Ž .Chemical Geology 152 1998 3–11

Patterns in N dynamics and N isotopes during primary successionin Glacier Bay, Alaska

Erik A. Hobbie ), Stephen A. Macko, Herman H. ShugartDepartment of EnÕironmental Sciences UniÕersity of Virginia, CharlottesÕille, VA 22903, USA

Abstract

The primary successional sequence in Glacier Bay, Alaska represents a 230-year record of the development of nitrogenŽ .N dynamics. Because of low inputs of N in precipitation and the absence of initial soil N pools, the pattern of Naccumulation is strongly biologically controlled. The simple successional sequence at Glacier Bay is dominated by two main

Ž .species Alnus sinuata and Picea sitchensis , thus the influence these species have on N dynamics is more easily deducedthan in more complex systems. Along a successional sequence in Glacier Bay, N mineralization rates, foliage and soil C:N,

15 Ž .and foliage and soil d N values in six sites ranging in age from 20 to 225 years old were examined. It is concluded that: 1Ž .Alnus sinuata and Dryas drummondii derived most of their N through the fixation of atmospheric N; 2 under conditions of

high N availability, differences among species in plant preference for ammonium or nitrate can be deduced from d15N

Ž . Ž .values; 3 over time, organic soil N separates into two isotopically distinct pools which differ in their turnover rate; 4 theŽ .transition from an alder-dominated to a spruce-dominated system results in slower N cycling; and 5 previous site

conditions are an important factor in explaining patterns in d15N values. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: N dynamics; Glacier Bay, Alaska; N isotopes

1. Introduction

Succession is a dominant paradigm in ecology.Vegetative changes are the most visible manifesta-tion of succession, but differences among soil re-sources can be critical in determining long-term pat-

Ž .terns of successional change Peet, 1992 . Primarysuccession refers to vegetative development on landthat has not previously supported vegetation, such aslava flows in Hawaii, sand dunes in Indiana, andland exposed by glacial retreat in Glacier Bay, AK.As only a small nutrient pool is initially available forplants in primary succession, the supply of nutrientsŽ .often nitrogen usually limits plant growth. Nitrogen

) Corresponding author.

is also unique among nutrients in the high degree towhich its cycling is biologically controlled. Thus, theinterplay between vegetation and N dynamics ismost evident during primary succession, where noappreciable reservoir of soil N exists prior to theestablishment of a plant community. The high degreeof biologic control over N dynamics, the lack ofprior N pools in primary succession, and the natureof N as the limiting nutrient in Glacier Bay shouldresult in a clear picture of how vegetation interactswith N dynamics during succession.

The gradual retreat of the glaciers over the last230 years in Glacier Bay has resulted in a series offorests whose successional age depends on the timeof glacial retreat. This sequence of forests has al-lowed a space-for-time substitution in successional

0009-2541r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 98 00092-8

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–114

Žstudies but see Fastie, 1994 for problems with this.technique . The forests that have developed are prob-

ably the best-studied example of primary successionin the world. The first colonizers are Populus tri-

Ž . Ž .chocarpa black cottonwood , Salix spp. willow ,and the nitrogen-fixing shrub Dryas drummondii.These pioneer species are quickly followed by the

Ž .N-fixing Alnus sinuata Sitka alder . Sitka alderdominates the early succession and is responsible forincorporating most of the nitrogen into the Glacier

Ž .Bay system Lawrence, 1979 . Alder and other de-ciduous species are eventually shaded out by Picea

Ž .sitchensis Sitka spruce , which dominates later suc-cession.

Recent work in Glacier Bay has focused on areassessment of soil nutrients and vegetational

Žbiomass during succession Bormann and Sidle,. Ž1990 , successional mechanisms Chapin et al.,.1994 , and computer modeling of forest dynamics

Ž .Weishampel, 1994 . This present study documentsthe results of an investigation of the interactionsbetween N dynamics and vegetation at six sitesranging in age from 20 to 230 years old along theGlacier Bay successional sequence. Of the six sites,

Ž .the youngest Site 1 is dominated by Dryas–Popu-lus–Salix, the next three by Alnus, and the two

Ž 15oldest by Picea. Data on C:N and N isotopes d N.values in foliage and soil along with N mineraliza-

tion rates in O horizon soils were used to identifycontrols on N supply to plants and N mineralizationin soils.

