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DIVISION S-7—FOREST & RANGE SOILS

Natural Isotopic Distribution in Soil Surface Horizons Differentiated by Vegetation

^a Dep. of Renewable Resources, Univ. of Alberta, Edmonton AB, Canada T6G 2E3
^b Soil and Water Sciences Program, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
^c Dep. of Earth Sciences, Dartmouth College, Hanover, NH 03755
^d Dep. of Geography, Univ. of California, Santa Barbara, CA 93106

^* Corresponding author (sylvie.quideau{at}ualberta.ca).

The isotopic composition of soil organic matter (SOM) is a useful tool for deciphering the different mechanisms underlying decomposition processes in soils. The objective of this study was to quantify the influence of oak (Quercus dumosa Nutt.) and pine (Pinus coulteri D. Don) vegetation on the isotopic variation occurring during decomposition by measuring ¹³C and ¹⁵N in selected litter and soil fractions. Soil samples obtained from A horizons of two lysimeter soils were separated by density and mineral size to isolate the floatable, fine silt, and clay fractions. These fractions as well as the litter samples were subjected to sequential chemical extractions to differentiate between polar and nonpolar extractives, acid-soluble carbohydrates, and acid-insoluble residues. The physical fractions varied by up to 3.5 for ¹³C and 4.7 for ¹⁵N, while acid-insoluble residues were depleted by 0.9 to 2.1 ¹³C as compared with the samples before extraction. Under oak, ¹³C and ¹⁵N content progressively increased from the litter to the floatable, fine silt, and clay fractions (by 4.7 for ¹³C and 4.9 for ¹⁵N). By comparison, under pine, enrichment of the clay fraction was 1.7 for ¹³C and 1.7 for ¹⁵N as compared with the initial litter. The greater enrichment in heavy isotopes under oak vegetation as compared with the pine could not be explained based on differences in litter inputs. Results suggested instead that variation in decomposition processes by vegetation type caused the differences in heavy isotope enrichment.

Abbreviations: SDEF, San Dimas Experimental Forest • SOM, soil organic matter

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