Chronic nitrogen additions reduce total soil respiration and microbial respiration in temperate forest soils at the Harvard Forest

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1 Forest Ecology nd Mngement 196 (24) Chronic nitrogen dditions reduce totl soil respirtion nd microil respirtion in temperte forest soils t the Hrvrd Forest Richrd D. Bowden,*, Eric Dvidson, Kthleen Svge, Chris Ari, Pul Steudler c Deprtment of Environmentl Science, Allegheny College, Medville, PA 16335, USA Woods Hole Reserch Center, P.O. Box 296, Woods Hole, MA 2543, USA c Mrine Biologicl Lortory, The Ecosystems Center, Woods Hole, MA 2543, USA Astrct At the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study, red pine nd mixed deciduous stnd showed immedite chnges in soil respirtion following nitrogen dditions (low N: 5 g N m 2 per yer; high N: 15 g N m 2 per yer) during the initil yer (1988) of the study. In the hrdwood stnd, soil respirtion rtes incresed fter N dditions in the first yer (control: 482. g C m 2 per yer; high N: g C m 2 per yer). This increse is ttriuted to incresed productivity in the hrdwood stnd compred to the pine stnd; N dditions re hypothesized to hve incresed either root or microil ctivity, or perhps oth. In the second yer, however, respirtion in the fertilized hrdwood plots ws not different from the control plot. In the pine stnd, nnul soil respirtion ws 21 nd 25% lower, respectively, in low N nd high N plots thn the control (429.9 g C m 2 per yer), with further reductions in the second yer. Weekly mesures of soil respirtion during summer 21 showed tht fter 13 yers of continuous nitrogen fertiliztion, soil respirtion in the high N plots during growing seson months ws suppressed y 41% in oth stnds. To investigte the possiility tht reduced microil ctivity contriuted to decresed totl soil respirtion, we incuted root-free soil nd mesured CO 2 fluxes. The pttern in verge respirtion for incuted soils ws similr to tht oserved from totl soil respirtion mesured in the field. Lortory respirtion rtes from the hrdwood high N nd pine high N soils were 43 nd 64%, respectively, lower thn rtes from control soils. This indictes tht nitrogen dditions hve reduced microil ctivity nd thus CO 2 production in the field. Declines in forest productivity mesured t oth sites, s well s sustntil tree mortlity oserved t the high N sites, my lso lower root ctivity nd rhizodeposition, nd re lso likely to reduce microil decomposition y reducing orgnic mtter ville to soil microes. # 24 Elsevier B.V. All rights reserved. Keywords: Soil respirtion; Cron dioxide; N deposition; C sequestrtion; Forest soils 1. Introduction * Corresponding uthor. Tel.: þ ; fx: þ E-mil ddresses: richrd.owden@llegheny.edu (R.D. Bowden), edvidson@whrc.org (E. Dvidson), ksvge@whrc.org (K. Svge), cri@uss.com (C. Ari), steudler@ml.edu (P. Steudler). Emissions of nthropogenic CO 2 (IPCC, 21) nd NO x (Korner, 2) into the tmosphere re of incresing importnce to considertions of ecologicl economics nd environmentl policy (e.g. Wigley /$ see front mtter # 24 Elsevier B.V. All rights reserved. doi:1.116/j.foreco

2 44 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) et l., 1996; Melillo nd Cowling, 22) ecuse of potentil impcts of ltered tmospheric chemistry on forest soil helth nd services. Glolly, soils contin Pg C, second only to C storge in the deep ocens (Rich nd Potter, 1995; Schimel, 1995). They lso contin the lrgest stores of nitrogen in terrestril ecosystems (Schlesinger, 1997). Cron fluxes in forest soils of the northern hemisphere re of prticulr interest to environmentl policy discussions ecuse revegettion of ndoned griculturl res is enling