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DIVISION S-7-FOREST & RANGE SOILS

Biogeochemical Cycling of Calcium and Magnesium by Ceanothus and Chamise

^a Soil and Water Sci. Program Dep. of Environ. Sci., Univ. of California, Riverside, CA 92521 USA
^b Dep. of Geography, Univ. of California, Santa Barbara, CA 93106 USA
^c USDA Forest Service, Pacific Southwest Exp. Stn., Riverside, CA 92507 USA

s.quideau{at}ipin.u-bordeaux.fr

Vegetation has long been recognized as a fundamental factor in soil formation, but vegetation and soils commonly covary in response to other environmental factors, confounding the specific effects of vegetation on soil properties. The lysimeter installation at the San Dimas Experimental Forest in southern California offers a rarely found opportunity for quantifying cation-cycling processes in a setting where all factors except vegetation are kept constant. The lysimeters were filled in 1937 with homogenized, fine sandy loam and planted in 1946 with chamise (Adenostoma fasciculatum Hook. and Arn.) and ceanothus (Ceanothus crassifolius Torr.). Comparison of the chamise and ceanothus lysimeters was best achieved by using the Ca/Mg ratio of the different cation pools and fluxes as an index. In 1987, the ceanothus exchangeable soil pool contained proportionally more Ca than Mg compared with chamise; that is, the Ca/Mg ratio in the ceanothus exchangeable soil pool was higher than that in chamise. Strong evidence supports vegetation influence on intra-system fluxes (weathering and biocycling) as the basis for these differences. First, more Ca than Mg was released by weathering under ceanothus than under chamise. Second, the ceanothus aboveground biomass exhibited a higher Ca/Mg ratio than the chamise. Third, differences between vegetation types widened with time since construction of the lysimeter installation in both the aboveground biomass and exchangeable soil pools. Differences in cation storage measured for the lysimeter chamise and ceanothus stands appear representative of natural chaparral communities throughout California, and may result in distinct Ca and Mg biogeochemical processes in associated ecosystems.

Abbreviations: ICP-MS, inductively coupled plasma mass spectrometry