Beyond Hydric Soils: Applying Process to Soil Patterns through Hydropedological Research in the Coastal Temperate Rainforest of Southeastern Alaska

Soils and hydrology are intricately linked in the development and functioning of ecosystems in the coastal temperate rainforest of southeastern Alaska. Abundant rainfall, slow decomposition and post-glacial substrates have formed a mosaic of well-drained and poorly drained soils with distinct vegetation communities. The interaction of geomorphology and soils influences the movement of water through the landscape and the development of these ecosystems through inorganic and organic transformations such as rock weathering and carbon turnover. Vertical changes in soil profiles also result from the influence of vegetation on physical and chemical transformations in soils. This complex terrestrial system has posed a challenge to terrestrial ecologists and soil scientists who have attempted to map ecosystem types and determine biogeochemical functions within these terrestrial ecosystems. Initial hydropedological research has revealed patterns associated with hydric soil formation along soil toposequences, but did not integrate soil conditions with transformations and transport of elements and compounds. We have established a hydropedomorphic template to determine common hydrologic and biogeochemical functions across a gradient of terrestrial vegetation types including uplands, forested wetlands, and poor fens. Hydrologic and biogeochemical measurements have established the range of conditions and patterns in homogeneous hydropedomorphic units. Broad vertical gradients in surface saturation in forested wetlands leads to abundant dissolved organic carbon production and export, while saturated poor-fens are important seasonal contributors of dissolved organic carbon and nitrogen. Uplands vary in their biogeochemical signatures according to the distribution of organic material in the soil surface and through weathering of lithologic deposits. This hydropedological classification can be used for broader modeling of regional patterns in the transformation and transport of material from the terrestrial to the freshwater and marine aquatic ecosystems.