these forests to serve s sinks for tmospheric C (Houghton, 1996; McGuire et l., 21; Gurney et l., 22), with sequestrtion rtes up to.3pgc per yer (Dixon et l., 1994; Ndelhoffer et l., 1999). Though there is considerle vriility nd uncertinty in the totl sink, it is estimted tht the C sink for estern US forests rnges from.4 to.35 Pg per yer (Houghton et l., 1999; Schimel et l., 2; Goodle et l., 22). Despite the importnce of forest soil C nd N storge, quntifictionofcndnstores,fluxes, nd interctions within terrestril ecosystems remin lrgely incomplete (McGuire et l., 21). Nitrogen fertiliztion in northern temperte zones hs een estimted to enhnce C storge y.3.5pgcperyer(townsend et l., 1996; Ndelhoffer et l., 1999). However, other estimtes suggest tht stimultory effects of N loding on ecosystems is not likely to ccount for significnt C storge (Korner, 2) nd my ctully reduce ecosystem productivity nd C storge (Aer et l., 1989; Schulze, 1989; Co nd Woodwrd, 1998). Soil respirtion is one of the primry fluxes of C etween soils nd the tmosphere, with glol relese of 75 Pg C per yer (Schlesinger nd Andrews, 2). Understnding controls on soil respirtion is criticl ecuse reltively smll chnges in respirtion rtes my drmticlly lter tmospheric concentrtions of CO 2 s well s rtes of soil C sequestrtion. In temperte forest soils, respirtion is influenced y numer of fctors, including sustrte qulity (Wring nd Running, 1998; Rout nd Gupt, 1989; Fog, 1988), temperture (e.g. Rustd et l., 21), soil moisture (e.g. Bowden et l., 1998; Svge nd Dvidson, 21), root iomss (Pregitzer et l., 2) nd microil ctivity nd iomss (Fisk nd Fhey, 21). Nitrogen dditions to forest soils hve shown vrile effects on soil CO 2 effluxes, including increses, decreses, or unchnged rtes (Slonius nd Mhendrpp, 1975; Brumme nd Beese, 1992; Cstro et l., 1994; Mttson, 1995; Hynes nd Gower, 1995; Vose et l., 1995; Bowden et l., 2; Burton et l., 24). With incresing rtes of nthropogenic N deposition (Vitousek et l., 1997), there is strong need to understnd links etween N inputs nd soil respirtion. Most studies reporting effects of N dditions on soil respirtion hve een conducted over reltively short periods, often within the first few yers of N dditions. Importntly, however, there is strong need to understnd long-term effects of N deposition on soil respirtion to etter predict impcts on soil C fluxes nd storge. The purpose of this study is to: (1) report initil nd long-term effects of experimentl N dditions on soil respirtion in temperte hrdwood stnd nd red pine plnttion t the Hrvrd Forest Longterm Ecologicl Reserch Site Chronic Nitrogen Amendment Study, nd (2) to exmine the role of N dditions on microil contriutions to totl soil respirtion (Tle 1). 2. Site description The study site is t the Hrvrd Forest, Petershm, Msschusetts, USA (4283 N, 7282 W). Two djcent 2.3 h stnds with well-documented histories were used (Bowden et l., 199; Aer et l., 1993): n pproximtely 55-yer-old mixed hrdwood stnd nd 75-yer-old red pine (Pinus resinos Ait.) plnttion (plnted 1926). Soils re of glcil origin, nd re stony- to sndy-loms clssified s Typic Dystrochrepts. Soils in oth stnds re mors, lthough the hrdwood stnd hs thicker orgnic horizon (6:5 1:3 cm (S.E., n ¼ 9)) thn the pine stnd (4:6 :3 cm (S.E., n ¼ 9)). Orgnic horizons in oth stnds re cidic, with men ph vlues of 3.3 in the hrdwood stnd nd 3.2 in the pine stnd. Men monthly tempertures rnge from 19 8C in July to 12 8C in Jnury; verge nnul precipittion is 112 cm (Mgill et l., 2). Totl nitrogen deposition to the forest is pproximtely.8 g m 2 per yer (wet:.6 g m 2 per yer; dry:.2 g m 2 per yer; Ollinger et l., 1993). Current forests developed following ndonment from pstures in the erly 19s, nd hve well-

