Short- and long-term effects of nutrient enrichment on salt marsh plant production and microbial community structure

Nutrient enrichment impacts ecosystems globally. Population history, especially past resource environments, of numerically dominant plant species may affect their responses to subsequent changes in nutrient availability. Eutrophication can also alter plant-microbe interactions via direct effects on associated microbial communities or indirect effects on dominant species' biomass production/allocation as a result of modified plant-soil interactions. We combined a greenhouse common garden and a field reciprocal transplant of a salt marsh foundation species (Spartina alterniflora) within a long-term, whole-ecosystem, nutrient-enrichment study to determine whether enrichment affects plant production and microbial community structure differently depending on plant population history. For the greenhouse portion, we collected 20 S. alterniflora genotypes-10 from an enriched creek that had received elevated nutrient inputs for 10 years and 10 from an unenriched reference creek-and reared them in a common garden for 1 year. For the field portion, we conducted a 2-year, fully crossed reciprocal transplant experiment with two gardens each at the enriched and unenriched sites; we examined the effects of source site (i.e. population history), garden site and plant genotype. After 2 years, plants in enriched gardens had higher above-ground biomass and altered below-ground allocation compared to plants in unenriched gardens. However, performance also depended on plant population history: plants from the enriched site had decreased above-ground and rhizome production compared to plants from the unenriched site, most notably in unenriched gardens. In addition, almost all above- and below-ground traits varied depending on plant genotypic identity. Effects of nutrient enrichment on the associated microbial community were also pronounced. Following 1 year in common garden, microbial community structure varied by plant population history and S. alterniflora genotypic identity. However, at the end of the reciprocal transplant, microbial communities differed primarily between enriched and unenriched gardens. Synthesis. Nutrient enrichment can impact plant foundation species and associated soil microbes in the short term. Most importantly, nutrient enrichment can also have long-lasting effects on plant populations and associated microbial communities that potentially compromise their ability to respond to changing resource conditions in the future.