Using atmospheric O2 and CO2 measurements to determine the stoichiometry of photosynthesis and respiration in a temperate forest

The O2:CO2 exchange ratio of the terrestrial biosphere (αb) is an important parameter in carbon sink calculations, but its value is not well constrained. We investigate the stoichiometry of O2 and CO2 at Harvard Forest in Petersham, Massachusetts over a span of six years, considering the covariation of O2 and CO2 in forest air during 6-hour periods to determine an average value for the O2:CO2 exchange ratio. This approach provides a way to determine the value of αb averaged across seasonal cycles and species assemblages. Our analysis produces an overall average exchange ratio of -1.06 ± 0.01. Comparing measurements within and above the canopy and during nighttime and daytime periods, we observe that atmospheric dynamics and canopy effects produce lower exchange ratios indicative of an enhanced forest signal at the low intake and for daytime periods. We also see an increase in the exchange ratio in the winter compared to the summer that may reflect changes in plant physiological processes or contamination by a fossil fuel signal. To determine whether our observed ratio is truly representative of αb, we use a simple model to estimate the range of variability in CO2 and O2 mixing ratios expected from local influence alone and use this as a criterion to isolate periods dominated by local exchange, yielding an average summer forest exchange ratio of -1.00 ± 0.02. Our analysis provides insight into the average value and variability of αb for temperate forests for use in calculation of the land carbon sink.