Extracellular Enzyme Activity Beneath Temperate Trees Growing Under Elevated Carbon Dioxide and Ozone

cause these plant tissues are the primary substrates for microbial metabolism in soil. Soil microorganisms are limited by the amount and type of plantOzone is a greenhouse gas that is accumulating in the derived substrates entering soil, and we reasoned that changes in the production and biochemical constituents of plant litter produced lower atmosphere, and elevated O3 has the potential to under elevated CO2 and O3 would elicit physiological changes in soil alter soil microbial communities through its influence microbial communities. To test this idea, we studied microbial activity on plant litter production. Elevated O3 can decrease beneath trembling aspen (Populus tremuloides Michx.), paper birch photosynthesis (Pye, 1988; Coleman et al., 1995a) and (Betula papyrifera Marsh.), and sugar maple (Acer saccharum Marsh.) net C gain (Skärby et al., 1987; Reich et al., 1990) in growing under experimental atmospheric CO2 (ambient and 522.7 L many woody plants, which should modify litter producL 1 ) and O3 (ambient and 54.5 nL L 1 ). To assess changes in microbial tion. For example, high levels of O3 often decrease root community function, we measured microbial biomass, respiration, and growth to a greater extent than foliage or stem growth the metabolism of root-derived substrates using BIOLOG GN micro(Manning et al., 1971; Blum and Tingey, 1977; Hogsett et plates. We also measured the activity of phosphatase, leucine aminoal., 1985). Elevated O3 also can modify the biochemical peptidase, -glucosidase, N-acetylglucosaminidase, cellobiohydrolase, phenol oxidase, and peroxidase enzymes, which are involved in plant composition of fine roots, wherein starch and soluble and fungal litter decomposition. Microbial biomass and respiration sugar concentrations can decline following exposure to were not significantly altered by elevated CO2 and O3. Cellobiohydroelevated O3 (Andersen et al., 1991). It is possible that delase activity significantly increased under elevated CO2; however, this creased growth and biochemical changes under elevated response was eliminated by elevated O3. N-acetylglucosaminidase activO3 could potentially mitigate increases in plant growth ity also increased under elevated CO2, but elevated O3 did not signifiand changes in tissue biochemistry resulting from elecantly alter this response. We found no difference in the metabolism vated atmospheric CO2. However, we do not understand of amino acids, organic acids, and simple carbohydrates, suggesting how these atmospheric gases will interact to alter the our experimental treatments did not alter the use of these substrates input of organic substrates to soil, which could potenby soil microorganisms. Our analysis indicates that changes in plant tially modify microbial activity in soil. growth in response to elevated CO2 and O3 alters microbial metabolism in soil. In a recent experiment, we observed that elevated CO2 significantly increased the biomass of living and dead fine roots, but the magnitude (83–113%) of this response differed between tree species and was elimiT increasing concentration of atmospheric CO2 nated by elevated O3 (King et al., 2001). Because microhas the potential to alter the cycling of C and N in bial growth is constrained by the type and amount of terrestrial ecosystems, because it can modify the producorganic substrates entering soil (Babiuk and Paul, 1970; tion and biochemistry of plant litter entering soil, which, Smith and Paul, 1990), the aforementioned changes in in turn, controls microbial activity. Elevated CO2 can belowground plant growth could potentially alter both increase photosynthesis (Kubiske et al., 1997; Curtis et substrate availability and microbial activity. The presal., 2000) and plant growth (Mikan et al., 2000; Zak et ence of substrates can induce the synthesis of specific al., 2000a), as well as alter the biochemical composition extracellular enzymes (Paul and Clark, 1996), and we of plant tissue produced above (Cotrufo et al., 1994) reasoned that the activity of key enzymes involved with and belowground (Cotrufo and Ineson, 1995). Roots are plant litter decomposition should respond to changes in an important source of organic substrates for microbial fine-root litter under elevated CO2 and O3. We also growth, and most studies suggest root production will reasoned that changes in microbial activity under eleincrease as CO2 accumulates in the Earth’s atmosphere vated CO2 and O3 would differ among plant taxa. To test (Pregitzer et al., 2000). Moreover, elevated CO2 can inthese ideas, we measured extracellular enzyme activities crease fine root C/N in some species (Betula pendula and the metabolism of labile, root-derived substrates and Picea sitchensis; Cotrufo and Ineson, 1995) and alter beneath contrasting temperate tree species grown under concentrations of starch, sugar, and total nonstructural experimental CO2 and O3 treatments. carbohydrates in others (Pinus taeda; Lewis et al., 1994). Therefore, the degree to which elevated CO2 modifies the production and chemistry of root and leaf litter MATERIALS AND METHODS should have direct effects on soil microorganisms, beExperimental Design Our study was conducted at a free-air CO2 and O3 enrichJ.L. Larson and D.R. Zak, School of Natural Resources & Environment (FACE) experiment located 25 km west of Rhinelander, ment, Univ. of Michigan, Ann Arbor, MI 48109; R.L. Sinsabaugh, Dep. of Earth, Ecological and Environmental Sciences, Univ. of Toledo, Toledo, OH 43606-3390. Received 9 Mar. 2001. *Corresponding Abbreviations: FACE, free-air CO2 and O3 enrichment; MBc, microauthor ([email protected]). bial biomass C; MUB, methylumbelliferone; PCA, principal components analysis. Published in Soil Sci. Soc. Am. J. 66:1848–1856 (2002).