Spatial and temporal variations in diurnal CO₂ fluxes of different Mediterranean ecosystems in Turkey

Diurnal rates of net CO₂ assimilation (P_N) and soil respiration (R_h) in the summer were measured in six Mediterranean ecosystems through an automatic continuous monitoring system. The six sites include a typical evergreen Pinus pinea L. forest with two co-occurring sclerophyllous shrubs (Pistacia terebinthus L. and Phillyrea latifolia L.), citrus (Citrus limon L.), corn (Zea mays L.), cotton (Gossypium hirsutum L.), soybean (Glycine max L.) and vineyard (Vitis vinifera L.). All six sites exhibited similar behaviour in that low soil water availability and high evaporative demand not only depressed P_N and R_h rates, but also changed the diurnal time course of their peak rates. Except for P. terebinthus and corn, P_N rates of all the species peaked in the morning (7:30-9:30), and R_h rates in all the sites were higher during the night than during the day (p < 0.05). Mean rates of net ecosystem emission (NEE) of CO₂ to the atmosphere were highest (-23.9 μmol CO₂ m^-2 s^-1) in the corn site and lowest (-0.6 μmol CO₂ m^-2 s^-1) in the vineyard site. On average, all six ecosystems were a net source of CO₂ to the atmosphere, due to R_h effluxes exceeding P_N rates. Net CO₂ assimilation and R_h explained 25% (for corn) to 87% (for citrus) of the diurnal fluctuations of the atmospheric CO₂ concentration. Multiple linear regression (MLR) models accounted for 42% (P. pinea) to 95% (cotton) of diurnal variations in P_N rates and 30% (forest) to 92% (citrus) of diurnal variations in R_h rates (p ≤ 0.001). The dependence of diurnal P_N and R_h rates on water vapour pressure deficit and soil water deficit for dry and hot summer days appeared to be major and needs to be re-examined for biogeochemical models of climate change effects on CO₂ dynamics of Mediterranean ecosystems.