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

Abstract

Core Ideas O horizons account for the majority of DOM entering B horizons in these sandy soils. There was less exchange with SOM in sugar maple stands compared to red pine stands. Microbial processing of DOM inputs appears to be important in the formation of SOM. 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 ∼95% of the C entering B horizons in sugar maple stands, but only ∼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.