Aluminum Control of Organic Carbon Cycling in Temperate Conifer Forest Ecosystems.

The release of dissolved organic matter (DOM) from the forest floor constitutes a significant flux of organic carbon (C) to the mineral soil. The majority of DOM is either oxidized or adsorbed to mineral surfaces within the soil profile. Metal complexation and mineral adsorption are key processes in removal of DOM from solution and soil C sequestration. In particular, ecosystem aluminum (Al) and iron (Fe) have been implicated as controls of DOM and soil C cycling, but the stabilization mechanisms are not well understood. Due in part to this lack of understanding, regional and landscape models of soil C cycling do not directly incorporate metal complexation and mineral adsorption mechanisms as parameters controlling soil C pool size and mean residence time. As such, we focused this project on understating the mechanistic control of soil minerals, particularly solution-state Al- and Fe- species and solid-state Al- and Fe-oxyhydroxides, on the cycling of organic carbon in temperate conifer forests of the western U.S. Our hypothesis was that both solution- and solid-state Al-species are primary controls of DOM and soil C biodegradation in temperate forest systems. We implemented coupled field and laboratory studies to understand the link between metal chemistry, mineral-DOM-microbe interactions and organic matter transformation and biodegradation in Pinus Ponderosa forests of Arizona. The project is organized around four research components aimed at: (i) quantifying the role of soil mineral assemblage and Al-humus complexation on soil C cycling; (ii) measuring the affect of complexation on metal and DOM physical chemistry; (iii) determining the behavior of metal-DOM complexes in soil profiles using sequential column leaching and biodegradation studies with DOM derived from ¹³C labeled Pinus ponderosa; and (iv) quantifying the abundance of Al-tolerant microbes in field soils and the response of microbial populations to metal-DOM complexes, and Al- and Fe-oxyhydroxides under laboratory conditions.