Decomposition of C-13-labelled standard plant material in a latitudinal transect of European coniferous forests: Differential impact of climate on the decomposition of soil organic matter compartments

C-13 labelled plant material was incubated in situ over 2 to 3 years in 8 c onifer forest soils located on acid and limestone parent material along a n orth-south climatic transect from boreal to dry Mediterranean regions in we stern Europe. The objectives of the experiment were to evaluate the effects of climate and the soil environment on decomposition and soil organic matt er dynamics. Changes in climate were simulated using a north-to-south casca de procedure involving the relocation of labelled soil columns to the next warmer site along the transect. Double exponential, decay-rate functions (for labile and recalcitrant SOM c ompartments) vs time showed that the thermosensitivity of microbial process es depended on the latitude from which the soil was translocated. Cumulativ e response functions for air temperature, and for combined temperature and moisture were used as independent variables in first order kinetic models f itted to the decomposition data. In the situations where climatic response functions explained most of the variations in decomposition rates when the soils were translocated, the climate optimised decomposition rates for the local and the translocated soil should be similar. Differences between thes e two rates indicated that there was either no single climatic response fun ction for one or both compartments, and/or other edaphic factors influenced the translocation effect. The most northern boreal soil showed a high ther mosensitivity for recalcitrant organic matter compartment, whereas the labi le fraction was less sensitive to climate changes for soils from more south ern locations. Hence there was no single climatic function which describe t he decay rates for all compartments. At the end of the incubation period it was found that the heat sum to achieve the same carbon losses was lower fo r soils in the north of the transect than in the south. In the long term, t herefore, for a given heat input, decomposition rates would show larger inc reases in boreal northern sites than in warm temperate regions. The changes in climate produced by soil translocation were more clearly ref lected by decomposition rates in the acid soils than for calcareous soils. This indicates that the physicochemical environment can have important diff erential effects on microbial decomposition of the labile and recalcitrant components of SOM.