PARENT SESSION
Oral Session # 68: Soil Ecology I: Communities, Respiration, and Nutrient Cycling.
Presiding: C Rumbaitis-del Rio
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 106.

Chronic NO₃^- deposition and lignin degradation in northern hardwood forests.

DeForest, Jared^*,1, Zak, Donald¹, Pregitzer, Kurt², ¹ University of Michigan, Ann Arbor, MI² Michigan Technological University, Houghton, MI

ABSTRACT- Human activity has globally doubled N input to terrestrial ecosystems. We reasoned that chronic N additions will alter the flow of C through the microbial foodweb by inhibiting the activity of lignin-degrading soil fungi, which, in turn, control ecosystem-level patterns of C and N cycling. We tested our hypothesis in a mature northern hardwood ecosystem in Michigan which has received experimental N additions (30 kg NO₃^--N ha^-1 y^-1) since 1994. In a laboratory study, we amended soils with ¹³C labeled vanillin, a product of lignin degradation, and tracked the flow of ¹³C through the microbial community. In addition, we simultaneously measured phenol oxidase activity, which is responsible for degrading lignin and vanillin. Phenol oxidase activity was significantly suppressed by 133% in N amended soils. In addition, we found N additions increased the recovery of ¹³C in fungal PLFA by 82%, but had no significant effect on recovery of ¹³C in bacteria PLFA or microbial respiration. Results suggest that chronic N additions suppressed lignin degradation, while it increased the ability of soil fungi to metabolize the products of lignin degradation.

Key words: carbon dynamics, anthropogenic nitrogen, extracellular enzymes, phospholipid fatty acid (PLFA)