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The world's soils contain three times as much carbon as the atmosphere. Thus, any changes in this carbon pool may affect atmospheric CO₂ levels with implications for climate change. Anthropogenic contributions to global carbon and nitrogen cycles have increased in the last century. Both temperature and nitrogen influence decomposition processes and are therefore critical in determining CO₂ return to the atmosphere. Kinetic theory predicts that the chemical composition of soil organic matter represents a dominant influence on the temperature response of decomposition. However, empirical observations and modeling indicate that this relationship is constrained by other factors. We address a number of research questions related to these factors, which are central to a thorough understanding of temperature sensitivity in decomposition. Specifically it offers one of the first empirical observations consistent with modeling in demonstrating increased temperature sensitivity for the uptake of carbon monomers over microbial cell membranes. Using NMR spectroscopy we were able to demonstrate how temperature response is directly related to the chemical composition of the organic material present. The thesis shows how increased soil nitrogen reduces temperature response. The key mechanism behind this observation, we suggest, is the influence of nitrogen on the chemical composition of organic matter, mediating a direct effect on temperature response. Given that nitrogen availability in terrestrial ecosystems has doubled relative to preindustrial levels, this observation may be vital in understanding the net effect of temperature increase on CO₂ return to the atmosphere. The proportion of carbon in plant litter transformed by microorganisms into biomass (carbon use efficiency; CUE) is a central factor determining global land-atmosphere CO₂ exchange. CUE was highly sensitive to whether carbon monomers or polymers were degraded; yet temperature had no clear effect on CUE. The majority of soil organic matter is comprised of polymers, highlighting the importance of using these as model substrates in studies of CUE. This thesis represents a major contribution to our understanding of the intrinsic and external controls acting on temperature sensitivity of decomposition, and thus to regulation of CO₂ return to the atmosphere under a changing climate.