Temperature dependence of carbon mineralisation: conclusions from a long-term incubation of subalpine soil samples

Carbon mineralisation from soil samples was analysed during a 104-day labor atory incubation at 5, 15 and 25 degrees C. The samples were taken from the upper horizon of each of two topographically different micro-sites (gully: A-horizon; ridge: Oe/Oa-layer) at the Stillberg Alp close to Davos in the Swiss Central Alps. On both the soils, carbon mineralisation rates decrease d substantially with incubation time (e.g. from 0.3 to 0.18 mg CO2-C d(-1) g(-1) organic carbon in the Oe-Oa-layer and from 0.6 to 0.2 mg CO2-C d(-1) g(-1) organic carbon at 25 degrees C in the A-horizon). Carbon mineralisati on was well described by a first-order kinetic two-compartment model and a functional temperature dependence of the rate constants. Both temperature m odels, the exponential pro-function and a quadratic function described the cumulative C-mineralisation correctly within one standard error of estimate (SE) of the measured values. However, the Q(10) model gave a slightly bett er fit to the data, and Q(10)-values of 2.5 and 2.8 were computed for the r ate constants of the organic layer and the A-horizon, respectively. While t he temperature dependence of the (time independent) rate constants of miner alisation appeared to be well-defined, this was not the case for Q(10) of t he instantaneous respiration rates, which were a non-linear function of inc ubation time. The general pattern of fluctuation of the instantaneous Q(10) -values was in accordance with the results computed by the models, and can be explained by the parallel decomposition of two different soil organic ma tter pools. To avoid the effects of the time of the respiration measurement on the calculated Q(10), it is recommended to analyse the whole time serie s in order to infer the temperature dependence of respiration, or at least to standardise the time at which soil respiration is measured. In a second part of the study, our laboratory results temperature effects were extrapol ated to the field, using measurements of soil temperature as driving variab les to a recently developed carbon balance model. Carbon mineralisation was roughly estimated to be 52-84 g C m(-2) year(-1) for the gullies and 70-12 5 g C m(-2) year(-1) for the ridges. Unexpectedly, the choice of the temper ature model had a great influence on the estimate of annual carbon minerali sation, even though models differed only little concerning the fit to the l aboratory incubation data. However, it could be shown that winter-time mine ralisation probably accounted for at least 22 and 40% of the whole-year min eralisation on the ridges and the gullies, respectively, and therefore, sho uld not be neglected in carbon-balance studies. (C) 2000 Elsevier Science L td. All rights reserved.