Soil sample size and physical properties matter in experimental studies of the moisture and temperature response of soil respiration

The influence of soil water and temperature on soil respiration is often studied using incubation experiments due to the challenges associated with field measurements. While incubations preserve most chemical and biological properties of the soil, they alter the physical environment. A critical issue is whether these alterations make incubation results unrepresentative of those under field conditions. To address this gap, we developed a multiscale model to explicitly resolve key processes controlling anaerobic CO₂ production and microbial respiration of dissolved and gaseous oxygen (O₂) in the pore space, which include heterogeneous microbial distribution and O₂ dissolution and diffusion. These processes are integrated into a macroscopic model to simulate CO₂ emissions in soil profiles. We applied the model to published incubation and field experiments to evaluate its accuracy and ability to predict the moisture and temperature sensitivity of CO₂ emissions. The model was then used to investigate how physical factors often overlooked in incubation experiments, such as soil depth, porosity, and alteration of soil structure, impact the moisture and temperature response of CO₂ emissions. Our results show that incubations substantially overestimate the temperature sensitivity of CO₂ emissions compared to that under field conditions, due to changes in the physical environment. Modifying soil structure also alters the moisture and temperature response of CO₂ emissions. These findings demonstrate the role of physical factors in regulating CO₂ emissions and underscore the need for caution when extrapolating incubation results to field conditions or using them to predict the response of soil carbon dynamics to global warming.