Storage and stability of mineral-associated soil organic matter pools in genetic horizons of harvested coniferous forest soils

Mineral soil organic matter (SOM) is associated with a suite of secondary minerals that can confer stability, resulting in the potential for long-term storage of carbon (C). Not all interactions impart the same level of stability, however; evidence is suggesting that SOM in certain mineral phases is dynamic and vulnerable to soil disturbance, such as forest harvesting. The objective of this research was to characterize SOM-mineral interactions in horizons of harvested soils of contrasting stand age. Sequential selective dissolutions representing increasingly stable SOM pools from soluble minerals (deionized water (DI)), non-crystalline (Na-pyrophosphate), poorly-crystalline minerals (HCl hydroxylamine), to crystalline secondary minerals (Na-dithionite HCl)) were carried out for Ae, Bf and BC horizons sampled from a young and mature forest site (35 and 110 years post-harvest) in Mooseland, Nova Scotia, Canada. Selective dissolution extracts were analyzed for dissolved organic carbon (DOC), its δ13C, Fe and Al. Initial isotopic analysis indicates that separate operational SOM pools were isolated: δ13C values of pyrophosphate-extracted non-crystalline (NC) phases were -27 to -28‰, similar to δ13C of bulk C and to plant-derived humic acids and fungal biomass, whereas the δ13C of DI extracts were more depleted in 13C (1-2 ‰). These SOM pools retained their isotopic signature through depth despite an enrichment in bulk SOM δ13C. NC dominated the C distribution for all horizons, followed by poorly crystalline (PC) minerals, and the C content of these two phases explained the variation in bulk C, while C in crystalline pools were similar for the two sites through depth. The mature site had twice as much C in the NC pool as the young site in the Bf horizons, supported by higher C/Fe+Al ratios, suggesting a change in loading following harvesting. Despite the destabilizing processes that occur with forest harvesting and evidence for the increased destabilization of NC and PC pools of SOM, those pools associated with crystalline OM remain stable, suggesting that the nature of mineral-SOM binding determines its stability and therefore its potential for long-term storage.