1. Individual plant genotypes as well as genotypic diversity can shape the structure and function of ecosystems; however, the abiotic environment may modify these genotypic influences on ecosystem-level responses. 2. To explore how the interactions between plant genotype, genotypic diversity and soil nutrient availability affect the structure and function of a temperate grassland ecosystem, we manipulated the genotypic diversity of a common perennial herbaceous plant, Solidago altissima (single genotype monoculture and diversity plots) and soil nutrient availability (+nitrogen, +phosphorus, +nitrogen and +phosphorus, unmanipulated control) in a common garden setting. We tracked temporal changes in ecosystem structure (e.g. leaf area index and net primary productivity) as well as a variety of ecosystem functions (e.g. net ecosystem carbon and water exchange and soil carbon efflux) over a growing season. 3. We found that variation in plant genotype identity consistently shaped ecosystem structure (above-ground net primary productivity) while it inconsistently altered several ecosystem functions across time. For instance, variation in plant genotype identity influenced net ecosystem carbon dynamics early in the growing season while it influenced water dynamics later in the growing season. The strength of the relationship between genotypic diversity and ecosystem function declined over the season and the relationship between ecosystem structure (above-ground net primary productivity) and function (net ecosystem carbon and water exchange) varied across treatments. Overall, there was a strong correlation between ecosystem structure and function across monoculture genotype plots but a weak relationship between ecosystem structure and function across mixed genotype plots. Surprisingly, soil nutrients did not influence ecosystem structure and had minimal impacts on carbon and water flux. 4. Our data suggest that plant genetic variation, and to some extent plant genotypic diversity, strongly influence ecosystem structure and function in an old-field ecosystem, but nutrient availability did not directly or interactively influence ecosystem structure or function.