3 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Tle 1 Annul rtes (g C m 2 per yer) of soil respirtion t control nd fertilized hrdwood nd red pine stnds, nd devition of fertilized plots from control plots, t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study Smpling yer Hrdwood Control Low N High N Control N Low N High N Rte Rte % chnge from Control Rte % chnge from Control Rte Rte % chnge from Control Rte % chnge from Control X þ þ S.E X þ S.E descried history (Bowden et l., 199). The hrdwood stnd is dominted y lck ok (Quercus velutin Lm.) nd red ok (Quercus rur L.) tht together comprised 76% of totl sl re of 21.8 m 2 h 1. The remining vegettion consists mostly of lck irch (Betul lent L.), pper irch (Betul ppyrifer Mrsh.), red mple (Acer rurum L.), nd Americn eech (Fgus grndifoli Ehrh.). The pine stnd is nerly pure red pine (98% of totl sl re of 51.9 m 2 h 1 ). 3. Methods 3.1. Plot estlishment In My 1988, three 3 m 3 m plots were estlished in ech stnd. One plot serves s control, one hs received 5 g N m 2 per yer (low N plots), nd one hs received 15 g N m 2 per yer (high N plots). In the first yer only, the low N plot received 3.7 g N m 2 per yer nd the high N plot received 12 g N m 2 per yer. Nitrogen, dded s NH 4 NO 3, ws pplied in liquid form with ckpck spryer in six equl dditions from April through Septemer. Untreted corridors t lest 5 m wide seprte plots. Ech plot ws divided into 5 m 5 m suplots, nd the outermost suplots in ech plot served s uffer strips nd were not smpled. Three interior suplots from ech plot were rndomly selected for mesurement of gseous fluxes. Soil smpling ws restricted to the remining suplots no soil ws removed from suplots used to mesure gs fluxes Field soil respirtion mesurements, yers 1 nd 2 Cron dioxide flux mesurements egn in My 1988 nd were conducted routinely for 2 yers (yer 1 ¼ , yer 2 ¼ ). Mesurements were mde t lest once per month from Mrch through Decemer, nd twice per month during most spring, summer nd fll months; fluxes were mesured t lest 2 weeks fter ech fertilizer ppliction. Soil respirtion mesurements were suspended when soils ecme snow-covered (pproximtely Decemer), nd resumed in the spring (April or My) when the upper 5 cm of soil egn to thw. Soil fluxes during winter were ssumed to e the verge of the finl Decemer estimte nd the first mesurement conducted in spring. Men spring soil tempertures ( 2.5 cm depth) in the pine stnd were 4.5 8C in yer 1 nd 1.5 8C in yer 2. In the hrdwood stnd, spring soil tempertures were 6.5 nd 8C in yers 1 nd 2, respectively. Soil elow 5 cm ws still frozen in the pine stnd, nd the upper 5 cm of soil in the hrdwood stnd hd thwed within the previous week. Soil cron dioxide fluxes were mesured using n in situ chmer incution technique (Bowden et l., 199; Rich et l., 1991). Chmers, constructed of 1.8 cm thick polyvinyl chloride (PVC) plstic, consisted of 28.7 cm dimeter 4: cm tll chmer top nd 5.2 cm tll lower portion (nchor) ttched to the orgnic horizon with stinless steel stkes. Anchors were pushed <1 cm into the orgnic horizon to minimize disturnce, nd the volume within ech nchor ws mesured. Anchors were removed during

