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Changes in the carbon stock of soil in response to climate change would significantly affect the atmospheric carbon dioxide concentration and consequently climate. The isotopes of carbon provide a means to study the temperature sensitivities of different soil carbon fractions. Where C3 vegetation has changed for C4, soil organic matter (SOM) from the different origins have different C-13/C-12 ratios. Relying on this feature, we took soil samples from a control field and a field where ordinary grain (C3) vegetation was replaced by maize (C4), 5 years ago. We measured the respiration rate and the C-13/C-12 ratio of the CO2 produced by the samples at different temperatures. Based on these measurements, we quantified that Q(10) was 3.4-3.6 for the total CO2 production while it was 2.4-2.9 at 20 degrees C for the maize-derived young carbon and 3.6 for the older C3-derived carbon. Our results suggest that climatic warming will accelerate especially the decomposition of the large pool of old soil carbon in these fields. (c) 2007 Elsevier Ltd. All rights reserved.