Translocation of Carbon from Surface Organic Horizons to the Subsoil in Coarse-Textured Spodosols: Implications for Deep Soil C Dynamics

Dissolved organic matter (DOM) translocated from O horizons has been viewed as the main source of subsoil carbon (C) in Spodosols. However, recent studies in other soil types have concluded that little O horizon DOM reaches the subsoil directly. We investigated whether C derived from O horizons contributes to deep-soil C stores in six Spodosol profiles under coniferous (red pine) or deciduous (sugar maple) forests. We used spectroscopic, stable-isotope and pyrolysis-gas chromatography/mass spectroscopy analysis of DOM and solid phase soil samples to assess whether O horizon C was transported directly to deep soil horizons or had been exchanged with mineral soil organic matter (SOM) pools. All three approaches indicated that O horizons are the source of the majority of DOM entering B horizons, with little contribution from desorbed SOM. Despite a greater stock of O horizon C in red pine stands, there was no difference between forest types in the quantity of C delivered to B horizons (18 g C m(-2) y(-1)). However, C derived from O horizons accounted for similar to 95% of the C entering B horizons in sugar maple stands, but only similar to 80% in red pine stands. Molecular analysis of DOM and SOM also showed a greater resemblance of B horizon SOM to DOM in sugar maple stands; however, it also clearly indicated the importance of microbial processing of input C in the formation of B horizon SOM. Our results demonstrate the strong connection between O horizons and deep soil C stocks in these sandy Spodosols and indicate that future changes in the quantity or quality of DOM entering the mineral soil have the potential to alter the delivery of surface C to depth.