4 46 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) the winter of , ut were left in plce throughout the yer in Incutions were conducted y plcing chmer tops, colored white to minimize solr heting, on the nchors nd holding them in plce with two 4.5 kg weights. Totl chmer volume, mesured when nchors were implnted 1 cm, ws pproximtely 5.4 L. Gs smples were withdrwn from the hedspce vi stinless steel luer-lock needle tht extended 2.5 cm into the hedspce. Fluxes were mesured six times (every 4 h) in 24 h period nd were timed to provide smpling t pproximte minimum nd mximum dily soil tempertures. During ech 3 min flux mesurement, hedspce ir smples (2 cm 3 ) were collected t the eginning of the incution nd t 1 min intervls therefter. Smples were withdrwn with 2 ml nylon syringes. Chmer tops were removed from nchors upon completion of ech flux mesurement. Smples were usully nlyzed within 4 dys of collection. Lortory tests showed tht syringes were inert to CO 2,showedno memory effect fter exposure to high concentrtions (197 ppm CO 2 -C) nd could store smples for up to 9 dys with less thn 4% chnge in the originl concentrtion. Cron dioxide ws nlyzed on Shimdzu Mini-2 gs chromtogrph (GC) equipped with 3 m 3:2mm dimeter stinless steel column filled with Poropk Q (8/1 mesh), nd 63 Ni electron cpture detector. Crrier gs (95% Ar, 5% CH 4 ) ws mintined t 2 ml min 1. Injection nd detector tempertures were 275 8C nd column temperture ws 5 8C. Smples nd stndrds were pssed through silic gel (8/1 mesh) trp to remove wter vpor nd were then crried into the GC vi stinless steel smpling vlve equipped with 1 ml smpling loop. Certified CO 2 stndrds (299.7, 11, nd 197 ppm CO 2 -C in N 2 )wereotinedfrom Scott Specilty Gses, Plumstedville, PA, USA. Cron dioxide concentrtions were corrected to STP conditions nd fluxes were clculted using the initil liner portion of the CO 2 concentrtion increse within the chmer (usully the first two smples over the 3 min incution). The minimum detection limit for individul flux mesurements ws pproximtely 2mgCO 2 -C m 2 h 1, depending upon comined field nd nlyticl precision on ech smpling dte. Coefficients of vrition of triplicte smples tken from the sme chmer t the sme time rnged from 2.4 to 6.1%. Annul CO 2 fluxes for ech tretment were clculted s the men of the nnul fluxes of the three chmers. Ech smpling dte ws considered the midpoint of smpling period, nd the nnul flux ws the sum of C respired during ll smpling periods. Amient ir (t 1 m), chmer ir, nd soil tempertures ( 2.5 nd cm) were mesured during ech flux mesurement. Air temperture ws mesured t one loction in ech stnd, nd chmer ir nd soil tempertures were mesured t one chmer in ech plot. Orgnic horizon smples (humic mteril, exclusive of litter overlying minerl soil) ðn ¼ 3 9Þ were grvimetriclly nlyzed for moisture during ech smpling (Bowden et l., 199) Field soil respirtion mesurements, summer, yer 13 During the summer of 21, soil respirtion ws mesured pproximtely weekly from June until Septemer using mnul soil chmer system with portle infrred gs nlyzer (Svge nd Dvidson, 23)(Tle 2). Fluxes were mesured etween 9: nd 12:, the time of dy when the verge flux of the diel cycle occurs (Dvidson et l., 1998). Six 1.2 cm dimeter 1 cm tll white polyvinyl chloride collrs were plced rndomly in ech of the tretment plots within ech stnd. Collrs were inserted pproximtely 2 4 cm into the soil, nd stood pproximtely 6 8 cm ove the soil surfce. A vented, 1 cm tll crylonitrile utdiene styrene plstic chmer top ws ttched to the IRGA nd plced over the soil collrs during ech soil respirtion mesurement, nd pump circulted ir from the chmer to the IRGA t.5 L min 1. A ckpckmounted Licor 6252 TM portle IRGA system ws used to record CO 2 concentrtions every 12 s over flux mesurement period of 5 min. The IRGA ws clirted efore ech mesurement using zero CO 2 stndrd nd known certified stndrd (523 ppmv CO 2 ). Liner regressions (concentrtion versus time) on concentrtions determined rtes of CO 2 flux Lortory soil incutions, yer 13 To ssess the possiility tht N fertiliztion hs depressed microil respirtion in the minerl soil horizon, we collected soils from ech of the plots

