Flanagan Lawrence B, Varney Gregory T
Department of Biology, Carleton University, 1125 Colonel By Drive, K1S 5B6, Ottawa, Ontario, Canada.
Oecologia. 1995 Jan;101(1):37-44. doi: 10.1007/BF00328897.
Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO concentrations and more negative δO values observed closer to the soil surface. The observed daily increases in δO values of forest CO were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO diffusing out of a leaf depends primarily on the O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the δO values of forest CO at night, because CO released from the soil has equilibrated with soil water which has a lower O content than leaf water. The observed relationship between diurnal changes in CO concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.
对在树脂松冠层内两个高度上每日采集的空气样本中的二氧化碳浓度和稳定氧同位素比率进行了测量。在所有三个采样日期的昼夜过程中，均观察到二氧化碳浓度和同位素组成发生了巨大变化。此外，森林冠层存在强烈的垂直分层现象，在靠近土壤表面处观察到更高的二氧化碳浓度和更负的δO值。观察到森林二氧化碳的δO值每日增加取决于相对湿度，这与光合气体交换过程中同位素分馏的模型预测一致。在光合气体交换过程中，进入叶片并与叶水达到平衡的一部分二氧化碳未被固定，而是以改变的氧同位素比率扩散回叶片外。从叶片中扩散出的二氧化碳的氧同位素比率主要取决于叶水的O含量，而叶水的O含量会随相对湿度而变化。相比之下，土壤呼吸在夜间导致森林二氧化碳的δO值下降，因为从土壤中释放的二氧化碳已与土壤水达到平衡，而土壤水的O含量低于叶水。在森林环境中观察到的二氧化碳浓度昼夜变化与氧同位素组成之间的关系，与一个气体混合模型一致，该模型考虑了与光合作用、呼吸作用以及森林与大气主体之间的湍流交换相关的二氧化碳通量的相对大小。
