Little is known about the functional roles of specific microbial decomposer taxa, despite their significant roles in ecosystem processes. We hypothesized that microbial decomposers vary in their abilities to use different resources under natural conditions in soil and that decomposers specialize in the breakdown of particular carbon substrates. To test this hypothesis, we added one of five organic compounds (glycine, sucrose, cellulose, lignin, and tannin-protein) directly to soils in conjunction with a nucleotide analog that labels active DNA. Fungi that proliferated in response to the substrates incorporated the nucleotide analog into their DNA and were likely to be utilizing the substrate for growth. We identified 11 fungal taxa whose relative abundances differed among substrates. One taxon, belonging to the genus Mortierella, increased in abundance under cellulose, while an unknown Basidiomycete was only present under lignin. Other taxa proliferated in response to several substrates. We also found that the community structure of active fungi shifted in response to glycine, sucrose, and cellulose. Diversity analyses further revealed that taxonomic richness was greater under cellulose (Shannon Index: 3.23±0.11 with 95% CI) and lignin (2.87±0.15) additions than the other treatments (2.34±0.16 to 2.64±0.13). These results support the hypothesis that fungal decomposers exhibit differing resource utilization capabilities under natural conditions and that some taxa have specialized functions. Resource partitioning may be important in structuring natural soil microbial communities, and microbial decomposers may not be functionally redundant. Therefore, ecosystem processes may be affected by shifts in soil biodiversity induced by global change.