Losses of soil organic matter (SOM) can lead to a decrease in soil quality, cause an increase in CO₂ emissions, thereby contributing to a rise in atmospheric CO₂ concentration, which in turn can affect the global climate. Microbial decomposition of SOM to CO₂ is one of the main processes by which SOM is lost. Breakdown of organic matter (OM) by solar irradiance (called “photodegradation”) can also contribute to decomposition, especially in dry ecosystems. Photodegradation has been studied in the field by measuring the mass loss of litter, but its contribution to CO₂ losses has not previously been determined at spatial and temporal scales appropriate for ecosystems. The main aim of this research was to examine the magnitude and drivers of the CO₂ efflux from terrestrial organic matter resulting from both microbial decomposition and photodegradation. Carbon dioxide fluxes were measured at a bare peatland in New Zealand using eddy covariance (EC) and a closed chamber. The EC system measured the total CO₂ flux, whereas the chamber only measured the biological component of the CO₂ flux. The abiotic irradiance-induced component of the CO₂ flux was obtained by subtracting the chamber flux from the EC flux, and by comparing day-and night-time EC measurements made under similar temperature and moisture conditions. Analogous comparisons were made using field data from a grassland site in California during the dry summer period when plants had senesced. To confirm that solar irradiance contributed to CO₂ effluxes from terrestrial OM, short incubations of OM in a small transparent flow-through chamber system (referred to as the “container”) were conducted. The container was also used to study the controls of photodegradation including the effects of irradiance intensity, wavelength and substrate species. On hot summer days, irradiance-induced CO₂ fluxes accounted for up to 58% and 90% of the total mid-day CO₂ flux at the peatland and grassland, respectively. Annual CO₂ production at the peatland was estimated to be 269 g C m-², of which 20% was due to photodegradation. At the grassland during the dry season (~3 months), approximately 27 g C m-² was lost as CO₂, of which 60% was due to photodegradation. Irradiance-induced CO₂ fluxes measured both in the field and in the container showed a very strong relationship with the intensity of solar irradiance. Higher fluxes were observed at greater temperatures, but temperature effects could not be separated from irradiance effects. Field data suggested that the dose response coefficient (=moles of CO₂ produced per unit of energy of incoming solar irradiance) did not differ between wet and dry conditions at the peatland. Per unit of energy, peat produced more CO₂ than grass litter in both the field and the container. Container measurements indicated the irradiance in the UV wavelength band was responsible for 14 % of the total irradiance-induced CO₂ flux. Per unit of energy, approximately 5 times as much CO₂ was produced in the field compared to the container fluxes. The causes for this difference are not known, and this observation highlighted the importance of conducting ecosystem-scale field experiments in addition to small-scale controlled experiments. The rate of CO₂ loss at the peatland resulting from microbial respiration was primarily controlled by the position of the water table, which in turn determined the thickness of the aerated peat layer. Greatest losses were observed in summer, when the water table was low and peat temperatures relatively high. Simple models previously applied in northern hemisphere peatlands predicted up to 86% of the variation in the observed daily averaged CO₂ fluxes based on peat temperature and depth to water table. The models were less successful at explaining the within-day variation of the CO₂ flux. To explain the complex variation in CO₂ fluxes at the within-day time scale, or if modelling is intended to increase understanding of the underlying processes of soil respiration, mechanistic models describing both CO₂ production at various depths and diffusion of CO₂ to the peat surface might be more appropriate. Carbon dioxide losses due to abiotic processes like photodegradation have generally been ignored in ecosystem-scale carbon exchange studies and models. The results of this study strongly suggest that this process should not be ignored for a variety of ecosystems where OM is exposed to high levels of solar irradiance for extended periods of time. The role of photodegradation in assisting microbial decomposition of complex OM is also poorly understood. To obtain reliable estimates of carbon cycling component fluxes, the contribution of photodegradation to OM decomposition and CO₂ losses should be quantified across a wide range of other ecosystems and the process should be incorporated into global carbon cycling models.