5 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Tle 2 Cumultive soil respirtion (g C m 2 ) during growing seson (June 6 Septemer 6, 21) from control nd fertilized hrdwood nd red pine forest plots, nd devition of fertilized plots from control plots, t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study Smpling yer Hrdwood Control Low N High N Control N Low N High N Rte Rte % chnge from control Rte % chnge from control Rte Rte % chnge from control Rte % chnge from control 1988 X þ þ S.E X þ S.E X S.E within ech stnd, nd mesured the rte of soil respirtion y root-free soils incuted in the lortory. A-horizon soils were collected with n okfield corer in erly July ner ech of the collrs used for soil respirtion smpling. Roots were removed, soils were pssed through 2 mm sieve, nd soils were mintined for 4 dys in the lortory t pproximtely 5 8C prior to incution. A susmple of ech of the soil smples ws ir-dried t 35 8C to determine field soil moisture content. Prior to incution, 2 g of fresh weight soil from ech smple ws djusted to field moisture content nd plced into.46 L (1 pt) cnning jr. Six replictes from ech of the tretment plots for ech stnd (36 totl) were incuted nd llowed to rech mient temperture of pproximtely 2 8C; the jrs were open just prior to the incution so tht hedspces would equilirte with mient ir. Jrs were then seled, mintined in the lortory t 2 8C, nd 5 ml of hedspce were collected t times, 6, 12 nd 18 min. Ech gs smple ws injected immeditely into Licor 6252 TM Infrred Gs Anlyzer. The flux rte of CO 2 ws clculted using liner regression on initil liner portion of the concentrtion curve. After incutions were complete, incuted soils were dried t 15 8C, nd respirtion rtes re reported on dry weight sis. All sttisticl tests were conducted using SigmStt Sttisticl Softwre Ver. 2. (SPSS Softwre Products, 1997). 4. Results Field soil respirtion, yers 1 nd 2 soil tempertures (Fig. 1) were not significntly different mong the plots within ech stnd. Orgnic horizon moisture content, however, did differ mong tretments. In the hrdwood stnd, the control plot (x ¼ 1:54 g H 2 O g soil 1 ) ws significntly drier (one-wy RMANOVA on men tretment vlues y dte, F ¼ 5:322, P < :9 over the yer) thn the high N plot (x ¼ 1:336 g H 2 O g soil 1 ). In the pine stnd, the control (1.17 g H 2 O g soil 1 ) nd high N (1.51 g H 2 O g soil 1 ) were significntly wetter (Friedmn one-wy RMANOVA on men tretment vlues y dte, chi-squre 11.7, P < :3) thn the low N (.93 g H 2 O g soil 1 ). Cron dioxide effluxes showed strong sesonl pttern (Fig. 2), with highest rtes oserved in July August in oth stnds, nd lowest rtes in erly winter. Rtes rnged from pproximtely 2 mg C m 2 h 1 during winter months, to over 15 mg C m 2 h 1 in the pine stnd nd over 2 mg C m 2 h 1 in hrdwood stnds in mid-summer. Soil respirtion ws nerly lwys higher in the hrdwood stnd thn the pine stnd. Fluxes exhiited positive exponentil reltionships with temperture nd negtive reltionships with moisture content of the orgnic horizon (Fig. 3). Exponentil reltionships etween soil temperture

