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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Atmospheric Nitrate Deposition, Microbial Community Composition, and Enzyme Activity in Northern Hardwood Forests

^a School of Natural Resources & Environment, 430 E. University, Univ. of Michigan, Ann Arbor, MI 48109-1115
^b School of Forest Resources and Environmental Science, Michigan Technological Univ., 1400 Townsend Dr., Houghton, MI 49931

^* Corresponding author (jdefores{at}umich.edu).

On a global scale, human activity has increased the atmospheric input of NO^–₃ to many terrestrial ecosystems. Anthropogenic NO^–₃ may be a potent modifier of ecosystem function, especially in temperate forests that are sometimes N limited. However, the impact of chronic N deposition on soil microorganisms is still poorly understood. Nitrate entering Lake States forests is rapidly assimilated by the microbial community and it is subsequently released as NH⁺₄. Because high levels of NH⁺₄ inhibit the activity of lignin-degrading soil fungi, we reasoned that chronic N additions could alter the composition and function of heterotrophic microbial communities in soil, and hence the ecosystem-level processes they mediate. We tested our hypothesis in four northern hardwood ecosystems in northern Michigan, which received experimental N additions (30 kg NO^–₃–N ha^–1 yr^–1) during the past 8 yr. We quantified microbial community function by measuring the activity of extracellular enzymes involved in plant litter degradation and described microbial community composition using phospholipid fatty acid (PLFA) analysis. Chronic N additions significantly suppressed ß-glucosidase activity by 24% in mineral soil and suppressed phenol oxidase activity by 35% in surface litter. We found no evidence that chronic N additions altered microbial community composition; NO^–₃ addition did not alter the relative abundance of bacterial, actinomycetal, fungal, or protozoan PLFAs. However, NO^–₃ additions significantly reduced microbial biomass by 18% relative to the control treatment. Results indicate that N additions broadly suppressed all microbial groups, not just the activity and abundance of lignin-degrading fungi.

Abbreviations: DOC, dissolved organic carbon • DON, dissolved organic nitrogen • FAME, fatty acid methyl esters • PLFA, phospholipid fatty acid

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