Persistence of Mineral-Associated Soil Organic Carbon in European Soil Profiles

Abstract:

Soil Organic Carbon (SOC) is a heterogeneous mixture of components that are not equally biologically available, including light, plant derived material, dissolved organic carbon, and mineral-associated organic matter (MOM). Radiocarbon ages of bulk SOC average across this heterogeneity. Of particular interest is whether there are small amounts of very old OC (the so-called ‘passive’ pool) that can mask the fact that much of the OM is much younger. MOM has been shown to be older than the light fraction and DOC, but MOM is also a mixture of old and young material. This study seeks to clarify the quantity of C persisting on millennial time scales at different depths in the soil profile, and what factors allow this fraction to be more persistent than other fractions.

We studied the fraction and age of C of the most chemically and physically stable fraction we could isolate from five European soils with differing land use, parent material, and soil type. First we isolated the MOM fraction by density, and then oxidized the MOM fraction with H₂O₂ to remove the labile C. The oxidation resistant residue was analyzed for C content and radiocarbon signature.

The oxidation procedure removed 70-95% of the MOM fraction; the residue had a consistently older radiocarbon signature than the initial MOM, indicating that the C removed was younger than the bulk average. This stable fraction ranged from 100 radiocarbon years Before Present (BP) in the top 5 cm, to 10,000 years BP at the 30-40 cm depth. Non-crystalline iron concentrations were correlated with the absolute amount of SOC protected from oxidation, but not its proportion or age. With the exception of a tilled cropland site, all examined profiles exhibited a nearly linear depletion in radiocarbon signature with depth in both the protected and oxidizable MOM, confirming that the most chemically and physically stable C is oldest at the deepest point in a 50 cm profile. Ongoing work on this study will further elucidate how the origin of the MOM (microbe-, plant-, or parent material-derived) relates to the radiocarbon age along the soil depth profile.