No diurnal variation in rate or carbon isotope composition of soil respiration in a boreal forest

Characterization of soil respiration rates and delta(13C) values of soil-respired CO2 are often based on measurements at a particular time of day. A study by Gower et al. (2001) in a boreal forest demonstrated diurnal patterns of soil CO2 flux using transparent measurement chambers that included the understory vegetation. It is unclear whether these diurnal patterns were solely the result of photosynthetic CO2 uptake during the day by the understory or whether there were underlying trends in soil respiration, perhaps driven by plant root allocation, as recently demonstrated in Mediterranean oak savannah. We undertook intensive sampling campaigns in a boreal Picea abies L. Karst. forest to investigate whether diurnal variations in soil respiration rate and stable carbon isotope ratio (delta C-13) exist in this ecosystem when no understory vegetation is present in the measurement chamber. Soil respiration rates and delta C-13 were measured on plots in which trees were either girdled (to terminate the fraction of soil respiration directly dependent on recent photosynthate from the trees), or not girdled, every 4 h over two 48-hour cycles during the growth season of 2004. Shoot photosynthesis and environmental parameters were measured concurrently. No diurnal patterns in soil respiration rates and delta C-13 were observed in either treatment, despite substantial variations in climatic conditions and shoot photosynthetic rates in non-girdled trees. Consequently, assessment of daily soil respiration rates and delta C-13 in boreal forest systems by single, instantaneous daily measurements does not appear to be confounded by substantial diurnal variation.