Optimum temperatures for soil respiration along a semi-arid elevation gradient in southern California

Soil respiration (R) has not been adequately studied at temperatures above 35 °C, which are common temperatures for soils in the southwestern United States and may be important for C dynamics in semi-arid regions. While frequently excluded from ecosystem models or set to 35 °C, the optimum temperature for soil R is poorly understood. Optimum temperatures are likely controlled by substrate availability, soil moisture content, and previous climate. To quantify the optimal temperature for soil R and hypothesized relationships, we collected soils from beneath and between plant canopies at three sites along a semi-arid elevation gradient. Processed soil samples were incubated at three soil moisture contents and soil R was measured at 6 temperatures, successively (25–55 °C). From these data, an activation energy for reaction kinetics and deactivation energy for enzyme functionality model was used to generate soil R curves from which two parameters were derived: Rmax, the maximum rate of soil R and Topt, the optimum temperature for soil R. Rmax was significantly greater for soils at the highest elevation and at medium soil moisture content. Topt was greater than 35 °C at all locations. In addition, Topt was significantly greater for soils with greater amounts of SOM but not significantly different along the elevation gradient or at different moisture contents. These results support inclusion of much higher optimum temperatures than currently used in many ecosystem and land-surface models and provide support for explaining variation in Topt as regulated by substrate quantity within a site and general insensitivity across climate differences.