HR: 09:30h
AN: B51B-07    [PDF]
TI: The Impact of Elevated CO2 on Soil C and N Cycling as Revealed by Decomposition of 15N-13C Labelled Residues in a Pasture Soil Exposed to FACE Conditions for 9 Years.
AU: * de Graaff, M
EM: mdegraaff@ucdavis.edu
AF: University of California-Davis, Dept. of Agronomy and Range Science, One shields avenue, davis, CA 95616 United States
AU: * de Graaff, M
EM: mdegraaff@ucdavis.edu
AF: Wageningen University, Dept. of Environmental Sciences, Laboratory of Soil Science and Geology, Wageningen, 6700 AA Netherlands
AU: Six, J
EM: jwsix@ucdavis.edu
AF: University of California-Davis, Dept. of Agronomy and Range Science, One shields avenue, davis, CA 95616 United States
AU: Harris, D
EM: dharris@ucdavis.edu
AF: University of California-Davis, Stable Isotope Facility, One shields avenue, Davis, CA 95616 United States
AU: Blum, H
EM: herbert.blum@ ipw.agrl.ethz.ch
AF: Swiss Federal Institute of Technology, Institute of Plant Sciences, Zurich, 555 Switzerland
AU: van Kessel, C
EM: cvankessel@ucdavis.edu
AF: University of California-Davis, Dept. of Agronomy and Range Science, One shields avenue, davis, CA 95616 United States
AB: The effect of prolonged elevated atmospheric CO2 on soil C and N cycling remains a widely debated topic. Elevated atmospheric CO2 may alter decomposition rates through changes in plant residue quality and through its impact on soil microbial activity. This study examines whether plant residues produced under elevated CO2 decompose differently than residues produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C-cycling under elevated CO2 made in coupled climate-SOM models. Trifolium repens and Lolium perenne residues, produced under elevated and ambient CO2, at two levels of N fertilizer were incubated in soil for 90 days. Soils and residues used for the incubation had been exposed to ambient and elevated CO2 under Free Air Carbon Enrichment (FACE)-conditions and had received 15N-N-fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens residues was unaffected by elevated atmospheric CO2 and rate of N fertilization; soil respiration and recovery of 15N in the soils was independent whether the residues were grown under ambient or elevated CO2. Any changes in litter C:N ratio due to elevated CO2, did not affect residue decomposition. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, it was not reflected in the rate of residue decomposition. Only respiration of L. perenne soil, following low N fertilization was enhanced after exposure to elevated CO2, irrespective of the addition of ambient or elevated residues. This increase in respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOM-C pools as described by multi-pool SOM models. The results appear not to confirm the assumption of a negative feedback induced in the C-cycle by elevated CO2 used in coupled climate-SOM models following an increase in elevated CO2. Moreover, this study showed no evidence for a positive feedback in the C-cycle following additional N fertilization.
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting