title: The invasive annual cheatgrass increases nitrogen availability in 24-year-old replicated field plots.
abstract: Previous studies comparing invaded and non-invaded sites suggest that cheatgrass (Bromus tectorum L.) causes soil N cycling to increase. Unfortunately, these correlative studies fail to distinguish whether cheatgrass caused the differences in N cycling, or if cheatgrass simply invaded sites where N availability was greater. We measured soil C and N concentrations and net and gross N-cycling rates on 24-year-old replicated field plots in a sagebrush-steppe ecosystem that had been plowed, fumigated, and seeded to different plant communities in 1984. Laboratory assays of soil collected throughout the soil profiles (0-60 cm) showed that soil NO3 (-), organic C and N, and net N mineralization, net nitrification, and soil respiration rates were all greater beneath cheatgrass than in sagebrush-perennial grass plots. In surface soils (0-10 cm), field and lab assays on five sampling dates during 2 years showed gross N mineralization, net N mineralization, and net nitrification rates were all faster beneath cheatgrass than in sagebrush-perennial grass plots. Modeling analyses based on soil respiration and gross N-cycling rates suggest that cheatgrass provides soil microbes with lower C:N substrates and that this could explain the faster N-cycling rates beneath cheatgrass. This is the first long-term replicated field study to conclusively show that cheatgrass created greater soil organic N pool sizes and stimulated N-cycling rates compared to similar-aged stands of sagebrush and native perennial grasses. Increased N-cycling rates may represent a positive plant-soil feedback that promotes continued dominance by cheatgrass, even in the absence of soil disturbance or fire.
authors.affiliation: Department of Biology, and the Ecology Center, Utah State University, 5305 Old Main Hill, Logan, UT, 84322, USA. john.stark@usu.edu.
authors.forename: John M
authors.initials: JM
authors.lastname: Stark
authors.affiliation: Department of Plants, Soils, and Climate, Utah State University, Logan, UT, 84322, USA.
authors.forename: Jeanette M
authors.initials: JM
authors.lastname: Norton
mesh_terms: D019306:Artemisia / Q000378:metabolism*; D031724:Bromus / Q000378:metabolism*; D017753:Ecosystem; D005390:Fires; D058865:Introduced Species; D058465:Nitrification; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D058458:Nitrogen Cycle; D006109:Poaceae / Q000378:metabolism; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology
chemical_list: D012987:Soil; D009584:Nitrogen