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

Abstract

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.