CRITICAL ZONE ARCHITECTURE AND THE REDISTRIBUTION OF SOIL METALS AND ORGANIC CARBON IN A NEW HAMPSHIRE HEADWATER CATCHMENT

Interactions between soils and subsurface flow paths create spatial variations in stream water chemistry in headwater catchments. These interactions also create morphological and chemical variations in soils at both the catchment and hillslope scales. We studied watershed 3 (WS3) at the Hubbard Brook Experimental Forest to better understand landscape variation in soil development and function. Nearly 100 soil profiles were fully described and analyzed, allowing assignation to a hydropedologic unit (HPU), a classification developed using landscape and morphological metrics that correspond with distinct water table regimes. Soil samples were analyzed for total organic C and extractable Al, Fe and the rare earth elements. Although podsolization (i.e. translocation of Al, Fe and C) is generally assumed to be a vertical process, we found both vertical and lateral redistribution in predictable patterns. Typical (horizontally layered) podzols were found in the majority of the catchment due to the dominance of vertical, unsaturated flow. However, lateral flow created four other HPU podzol types with distinct morphology and chemistry. Near the catchment ridge, sharply contrasting lateral podzols together mirrored the horizonation found in a typical podzol but in a downslope, not vertical, sequence. Thicker spodic horizons of laterally accumulating soils generally reflected larger pools of metals and subsoil organic C. The spatial distribution of positive cerium-anomalies (Ce/Ce*) in soil profiles proved to be a consistent hydropedologic indicator of lateral flow and seasonally high water table in three hillslopes. This variation across the landscape coincided with variation in both stream solutes and soil carbon accumulation. Ongoing work is investigating the lateral movement of weathering products from the catchment ridge downward through a sequence of HPUs.