Nutrient enrichment alters the magnitude and timing of fungal, bacterial, and detritivore contributions to litter breakdown

Microbial decomposers and metazoan detritivores rely on and affect processing rates of detrital C in streams. Nutrient enrichment can affect the biomass of both groups, but we lack a quantitative understanding of how these groups affect breakdown rates of C under elevated nutrient conditions. We quantified the relative contribution of decomposers (fungi, bacteria) and detritivores (shredders) to breakdown rates in a reference vs a nutrient-enriched stream. We measured breakdown rates and litter-associated shredder, fungal, and bacterial biomass on Rhododendron maximum L. and Acer rubrum L. and modeled the contribution of these groups over the decay sequence for R. maximum and for a single time period for A. rubrum. We used published metabolic parameters for fungi and bacteria under high-and low-nutrient conditions and shredder feeding rates based on organisms and leaf litter from our study area. Breakdown rates of both litter types were higher (similar to 4-5x) in the treatment than reference stream and were associated with large changes in biomass of fungi, bacteria, and shredders. Changes in microbial metabolic parameters (growth rate, growth efficiency) affected estimates of mass loss. Fungal and shredder contributions were roughly equivalent and greater than bacterial contributions under reference and treatment conditions, but nutrient enrichment accelerated the colonization sequence of fungi and shredders, and shredders had relatively greater effects on mass loss in later stages of breakdown. Our results suggest facilitation of detritivores by decomposers and large combined effects on breakdown with nutrient enrichment. Decomposer contributions could exceed those of detritivores with changes in microbial metabolic rates. Large increases in breakdown rates could be attributed to biotic response to nutrient enrichment, are sensitive to microbial metabolic rates, and have the potential to alter residence times of litter in streams.