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 spat ial scales in a black spruce forest in northern Canada: CO2 released from t he forest floor and CO2 released from the entire ecosystem at night. Despit e wide variation in the delta(13)C values of organic matter among above-gro und plant species, and along a continuum from moss through to the mineral s oil, the carbon isotope ratio of respired CO2 was quite similar to the delt a(13)C value for the dominant black spruce foliage. The CO2 released from t he forest floor during the fall was slightly enriched in C-13 compared to C O2 respired by the entire ecosystem, perhaps because soil respiration contr ibutes a larger percentage to total ecosystem respiration later in the year as the soil warms. Shortterm changes in the oxygen isotope ratio of precip itation and variation in enrichment of O-18 during evaporation and transpir ation had significant effects on the delta(18)O value of respired CO2. Chan ges in the oxygen isotope ratio of water in moss tissue can have a large ef fect on total ecosystem respired CO2 both because a large surface area is c overed by moss tissue in this ecosystem and because the equilibration betwe en CO2 diffusing through the moss and water in moss tissue is very rapid. D uring the summer we observed that the delta(18)O value of CO2 respired from the forest floor was relatively depleted in O-18 compared to CO2 respired from the entire ecosystem. This was because water in black spruce foliage h ad higher delta(18)O values than moss and soil water, even at night when tr anspiration had stopped.