Interpreting diel hysteresis between soil respiration and temperature

Increasing use of high-frequency automated soil respiration measurement techniques has revealed complex diel relationships between soil respiration and temperature. Soil surface flux is often lagged from soil temperature by several hours, which results in semi-elliptical hysteresis loops when surface flux is plotted as a function of soil temperature. Both biological and physical explanations have been suggested for hysteresis patterns, and there is currently no consensus on their causes or how such data should be analyzed to interpret respiration temperature sensitivity. In this study we employ a one-dimensional soil CO2 and heat transport model to demonstrate the theoretical basis for lags between surface flux and soil temperatures, and to examine the influence of soil properties and environment on lag times and hysteresis patterns. We demonstrate that diel phase lags between surface flux and soil temperature are a mainly the result of heat and CO2 transport processes. Other factors that vary on a diel basis, such a carbon substrate supply and atmospheric CO2 concentrations, can modify lag times and hysteresis patterns to varying degrees but are not required to explain the existence of hysteresis between surface flux and soil temperature. Physical transport processes are especially sensitive to soil moisture, and physical effects of soil moisture on CO2 transport may easily be confused with biological effects. Consideration of heat and CO2 transport processes are essential for correct interpretation of diel soil respiration patterns and for determining the true temperature sensitivity of soil respiration using these high time resolution data.