of the terrestrial biospheric carbon sink is the large error associated with estimating the global terrestrial biosphere's oxidative ratio (OR).  The OR of the terrestrial biosphere is the ratio of moles O₂/moles CO₂ in the gas exchange fluxes between the terrestrial biosphere and the atmosphere, and is a key parameter in estimations of the size of the terrestrial carbon sink made using O₂ and CO₂ measurements. Global OR is commonly assumed to be 1.1±0.05. However, it is likely that there is significant variation in OR at the ecosystem level due to variations in plant biochemical composition and changes in ecosystem structure due to land use change.

OR has traditionally been measured by tracking O₂ and CO₂ gas concentrations within an ecosystem.  If a long-term, average OR value is desired, this approach requires long-term ecosystem gas monitoring, which is labor-intensive.  Additionally, the atmosphere in many ecosystems of interest is affected by CO₂ from fossil fuel combustion, making it challenging to determine if O₂/CO₂ ratios measured reflect the true ecosystem OR.  Air mass back trajectories can be calculated to determine which samples are most likely contaminated, but this adds another layer of complexity to the measurement.

Results/Conclusions

We have developed and intercompared three fast and accurate techniques to measure ecosystem OR.  All three of these techniques are based on the relationship between OR and the ecosystem organic matter carbon oxidation state (C_ox), a parameter which can be measured using an elemental analyzer (e.g. Costech ECS 4010), a bomb calorimeter, or a nuclear magnetic resonance spectrometer.  Our three techniques are intercomparable to ±0.011 OR units, with the majority of error due to organo-mineral interactions in soils.  OR measurements in aboveground biomass (made using an elemental analyzer or bomb calorimeter) are accurate to better than ±0.006.   We describe in this presentation the logistics of making OR measurements via our new techniques and we show an overview of high precision OR measurements for three ecosystems (forest with and without CO₂ enrichment and row-crop agriculture).