2. Methods

ŽGlacier Bay is located in south-east Alaska 598N,.1368W , at the northern end of the temperate rainfor-

est that stretches from Oregon to southern Alaska.Precipitation averages 183 cm yearly, with 60% ofprecipitation falling from August to November. Ithas a cool maritime climate, with an average summertemperature at Bartlett Cove of 128C and an averagewinter temperature of y1.78C.

Five sites were selected in 1991, representing thethree major successional stages and a range of site

Ž .ages. A sixth site Site 4 was added in 1992 toŽrepresent the transition from alder to spruce Fig. 1,

.Table 1 . Three of the locations chosen, Sites 1, 2,

and 5, had been previously investigated by FastieŽ .1994 for successional history. Sites 3 and 6 were

Ž .stem-mapped by Weishampel 1994 in a modelingstudy of forest dynamics.

Site 1, Muir Inlet, is sparsely vegetated by plantswith light, wind-dispersed seeds. Woody vegetationconsisted of scattered Salix spp. and P. trichocarpaaveraging 40 cm high, interspersed with mats of thenitrogen-fixing shrub D. drummondii. Dryas occu-pied around 10% of the ground surface area. Noorganic horizon is present, unlike all other sites. Site2, Goose Cove, is dominated by A. sinuata withsome P. trichocarpa as well as minor numbers of S.sitchensis, S. alaxensis, and Pi. sitchensis. Site 3,Adams Inlet, is fairly similar to the Goose Cove site,although Salix spp. are more prominent. Alder stemsare somewhat larger than at Goose Cove, averaging

Ž .5.5 cm versus 4.5 cm diamater at breast height dbh .Site 4, Muir Point, is the most heterogeneous invegetation composition, with large spruce, -40 cmdbh, patchily interspersed within dense alder. Largespruce have shaded out other trees immediately be-neath them. Site 5, Beartrack Cove, is dominated byspruce up to 50 cm dbh, with some western hem-

Ž .locks Tsuga heterophylla in the understory. Vac-cinium oÕalifolium and occasional alder are alsopresent at low densities. A thick moss mat covers theground at this site as well as Site 6, Bartlett Cove.Site 6 is dominated by spruce up to 90 cm dbh. Fewspruce less than 20 cm dbh are present, althoughhemlock occur at all sizes up to 40 cm dbh. Vac-cinium covers up to 50% of the ground surface.

At each site, all woody stems greater than 1 cmdbh were recorded. The plot size for vegetationsampling was 600 m2 for Site 1 and 900 m2 for all

Ž .other sites Table 1 . Foliage samples were collectedfrom five specimens of Dryas, alder, willow, cotton-wood, and spruce. Organic soil horizons at each site

Ž .were also sampled ns5 . Samples were oven-driedat 408C. Vegetation samples were ground in a Wileymill to pass through a 40 mesh screen prior toanalysis. Soil O horizons were separated from litterlayers, passed through a 1 mm sieve and subse-quently ground with a mortar and pestle. Percentcarbon, percent nitrogen, and C:N ratios were deter-mined on a Carlo Erba Nitrogen Analyzer 1500.

Ž .Samples ns5 for stable isotope analysis wereprepared using the Dumas sealed-tube method

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–11 5

Fig. 1. Glacier Bay map, with location of study sites indicated by arrows. Dates indicate location of glacial termini. Modified fromŽ .Weishampel 1994 .

Ž .Macko, 1981 . After cryogenic separation of CO2Ž 15 .and N , nitrogen stable isotope ratios d N values2

were determined on a VG Prism Series II isotoperatio mass spectrometer. Reproducibility of measure-ments on duplicate samples was "0.1‰. Stableisotope abundances are reported as

d 15 Ns R rR y1 =1000,Ž .sample standard

where Rs15Nr14 N of either the sample or theŽ .reference standard atmospheric N .2

Nitrogen mineralization rates were estimated us-Žing in situ soil incubations, or ‘buried bags’ Nadel-

.hoffer et al., 1983 . After removal of the litter layer,soil cores were taken through the organic and min-eral soil to a depth of approximately 10 cm in themineral soil, using bulb corers with an inside diame-ter of 12.8 cm. The soil cores were separated intoorganic and mineral horizons, and then placed intogas-permeable 1 mille thick polyethylene bags be-fore being reinstated into the soil. The cores were

