Effects of elevated CO2, temperature, and soil water availability on biological N2-fixation in a constructed old field community
Principal investigator: Charles T. Garten Jr.
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Effects of elevated CO2, temperature, and soil water availability on biological N2-fixation in a constructed old field community Principal investigator: Charles T. Garten Jr. |
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Project goal Quantify and understand the role of asymbiotic and symbiotic N2-fixation in a constructed old field community, the effects of manipulated environmental factors (CO2 concentration, temperature, and soil water availability) on N2-fixation, and the significance of N2-fixation for plant community response to altered environmental conditions. Ecosystem being studied A constructed old field ecosystem meant to represent abandoned agricultural land in eastern Tennessee. Old field ecosystems are common and widespread around much of the world. The aboveground plant community that characterizes old fields is typically dominated by short-statured, relatively short-lived plants. This facilitates whole-ecosystem manipulation and analysis within reasonable, but ecologically meaningful, time frames. The diverse species composition of old-fields enables the study of how different species respond to different environmental drivers, and to consider how biotic interactions, such as competition or facilitation, affect the response of the community to interacting global change factors.
Results Measurements of biological nitrogen fixation (BNF) in the constructed old fields indicated higher soil moisture increased both asymbiotic and symbiotic N2-fixation. The estimated maximum annual amount of asymbiotic N2-fixation in surface soils (i.e., less than 2 mg N per square meter per year) was negligible relative to symbiotic N2-fixation by resident legumes or other local sources of N input. Semi-quantitative measurements of symbiotic N2-fixation by Trifolium and Lespedeza indicated that more than 80% of their aboveground tissue N concentration was derived from BNF which may partly explain the success of legumes in competition with non-N2-fixing plants during the first 2 years of the experiment. Two non-N2-fixing species had N isotope signatures that indicated some of their N acquisition was by the root uptake of symbiotically fixed N (i.e., rhizosphere transfer of N from N2-fixing plants). It was estimated that approximately 50% of aboveground N during the 2003 growing season was derived from symbiotic N2-fixation by the two legume species in the old field community. Biological N2-fixation may be a key determinant of plant community response to changes in atmospheric CO2 concentration, temperature, and soil water availability in the constructed old fields. Why this is important Interactive effects of altered atmospheric CO2 concentration, warming, and altered soil water availability have been studied in relatively few ecosystems despite the likelihood that global climatic change will include multiple environmental effects. Nitrogen-fixing species could be at a competitive advantage, relative to non-N2-fixing plants, under some climatic change scenarios. A greater success of legumes in a future climate could have important effects on herbaceous-plant community composition and ecosystem biogeochemical cycling.
Methods Artificial (constructed) old field communities were established in 4-m diameter open-top chambers as part of the old field community clmate and atmospheric manipulation study at the DOE Oak Ridge National Laboratory in east Tennessee. The following species were planted during July, 2002, and April, 2003: (1) Trifolium pratense, (2) Lespedeza cuneata, (3) Plantago lanceolata, (4) Festuca elatior, (5) Andropogon virginicus, (6) Solidago canadensis, and (7) Dactylis glomerata. Both Trifolium and Lespedeza are N2-fixers. The remaining five species are non-N2-fixing and rely solely on root uptake of N from the soil. The experiment was based on a split plot design with two levels (elevated and ambient) of CO2 concentration and temperature. Elevated air temperatures are maintained at about 3 degrees C above ambient and elevated CO2 treatments are maintained at about 300 ppm above ambient. Two moisture regimes are also applied within each chamber (i.e., the split-plot). Water is added to half the plot to maintain approximately 5% difference in surface soil volume water content. The experimental treatments commenced in May, 2003. Potential asymbiotic N2-fixation was measured using surface (12 cm deep) mineral soil samples and acetylene (C2H2) reduction and/or 15-N-tracer methods in laboratory incubations. Symbiotic N2-fixation was measured by 15-N-natural abundance methods using plant samples collected at the end of the 2003 growing season. The isotopic fractionation associated with N2-fixation was determined by growing legumes in a greenhouse under N-free conditions. Future plans include 15-N-labeling of individual plants during the 2006 growing season to determine the rhizodeposition of legume-derived N and its transfer to non-N2-fixing species under the different experimental treatments. Recent key publications There are no publications yet from this young project. Other program publications are listed on the Publications Page. Personnel Charles T. Garten Jr., Oak Ridge National Laboratory Funding period: May 2004 to present |
