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Harvard Forest Symposium Abstract 2010

  • Title: 15N Tracer Recoveries in the Harvard Forest Chronic N Plots 16 Years after Labeling as Constraints on Nitrogen Deposition Effects on Forest Carbon Storage
  • Primary Author: Knute Nadelhoffer (University of Michigan (all campuses))
  • Additional Authors: Alexis DeGabriele (University of Michigan (all campuses)); Serita Frey (University of New Hampshire - Main Campus); Michael Grant (University of Michigan); Jim LeMoine (University of Michigan (all campuses))
  • Abstract:

    We analyzed forest floor, soil, and biomass samples collected from the Chronic N Addition Study plots in 2008 for 15N, after 20 years of N additions (0 and 50 kg N ha-1 yr-1) and 16 years following 2 years of 15N labeling, in order to track long-term fates of tracer additions and assess N deposition effects on forest C balances. Fertilizer was applied at 50 kg NH4NO -N ha-1 yr-1 to 30m x 30m plots in a red pine and an oak forest from 1988 through the present. 15N tracers were applied in 1991 and 1992 as either 15NH4 or 15NO3 to halves of control (background ~8 kg N ha-1 yr-1) and fertilized (background + fertilizer = 58 kg N ha-1 yr-1) plots. Sample 15N values, N pool sizes, and mass balances were used to trace decadal scale movements of N deposition into ecosystem pools under background and elevated N additions and to estimate the effects of chronic N additions on forest ecosystem C storage. Tree biomass was a weak but persistent sink for N additions, with 15N tracer recoveries in total tree biomass (wood, bark, roots, and foliage) after 16 years ranging from 8.1 to 14.9% of total tracer additions across forest types, N addition rates, and tracer forms (15NH4 and15NO3). Tracer recoveries in fine roots decreased from 1992 through 2008 in both oak and pine forests under all treatments, whereas recoveries in foliage decreased in oak but either remained constant or increased slightly between 1998 and 2008 in red pine. Woody tissues continued to accumulate 15N through the 16 years following tracer additions; percent tracer recoveries in pine forest wood+bark increased between 1998 and 2008 from 1.4 to 3.0% under ambient deposition and from 1.6 to 3.0% with fertilization. Percent recoveries in oak plots increased from 4.2 to 6.0% under ambient deposition and from 6.0 to 6.6% with fertilization. Also, 15N was mobilized from older wood to younger wood and bark across all treatments, even as the total recovery in wood increased through time. Overall, tracer recoveries in soils and forest floors were much higher than in tree biomass, ranging from 49 to 101% of additions across forest types and N addition rates (Figures 1 and 2). Stoichiometric analyses of these recoveries suggest that N additions are contributing to soil C accumulation to a greater extent than to biomass accumulation in these forests.

  • Research Category: Forest-Atmosphere Exchange
    Group Projects
    Large Experiments and Permanent Plot Studies
    Soil Carbon and Nitrogen Dynamics

  • Figures:
  • KN_HFLTER2010_Fig1.pdf
    KN_ HFLTER2010_Fig2.pdf