Table 1Ž .Site description summaries for the six sites, with years since deglaciation site age and the current dominant woody vegetation

Site no., name, approximate Dominant vegetation Basal area2 y1Ž .years since deglaciation m ha

Ž .1. Muir Inlet 20 yr Dryas, Salix, Populus 0Ž .2. Goose Cove 55 yr Alnus 30.2Ž .3. Adams Inlet 90 yr Alnus 27.9

Ž .4. Muir Point 110 yr Alnus, Picea 30.8Ž .5. Beartrack Cove 165 yr Picea, Tsuga 60.6

Ž .6. Bartlett Cove 225 yr Picea, Tsuga 78.5

Basal area per hectare represents the cross-sectional area of trunks at 1.4 m above the ground.

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–116

disturbed as little as possible to preserve soil struc-ture and in situ soil conditions. Incubations were for

Ž .28 days in 1991 ns9 per site and 24 days in 1992Ž .ns5 per site , and were begun on June 15 in 1991and June 11 in 1992. Only pre-incubation ammo-nium and nitrate concentrations were obtained for1992, and only post-incubation concentrations wereobtained for 1991. We assumed pre-incubation con-centrations to be roughly equivalent in 1991 and1992, which allowed us to estimate 1992 pre-incuba-tion ammonium and nitrate levels from ammoniumand nitrate levels after 4 weeks of incubation in 1991to arrive at estimates for mineralization.

Residence times for nitrogen in the O horizonwere calculated by dividing the percentage of soilnitrogen by the mineralization rate, and then dividingby the fraction of yearly degree days above O8Cduring mineralization.

Results for C:N and d15N were analyzed with a

one-way, two-tailed ANOVA at a significance levelof 0.05. Separation of means was done using theScheffe f-test. Means are reported plus or minus one´standard deviation.

3. Results and discussion

Significant differences in d15N were observed

among species within sites and for the same speciesŽ .among sites Table 2, Fig. 2 . Variations among

species at individual sites probably resulted from

Fig. 2. Relationship between site age and nitrogen isotope valuesŽ .‰ of foliage for 5 species of plants and O horizon soil.

different preferences for ammonium, nitrate, andfixed N , whereas across-site variations of a single2

species may have resulted from differences in thed

15N of available soil N.15 ŽThe d N values of Dryas and Alnus -2‰ to

.-1‰ indicated that most of the nitrogen for thesespecies is derived from fixation. Alnus grown in

15 ŽN-free media had a d N of y1.9‰ Beaupied et. 15al., 1990 . Bulk organic horizon d N was relatively

constant over succession, which is not surprising asalmost all soil N is recently derived from fixation.Presumably, insufficient time had elapsed for site

Table 2Ž .Nitrogen isotope values ‰ and standard deviations for 5 species of plants and the soil O horizon for each of 6 locations

Species Site no.

Site 1 Site 2 Site 3 Site 4 Site 5 Site 6

Ž .age yr

20 55 90 110 165 22515

d NPicea n.p. 0.4"0.6 y2.1"0.2 y2.9"0.2 y5.0"0.2 y5.7"0.5Alnus n.p. y2.1"0.8 y1.5"0.4 y1.8"0.2 y1.3"0.7 y1.6"0.4Dryas y1.3"0.7 n.p. n.p. n.p. n.p. n.p.Salix y8.2"1.1 y2.1"0.8 y2.5"1.3 y3.6"0.0 n.p. n.p.Populus y7.0"3.5 y1.3"1.0 y2.7"1.9 y2.8"1.1 n.p. n.p.O Horizon n.p. 0.1"0.9 0.3"1.4 y0.9"1.2 1.8"0.7 y0.2"1.9

Ž .The age years of each location is also given.n.p.sspecies or horizon not present at that site.

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–11 7

differences in fractionation in the O horizon to leadto major shifts in isotopic composition. In contrast toAlnus d

15N signatures, the d15N of Picea declined

Ž .significantly ns25, dfs20, p-0.0001 duringsuccession and probably reflects the signature ofmineralized soil N.

