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ECOSYSTEMS Ecosystems 2000 3 238 253 DOI 10 1007 s100210000023 r 2000 Springer Verlag Long Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests Alison H Magill 1 John D Aber 1 Glenn M Berntson 1 William H McDowell 2 Knute J Nadelhoffer 3 Jerry M Melillo 3 and Paul Steudler3 1Complex Systems Research Center University of New Hampshire Durham New Hampshire 03824 2Department of Natural Resources University of New Hampshire Durham New Hampshire 03824 3The Ecosystem Center Marine Biological Laboratory Woods Hole Massachusetts 02354 USA ABSTRACT This article reports responses of two different forest ecosystems to 9 years 1988 96 of chronic nitrogen N additions at the Harvard Forest Petersham Massachusetts Ammonium nitrate NH4NO3 was applied to a pine plantation and a native deciduous broad leaved hardwood forest in six equal monthly doses May September at four rates control no fertilizer addition low N 5 g N m 2 y 1 high N 15 g N m 2 y 1 and low N sulfur 5 g N m 2 y 1 plus 7 4 g S m 2 y 1 Measurements were made of net N mineralization net nitrification N retention wood production foliar N content and litter production soil C and N content and concentrations of dissolved organic carbon DOC and nitrogen DON in soil water In the pine stand nitrate losses were measured after the first year of additions 1989 in the high N plot and increased again in 1995 and 1996 The hardwood stand showed no significant increases in nitrate leaching until 1995 high N only with further increases in 1996 Overall N retention efficiency percentage of added N retained over the 9 year period was 97 100 in the control and low N plots of both stands 96 in the hardwood high N plot and 85 in the pine high N plot Storage in aboveground biomass fine roots and soil extractable pools accounted for only 16 32 of the added N retained in the amended plots suggesting that the one major unmeasured pool soil organic matter contains the remaining 68 84 Short term redistribution of 15N tracer at



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