Spatial And Temporal Variation In The Carbon And Oxygen Stable Isotope Ratio Of Respired CO2 In A Boreal Forest Ecosystem

We measured the stable isotope ratio of respired carbon dioxide at two spatial scales in a black spruce forest in northern Canada: CO₂ released from the forest floor and CO₂ released from the entire ecosystem at night. Despite wide variation in the δ13C values of organic matter among above-ground plant species, and along a continuum from moss through to the mineral soil, the carbon isotope ratio of respired CO₂ was quite similar to the δ¹³C value for the dominant black spruce foliage. The CO₂ released from the forest floor during the fall was slightly enriched in ¹³C compared to CO₂ respired by the entire ecosystem, perhaps because soil respiration contributes a larger percentage to total ecosystem respiration later in the year as the soil warms. Short-term changes in the oxygen isotope ratio of precipitation and variation in enrichment of ¹⁸O during evaporation and transpiration had significant effects on the δ¹⁸O value of respired CO₂. Changes in the oxygen isotope ratio of water in moss tissue can have a large effect on total ecosystem respired CO₂ both because a large surface area is covered by moss tissue in this ecosystem and because the equilibration between CO₂ diffusing through the moss and water in moss tissue is very rapid. During the summer we observed that the δ¹⁸O value of CO₂ respired from the forest floor was relatively depleted in ¹⁸O compared to CO₂ respired from the entire ecosystem. This was because water in black spruce foliage had higher δ¹⁸O values than moss and soil water, even at night when transpiration had stopped.