Unifying soil respiration pulses, inhibition, and temperature hysteresis through dynamics of labile soil carbon and O2

Event-driven and diel dynamics of soil respiration (R-s) strongly influence terrestrial carbon (C) emissions and are difficult to predict. Wetting events may cause a large pulse or strong inhibition of R-s. Complex diel dynamics include hysteresis in the relationship between R-s and soil temperature. The mechanistic basis for these dynamics is not well understood, resulting in large discrepancies between predicted and observed R-s. We present a unifying approach for interpreting these phenomena in a hot arid agricultural environment. We performed a whole ecosystem wetting experiment with continuous measurement of R-s to study pulse responses to wetting in a heterotrophic system. We also investigated R-s during cultivation of Sorghum bicolor to evaluate the role of photosynthetic C in the regulation of diel variation in R-s. Finally, we adapted a R-s model with sensitivity to soil O-2 and water content by incorporating two soil C pools differing in lability. We observed a large wetting-induced pulse of R-s from the fallow field and were able to accurately simulate the pulse via release of labile soil C. During the exponential phase of plant growth, R-s was inhibited in response to wetting, which was accurately simulated through depletion of soil O-2. Without plants, hysteresis was not observed; however, with growing plants, an increasingly significant counterclockwise hysteresis developed. Hysteresis was simulated via a dynamic photosynthetic C pool and was not likely controlled by physical processes. These results help characterize the complex regulation of R-s and improve understanding of these phenomena under warmer and more variable conditions.