Decade-scale changes of soil carbon, nitrogen and exchangeable cations under chaparral and pine

Four large lysimeters on the San Dimas Experimental Forest, each filled 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 which soils were investigated are scrub oak (Quercus dumosa Nutt.) , chamise (Adenostoma fasciculatum Hook. and Am.), ceanothus (Ceanothus crassifolia Torr.), and Coulter pine (Pinus coulteri B. Don). A mass-balance approach was used to measure changes in C, N, exchangeable base cations, and exchangeable acidity to a depth of 1 m in the mineral soils over 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 mm3). more than doubling the original amount. Since the source of C in these soils is the photosynthetic fixation of atmospheric CO,, the mass of C accumulated reflects the magnitude of the CO2 sink provided by chaparral soils in their initial stages-of formation. The calculated rate of soil C accumulation is as much as 0.09 kg mm3 yr-‘. The increase in N was highest in the soil under ceanothus (0.12 kg mP3), the only Nz-fixing species in this study. Exchangeable Ca increased by 25.7 mol mm3 in the soil under oak, while the maximum increase in exchangeable Mg was 5.5 mol m -3 also under oak. Exchangeable Na was leached from all of the soils (a maximum of 2.4 mol mP3 lost from under chamise and ceanothus) and K was slightly depleted.