Long-Term Nitrogen Additions andNitrogen Saturation in TwoTemperate ForestsAlison H. Magill,1* John D. Aber,1Glenn M. Berntson,1William H. McDowell,2Knute J. Nadelhoffer,3Jerry M. Melillo,3and Paul Steudler31Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824;2Department of NaturalResources, University of New Hampshire, Durham, New Hampshire 03824;3The Ecosystem Center, Marine Biological Laboratory,Woods Hole, Massachusetts 02354, USAABSTRACTThis article reports responses of two different forestecosystems to 9 years (1988–96) of chronic nitrogen(N) additions at the Harvard Forest, Petersham,Massachusetts. Ammonium nitrate (NH4NO3) wasapplied to a pine plantation and a native deciduousbroad-leaved(hardwood)forestinsix equalmonthlydoses (May–September) at four rates: control (nofertilizer addition), low N (5gNm-2y-1), high N (15gNm-2y-1), and low N ⫹ sulfur (5gNm-2y-1plus7.4gSm-2y-1). Measurements were made of net Nmineralization, net nitrification, N retention, woodproduction, foliar N content and litter production,soil C and N content, and concentrations of dis-solvedorganiccarbon (DOC)andnitrogen (DON)insoil water. In the pine stand, nitrate losses weremeasured after the first year of additions (1989) inthe high N plot and increased again in 1995 and1996. The hardwood stand showed no significantincreases in nitrate leaching until 1995 (high Nonly), with further increases in 1996. Overall Nretention efficiency (percentage of added N re-tained) over the 9-year period was 97–100% in thecontrol and low N plots of both stands, 96% in thehardwood high N plot, and 85% in the pine high Nplot. Storage in aboveground biomass, fine roots,and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots,suggestingthattheonemajor unmeasuredpool,soilorganic matter, contains the remaining 68–84%.Short-term redistribution of15N tracer at naturalabundance levels showed similar division betweenplantand soilpools.Directmeasurementsofchangesin total soil C and N pools were inconclusive due tohigh variation in both stands. Woody biomass pro-duction increased in the hardwood high N plot butwas significantly reduced in the pine high N plot,relative to controls. A drought-induced increase infoliar litterfall in the pine stand in 1995 is onepossible factor leading to a measured increase in Nmineralization, nitrification, and nitrate loss in thepine high N plot in 1996.Key words: ammonium nitrate; biomass produc-tion; foliar chemistry; net mineralization; net nitrifi-cation; nitrogen deposition;nitrogensaturation;soilsolution chemistry.INTRODUCTIONN deposition was first proposed as a possible threatto forest ecosystems nearly 15 years ago (Nihlga˚rd1985). Since that time, a number of coordinatedstudies in Europe under the NITREX and EXMANprograms (Wright and van Breemen 1995; Wrightand Rasmussen 1998) and individual experimentsin the US (for example, Christ and others 1995;Magill and others 1996, 1997; McNulty and others1996; Peterjohn and others 1996; Fenn and others1998; Norton and Fernandez 1999) have providedmuch of the data necessary for an increased under-Received 2 April 1999; Accepted29 October 1999.*Corresponding author; e-mail: [email protected] (2000) 3:238–253DOI: 10.1007/s100210000023ECOSYSTEMSr2000 Springer-Verlag238standing of the process of N saturation. In a recentarticle (Aber and others 1998), the results of Euro-pean and North American studies were summarizedinto an inclusive theory where N saturation isdefined as a series of nonlinear changes in keyecosystem processes in response to elevated nitro-gen inputs. The primary indicators of N saturationinclude increases in nitrate leaching losses, netnitrification rates, and foliar N content, and aninitial increase and subsequent decline in both netnitrogen mineralization and net primary productiv-ity. Observation and measurement of such changescan be used to estimate the stage of N saturation,with each stage being characterized by a set ofecosystem response variables (see Aber and others1989, 1998).In contrast to the similarities observed among theN deposition experiments mentioned above is asignificant amount of variation in the sensitivity ofdifferent forest types to N deposition. For example,it appears that conifer stands move through thestages of N saturation more rapidly than broad-leaved deciduous forests (Stoddard 1994; Aber andothers 1998). Where a forest lies within the con-tinuum of N saturation is a function of severalfactors including community type, soil properties(especially C:N ratios; Gundersen and others 1998a;Gundersenandothers 1998b), land-usehistory,andthe rate and duration of N loading (Aber and others1998). Still, little is known about the mechanismsthat drive N retention and control the rate at whichN saturation proceeds.The Chronic Nitrogen Amendment experiment atthe Harvard Forest in central Massachusetts (USA)is one of the longest running N addition experi-ments to date and has provided some of the essen-tialdata forunderstandingecosystemresponsesto Nadditions (Aber and others 1993; Castro and others1995; Currie and others 1996; Magill and others1997; McDowell and others 1998; Magill and Aber1998, Berntson and Aber forthcoming; Nadelhofferand others 1999a; Yano and others 1998). Thecomparison of two forest types that have markedlydifferent sensitivities to N deposition, coupled withan experimental gradient of N addition rates, maketheChronicN Amendment experimentanexcellentplatformfor elucidatingthe mechanismsof Nimmo-bilization in response to N deposition.This article summarizes 9 years (1988–96) offorest ecosystem N cycling, N and C storage, andforest productivity in the Chronic Nitrogen Amend-ment experiment. The long-term record of N fluxesand storage allowed us to examine several impor-tant questions including: (a) how does interannualclimatic variability, including extreme events, suchas drought, influence the ontogeny of N saturation;(b) can we discern whether rates of N deposition orcumulative N deposition have a greater impact onthe progression of N saturation?; (c) do differentforest types show a convergence or a divergence inresponses to long-term N deposition?METHODSStudy Site and Experimental DesignThe Chronic Nitrogen Addition experiment is lo-cated at the Harvard Forest in central Massachusetts(42o30’N, 72o10’W) and is one of the core experi-ments of the Harvard Forest Long-Term