PARENT SESSION
Organized Oral Session 48: Nitrogen cycling in terrestrial ecosystems: New insights from isotopes and experiments
Organizer(s): MC Mack, P Templer, and K Nadelhoffer
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

Elevated CO₂ increases plant foraging for soil resources: Evidence from a field ¹⁵N tracer experiment.

Holmes, William^*,1, Zak, Donald¹, Pregitzer, Kurt², King, John², Loya, Wendy², Ellsworth, David¹, ¹ University of Michigan, Ann Arbor, MI² Michigan Technological University, Houghton, MI

ABSTRACT- In north temperate forests, enhanced tree growth under elevated atmospheric CO₂ could be limited by nutrient availability as well as tissue damage from O₃ exposure. At the FACTS II FACE experiment in Wisconsin, elevated CO₂ led to consistently greater NPP in aspen, aspen-birch and aspen-maple communities for several growing seasons. Elevated tropospheric O₃ generally counteracted this response. Previous measurements of gross rates of N mineralization and immobilization suggested that elevated CO₂ increased N availability to plants, whereas elevated O₃ may have reduced N availability due to greater microbial N demand. To better assess treatment effects on plant N acquisition and ecosystem N cycling, we conducted an ecosystem scale ¹⁵N tracer experiment wherein we applied ¹⁵NH₄Cl (0.015 g ¹⁵N/m²) directly to forest floor and sampled soil and biomass pools one year later. The majority of applied ¹⁵N (56-75%) was recovered in forest floor and soil organic matter. Preliminary analysis of leaves from aspen and aspen-birch communities indicates ¹⁵N recovery within canopy biomass increased under elevated CO₂ (+68%) and elevated CO₂ + O₃ (+99%); recovery decreased under elevated O₃ (-29%). Our results provide clear evidence that elevated CO₂ increased the ability of plants to forage for soil N, which is likely the result of greater fine root production and greater exploitation of soil for resources.

Key words: nitrogen cycling, elevated CO2, ozone, FACE