The d15N of willow and cottonwood are both

Žsignificantly less ns20, dfs16 for both, ps.0.002 for cottonwood, p-0.0001 for willow at Site

Ž .1 age 20 years than at older sites. Willow and15 Žcottonwood d N are also significantly less ns15,

.dfs12, p-0.0001 than the nitrogen-fixing shrubDryas at this site. This suggests that precipitation isa main N source at Site 1 for the non-nitrogen fixingplants, unlike later sites where mineralized organic Nis the dominant source. Some of willow and cotton-wood nitrogen is undoubtedly derived from the min-eralization of fixed organic N from Dryas, however,the absence of an O horizon and the sparse nature ofthe vegetation cover suggests that the mechanism bywhich this occurs is probably quite different thanthat of later sites. Although d

15N for precipitation Nwas not measured, it is generally depleted in 15N,

15 Žwith d N values from 0‰ to y10‰ Nadelhoffer.and Fry, 1994 .

Site 2 was the only location where plants solelydependent on soil N for nitrogen differed signifi-cantly in 15N abundances, with spruce d

15N valuesŽ .being significantly higher ns20, dfs16 than wil-

Ž . Ž .low ps0.005 and cottonwood values ps0.02 .This could result from Sitka spruce preference for

Žammonium over nitrate as an N source Ingestad,. Ž .1979 . Ingestad 1979 indicated that Sitka spruce

preferentially took up 20% nitrate and 80% ammo-nium, even as the supply ratio of nitrate to ammo-nium varied from 20%r80% to 60%r40%. Al-though direct evidence at this location of the prefer-ence by willow and cottonwood for ammonium ornitrate is lacking, plants from ruderal habitats such asthese two species generally have high nitrate reduc-

Ž .tase activities Lee and Stewart, 1978 , indicating theability to use nitrate effectively. Isotopic discrimina-tion during nitrification should result in a higher

15 Žd N signature for ammonium than for nitrate De-.lwiche and Steyn, 1970 .

At Sites 5 and 6, alder d15N was significantly

Ždifferent from spruce ns10, dfs8, p-0.001 at.Site 5, ps0.01 at Site 6 . This was attributed to

alder using atmospheric N as a main nitrogen source.2

However, in Sites 3 and 4, non-nitrogen fixers hadsignatures similar to alder. At these two sites, miner-alized soil N should resemble fixed N in d

15Nsignature.

Several alternative explanations are available as towhy spruce differs from willow and cottonwood in15N abundance at Site 2. One possibility is isotopicdiscrimination for ammonium during uptake. Thishas been observed in rice plants at high levels of

Ž .applied N Yoneyama et al., 1991 . Isotopic discrim-ination on uptake has been generally discounted infield situations in that no discrimination should occurwhere nitrogen is limiting. However, if rooting depths

15 Ždiffer and soil d N varies with depth Nadelhoffer.and Fry, 1988 , the observed pattern could occur.

This difference between spruce and willow is notseen at any other site and therefore is not consistentwith this explanation.

Information on d15N is often used to determine

the fraction of N supply of a nitrogen-fixing plantderived from atmospheric fixation of N . In order to2

calculate this fraction, three d15N values are typi-

cally used: the d15N of the nitrogen fixer grown in a

medium free of mineral N, the d15N of the nitrogen

fixer in the environment under study, and the d15N

of a reference plant that does not fix nitrogen. Theresults here suggest that caution is needed in usingthis method in environments high in available N, asthese environments may permit isotope discrimina-tion to be expressed based on preference for ammo-nium and nitrate. Unless the presumed nitrogen-fix-ing plant and the reference plant take up the sameratio of ammonium to nitrate, incorrect estimates offixation will occur.

Spruce is the only non-nitrogen fixer presentacross most sites. Therefore its d

15N can best indi-cate how N dynamics change from early to latesuccession. Spruce 15N abundances decline with in-creasing site age whereas soil d

15N remains rela-tively constant. This discrepancy suggests the utiliza-tion of two isotopically distinct soil N pools and therelative contribution of these pools to the total min-eralized N. In early succession, most N is in a labileŽ . 15easily degraded , organic pool, and d N of mineral-ized N closely resembles total soil d

15N. As therelative size of the organic N pool in a refractoryŽ .not readily degraded form increases over time, the

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–118

differences in isotopic signature between soil N andmineralized N increase. Easily mineralized soil N,which will be reflected in foliage d

15N, becomesdecoupled from overall soil N as soil developmentproceeds. The trend of increasing divergence be-tween total soil d

15N and foliage d15N with increas-

ing site age reflects the increase in the size of Npools with long turnover times. Other researchersŽ .Paul and Juma, 1981; Parton et al., 1987 haveproposed similar schemes for models of N dynamics,where soil N is broken down into two, three, or fourseparate pools that differ in how rapidly they aremineralized.

