Diurnal hysteresis between soil CO₂ and soil temperature is controlled by soil water content

Recent years have seen a growing interest in measuring and modeling soil CO 2 efflux, as this flux represents a large component of ecosystem respiration and is a key determinant of ecosystem carbon balance. Process‐based models of soil CO 2 production and efflux, commonly based on soil temperature, are limited by nonlinearities such as the observed diurnal hysteresis between soil CO 2 concentration ([CO 2 ]) and temperature. Here we quantify the degree to which hysteresis between soil [CO 2 ] and soil temperature is controlled by soil water content in a montane conifer forest, and how this nonlinearity impacts estimates of soil CO 2 efflux. A representative model that does not consider hysteresis overestimated soil CO 2 efflux for the entire growing season by 19%. At high levels of soil water content, hysteresis imposes organized, daily variability in the relationship between soil [CO 2 ] and soil temperature, and at low levels of soil water content, hysteresis is minimized.