Monday, August 4, 2008

PS 4-65: Elevated atmospheric CO2 concentrations do not alter net nitrogen mineralization rates in a CO2 enriched sweetgum forest

Aimée T. Classen1, Colleen M. Iversen2, Caitlin R. Guthrie3, and Richard J. Norby1. (1) Oak Ridge National Laboratory, (2) University of Tennessee, Knoxville, (3) Pomona College


Carbon dioxide concentrations ([CO2]) in the atmosphere have increased by 36% in the last 250 years. Previous research has demonstrated that elevated levels of atmospheric [CO2] increase forest production and N demand in temperate ecosystems, and soil N availability may constrain the response of forests to elevated [CO2]. Understanding how soil N cycling responds to elevated [CO2] in forests will enable scientists to make better predictions of how forests will respond to climatic changes in the future. We took advantage of a long-term Free-Air CO2-Enrichment experiment at Oak Ridge National Laboratory (ORNL) to test our hypothesis that elevated [CO2] would decrease net N mineralization. We incubated soil cores during the growing season of 2007 for one-month intervals, and measured the difference in NH4 and NO3 concentrations and inorganic N leaching from the beginning to the end of the incubation. 


Our results indicate that there is no difference in net N mineralization rates, net N nitrification rates, or leaching between elevated and ambient plots. However, there was a significant difference in leaching rates for both treatments across the four months. These results may indicate that increased forest production under elevated [CO2] has not altered the net amount of N available for tree uptake at ORNL FACE. Alternatively, the lack of response to elevated [CO2], and concurrent temporal response could reflect the low levels of precipitation (the third lowest on record since 1895) observed over the course of this study. We will also present preliminary results on a gross nitrogen mineralization experiment.