9316378 Graham The use of soils for detailed geomorphic and environmental interpretations is limited by lack of precise knowledge of pedogenic rates and trends as a function of various environmental factors. This research will quantify decade-scale soil genesis processes by analyzing a unique on-going experiment started =50 years ago at the San Dimas Experimental Forest in Southern California. Large (5.3m x 5.3m x 2.1m deep) earthen-walled pits were filled with homogenized soil material and planted with monocultures constituting an imposed biosequence of four native plant species (chamise, ceanothus, scrub oak, and Coulter pine). A chronosequential record of the initial stages of soil formation is provided by archived samples from each 7.5 cm layer of original fill material and soil samples taken at incremental depths in 1958, 1975, and 1987 under each type of vegetation. The objective of this research is to quantify the biotic, atmospheric, and pedogenic influences on the initial homogeneous fill material as a function of time for each of the soil-vegetation systems. A mass balance approach will be used to quantify gains, losses, and redistributions of constituents in the soils developed after 12, 29, and 41 years under the four different plant species. Constituents to be analyzed include major rock-forming elements, clay, silt, pedogenic Fe oxides, exchangeable cations, C, N, and heavy metals. Calculations will be based on the archived original parent material samples corresponding to soil horizons that developed and were sampled in subsequent years. Biomass quantity and composition will be measured and included in the mass balance. Long-term measurements of atmospheric wet- and dry-fall exist for the site and will also be included. Radiogenic isotopes will be used to distinguish the origin and disposition of various soil constituents. Distinctive Sr, Nd, and Pb isotopic rations in soils and tree rings will serve as tracers to indicate atmospheri c and mineral weathering sources of elements. The combination of mass balance and radiogenic isotope tracer methods will yield a detailed accounting of processes responsible for additions, losses, transfers, and transformations in the soil-vegetation systems. Precise quantification and interpretation is warranted because the San Dimas soil-vegetation plots are likely the most closely constrained and well documented soil genesis experiments in existence. They fill a critical gap between experimental studies that operate over several years and traditional soil chronosequence studies that address time scales of 102 to 106 years. Results of this research will yield a much more precise understanding of the initial stages of soil evolution than currently exists. This knowledge can be used to improve interpretations of fault movement histories from soils and to predict sources and sinks of elements, including heavy metal contaminants and plant nutrients, on a time scale commensurate with human-induced effects on atmospheric chemistry.
