title: Long-term nitrogen addition causes the evolution of less-cooperative mutualists.
abstract: Human activities have altered the global nitrogen (N) cycle, and as a result, elevated N inputs are causing profound ecological changes in diverse ecosystems. The evolutionary consequences of this global change have been largely ignored even though elevated N inputs are predicted to cause mutualism breakdown and the evolution of decreased cooperation between resource mutualists. Using a long-term (22 years) N-addition experiment, we find that elevated N inputs have altered the legume-rhizobium mutualism (where rhizobial bacteria trade N in exchange for photosynthates from legumes), causing the evolution of less-mutualistic rhizobia. Plants inoculated with rhizobium strains isolated from N-fertilized treatments produced 17-30% less biomass and had reduced chlorophyll content compared to plants inoculated with strains from unfertilized control plots. Because the legume-rhizobium mutualism is the major contributor of naturally fixed N to terrestrial ecosystems, the evolution of less-cooperative rhizobia may have important environmental consequences.
authors.affiliation: Department of Biology, St. Ambrose University, 518 West Locust St, Davenport, Iowa, 52803; Kellogg Biological Station and Department of Plant Biology, Michigan State University, 3700 E. Gull Lake Drive, Hickory Corners, Michigan, 49060. weesedylanj@sau.edu.
authors.forename: Dylan J
authors.initials: DJ
authors.lastname: Weese
authors.forename: Katy D
authors.initials: KD
authors.lastname: Heath
authors.forename: Bryn T M
authors.initials: BT
authors.lastname: Dentinger
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Lau
mesh_terms: D005075:Biological Evolution; D018533:Biomass; D002734:Chlorophyll / Q000032:analysis; D017753:Ecosystem; D007887:Fabaceae / Q000382:microbiology* / Q000502:physiology; D005308:Fertilizers; D009584:Nitrogen / Q000737:chemistry*; D058458:Nitrogen Cycle; D010802:Phylogeny; D012231:Rhizobium / Q000235:genetics* / Q000502:physiology; D013559:Symbiosis / Q000235:genetics*; D029921:Trifolium / Q000382:microbiology / Q000502:physiology
chemical_list: D005308:Fertilizers; D002734:Chlorophyll; D009584:Nitrogen
keywords: Adaptation; coevolution; nitrogen deposition; resource mutualism; symbiosis