U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Ecosystem response to elevated tropospheric CO2 and O3 is regulated by plant-microbe interaction in soil

Principal investigator:   Donald R. Zak

Project goal

Determine if (and how) interactions between plants and microbes in the soil can regulate the effects of elevated carbon dioxide (CO2) and/or ozone (O3) on forest structure and functioning.

Rhinelander FACE

Top: aerial view of the DOE Wisconsin FACE (free-air CO2 enrichment) experiment. Bottom: one of the experimental "rings" showing relative locations of the different constructed communities.

Ecosystem being studied

Constructed stands of (a) the pioneer species trembling aspen (Populus tremuloides), (b) a mixture of aspen and another pioneer species paper birch (Betula papyrifera), and (c) a mixture of aspen and the late successional species sugar maple (Acer saccharum). These three species are important components of many northern hardwood forests and make up about 60% of pulpwood harvests in the Great Lake States. Trembling aspen is the dominant angiosperm tree species in the boreal forest, and is the most widely distributed tree species in North America.


Elevated CO2 has increased plant growth by 30% and belowground detritus production an equivalent amount, but elevated O3 largely counteracted this effect. Greater belowground litter inputs with elevated CO2 increased fungal abundance by 75% and the fungal degradation of cellulose by 40%; elevated O3 eliminated this effect.

Plants growing in elevated CO2 took up 1.7 times more soil nitrogen than plants growing in the ambient atmosphere, a response that was directly proportional to the growth increase produced by elevated CO2. Thus, plants more effectively foraged for this essential nutrient in elevated CO2. Stable isotope analysis revealed that elevated CO2 in combination with elevated O3 reduced the formation of total and decay-resistant soil organic matter by 50% (relative to amounts formed in elevated CO2 with ambient O3).

Data from this experiment indicate that increasing tropospheric O3 concentration can negate the positive effect of increasing CO2 concentration on the growth of aspen, paper birch, and sugar maple. Increases in CO2 and O3 concentrations can alter the types of microorganisms in soil and the rates at which they supply essential nutrients to trees and transform soil organic matter.

Why this is important

During the next 40-50 years, concentrations of both CO2 and O3 may increase to levels used in this experiment, largely as a result of fossil fuel use. Because the concentrations of both gases are increasing, forecasting the effects of fossil fuel use on the structure and functioning of important forest ecosystems will depend on knowledge gained from studies involving both gases. This experiment is the major source of such knowledge for hardwood forests.

The project uses the following conceptual model: (1) elevated CO2 and O3 can affect tree photosynthesis, growth, and biomass partitioning, perhaps in tree-species-specific ways and with the CO2 and O3 effects often opposing each other; (2) changes in tree physiology and growth can affect both leaf and fine-root litter production and biochemistry; (3) changes in litter production and biochemistry can alter the amount and quality of organic substrates available for growth and maintenance by soil microbes; (4) changes in those organic substrates can affect the composition and functioning of communities of both soil animals and microbes; (5) changes in soil animal and microbial communities can in turn affect the pools of plant-available soil nitrogen and the amount and chemical composition of soil organic matter (SOM); and (6) changes in SOM composition can affect SOM longevity.


This project is part of the DOE Rhinelander, Wisconsin, FACE [free-air CO2 enrichment] experiment, which consists of twelve 30-m-diameter rings, assigned to factorial treatments of CO2 concentration (ambient and 560 ppm) and O3 concentration (ambient and approximately 1.5 x ambient), replicated three times for each treatment combination (12 rings total). Half the rings contain five trembling aspen genotypes of differing sensitivity to CO2 and O3, one quarter of each ring contains aspen and paper birch, and the other quarter contains aspen and sugar maple.

In this project, measurements of fine root demography, fine root biochemical composition, microbial extracellular enzyme activity, microbial community composition, and an array of soil carbon and nitrogen cycling processes are periodically conducted. Molecular methods and analysis of stable isotopes of carbon are used to quantify microbial community composition and functioning in soil.


Donald R. Zak, University of Michigan

Kurt S. Pregitzer, University of Nevada, Reno (subaward)

Funding period:   August 1999 to present