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Harvard Forest Symposium Abstract 2005

• Title: Seeing the Forest for the Soils: Fate of an 15N Foliar N Addition in a Mature Spruce-Hemlock Stand, Howland, Maine
• Primary Author: Bryan Dail (University of Maine)
• Additional Authors: John Aber (University of New Hampshire - Main Campus); Eric Davidson (University of Maryland - Center for Environmental Science); David Hollinger (USDA Forest Service)
• Abstract:

  Human activities have more than doubled the quantity of biologically available nitrogen (N) in terrestrial ecosystems; however these inputs are spread unevenly and are concentrated in areas of intensive agriculture, industry, and population centers (Galloway et al., 1995; Townsend et al., 1996). In ecosystems where N supply exceeds biological demand, there is potential for ecosystem leaching and loss of soil nutrients and eutrophication of fresh and marine waters. In N limited systems, however, additional N inputs may increase plant productivity and thus, carbon (C) storage (Vitousek and Howarth, 1991). How much anthropogenic N contributes to long term C storage in woody plant biomass (which has the added benefit of being a valuable product) as opposed to becoming immobilized in other ecosystem compartments with higher turnover or less obvious value, such as soil, is still debated (Aber et al., 1995; Nadelhoffer et al., 1999; Schindler and Bayley 1993). Many N manipulation studies suggest that soil, and not plant biomass, is the primary sink for experimentally increased N (Aber et al., 1995; Nadelhoffer et al, 1999) and thus infer that the resultant C storage attributed to increased N availability worldwide would not explain much of the “unexplained” C sink in the northern hemisphere. The residence of added N in soil, others have argued, could be a result of the N being added directly to soils, which accounts for only part of ecosystem inputs. Forest canopies are known to assimilate wet and dry N deposition and this uptake mechanism may account for as much as 30% of the annual growth requirement in some trees (Schultze 1989, but see Bowden et al., 1989; Boyce et al, 1996). We sought to investigate the importance of foliar uptake in a mature spruce-hemlock forest in central Maine.
  
  
  In 2001 we began a wet foliar addition of 18-20 kg N/ha to a 21 ha plot centered on a Eddy covariance tower. The addition was as dissolved ammonium nitrate and was added 5-6 times during the growing season only. In 2 0.3 ha subplots, we made an additional foliar spray of 1% enriched ammonium or nitrate. In 2003 we sampled forest floor, soil, green foliage and woody biomass in the 15N subplots and determined ecosystem N pool sizes to estimate the fate of three years of N addition to this N limited site. Bulk N flux measurements made by TF collectors showed that 30-45% of experimental N was washed from the canopy on an annual basis and that about 30% of this flux was N spray that missed the canopy on the day of the application. Another 5-10% was lost from the canopy by gaseous processes (H. Seivering pers. comm.) yet only 3-6% was recovered in green foliage and bole wood. The largest recovery in measured pools was found in the O horizon (41% ammonium; 65% nitrate) and in recognizable litter (22% ammonium; 6% nitrate). 15N was not detected in underlying mineral soils and as yet has not been calculated for roots or woody biomass in the canopy. This latter sink may be important as red spruce branch and twig N assimilation has previously been shown to account for as much as 2-8% of annual N requirements of mature trees (Boyce et al., 1996).We were able to recover roughly 70 to 80% of our added label in the measured pools and thus the remaining N pools or ecosystem losses (leaching, denitrification) may yet confirm that soils are the short term sink for additional N.
  
  
  

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