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 laboratory 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 decreased 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 mineralisation was well described by a first-order kinetic two-compartment model and a functional temperature dependence of the rate constants. Both temperature models, 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 better fit to the data, and Q(10)-values of 2.5 and 2.8 were computed for the rate constants of the organic layer and the A-horizon, respectively. While the temperature dependence of the (time independent) rate constants of mineralisation appeared to be well-defined, this was not the case for Q(10) of the instantaneous respiration rates, which were a non-linear function of incubation 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 matter 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 series 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 extrapolated to the field, using measurements of soil temperature as driving variables 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 70-125 g C m(-2) year(-1) for the ridges. Unexpectedly, the choice of the temperature model had a great influence on the estimate of annual carbon mineralisation, even though models differed only little concerning the fit to the laboratory incubation data. However, it could be shown that winter-time mineralisation probably accounted for at least 22 and 40% of the whole-year mineralisation on the ridges and the gullies, respectively, and therefore, should not be neglected in carbon-balance studies. (C) 2000 Elsevier Science Ltd. All rights reserved.