Natural abundance of 13C in CO2 respired from forest soils reveals speed of link between tree photosynthesis and root respiration.

Soil respiration from a boreal mixed coniferous forest showed large seasonal variation in natural abundance of 13C, ranging from -21.6‰ to -26.5‰. We tested if weather conditions could explain this variation in δ13C of respired CO2, and found that the air relative humidity 1-4 days before the days of CO2 sampling best explained the variation. This suggested that high δ13C values were caused by effects of air humidity on isotope fractionation during photosynthesis and that it took 1-4 days for the C from canopy photosynthesis of 20-25 m trees to become available for root/rhizosphere respiration. We calculated that these new photoassimilates could account for at least 65% of total soil respiration.