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Harvard Forest Symposium Abstract 2010

• Title: Carbon Balance and Dynamics in Response to Twenty Years of Nitrogen
• Primary Author: Serita Frey (University of New Hampshire - Main Campus)
• Additional Authors: Richard Bowden (Allegheny College); Edward Brzostek (Indiana University - Bloomington); Bruce Caldwell (Oregon State University); Susan Crow (University of Hawaii - Manoa); Michelle Day (University of New Hampshire - Main Campus); Adrien Finzi (Boston University); Kate Lajtha (Oregon State University); Jim LeMoine (University of Michigan (all campuses)); Rich MacLean (University of New Hampshire - Main Campus); Mary Martin (University of New Hampshire - Main Campus); William McDowell (University of New Hampshire - Main Campus); Knute Nadelhoffer (University of Michigan (all campuses)); Scott Ollinger (University of New Hampshire - Main Campus); Pamela Templer (Boston University)
• Abstract:

  Anthropogenic emissions of reactive nitrogen to the atmosphere (primarily due to fertilizer production, ammonia production for industrial purposes, and fossil fuel combustion) and the subsequent deposition to Earth’s surface have increased nearly 200% since the beginning of the industrial revolution and are projected to double yet again by 2050 (Gallaway et al. 2008). Nitrogen deposition rates over large regions of the world now exceed 10 kg N ha-1 yr-1, more than an order of magnitude higher than natural background levels (Galloway et al. 2008). This fertilization of historically nitrogen-limited ecosystems has the potential to saturate biotic demand for nitrogen thereby producing a cascade of negative impacts on ecosystem processes. Following two decades of research on the nitrogen saturation phenomenon, known effects of nitrogen deposition on temperate ecosystems include increased nitrate leaching to groundwater, enhanced trace gas (i.e., nitrous and nitric oxide) emissions to the atmosphere, altered ecosystem carbon storage, nutrient imbalances (e.g., declines in tree Ca:Al and Mg:N ratios), shifts in plant community composition, and ecosystem (namely forest) decline.
  
  
  
  The recent scientific discussion on nitrogen deposition effects has focused on the potential relationship between nitrogen deposition and carbon sequestration (Mack et al. 2004; Magnani et al. 2007; de Vries et al. 2009). It appears that this relationship is typically positive for temperate and boreal forests, with above-ground accumulation of carbon in forests generally falling within the range of 15 to 40 kg C stored per kg N added via deposition (de Vries et al. 2009). Carbon storage in forest soils also tends to be enhanced by N deposition (Pregitzer et al. 2008; Zak et al. 2008), with the storage response ranging from 5 to 35 kg C per kg N added (de Vries et al. 2009).
  
  
  
  The Harvard Forest Chronic Nitrogen Addition Study is one of the few on-going, long-term N saturation experiments (Aber et al., 1989, 1998; Aber and Magill, 2004). Located in an old red pine plantation and a mixed hardwood forest the treated plots have received 50 and 150 kg N ha-1 yr-1, as NH4NO3, in six equal monthly applications during the growing season each year since the start of the experiment in 1988. Regular measurements have been made over the past 20 years to assess woody biomass production and mortality, foliar chemistry, litter fall, and soil N dynamics. Less frequent measurements of soil C pools, soil respiration, fine root dynamics, and microbial biomass and community structure have been made. An intensive sampling campaign was carried out in fall 2008 with a focus on evaluating how the long-term N additions have impacted ecosystem C storage and N dynamics. Our primary objective was to assess the amount of C and N stored in wood, foliage, litter, roots, and soil (to a depth of ~50 cm). Results from Harvard Forest indicate that chronic nitrogen additions to a mixed hardwood forest stand have increased forest floor mass and soil carbon stocks across the soil profile; suppressed litter decay; decreased microbial biomass, especially the fungal component; decreased microbial enzyme activity, particularly for enzymes responsible for protein and lignin decay; and altered microbial community structure. These results, along with those from several other recent studies, suggest that microbial responses to long-term nitrogen additions (either via fertilization or nitrogen deposition) play an important role in the observed carbon accumulation in these soils.
  
  
  
  References
  
  Aber JD and Magill AH. 2004. Chronic nitrogen additions at the Harvard Forest (USA): the first 15 years of a nitrogen saturation experiment. Forest Ecology & Management 196: 1-5.
  
  
  
  Aber JD, Nadelhoffer KJ, Steudler P, and Melillo JM 1989. Nitrogen saturation in northern forest ecosystems—hypotheses and implications. BioScience 39: 378–386.
  
  
  
  Aber JD, McDowell WH, Nadelhoffer KJ, Magill A, Berntson G, Kamakea M, McNulty SG, Currie W, Rustad L, Fernandez I. 1998. Nitrogen saturation in temperate forest ecosystems: hypotheses revisited. BioScience 48: 921–934.
  
  
  
  de Vries W, Solberg S, Dobbertin M, Sterba H, Laubhann D, van Oijen M, Evans C, Gundersen P, Kros J, Wamelink GWW, Reinds GJ, and Sutton MA. 2009. The impact of nitrogen deposition on carbon sequestration by European forests and heathlands. Forest Ecology & Management 258: 1814-1823.
  
  
  
  Galloway JN, Townsend AR, Erisman JW, Bekunda M, Cai Z, Freney JR, Martinelli LA, Seitzinger SP, and Sutton MA. 2008. Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320: 889-892.
  
  
  
  Mack MC, Schuur EAG, Bret-Harte MS, Shaver GR, and Chapin FS. 2004. Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization. Nature 431: 440-443.
  
  
  
  Magnani F, Mencuccini M, Borghetti M, Berbigier P, Berninger F, Delzon S, Grelle A, Hari P, Jarvis PG, Kolari P, Kowalski AS, Lankreijer H, Law BE, Lindroth A, Loustau D, Manca G, Moncrieff JB, Rayment M, Tedeschi V, Valentini R, Grace J, 2007. The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 848–850.
  
  
  
  Pregitzer KS, Burton AJ, Zak DR, and Talhelm AF. 2008. Simulated chronic N deposition increases carbon storage in northern temperate forests. Global Change Biology 14:142–153.
  
  
  
  
  
  

• Research Category: Large Experiments and Permanent Plot Studies
  Soil Carbon and Nitrogen Dynamics