Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA.

Academic Article

Abstract

  • Humans have altered the global and regional cycles of nitrogen more than any other element. Alteration of N cycling patterns and processes in forests is one potentially negative outcome of accelerated N deposition worldwide. To assess potential impacts of N deposition on temperate forests, a series of chronic nitrogen additions in two contrasting forest types (red pine plantation and mixed hardwood stand) were designed as a core experiment of the Harvard Forest (HF) Long-term Ecological Research (LTER) program. This paper describes the chronic N experimental study site in detail and presents the long-term baseline measurements established at the beginning of treatments in 1988.Results reported here continue or accelerate trends presented in previous papers. Losses of inorganic N remain high in the high N plots (higher in pines than hardwoods) and low N plots in the pine stand also have measurable DIN losses. Foliar and fine root N concentrations are elevated significantly. Mortality of red pine reached 56% by 2002 in the pine high N plot, and biomass accumulation has stopped altogether. The high N hardwood stand shows increased ANPP, but excess N availability and a severe drought in 1995 contributed to mortality of 72% of red maple trees by 2002. Species importance and litterfall patterns were altered in several plots after 1995. Roots, foliage and wood have diminished as net sinks for added N, re-emphasizing the role of soils in N retention. Two mechanisms for large net retention of added N were suggested in a review paper in 1998. Of these, abiotic immobilization is supported by a growing set of papers, while assimilation and re-exudation by mycorrhizae is suggested by increased DON concentrations.
  • Authors

  • Magill, AH
  • Aber, John
  • Currie, WS
  • Nadelhoffer, KJ
  • Martin, Mary
  • McDowell, William
  • Melillo, JM
  • Steudler, P
  • Status

    Publication Date

  • July 2004
  • Has Subject Area

    Published In

    Keywords

  • ammonium nitrate
  • biomass production
  • foliar chemistry
  • nitrogen deposition
  • nitrogen saturation
  • soil solution chemistry
  • Digital Object Identifier (doi)

    Start Page

  • 7
  • End Page

  • 28
  • Volume

  • 196
  • Issue

  • 1