PNNL

Abstract

Despite the importance of wetlands for water quality and carbon storage, biogeochemical processes in these systems remain poorly understood. In particular, little is known about the variability in carbon content and processing among different wetland types. To determine how differences in hydrogeomorphic setting influence soil organic carbon (SOC) processing, we compared carbon content and composition between depressional and slope wetlands located in the Colorado Rocky Mountains. Depressional wetlands were characterized by seasonally declining water tables, high clay content, and thick organic layers. Slope wetlands had relatively consistent groundwater inputs, coarser soil textures, and thinner organic layers. Carbon dating suggests subsurface SOC in slope wetlands was older than that from comparable depths in depressional wetlands. Analysis of SOC composition by solid-state 13C nuclear magnetic resonance spectroscopy demonstrated a higher prevalence of aliphatic compounds in depressional wetlands, especially at depth, whereas subsurface carbon content in slope wetland soils was dominated by aromatic compounds. We propose that the higher clay and OC contents and long hydraulic residence times in depressional wetlands favor anaerobic conditions, which limit decomposition and promote the preservation of highly reduced organic compounds such as aliphatics. The results presented in this work provide an enhanced understanding of the influence of hydrogeomorphic class on carbon processing in subalpine wetlands and suggest that hydrogeomorphic class should be considered in wetland carbon modeling and water management practices.