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: CO 2 released from the forest floor and CO 2 released from the entire ecosystem at night. Despite wide variation in the δ 13 C 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 CO 2 was quite similar to the δ 13 C value for the dominant black spruce foliage. The CO 2 released from the forest floor during the fall was slightly enriched in 13 O compared to CO 2 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 18 O during evaporation and transpiration had significant effects on the δ 18 O value of respired CO 2 . Changes in the oxygen isotope ratio of water in moss tissue can have a large effect on total ecosystem respired CO 2 both because a large surface area is covered by moss tissue in this ecosystem and because the equilibration between CO 2 diffusing through the moss and water in moss tissue is very rapid. During the summer we observed that the δ 18 O value of CO 2 respired from the forest floor was relatively depleted in 18 O compared to CO 2 respired from the entire ecosystem. This was because water in black spruce foliage had higher δ 18 O values than moss and soil water, even at night when transpiration had stopped. DOI: 10.1034/j.1600-0889.1999.00018.x