Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy decreased on average 7-10 mu mol mol(-1) below tropospheric baseline values measured at Barbados. Within the main part of the canopy (greater than or equal to 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO2] immediately above the forest floor of about 50 mu mol mol(-1). Temporal and spatial variations in [CO2](canopy) were reflected in changes of delta(13)C(canopy) and delta(18)O(canopy) values. Tight relationships were observed between delta(13)C and delta(18)O of canopy CO2 during both seasons (r(2) > 0.86). The most depleted delta(13)C(canopy) and delta(18)O(canopy) values were measured immediately above the forest floor (delta(13)C = -16.4 parts per thousand; delta(18)O = 39.1 parts per thousand SMOW). Grandients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0 parts per thousand and 6.3 parts per thousand for delta(13)C, and between 1.0 parts per thousand, and 3.5 parts per thousand, for delta(18)O. The delta(13)C(leaf) and calculated c(i)/c(a) of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20 parts per thousand of the differences in delta(13)C(leaf) within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c(i)/c(a). Plotting 1/[CO2] vs. the corresponding delta(13)C ratios resulted in very tight, linear relationships (r(2) = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the delta(13)C of respired sources were close to the measured delta(13)C of soil respired CO2 and to the delta(13)C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Delta(e), were calculated as 20.3 parts per thousand during the dry season and as 20.5 parts per thousand, during the wet season.