QG&G BIRKELAND AWARD: CONTROLS ON GLOSSIC HORIZON DEVELOPMENT IN ALFISOLS OF THE GREAT LAKES REGION

Data recently obtained from a Glossudalf in northern Michigan indicate that loss of clay from the upper argillic horizon, which then forms a glossic (B/E or E/B) horizon, is triggered by free iron (Fe) oxide eluviation. This finding runs counter to the long-held notion that acidification and desaturation of the exchange complex drives argillic horizon degradation and clay remobilization in Alfisols and similar soils. In this talk, I present preliminary findings on the mechanisms and processes underlying Fe eluviation from research on six additional Glossudalfs in Michigan. These data better elucidate the controls on glossic horizon genesis. Field observations and selective Fe extraction data suggest that episodic stagnation at the upper boundary of the argillic horizon is the main driver of Fe oxide eluviation in the degrading (glossic) zone. Incubation experiments allowing litter leachates collected from representative sites to interact with intact (non-degraded) illuvial materials further suggest that the kinetics of this process are likely modulated both by the content and mineralogy of Fe oxides, as well as the quantity and quality of dissolved organic matter (DOM) delivered to the subsoil. Mixed (coniferous-deciduous) litter leachate more effectively reduced and dissolved Fe oxides than did litter leachate from deciduous forests, especially when the illuvial materials contained less total free Fe and more poorly crystalline Fe phases. Molecular analysis of DOM produced over the leaf-off period also shows that mixed forest litter consistently contributes more and higher average molecular weight compounds to the soil solution. Together these findings imply that although a similar mechanism may be responsible for Fe oxide eluviation, the processes involved are enhanced in soils where the conditions are more conducive for Fe-DOM interactions. Additional work will focus on implementing geospatial procedures to interpret regional soil variability in context of these findings.