资源环境科技发展态势分析平台

Measurements of atmospheric O₂ have been used to quantify large-scale fluxes of carbon between the oceans, atmosphere and land since 1992 (Keeling and Shertz, 1992). With time, datasets have grown and estimates of fluxes have become more precise, but a key uncertainty in these calculations is the exchange ratio of O₂ and CO₂ associated with the net land carbon sink (α_B). We present measurements of atmospheric O₂ and CO₂ collected over a 6-year period from a mixed deciduous forest in central Massachusetts, USA (42.537^∘ N, 72.171^∘ W). Using a differential fuel-cell-based instrument for O₂ and a nondispersive infrared analyzer for CO₂, we analyzed airstreams collected within and ∼5 m above the forest canopy. Averaged over the entire period of record, we find these two species covary with a slope of $-\mathrm{1.081}\pm \mathrm{0.007}$ mol of O₂ per mole of CO₂ (the mean and standard error of 6 h periods). If we limit the data to values collected on summer days within the canopy, the slope is $-\mathrm{1.03}\pm \mathrm{0.01}$. These are the conditions in which biotic influences are most likely to dominate. This result is significantly different from the value of −1.1 widely used in O₂-based calculations of the global carbon budget, suggesting the need for a deeper understanding of the exchange ratios of the various fluxes and pools comprising the net sink.