The effect of warming on the CO2 emissions of fresh and old organic soil from under a Sitka spruce plantation

Under strictly controlled conditions we investigated the medium-term response of heterotrophic soil respiration to temperature. Soil fractions from a Sitka spruce plantation were sampled to represent “fresh” (i.e. shallower and containing more labile substrates) and “old” (i.e. deeper and presumed to contain more recalcitrant substrates) carbon sources. CO2 efflux and its isotopic (δ13C) signature were measured using a Tunable Diode Laser. “Fresh” soil showed substantially higher CO2 effluxes and its soil respiration was more sensitive to temperature in the range 5–10 °C than “old”. After a 56 day incubation at 20 °C the soils were re-tested. The CO2 fluxes were now lower but the sensitivity to temperature had increased in both ”fresh” and “old” soils. Moreover, after this 56 day incubation the δ13C signal of the CO2 from the “fresh” soil had declined, indicating a change in the substrate utilised for respiration.