Accuracy and precision in measurements of biomass oxidative ratios

One fundamental property of the Earth system is the oxidative ratio (OR) of the terrestrial biosphere, or the mols CO2 fixed per mols O2 released via photosynthesis. This is also an essential, poorly constrained parameter in the calculation of the size of the terrestrial and oceanic carbon sinks via atmospheric O2 and CO2 measurements. We are pursuing a number of techniques to accurately measure natural variations in above- and below-ground OR. For aboveground biomass, OR can be calculated directly from percent C, H, N, and O data measured via elemental analysis; however, the precision of this technique is a function of 4 measurements, resulting in increased data variability. It is also possible to measure OR via bomb calorimetry and percent C, using relationships between the heat of combustion of a sample and its OR. These measurements hold the potential for generation of more precise data, as error depends only on 2 measurements instead of 4. We present data comparing these two OR measurement techniques.