Carbon sequestration in clay mineral fractions from 14C-labeled plant residues

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 (CO2)-C-14. 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 mu m) was separated into coarse, medium, and fine clay size-fractions (0.2-2.0,0.02-0.2, and <0.02 mum, 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 day fractions (3.70 and 3.93%, respectively) was similar. Scintillation analysis indicated an increase in C-14 specific activity in all fractions after 360 d of incubation. For both sampling times, 0 and 360 d, the highest C-14 specific activity occurred in the fine clay fraction (847.2 and 1529 Bq g(-1) C, respectively), whereas the lowest C-14 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.