Influence of 13C-enriched foliage respired CO2 on δ13C of ecosystem-respired CO2

The delta C-13 of slash pine foliage dark- respired CO2 ( delta C-13(F)) was measured at temperatures ranging from 6.6 degrees to 30 degrees C. The delta C-13(F) was C-13- enriched initially after dark by as much as 9 parts per thousand and declined toward an asymptotic value after a few hours in the dark. Maximum enrichment in delta C-13(F) occurred at the lowest temperatures and the decline in delta C-13(F) toward the asymptotic value took longer at lower temperatures. The effect of this temporal variation in foliage enriched CO2 on determination of the delta(CO2)-C-13 of ecosystem respiration ( delta C-13(R)) was investigated using two approaches by measuring the nighttime CO2 concentration and delta(CO2)-C-13 ratio in a 30- m- tall pine forest in Florida, United States. The first approach ( time series) consisted of making measurements at a single height at midcanopy from 1 hour after sunset until 0200 local time. The second approach ( profile) consisted of making measurements at multiple heights from ground level to the top of the canopy synoptically prior to dawn. For each method a Keeling plot was used to estimate the delta C-13(R). The delta C-13(R) values determined with the time series were C-13-enriched relative to those determined from profiles on average by 1.0 parts per thousand and by as much as 3.1 parts per thousand. A mass balance was used to determine the impact of the nonconstancy of delta C-F on the overall Keeling plot intercepts. For the months examined, results indicate that ( 1) the enrichment in the Keeling intercepts determined from time series compared to those determined from profiles was consistent with the addition of nighttime foliage respired C-13- enriched CO2 and ( 2) Keeling plots determined from midcanopy heights appear linear despite the addition of temporally variable source delta(CO2)-C-13 ( range: 4 parts per thousand). A common approach that specifies the time ( predawn) and spatial distribution ( multiple heights) for CO2 collection is suggested to improve intercomparisons of delta C-13(R) from different ecosystems.