DECADE-SCALE CHANGES OF SOIL CARBON, NITROGEN AND EXCHANGEABLE CATIONS UNDER CHAPARRAL AND PINE

Four large lysimeters on the San Dimas Experimental Forest, each fille d with similar parent material and planted with monocultures of native species in 1946, provide a unique opportunity to quantify short-term effects of plant species on soil properties. The four species under wh ich soils were investigated are scrub oak (Quercus dumosa Nutt.), cham ise (Adenostoma fasciculatum Hook. and Am.), ceanothus (Ceanothus cras sifolia Ton.), and Coulter pine (Pinus coulteri B. Don). A mass-balanc e approach was used to measure changes in C, N, exchangeable base cati ons, and exchangeable acidity to a depth of 1 m in the mineral soils o ver a 41-year period, The C content increased in all of the soils, but the greatest change was in the soil under oak (3.7 kg m(-3)), more th an doubling the original amount. Since the source of C in these soils is the photosynthetic fixation of atmospheric CO2, the mass of C accum ulated reflects the magnitude of the CO2 sink provided by chaparral so ils in their initial stages of formation. The calculated rate of soil C accumulation is as much as 0.09 kg m(-3) yr(-1). The increase in N w as highest in the soil under ceanothus (0.12 kg m(-3)), the only N-2-f ixing species in this study. Exchangeable Ca increased by 25.7 mol m(- 3) in the soil under oak, while the maximum increase in exchangeable M g was 5.5 mol m(-3) also under oak. Exchangeable Na was leached from a ll of the soils (a maximum of 2.4 mol m(-3) lost from under chamise an d ceanothus) and K was slightly depleted.