Prescribed fire was returned into over-stocked ponderosa pine stands on the Mogollon Rim of Arizona for the purpose of restoring fire into the ecosystem and removing fuel buildups. Prescribed fires have been ignited at intervals of 1, 2, 4, 6, 8, and 10 years to determine the best fire return interval for Southwest ponderosa pine ecosystems. Two sites were treated; one on volcanic-derived soils, and the other on sedimentary-derived soils near Flagstaff, Arizona, starting in 1976 and 1977, respectively. Samples from upper 5 cm of the A horizons were analyzed for total carbon and nitrogen using an elemental analyzer. Soil carbon and nitrogen levels were highly variable and exhibited an increasing, but inconsistent, concentration trend related to burn interval. High spatial variability measured within treatments is probably due to micro-site differences (location of samples in the open, under large old-growth trees, in small-diameter thickets, in pole-sized stands, next to downed logs, etc.). Stratification of samples by micro-site differences could possibly reduce the within plot variability but add considerable complexity to the sampling design.