6 48 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Hrdwood o C g H 2 O g-soil o C g H 2 O g-soil /1/88 7/1/88 11/1/88 3/1/89 7/1/89 11/1/89 3/1/9 7/1/9 Control Low N High N Fig. 1. Soil temperture ( 2.5 cm depth) nd moisture (orgnic horizon) in control nd fertilized hrdwood nd red pine forest stnds ( ) t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study. nd soil respirtion were very highly significnt ðp < :1Þ in ll plots, with r 2 vlues rnging from.72 to.86. An overll pttern of decresed respirtion with incresed orgnic horizon moisture content is pprent, ut individul reltionships mong fluxes nd moisture within ech tretment during the study period were less cler. Exponentil decy reltionships etween respirtion nd soil moisture over the smpling period were significnt in two of the six plots, with r 2 vlues rnging from.13 to.35. Nitrogen fertiliztion showed different effects in the two stnds during the first yer of fertiliztion. In the hrdwood stnd (Fig. 4), N fertiliztion incresed nnul CO 2 effluxes y 26 nd 24% (one-wy RMA- NOVA, F ¼ 9:348, P < :1, Tukey s test) in the low N nd high N plots (Tle 1), respectively, over the control plots (482:1 4:94 g C m 2 per yer). In contrst, pine stnd control plot soil respirtion (429:87 41:89 g C m 2 per yer) ws 2% higher (Friedmn one-wy RMANOVA, chi-squre , P < :1) thn the high N plot, though not significntly different thn the low N plot. In the second yer of fertiliztion, the hrdwood stnd egn to show depression in soil respirtion due to N fertiliztion; low N nd high N rtes did not differ significntly (one-wy RMANOVA, F ¼ 7:53,

7 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Hrdwood Control Low N High N /1/88 7/1/88 11/1/88 3/1/89 7/1/89 11/1/89 3/1/9 7/1/ Control Low N High N /1/88 7/1/88 11/1/88 3/1/89 7/1/89 11/1/89 3/1/9 7/1/9 Fig. 2. Soil respirtion ( ) in control nd fertilized hrdwood nd red pine forest stnds t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study. P < :4, Tukey s test) from control rtes, lthough respirtion in the high N plot ws significntly lower (27%) thn the low N plot. In the pine stnd, respirtion in oth fertilized plots were significntly lower thn the control (one-wy RMANOVA, F ¼ 15:567, P < :1, Tukey s test) (Fig. 5) Field soil respirtion, yer 13 Men growing seson soil respirtion (Tle 2, Fig. 5) in the N mended plots ws further reduced in 21 in the hrdwood stnd compred to results oserved in the smpling yer. The high N plot exhiited significntly lower respirtion rtes thn the control plot (one-wy ANOVA, F ¼ 9:471, P < :2, Tukey s test) (Fig. 6). The soil respirtion pttern during 21 in the Stnd ws nerly identicl to the pttern oserved in the smpling seson, with oth fertilized plots exhiiting significntly lower rtes thn those oserved in the control (one-wy ANOVA, F ¼ 17:85, P < :1, Tukey s test).

8 5 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Soil Temperture o C 25 Soil Moisture g H 2 O g-soil -1 H Control H Low N H High N P Control P Low N P High N H Control H Low N H High N P Control P Low N P High N Fig. 3. Reltionships etween soil respirtion nd soil temperture ( 2.5 cm depth) nd soil moisture (orgnic horizon) in control nd fertilized hrdwood nd red pine forest stnds t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study. H: hrdwood, P: pine. Temperture reltionships (P < :1 for ll plots) hrdwood control: flux ¼ 13:2 þ 32:25 ð:86tempertureþ, r 2 ¼ :72; hrdwood low N: flux ¼ :173 þ 2:69 ð:1164tempertureþ, r 2 ¼ :86; hrdwood high N: flux ¼ 3:8 þ 16:28 ð:1198tempertureþ, r 2 ¼ :86; pine control: flux ¼ 28:6 þ 52:66 ð:527tempertureþ, r 2 ¼ :75; pine low N: flux ¼ 18:7 þ 44:35 ð:55tempertureþ, r 2 ¼ :73; pine high N: flux ¼ 31:7 þ 51:11 ð:477tempertureþ, r 2 ¼ :73. Moisture reltionships hrdwood control: flux ¼ 61:3 þð473 e 6:8747moisture Þ, r 2 ¼ :29, P ¼ :58; hrdwood low N: flux ¼ 93:4 þð29:3e :3844moisture Þ, r 2 ¼ :35, P ¼ :25; hrdwood high N: flux ¼ 44:5 þð199:9e :3248moisture Þ, r 2 ¼ :13, P ¼ :318; pine control: flux ¼ 51:9 þð17:9e :2775moisture Þ, r 2 ¼ :18, P ¼ :184; pine low N: flux ¼ 22: þð147:3e 1:4179moisture Þ, r 2 ¼ :26, P ¼ :78; pine high N: flux ¼ 49:2 þð645:8e 6:4395moisture Þ, r 2 ¼ :33, P ¼ :33.