Because most ecosystem N is in the soil pool, theO horizon d

15N data suggest that the average d15N

of the overall ecosystem remains relatively constant,reflecting the primary nitrogen source, atmospheri-cally-fixed nitrogen. Denitrification losses from simi-lar forests in the Pacific Northwest are less than 0.3

y1 y1 Ž .kg ha yr Binkley et al., 1992 , and computedstreamwater losses in Glacier Bay are less than 1 kg

y1 y1 Ž .ha yr for mineral N Stottlemeyer, 1990 . Thedecline in d

15N of spruce indicates that labile, bio-logically available nitrogen becomes isotopicallymore depleted in 15N during succession. Isotopicallyheavy nitrogen may be sequestered in either deep

Ž .soils Nadelhoffer and Fry, 1988 , standing wood, orin recalcitrant litter and soil fractions. The decline ind

15N of labile N over succession probably resultsfrom fractionations during mineralization and subse-quent recycling between soil and vegetation.

The C:N ratio in plant foliage and soils canindicate differences in the use of N as well as theunderlying availability of N for plant uptake in that

Ž .site Fig. 3 . Alder C:N ratios varied little amongsites, confirming its independence from the soil forits N supply. All other species showed some signifi-cant within-site variation. Willow appeared the mostsensitive to site conditions, with over a twofolddecline in C:N from Site 1 to Site 2. CottonwoodC:N was also lower at Site 2 compared to Site 1, butunlike willow, had a relatively constant C:N acrossSites 2–4. Spruce C:N was relatively constant across

Ž .the three alder-dominated Sites 2–4 and the youngerspruce-dominated Site 5; however C:N in the older

Ž . Žspruce site Site 6 was significantly higher ns25,.dfs20, ps0.0002 , indicating nitrogen limitation

in this site. This nitrogen limitation does not simply

Fig. 3. Relationship between site age and C:N ratio of foliage of 5species of plants and O horizon soil.

reflect the influence of the organic horizon, as the OŽhorizon C:N was significantly higher ns22, dfs

. Ž .17, p-0.0001 in both spruce-dominated sites 5–6Ž .than in the three alder-dominated sites 2–4 .

Willow displayed the greatest variability in C:Namong sites, presumably because it cannot effec-tively translate increased N uptake into new growth.The low C:N in willow foliage at Site 2 may resultfrom luxury consumption, which is a common fea-ture of species adapted to nutrient-poor environmentsŽ .Chapin, 1980 . In contrast, cottonwood and sprucemaintain relatively constant C:N over most sites.Comparison of tree ring growth records for thesespecies with mineralization data could confirmwhether willow responds less than other species toincreases in N supply.

The increase in soil C:N with the change fromalder-dominated sites to spruce-dominated sitesshows the vegetational influence on soil quality. The

Ž .increase in soil C:N from Site 4 age 110 years toŽ .Site 5 age 165 years did not result in a correspond-

ing increase in the C:N of spruce foliage in Site 5;Ž .however, by Site 6 age 225 years C:N of spruce

had increased, indicating severe nitrogen limitation.The C:N ratios of the soils in Sites 5 and 6 weresimilar, 29.0 and 30.9, respectively. Spruce foliageC:N is a more sensitive indicator of N availabilitythan soil C:N. Because the spruce foliage C:N has

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Table 3O horizon N mineralization

Site Net min. Net ammon. Net nitrif. %N in Soil Mineralized N Initial TurnoverŽ .age mmol N mmol N mmol N mmol nitrate time yr

y1 y1 y1 y1Ž . Ž . Ž . Ž . Ž .yr 100 g soil 100 g soil 100 g soil 1000 g N mmol Ny1 y1 y1 y1 y1Ž .month month month month 100 g soil

55 1059 394 665 0.70 1510 19"1 890 1821 431 1390 1.43 1270 27"54 10

165 553 184 369 0.80 690 0"0 18225 437 132 305 0.95 460 1"1 27

Min.smineralization.Ammon.sammonification.Nitrif.snitrification.%N in soil combined with net mineralization data to produce column 8, turnover time, assuming mineralization is proportional to degree–days above 08C.

