Radiocarbon content and stable carbon isotopic ratios of individual fatty acids in subsurface soil: Implication for selective microbial degradation and modification of soil organic matter

A subsurface soil sample was studied for molecular composition and stable carbon isotopic ratios (delta C-13) of fatty acids (FAs) using a capillary gas chromatography (GC) and GC/combustion/isotope ratio mass spectrometer, respectively. Compound specific radiocarbon analyses of FAs were also conducted using an off-line preparative capillary GC/accelerator mass spectrometer. Molecular distribution of FAs (C-14-C-32) is characterized by even carbon numbered predominance with two maxima at n-C-16 and n-C-28, being similar to that of plant leaf. However, branched chain C-15 and C-17 acids were abundant in soil, indicating bacterial degradation and modification of soil organic carbon. The delta C-13 of FAs (-35.2 to -23.0 parts per thousand) are within the range of plant leaf delta C-13 (-36.4 to -31.2 parts per thousand), except for heavy values (-23.0 to -28.1%o) of C,4 to C-18. The heavier delta C-13 values are most likely interpreted by microbial re-synthesis of shorter chain FAs in soil. We also report, for the first time, a significant diversity of Delta C-14 values (+17 to +127 parts per thousand) in soil FAs, with higher values (+96 to +127 parts per thousand) for saturated and unsaturated C-18 and lower values (+16 to +19 parts per thousand) for longer chain C-28 and C-30. The higher radiocarbon values can be explained by a combination of selective microbial decomposition of shorter chain FAs in subsurface soil, and the subsequent deposition and mixing of modern carbon with old carbon in soil. The modern carbon may be transported downward in the form of shorter chain FAs and other water-soluble organic compounds by meteoric water and/or tree roots, followed by microbial re-synthesis of lipids in soil. This study implies that microbial degradation and modification of soil organic matter play an important role in geochemical processes which control the carbon cycle on the Earth.