Examining mineral-associated soil organic matter pools through depth in harvested forest soil profiles
Mineral-associated organic matter is associated with a suite of soil minerals that can confer stability, resulting in the potential for long-term storage of carbon (C). Not all interactions impart the same level of protection, however; evidence is suggesting that C in certain mineral pools is dynamic and vulnerable to disturbance in the decades following harvesting. The objective of this research was to describe and characterize organic matter-mineral interactions through depth in horizons of
... th in horizons of soils of contrasting stand age. Sequential selective dissolutions representing increasingly stable mineral-associated organic matter pools from water soluble minerals (deionized water), organo-metal complexes (Na-pyrophosphate), poorly-crystalline minerals (HCl hydroxylamine), and crystalline secondary minerals (Nadithionite HCl)) were carried out for A e , B f and BC horizons sampled from a Young and Mature forest site (35 and 110 years post-harvest) in Mooseland, Nova Scotia, Canada. Sequential selective dissolution extracts were analyzed for C, δ 13 C, iron (Fe) and aluminum (Al). Organo-metal complexes (OMC) were the largest mineral-associated OM pool in all horizons. This pool dominated the C distribution in B horizons (~60-70% of B f bulk C), with a minor contribution from poorly-crystalline (PCrys), crystalline (Crys) minerals and water soluble (WS) associations. C in OMC and PCrys pools explained the variation in bulk C in horizons through depth at both sites. Twice as much C in OMC pools was measured at the Mature site compared to the Young site in the B f horizons, supported by higher C:(Fe+Al) ratios. Isotopic analysis indicated that this extraction procedure isolated distinct mineralassociated OM pools. δ 13 C signatures of pyrophosphate-extracted OMC pools ranged from -27‰ to -28‰, similar to δ 13 C of bulk C and to plant-derived humic acids and associated biomass. The water soluble phase (mean δ 13 C = -29 ‰) was up to 2 ‰ more depleted, whereas the δ 13 C of Crys pools were more enriched in 13 C (-13‰ to -16 ‰) compared to bulk soil. The results from this study suggest that association with minerals does not necessarily confer stability: organo-metal pools dominate in podzol horizons through depth, and contribute most to C storage, but are potentially susceptible to destabilization following the physical changes resulting from forest harvesting disturbance. Fig 4. Plot of pedogenic ratios for Young (solid line) and Mature (broken line) through depth, which are indexes of a) weathering; and b) organic complexation. Note that strong weathering is indicated by a low ratio in a). In both diagrams, the site with stronger complexation or weather has a darker line.