Identifying Controls on Surface Carbon Dioxide Efflux in a Semi-arid Ecosystem

This study focused on soil respiration, the least constrained process in the terrestrial carbon cycle, and a source of uncertainty for model predicted ecosystem response to climate change. Respiration processes that make up soil respiration respond to environmental influencing factors, temperature, and moisture. Respiration responses vary by site, time of year, and also year to year, confounding determination of individual influence of the factors, making soil respiration difficult to model. This research took advantage of the climate characteristics of a semi-arid system, in order to observe soil temperature and moisture influence on soil respiration. Field measurements of soil surface CO2 efflux taken using an automated closed dynamic chamber system approximated soil respiration. Analysis of the growing season pattern of soil respiration compared to patterns of environmental variables supported a conceptual model in which soil respiration exhibits variable dependency on both temperature and moisture. A series of mathematical models of soil respiration were evaluated from the literature that featured various moisture dependency modifications to common temperature-dependent relationships. Model performance was assessed by model regression fit to the seasonal trend in soil respiration and the model consistency with conceptual understanding of soil respiration function in the system. The best model fit to the data also reflected the conceptual model of soil respiration, that soil moisture status determines the degree of temperature dependence of soil respiration. The field observations reflected trends seen in other semi-arid systems, particularly those that experience growing season water limitation in the summer due to lack of access to or availability of deep soil moisture. The research supported common themes observed in all semi-arid ecosystems: the peak soil respiration occurs in spring when warm and wet conditions converge, and the temperature dependency of soil respiration declines as soil moisture declines.