EFFECT OF TIMBER HARVEST ON SOIL CARBON STORAGE AT BLODGETT-EXPERIMENTAL-FOREST, CALIFORNIA

Four plots from a mixed conifer forest were similarly cleared, burned, and replanted at various times over 17 years; a plot logged 79 years before sampling was used as a control. The plots had similar slope (2 to 15%, midslope position), aspect (south to southeast), and soil type (Holland series: mesic Haploxeralf; a Gray Brown Luvisol in the Canad ian classification system). Twenty sites at each plot were sampled vol umetrically by horizon to 20 cm below the organic - mineral soil bound ary. Samples were analyzed for bulk density, organic C, and total N. T here was an initial loss (15%) of organic C from the soil within 1 to 7 years, likely the result of oxidation (burning and decomposition) an d erosion. For 17 years of forest regrowth, the soil continued to lose C (another 15%), probably owing to decomposition of slash material an d possibly erosion, despite the slight accumulation of new litter and roots. After 80 years of regrowth, rates of carbon accumulation exceed ed fates of loss, but carbon storage had declined and was not likely t o recover to preharvest levels. Timber harvest and site preparation dr amatically altered soil C and N distribution, in which C/N ratios afte r site preparation were initially high throughout the upper 20 cm. Sub sequently, C/N ratios became lower with depth and with recovery age. A lthough stocks of C and N varied considerably among the plots and did not change consistently as a function of recovery age, the C/N ratios did vary systematically with recovery age. We hypothesize that the amo unt of C ultimately stored in the soil at steady state depends largely on N reserves and potentials, which appear to vary with erosion, inte nsity of burning, and site treatment.