PARENT SESSION
Oral Session #58: Elevated CO₂. Presiding: A. Finzi.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Madison Ballroom C.

Effects of CO₂ and O₃ enrichment on microbial metabolism of ¹³C-labeled compounds in forest soils.

PHILLIPS, REBECCA¹, ZAK, DONALD¹, HOLMES, WILLIAM¹, ¹

ABSTRACT- Elevated atmospheric CO₂ and O₃ are potent modifiers of plant growth, presenting the possibility that they also could alter the input of plant-derived substrates for microbial metabolism in soil. We used ¹³C-labeled cellobiose and N-acetylglucosamine to determine whether changes in plant growth under elevated CO₂ and O₃ have altered the microbial metabolism of plant and fungal cell walls. We added these isotopically labeled substrates to soil collected beneath Populus tremuloides, Betula papyrifera, and Acer saccharum growing under factorial CO₂ and O₃ treatments (ambient and elevated) in a FACE experiment located in northern Wisconsin. The addition of ¹³C-labeled substrate substantially increased the ¹³C of microbial respiration, but it did not alter its rate. The amount of respired ¹³C from both substrates was greatest in the elevated CO₂ treatment (1 g ¹³C-CO₂ g^-1) and lowest in the elevated O₃ treatment (0.6 g ¹³C-CO₂ g^-1), relative to ambient control (0.7 g ¹³C-CO₂ g^-1). This response was similar beneath each plant species. Recovery of ¹³C was greatest in soil organic matter (ca. 60%), and there were no significant differences among treatments; recovery in DOC (ca. 10%) also was similar among treatments. Changes in the ¹³C of microbial PLFA indicated that both substrates were primarily metabolized by soil bacteria, suggesting that elevated CO₂ increased the rate at which these substrates are metabolized by bacterial populations in soil.

KEY WORDS: PLFA, O₃ , CO₂ , soils