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Harvard Forest Symposium Abstract 2011

  • Title: Long-term N addition alters forest floor root mass in hardwood and pine forests
  • Primary Author: Richard Bowden (Allegheny College)
  • Additional Authors: Serita Frey (University of New Hampshire - Main Campus); Jim LeMoine (University of Michigan (all campuses)); Knute Nadelhoffer (University of Michigan (all campuses)); Sarah Wurzbacher (Allegheny College)
  • Abstract:

    Forests typically respond to nitrogen additions with increased productivity, hence a long-held paradigm is that chronic additions of anthropogenically-derived atmospheric deposition would have positive ecosystem effects. However, twenty years of work at the Harvard Forest Chronic N Deposition plots, and in other forest ecosystems as well, have shown that that this simplistic view is incomplete. For example, enhanced ammonium uptake increases soil acidity, leading to mobilization of aluminum and loss of nutrient cations (e.g. Mg2+, Ca2+, and K+ ) both of which influence root mass, turnover, and activity.







    To address long-term (20 year) impacts of N additions on forest root mass, we removed O-horizon (forest floor) samples from the Chronic N hardwood and red pine stands to quantify the total mass of roots. In 2008, we collected three to six 20x20cm O horizon samples from of each of the Control, Low N (50 kg N ha-1y-1), and High N (150 kg N ha-1y-1) plots, in both the hardwood and red pine stands. Samples were frozen immediately after collection. For analysis, forest floor samples were thawed, and roots were extracted manually from each sample, washed, and divided into the following diameter classes: >2 mm, >1,<2mm, <1mm; roots were kept moist during the sorting process, and subsequently dried (105oC) for mass analysis.







    We found that long-term N additions had contrasting results in the two forests. In the hardwood plots, total root mass (< 2mm) increased from 0.167 + 0.026 (S.E.) kg m-2 in the control plot to 0.434 + 0.170 kg m-2 in the high N plots. In contrast, in the red pine stand, roots declined from 0.074 + 0.011 (S.E.) kg m-2 in the control plot to 0.031 + 0.016 kg m-2 in the high N plots. These data are in agreement with data for aboveground productivity at the Chronic N plots, which show stimulated growth in the hardwoods and severe growth declines and enhanced mortality in the pine stand.



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

  • Figures:
  • HF 20 year FF root mass.pdf