Temperate forests play an important role in the global
carbon cycle, and are thought to currently be a sink for
atmospheric CO2. However,we lack understanding of
the drivers of forest carbon accumulation and loss,
hampering our ability to predict carbon cycle
responses to global change. In this study,we used CO2
flux and radiocarbon (14C) measurements to investigate
the role of seasonal drivers on soil respiration.
Radiocarbon measurements of CO2 evolved during
incubation of fine roots and root-free soils at the
beginning and end of the growing season (April and
August) showed that these two soil respiration sources
(fine roots vis-a`-vis soils) have different mean residence
times that stayed constant between seasons.
Radiocarbon measurements show that root respiration
was made up of carbon fixed 3–5 years prior to
sampling, and that heterotrophic respiration was
made up of carbon fixed 7–10 years prior. The difference
in radiocarbon signature between the two soursources
allowed us to partition autotrophic and
heterotrophic respiration sources for soil respiration
measurements in the field. We observed a small but
significant increase in D14C of soil respiration between
April and August, suggesting an increase in heterotrophic
respiration sources over the growing season.
Using a two end-member mixing model, we estimate
that 55 ± 22% of soil respiration originated from
autotrophic (root) sources in April, but their contribution
dropped to 38 ± 21% in August. These findings
suggest that the contribution of root respiration
increases at a time of high productivity and/or as a
result of relatively low microbial respiration in the
early spring in this old-growth coniferous forest.
