Gabriele Broll

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 258C. The samples were taken from the upper horizon of each of two topographically diAerent 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 ˇ1 g ˇ1 organic carbon in the Oe‐Oa-layer and from 0.6 to 0.2 mg CO2‐ Cd ˇ1 g ˇ1 organic carbon at 258C in the A-horizon). Carbon mineralisation was well described by a first-order kinetic twocompartment model and a functional temperature dependence of the rate constants. Both temperature models, the exponential Q10-function and a quadratic function described the cumulative C-mineralisation correctly within one standard error of estimate (SE) of the measured values. However, the Q10 model gave a slightly better fit to the data, and Q10-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 Q10 of the instantaneous respiration rates, which were a non-linear function of incubation time. The general pattern of fluctuation of the instantaneous Q10-values was in accordance with the results computed by the models, and can be explained by the parallel decomposition of two diAerent soil organic matter pools. To avoid the eAects of the time of the respiration measurement on the calculated Q10, 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 eAects 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 ˇ2 year ˇ1 for the gullies and 70‐125 g C m ˇ2 year ˇ1 for the ridges. Unexpectedly, the choice of the temperature model had a great influence on the estimate of annual carbon mineralisation, even though models diAered 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. 7 2000 Elsevier Science Ltd. All rights reserved.

Humus Forms in the Forest-Alpine Tundra Ecotone at Stillberg (Dischmatal, Switzerland): Spatial Heterogeneity and Classification

In the forest-alpine ecotone at Stillberg (Dischmatal/Switzerland) the morphology of humus forms and the spatial variability of organic layer properties were investigated. At northeast-exposed gully sites mulls with high acidity in the Ahorizon occur. They were classified after the Canadian classification of humus forms as Rhizomulls. Mors occur on ridges and on their east- and north-exposed aspects. They can be differentiated by the ratio between the thickness of the Fhorizon and the combined thickness of the F- and H-horizon. The relative thickness of the F-horizon increases significantly in the order: east aspects < ridges < north aspect. The humus forms of the east aspects and the ridges were classified as Humimors and those of the north aspects as Hemimors. The Canadian classification was suitable to describe the properties of the horizons and to classify the humus forms. The results of a grid sampling at the study sites and the computation of nonergodic correlograms show that the spatial variability of organic-layer thickness, bulk density, and moisture is high (CV around 50%), with a pronounced small-scale heterogeneity (range usually below 2 m and more than 50% variability occurs within 0.3 m). Only 33% of the variance of organic-layer thickness were explained by site and vegetation structure, but in spite of the low percentage both proved to be a significant factor. In the forest-alpine tundra ecotone about 30 to 35 soil samples per site are needed for a reliable estimation of the mean of the organic-layer thickness.

Earthworms and dehydrogenase activity of urban biotopes

Abstract Earthworm populations and biomass were determined on four urban sites, representing different stages of secondary succession. Moreover, dehydrogenase activity of the topsoils was measured on three of these sites. Formerly, the sites considered were used as garden, grassland or arable land. The soil properties of all sites investigated illustrate the former land use and in some cases the effects of succession. Higher densities of earthworms were found in biotopes, which were used for horticulture. Fewer individuals occur on former agricultural land (grassland and arable land). Biomass of earthworms and dehydrogenase activity decrease from ancient garden land to grassland and arable land. The results are discussed with special regard to the actual site characteristics, such as soil properties and litter quality.