Role of N2-fixation in Constructed Old-field Communities Under Different Regimes of [CO2], Temperature, and Water Availability
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Nitrogen fixation was measured in constructed old-field ecosystems that were exposed for 3 years to different combinations of elevated atmospheric [CO2] and temperature (300 ppm and 3°C above ambient, respectively), and ambient or reduced soil moisture (corresponding to 25 or 2 mm rainfall per week). The old-fields included seven planted herbaceous annual and perennial species, including two legumes (Trifolium pratense and Lespedeza cuneata). Potential asymbiotic N2-fixation by soils, measured in laboratory incubations, was significantly less under the “dry” treatment but was estimated to contribute little overall to annual ecosystem N budgets. Foliar N concentrations declined significantly under elevated [CO2]. Effects of the three environmental factors on the mean (±SE) fraction of legume N derived from atmospheric N2 (FN dfa) varied from year-to-year, and FN dfa ranged from 0.64 ± 0.05 to 0.94 ± 0.03 depending on species and growing season. High rates of symbiotic N2-fixation (4.6–12 g N m−2 y−1) that annually contributed from 44% to 51% to the aboveground N stock in the old-field community was an important process driving changes in species composition over the 3-year experiment. Lespedeza biomass increased over time at the expense of several other species, including the other N-fixer, Trifolium. The dominance of Lespedeza in our ecosystem was due to high symbiotic N2-fixation rates, as well as shading effects on other species. The high symbiotic N2-fixation rates were largely independent of manipulations of [CO2], temperature, and water. The relatively high percentage of the aboveground N stock in this ecosystem contributed by symbiotic N2-fixation suggests that non-legume species may have benefited indirectly via reduced community demands on soil N supplies. Species-specific traits were important in the constructed ecosystems, indicating that multi-species studies are required for understanding complex interactions among environmental factors and dynamic changes in community composition.
KeywordsLespedeza legumes 15N natural abundance method multi-factor experiments climate change constructed ecosystems
This research was sponsored by the U.S. Department of Energy, Office of Science, Biological and Environmental Research/Program for Ecosystem Research under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC. We wish to thank B. Lu, E.C. Engel, P. Allen, C. Reilly Sheehan, S. Wan, S. Freyaldenhoven, and K. Cox for their valuable assistance in the laboratory and/or field. We would also like to thank the ORNL/UT Ecosystem Ecology lab group (C. Iversen, H. Castro, O. Dermody, C. Campany, D. Weston, K. Sides, E. Austin, E. Felker-Quinn) for insightful comments on the draft manuscript.
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