( )E.A. Hobbie et al.rChemical Geology 152 1998 3–1110

increased but O horizon C:N has not, we concludethat the degree of N limitation and the proportion ofsoil N that is bound up in slowly recycling fractionsincreases between these two sites. It is suggested thatthe approximately 60 additional years over whichspruce has dominated the Bartlett Cove site com-pared to the Beartrack Cove site has favored thebuildup of more refractory soil N, which in turn hasled to a more N-limited system.

ŽOnly the two younger alder-dominated sites 2.and 3 had pre-incubation nitrate levels greater than

1 mgr100 g soil, although all sites showed substan-Ž .tial nitrification upon incubation Table 3 . Net min-

eralization was greater and soil N turnover was fasterin the alder-dominated Sites 2 and 3 than in spruce-dominated sites. Residence time of N in the Ohorizon also increased steadily with successional age.

The high initial levels of soil nitrate seen in Sites2 and 3 suggest that nitrifiers are able to competesuccessfully with other soil microbes and plants forammonium. This may arise because alder dominatesin these two sites, generating substantial amounts ofN through N fixation, with consequent less vegeta-tive demand on the soil mineral N pool. Bormann

Ž .and Sidle 1990 also found substantial soil nitrate ina 62-year-old, alder-dominated site.

The general trend of mineralization results indi-cates a decreasing rate of nitrogen turnover withincreasing successional age, and particularly with thedominance of spruce in later succession. This generaltrend may translate into less total mineral nitrogenavailable for uptake, depending on the amount ofnitrogen present at each site. Previous researchers in

ŽGlacier Bay Crocker and Major, 1955; Ugolini,.1968; Bormann and Sidle, 1990 found that most

nitrogen accumulation occurred during the period ofalder dominance, which would suggest that the slowerN mineralization in later succession would lead toless N available for uptake. The decrease in the rateof nitrogen turnover is in agreement with the hypoth-esis that the fraction of refractory nitrogen increaseswith successional age.

Because alder litter is quickly mineralized, rela-tively rapid soil N turnover is seen when alderdominates sites. The dominance of spruce in latersuccession in Glacier Bay results in the replacement

Ž .of readily decomposable alder leaves Mikola, 1958by more slowly decomposed, lignin-rich needles. In

addition, because spruce needles are retained formore than one year, the proportion of litter from leaftissues declines. Both of these factors result in atransition to a system where refractory nitrogen poolsdominate.

4. Conclusions

Significant differences in the d15N signatures in

plant tissues of woody species at the same site areattributed to patterns of N utilization of ammonium,nitrate or symbiotically fixed nitrogen. Significantdifferences within a species among sites of differentages appear to be a consequence of the d

15N of theavailable soil N. The C:N in plant foliage demon-strates a significant pattern of among-species andamong-site variation that is largely attributed to thepattern of species response to N availability, and tothe pattern of N availability at sites of differing ageand history of development. Nitrogen mineralizationrates in the soils also suggest a pattern of reducedmineral N availability for older, spruce-dominatedsites. From d

15N values and C:N ratios, the gradualformation during succession of soil N pools differingin isotopic signature and refractibility is deduced.

The comparative youth of the primary succes-sional sequence of Glacier Bay, and the consequentability to estimate the previous site history of thisunique area were simplifying factors in the formationof the above conclusions. As the site history is notwell understood in most studies of N dynamics, theusefulness of N isotopic measurements will be in-creased by determining d

15N values in a greatervariety of ecosystem pools, particularly soil ammo-nium and nitrate. Relationships among plants, de-composers, and soil N pools will be thereby re-vealed. Comparisons among measured d

15N signa-15 Žtures and model-predicted d N values Hobbie et

. 15al., in prep. will allow d N values to be used in amore quantitatively useful way. This should quicklylead to an enhanced understanding of the fundamen-tal controls in nitrogen dynamics.

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