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DIVISION S-2—SOIL CHEMISTRY

Carbon Sequestration in Clay Mineral Fractions from ¹⁴C-Labeled Plant Residues

^a Dep. of Agronomy, Iowa State University, Ames, IA 50011
^b USDA-ARS Appalachian Farming Systems Research Center, 1224 Airport Rd., Beaver, WV 25813
^c USDA-ARS National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

^* Corresponding author (jgonzalez{at}afsrc.ars.usda.gov).

An understanding of organic C dynamics in soils is necessary to develop management options to enhance soil organic C sequestration. The objective of this research was to study the distribution of newly formed humic materials into mineralogically distinct clay-size fractions of a silt loam soil. Oats (Avena sativa L. cv. Ogle), grown under simulated no-tillage conditions, were pulse labeled with ¹⁴CO₂. After senescence, the labeled surface residue was removed and replaced with unlabeled residue and the labeled roots were allowed to decompose in the soil for 360 d. The soil clay fraction (<2 µm) was separated into coarse, medium, and fine clay size-fractions (0.2–2.0, 0.02–0.2, and <0.02 µm, respectively) by centrifugation. X-ray diffraction indicated that quartz, illite, and kaolinite were the dominant mineral phases in the coarse clay fraction while smectite was the dominant mineral phase in the fine clay fraction. The organic C content in the coarse and fine clay fractions (3.70 and 3.93%, respectively) was similar. Scintillation analysis indicated an increase in ¹⁴C specific activity in all fractions after 360 d of incubation. For both sampling times, 0 and 360 d, the highest ¹⁴C specific activity occurred in the fine clay fraction (847.2 and 1529 Bq g^-1 C, respectively), whereas the lowest ¹⁴C specific activity occurred in the coarse clay fraction (565.8 and 770.9 Bq g^-1 C, respectively). The results suggest that new humic materials are preferentially forming or accumulating on smectite surfaces.

Abbreviations: POM, particulate organic matter • SEM, scanning electron microscopy • SOM, soil organic matter • XRD, X-ray diffraction

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