Despite the large quantity of research on the sensitivity of soil organic matter (SOM) degradation to temperature change, there is still no consensus as to whether soil efflux Of C02 Will increase with elevated temperatures. Understanding the mechanisms that controIC02efflux from the soil is therefore critical for predicting the response of ecosystems to climate change, particularly in polar regions where warming is occurring at rates 2-3 times greater than in temperate regions. This thesis considered two methodologies for evaluating whether soil efflux0f C02 is driven by recent inputs from plants and, secondly, the degree of temperature sensitivity. Firstly, Part I (Chapter 2) considered basal C02 and nutrient limitations in these soils. Secondly, Part 11 (Chapters 3-6) utilized 14C -labelled glucose and amino acids to assesst he mineralization of low molecular weight (MW) C in soils from the UK and Arctic and the temperature sensitivity of mineralization. UK and Svalbard soils had similar respiration rates per gram of 0 horizon and metabolism of low MW C with an exponential decay coefficient ki averaging < 1.5 h in the laboratory. However, cycling of C through the microbial biomass was significantly slower in Svalbard soils than in UK grassland soils, though when Svalbard soils were incubated at 20 OC, turnover was similar to UK field rates. Respiration measurements undertaken on an area basis were also significantly different, with the UK having much greater respiration rates, highlighting the importance of the sampling methodology when considering results. Both soils were substrate limited, indicating that respiration rates were dominated by recent contributions of labile C from plants. Temperature changes did cause an increased respiratory loss of SOM, but not to the magnitude expected with the Qjo value being < 2. Temperature dependence of SOM degradation was shown to be C pool dependent; with mineralization of labile substrate-C insensitive to temperature, but microbial biomass-C turnover and mineralization of higher MW SOM fractions increased with elevated temperatures. Partitioning of 14C changed with elevated temperatures, with more C utilized for respiration rather than growth, indicating a possible decreasein efficiency of growth at higher temperatures. Further research is needed in the temperature-dependence of SOM degradation, particularly turnover of high MW SOM, in the laboratory and field.