Arctic landscapes are characterized by a diversity of ecosystems, which differ in plant species composition, litter biochemistry, and biogeochemical cycling rates. Tundra ecosystems differing in plant composition should contain compositionally and functionally distinct microbial communities that differentially transform dissolved organic matter as it moves downslope from dry, upland to wet, lowland tundra. To test this idea, we studied soil microbial communities in upland tussock, stream-side birch-willow, and lakeside wet sedge tundra in arctic Alaska, USA. These are a series of ecosystems that differ in topographic position, plant composition, and soil drainage. Phospholipid fatty acid (PLFA) analyses, coupled with compound-specific 13C isotope tracing, were used to quantify microbial community composition and function; we also assayed the activity of extracellular enzymes involved in cellulose, chitin, and lignin degradation. Surface soil from each tundra ecosystem was labeled with 13C-cellobiose,13C-N-acetylglucosamine, or 13C-vanillin. After a five-day incubation, we followed the movement of 13C into bacterial and fungal PLFAs, microbial respiration, dissolved organic carbon, and soil organic matter. Microbial community composition and function were distinct among tundra ecosystems, with tussock tundra containing a significantly greater abundance and activity of soil fungi. Although the majority of 13C-labeled substrates rapidly moved into soil organic matter in all tundra soils (i.e., 50-90% of applied 13C), microbial respiration of labeled substrates in wet sedge tundra soil was lower than in tussock and birch-willow tundra; approximately 8% of 13C-cellobiose and approximately 5% of 13C-vanillin was respired in wet sedge soil vs. 26-38% of 13C-cellobiose and 18-21% of 13C-vanillin in the other tundra ecosystems. Despite these differences, wet sedge tundra exhibited the greatest extracellular enzyme activity. Topographic variation in plant litter biochemistry and soil drainage shape the metabolic capability of soil microbial communities, which, in turn, influence the chemical composition of dissolved organic matter across the arctic tundra landscape.