Controlling Factors On Temperature Sensitivity of Soil Respiration At the Soil Profile Scale.

Sensitivity of CO₂ efflux from soil organic matter (SOM) decomposition to temperature is a pivotal parameter in global C cycle models. Under field conditions factors such as soil moisture, O₂ and CO₂ contents interact with SOM temperature sensitivity possibly leading to different temperature responses than anticipated from biochemical kinetics. At the soil profile scale, driving factors can change independently with depth, making a depth-specific assessment of temperature sensitivity essential. The objectives of this study were to assess 1) the temporal variation of CO₂ production at different depths and the contribution of subsoil CO₂ production to total soil respiration and 2) the controlling factors of SOM decomposition and its temperature sensitivity at different depths. We measured CO₂, O₂, temperature, and soil water content (θ%vol) in two plots at 15, 30, 50, 70, and 90cm depths at the University of California student farm in Davis from April, 2008 till September 2009. We calculated CO₂ flux and concentration gradients and compared temperature sensitivity between top and subsoil by fitting an exponential Q₁₀ model to the data. The A horizon had the largest daily CO₂ fluctuations (average CO₂ concentration =0.2%) during spring and winter whereas in the C2 horizon, CO₂ production (average CO₂ concentration =0.8%) followed the soil temperature variation, with lowest CO₂ production in winter and highest in summer. Under non-limiting moisture (θ>20%) the Q₁₀ value for the A horizon was 5.33 (0.93µmol CO₂ m³ s^-1) decreasing to 1.29 (0.57µmol CO₂ m³ s^-1) at θ<10%. In the C2 horizon at θ>20%, Q₁₀ was 3.13 (0.19µmol CO₂ m³ s^-1) indicating that it is less sensitive to temperature variations than topsoil when moisture is non-limiting. Moisture content in the subsoil did not decrease below 15% and Q₁₀ was 1.85 (0.45µmol CO₂ m³ s^-1) at this moisture. In conclusion, this study showed decreasing temperature sensitivity with soil depth and that under Mediterranean conditions, CO₂ production from subsoils is important in determining soil CO₂ efflux.