Wednesday, November 15, 2006 - 10:30 AM

Physicochemical Controls on the Dynamics of Soluble C in Variable-Charge Forest Soils.

Brian D. Strahm and Robert Harrison. Univ of Washington, College of Forest Resources, Box 352100, Seattle, WA 98195

Soils with variable charge components are prevalent in the tropics in which intense weathering has left high concentrations of oxides in situ.  Outside of such heavily weathered soils, similar physicochemical properties have been observed in soils of volcanic influence.  Despite accounting for only a small percentage of total land area, these soils are often heavily utilized for intensive forest management in the Pacific Northwest.  As such, it is important to examine possible feedback mechanisms between land use practices and the potential of these systems to retain nutrients and sequester C. This study was conducted to determine how the mobility and bioavailability of low molecular weight organic acids (LMWOAs) are influenced by changes in the dynamics at the soil-solution interface for soils with variable-charge components.  Sorption isotherms of three LMWOAs, one amino acid (glycine), one mono-carboxylic acid (propionic acid) and one di-carboxylic acid (malonic acid), were generated for three forest soils with variable charge components using batch equilibration techniques.  Prior to a 1 h equilibration, all soils were saturated with KCl and soils and solutions were adjusted to one of three pH levels (4, 6, 8 0.1).  Bioavailability of the LMWOAs was assessed through the generation of desorption isotherms for each combination as well as through laboratory incubations.  Results indicate sorptive affinities and maximum sorptive capacities that are directly related to commonly measured and reported soil properties, including mineralogy and pH, making it easier to identify sites with such capabilities.  In that context, land use practices that influence both the pH regime of a soil, as well as the production of soluble C should be carefully considered as a potentially mechanism to retain C and nutrients on a site through physicochemical interactions with the mineral surface.