Role of N sub(2)-fixation in Constructed Old-field Communities Under Different Regimes of [CO2], Temperature, and Water Availability

• Published in: Ecosystems (New York) Vol. 11; no. 1; pp. 125 - 137
• Main Authors: Garten, CT Jr, Classen, A T, Norby, R J, Brice, D J, Weltzin, J F, Souza, L
• Format: Journal Article
• Language: English
• Published: 01.02.2008
• Subjects: Lespedeza; Lespedeza cuneata; Trifolium; Trifolium pratense
• ISSN: 1432-9840

Nitrogen fixation was measured in constructed old-field ecosystems that were exposed for 3 years to different combinations of elevated atmospheric [CO sub(2)] and temperature (300 ppm and 3 degree 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 N sub(2)-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 [CO sub(2)]. Effects of the three environmental factors on the mean ( plus or minus SE) fraction of legume N derived from atmospheric N sub(2) (FN dfa) varied from year-to-year, and FN dfa ranged from 0.64 plus or minus 0.05 to 0.94 plus or minus 0.03 depending on species and growing season. High rates of symbiotic N2-fixation (4.6-12 g N m super(-2) y super(-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 N sub(2)-fixation rates, as well as shading effects on other species. The high symbiotic N sub(2)-fixation rates were largely independent of manipulations of [CO sub(2)], temperature, and water. The relatively high percentage of the aboveground N stock in this ecosystem contributed by symbiotic N sub(2)-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.