BASE CATION BIOGEOCHEMISTRY AND WEATHERING UNDER OAK AND PINE - A CONTROLLED LONG-TERM EXPERIMENT

Large earthen-walled lysimeters at the San Dimas Experimental Forest i n southern California present a unique opportunity to assess vegetatio n effects on biogeochemical processes and cation release by weathering in controlled soil-vegetation systems where archived samples of soil parent material are available for comparison. The lysimeters were fill ed in 1937 with homogenized fine sandy loam derived on site from the w eathering of diorite, and planted in 1946 with scrub oak (Quercus dumo sa) and Coulter pine (Pinus coulteri). Changes in base cation contents were measured in above-ground biomass, and total and exchangeable soi l pools to a depth of 1 meter. All cations in the non-exchangeable soi l pool decreased relative to the initial fill material, indicating rel ease by weathering. Sodium and K were depleted from both exchangeable and non-exchangeable pools of the soils. Plant uptake of Na was minima l, whereas K storage in vegetation exceeded the loss from the exchange able soil pool. In both soil-vegetation systems, but especially for oa k, there was an increase in exchangeable Ca and Mg. For all base catio ns, storage in above-ground biomass was greater for oak, whereas losse s by weathering from the non-exchangeable soil poor were greater under pine. Strong evidence supports biocycling as a controlling mechanism resulting in greater Ca and Mg release by weathering under pine. In ad dition, decreases in non-exchangeable Ca and Mg were strongly correlat ed to decrease in Si under oak, whereas no correlation was observed un der pine. We conclude that weathering reactions or stoichiometry diffe red between vegetation types.