Theoretical model of the abiotic component of soil CO2 tracer efflux in C pulse-labeling experiments on plant-soil systems

For measurement of the time lag between photosynthesis and CO₂ efflux from soil, the carbon isotope pulse-labeling technique is considered as the most suitable. However, an interference from the abiotic tracer CO₂ component is identified as a key difficulty for obtaining accurate results with this technique. Guidelines on how to reduce this interference are therefore urgently needed. The flux of abiotic ¹³CO₂ tracer into soil during the labeling stage, and its return to atmosphere during the monitoring stage was modeled numerically, and the labeling stage also analytically. The controls of the abiotic interference were investigated using these models. The amount of the abiotic tracer component and the time distribution of its rate of return to the atmosphere, were predicted by these models. The main model parameters were D_m (=the ratio between the soil ¹³CO₂ diffusivity and the retardation factor), and the ¹³CO₂ concentration at the soil-atmosphere interface during the labeling stage (S₁₃), while background ¹³CO₂ soil production parameters were unnecessary. The presented models guide the selection of experimental parameters for minimization of the abiotic interference. With parameterization for a particular case, the present numerical model provides a preliminary order-of-magnitude estimate of the abiotic component, which would indicate if this interference is of significance.