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Variations in soil organic matter accumulation across an elevation can be used to explain the control of substrate supply and variability on soil metabolic activity. We investigated geographic changes in soil organic matter and metabolic rates along an elevation gradient (289-2,489 m) in the Santa Rosa Mountains, California, USA from subalpine and montane pine forests through chaparral to desert. From base (289 m) to summit (2,489 m), 24 sites were established for collecting soil samples under canopies and inter-canopy spaces, at 0-5 and 5-15 cm soil depths increments. Soil organic matter (SOM) content was determined using weight loss on ignition at 550 degrees C and soil CO2 efflux (R) was measured at day 5 (R-5) and day 20 (R-20) of incubation. Changes in SOM content along the elevation gradient showed a significant relationship (P<0.05) but R-5 and R20 were not related to either elevation or SOM content. However, the ratio of R and SOM (R-5/SOM) showed a strong relationship across the mountains at both soil depths. R-5/SOM, as an indicator of carbon use efficiency, may be applicable to other semi-arid transects at larger scale modeling of soil metabolic processes.