A growing body of genes-to-ecosystems research has documented the ecosystem-level consequences of intraspecific variation in plants caused in large part by variation in chemical composition. Understanding how this genetic variation in trees might interact with elevated nutrients resulting from increases in anthropogenic deposition may give us insight into how future riparian forests might influence adjacent streams through leaf-litter deposition. We examined the effects of tree genotype, soil nutrient environment, and their interaction on aspen (Populus tremuloides) litter chemistry and aquatic decomposition. We used litter collected from 5 aspen genotypes grown in a common garden under low and high nutrient availability and monitored decomposition over 112 d in a woodland stream. Genotype, environment, and genotype × environment interactions influenced litter chemistry and decomposition dynamics. Genotype and environment both strongly influenced initial litter chemistry, with significant genotype × environment interactions for bound condensed tannins and C∶N. Consistent with effects on litter chemistry, decomposition rates were significantly affected by genotype, environment, and genotype × environment interactions. These results suggest that future changes in intraspecific genetic variation of tree species and deposition of nutrients because of shifts in climate, land use, and related factors may influence decomposition processes in terrestrial systems and in the aquatic systems with which they are coupled via material transfer.