PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Long-term redistribution of 15N tracers used to assess C/N interactions in vegetation, soil, and woody debris under varying N inputs in two temperate forests.
Currie, William1, Nadelhoffer, Knute2, Colman, Benjamin2, 1 2
ABSTRACT- A key question in the study of forest C and N cycling is whether elevated N deposition is likely to increase C storage, particularly in living and dead wood. We recovered 8 year old 15N tracers in two temperate forests under ambient and N-amended conditions to test predicted 15N redistributions (made by the TRACE model) among vegetation and soil pools and among woody and non-woody pools. We measured 15N in foliage, roots, living bolewood at multiple heights, in canopy branches, soils, and fine woody debris. To provide a broader context for woody debris, we also related large-scale patterns in coarse woody debris to contrasting histories of land use and disturbance in the two forests. After 8 years, recovery of 15N decreased in foliage, while it increased in living wood since 1992 (the last year of tracer applications). Increases in N content of fine woody debris accounted for only 0.4% of elevated N inputs. N storage in living wood showed complex patterns with height and form of 15N tracer used. Patterns in woody debris indicated that plant-soil cycling operating over a decadal time scale or longer, rather than detrital immobilization, controls C:N ratios and N pool sizes in woody debris. Coarse woody debris stored 14 to 20 Mg C/ha and 146 to 155 kg N/ha while size and decay classes exhibited strong relationships to land use histories.
KEY WORDS: 15N tracers, Harvard Forest, C sequestration, N retention