Articles | Volume 11, issue 12
Biogeosciences, 11, 3323–3337, 2014
Biogeosciences, 11, 3323–3337, 2014

Research article 23 Jun 2014

Research article | 23 Jun 2014

Nitrogen inputs and losses in response to chronic CO2 exposure in a subtropical oak woodland

B. A. Hungate1, B. D. Duval1,5, P. Dijkstra1, D. W. Johnson2, M. E. Ketterer3, P. Stiling4, W. Cheng6, J. Millman7, A. Hartley8, and D. B. Stover9 B. A. Hungate et al.
  • 1Department of Biological Sciences and Center for Ecosystem Science Society, Northern Arizona University, Flagstaff, AZ 86011, USA
  • 2Department of Environmental and Resource Sciences, University of Nevada-Reno, Reno, NV 89557, USA
  • 3Department of Chemistry and Biochemistry, Northern Arizona University, Flagstaff, AZ 86011, USA
  • 4Department of Biology, University of South Florida, Tampa, FL, USA
  • 5US Dairy Forage Research Center, USDA-ARS, Madison, WI 53706, USA
  • 6Faculty of Agriculture, Yamagata University, 1–23, Wakaba-cho, Tsuruoka, Yamagata, 997-8555, Japan
  • 7Horace Mann Bronx Campus, Bronx NY 10471, USA
  • 8Department of Marine and Ecological Sciences, Florida Gulf Coast University, Fort Myers, FL 33965-6565, USA
  • 9Department of Energy, Office of Biological and Environmental Research, US Department of Energy, Washington, DC 20585, USA

Abstract. Rising atmospheric CO2 concentrations may alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 years of elevated atmospheric CO2 concentrations. We also explored the role of herbivory, carbon limitation, and competition for light or nutrients in shaping the response of N fixation to elevated CO2. Elevated CO2 did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term 15N tracer indicated that elevated CO2 increased leaching losses. Elevated CO2 had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO2 tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response of N fixation to elevated CO2. These results indicate that positive responses of N fixation to elevated CO2 may be transient and that chronic exposure to elevated CO2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO2 may overestimate future N accumulation in the biosphere.

Final-revised paper