Natural isotopic distribution in soil surface horizons differentiated by vegetation

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 delta(13)C and delta(15)N 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 delta(13)C and 4.7parts per thousand for delta(15)N, while acid-insoluble residues were depleted by 0.9 to 2.1parts per thousand delta(13)C as compared with the samples before extraction. Under oak, C-13 and N-15 content progressively increased from the litter to the floatable, fine silt, and clay fractions (by 4.7parts per thousand for delta(13)C and 4.9parts per thousand for delta(15)N). By comparison, under pine, enrichment of the clay fraction was 1.7parts per thousand for delta(13)C and 1.7parts per thousand for delta(15)N 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.