Soil respiration in forest ecosystems plays a key role in the assessment of the net ecosystem C balance. Photosynthetic activity supplying carbohydrates from leaves to roots is an important driver of soil respiration. However, the immediate direct linkage between above- and below-ground is still uncertain. Here, we studied the effects of assimilated C supply on the diurnal pattern of soil respiration and its C isotope value (δ¹³C).

A pulse-labelling experiment was carried out in three Quercus serrata seedlings during the period of peak biomass. The δ¹³C of soil respiration was monitored at a high time resolution with an open dynamic chamber system using a cavity ring-down isotope CO₂ analyzer (Picarro Inc.).

The time lag between C assimilation and soil respiration to the first high peak ranged from 23 to 26 hours after labeling. The δ¹³C of soil respiration showed a diurnal pattern, exhibiting significantly higher δ¹³C values during the morning time relative to those during the night time. Also, after the first high peak, the δ¹³C value decreased as time goes by, keeping the diurnal pattern. The variability of δ¹³C was correlated with the PAR and soil temperature. These results indicated that the respiratory utilization might be determined by the availability and inflow of assimilated C from leaves. We will discuss in more detail soil-plant-atmosphere continuum.