HR: 17:00h
AN: B34A-05    [Abstracts]
TI: Modelling Plant and Soil Nitrogen Feedbacks Affecting Forest Carbon Gain at High CO₂
AU: * McMurtrie, R E
EM: r.mcmurtrie@unsw.edu.au
AF: School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, NSW 2052, Australia
AU: Norby, R J
EM: norbyrj@ornl.gov
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831- 6422, United States
AU: Franklin, O
EM: franklin@iiasa.ac.at
AF: Institute for Applied Systems Analysis, IIASA, Laxenburg, AUT 2361, Austria
AU: Pepper, D A
EM: d.a.pepper@unsw.edu.au
AF: School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, NSW 2052, Australia
AB: Short-term, direct effects of elevated atmospheric CO₂ concentrations on plant carbon gain are relatively well understood. There is considerable uncertainty, however, about longer-term effects, which are influenced by various plant and ecosystem feedbacks. A key feedback in terrestrial ecosystems occurs through changes in plant carbon (C) allocation patterns. For instance, if high CO₂ were to increase C allocation to roots, then plants may experience positive feedback through improved plant nutrition. A second type of feedback, associated with decomposition of soil-organic matter, may reduce soil-nutrient availability at high CO₂. This paper will consider mechanistic models of both feedbacks. Effects of high CO₂ on plant C allocation will be investigated using a simple model of forest net primary production (NPP) that incorporates the primary mechanisms of plant carbon and nitrogen (N) balance. The model called MATE (Model Any Terrestrial Ecosystem) includes an equation for annual C balance that depends on light- saturated photosynthetic rate and therefore on [CO₂], and an equation for N balance incorporating an expression for N uptake as a function of root mass. The C-N model is applied to a Free Air CO₂ Exchange (FACE) experiment at Oak Ridge National Laboratory (ORNL) in Tennessee, USA, where closed-canopy, monoculture stands of the deciduous hardwood sweetgum ( Liquidambar styraciflua) have been growing at [CO₂] of 375 and 550 ppm for ten years. Features of this experiment are that the annual NPP response to elevated CO₂ has averaged approximately 25% over seven years, but that annual fine-root production has almost doubled on average, with especially large increases in later years of the experiment (Norby et al. 2006). The model provides a simple graphical approach for analysing effects of elevated CO₂ and N supply on leaf/root/wood C allocation and productivity. It simulates increases in NPP and fine-root production at the ORNL FACE site that are consistent with experimental measurements. Increased below-ground C allocation has been observed at other forest high-CO₂ experiments including the Duke FACE and Flakaliden experiments in North Carolina, USA, and Sweden, respectively. This result is predicted by MATE, but not by biogeochemical-cycling models such as our plant-soil model G'DAY (Generic Decomposition And Yield), which predicts instead that a positive CO₂ response will enhance litter quantity, and hence will increase soil N immobilisation and reduce the pool of N available for plant uptake (Pepper et al. 2007). We will use G'DAY and MATE to determine the sensitivity of the modelled CO₂ response to key model parameters on contrasting timescales. One conclusion is that models are required that simulate both increased N uptake at high CO₂ as a consequence of increased root production and soil N-cycling feedbacks. References: Norby RJ, Wullschleger SD, Hanson PJ, Gunderson CA, Tschaplinski TJ, Jastrow JD (2006) CO₂ enrichment of a deciduous forest: the Oak Ridge FACE experiment. Ecological Studies 187: 231-251 (Springer-Verlag, Berlin). Pepper DA, Eliasson PE, McMurtrie RE, Corbeels M, Ågren GI, Strömgren M, Linder S (2007) Simulated mechanisms of soil N feedback on the forest CO₂ response. Global Change Biology 13: 1265-1281.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0466 Modeling
DE: 0469 Nitrogen cycling
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting