PARENT SESSION
Oral Session #98: Carbon and Nitrogen Cycling: N-addition, N-flux.
Presiding: C. Hoover
Friday, August 9. 9:00 AM to 12:00 PM. Greenlee Meeting Room, TCC.
The weighted C/N stoichiometries of ecosystem pools that retain elevated N deposition in temperate forests .
Currie, William*,1, Nadelhoffer, Knute2, Gundersen, Per3, 1 University of Maryland Center for Environmental Science Appalachian Laboratory, Frostburg, MD2 The Ecosystems Center, Woods Hole, MA3 Danish Forest and Landscape Research Institute, Horsholm, Denmark
ABSTRACT- Projections of the weighted C/N stoichiometry of N retention at the ecosystem level were made using a biogeochemical model that was tested through model-data comparisons of 15N tracer recoveries at three forest sites. Sites included pine and oak forests at the Harvard Forest, MA, mixed deciduous-coniferous forest at Bear Brooks Watershed, ME, and a spruce forest at Klosterhede, Denmark. The weighted C/N ratio was calculated from the C/N ratio in each ecosystem pool where N inputs were retained, together with the proportion of N inputs retained in each pool. Different forests retain elevated N inputs to varying degrees in living wood, foliage, fine roots, dead wood, litter, and soil humus and organic matter. Within a forest, a 15N-labelled pulse of N inputs redistributes among these pools over time. Recovery of large-scale 15N tracers allowed verification and refinement of model calculations of proportions of N inputs entering each ecosystem pool, both initially and over time scales up to 9 years. In these forests, projected values of weighted C/N stoichiometries of N retention ranged from 20 to 43 over the period of 1 to 100 yr, differing among forest types and rates of N input. Weighted C/N stoichiometries of the retention of a labelled cohort of N inputs showed complex patterns over time. The weighted C/N first increased over 5 to 50 yr, as some soil-immobilized N was taken up into trees, then generally decreased as N in litterfall became incorporated into stable pools of soil organic matter. Projections indicated that overall long-term C sequestration resulting from elevated N deposition will be limited by the narrow C/N ratios of most ecosystem pools where N is retained.
KEY WORDS: stable isotope, nitrogen retention, carbon sequestration, temperate forests