9 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) g C m -2 y -1 g C m -2 y Lortory soil respirtion Root-free soil respirtion rtes in the lortoryincuted soils during summer of yer 13 (Fig. 7) were greter in the control soil thn the high N plot soil for oth the hrdwood nd pine stnds. In the hrdwood soil, respirtion in the high N plot ws significntly lower thn either the control or low N plots (one-wy ANOVA, F ¼ 4:93, P < :38, Duncn s multiple rnge). In the pine soil, respirtion in the control plot ws significntly greter thn either the low N or high N plots (one-wy ANOVA, F ¼ 14:492, P < :1, Tukey s test). 5. Discussion Hrdwood, Control Low N High N Fig. 4. Annul soil respirtion ( , ) in control nd fertilized hrdwood nd red pine forest stnds t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study. Lower respirtion rtes in the pine stnd (429.9 ðyer 1Þ 48:5 (yer 2) g C m 2 per yer) compred to the hrdwood stnd (482. ðyer 1Þ 533:6 (yer 2) g C m 2 per yer) my reflect generlly lower rtes of gross primry productivity y the pine stnd. The first 2 yers of our results fit within the wide rnge of results reported in the literture for soil respirtion responses to N dditions. For exmple, incresed respirtion in the hrdwood stnd grees with results for young spen cuttings (Pregitzer et l., 2) s well s for those studies tht report positive correltion etween root N concentrtions nd soil respirtion (e.g. Zogg et l., 1996; Burton et l., 22). Decresed respirtion is consistent with numer of studies, including those in mixed hrdwoods (Bowden et l., 2; Fisk nd Fhey, 21; Burton et l., 24), s well s in coniferous forests (Mttson, 1995; Mier nd Kress, 2) Looking t growing seson months only, respirtion ws reduced 14% in the low N plot nd 41% in the high N plot in 21, which is sustntilly greter thn the reductions oserved in yer 2 of the study. Suppression of soil respirtion ws lso pronounced in the pine stnd fter 13 yers, ut rtes of suppression (34 41%) were essentilly identicl to those rtes mesured in yer 2. We cnnot explin completely the pttern tht we oserved in the first 2 yers, ut we suspect tht incresed soil respirtion ws relted to the increses in oveground iomss woody increments nd litterfll, s well s to stnding root iomss in fertilized plots oserved y Mgill et l. (1997). It is likely tht dded C from oveground litter nd from decomposing roots stimulted heterotrophic respirtion, nd the greter mss of roots nd thus root exudtion enhnced utotrophic respirtion in the fertilized plots. Recent work (Hogerg et l., 21) hs shown rpid link etween photosynthetic production nd elowground C lloction. The reversl of the N ddition response in the hrdwood stnd during the second yer my reflect chnges in root ctivity ssocited with nutrient uptke. A lrge frction of root respirtion is llocted to N ssimiltion (Bloom et l., 1992), ut with lrger doses of N redily ville for uptke, energetic costs of N ssimiltion my hve een reduced. The erly response in the pine stnd my hve een driven y direct impcts of N dditions on the soil microil community, s well s y chnges in overll forest growth. Wheres oveground litterfll nd stnding root iomss were greter in the fertilized plots thn in the control during

10 52 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Hrdwood Control Low N High N 6/1/21 7/1/21 8/1/21 9/1/ Control Low N High N 6/1/21 7/1/21 8/1/21 9/1/21 Fig. 5. Soil respirtion in control nd fertilized hrdwood nd red pine forest stnds during the 21 growing seson t the Hrvrd Forest Long-term Ecologicl Reserch Site Chronic Nitrogen Amendment Study. the period from 1989 to 1993, the woody oveground increment ws lower in the high N plot thn either the control or low N plots. An dditionl fctor, especilly in the pine stnd, my e the response of the ectomycorrhizl fungl community, which hs een shown to e quite sensitive to N inputs (Hrley nd Smith, 1983). Lortory incution of root-free soil shows tht heterotrophic respirtion from the microil community in fertilized plots is reduced. Although Micks et l. (24) did not find microil respirtion response immeditely (within dys) following N fertilizer ddition, Compton et l. (24) found tht chronic N dditions decresed microil iomss nd diversity,

11 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Hrdwood 8 Hrdwood 1 5 mg C kg-soil -1 h Control Low N High N Fig. 6. Men rtes of soil respirtion in control nd fertilized hrdwood nd red pine forest stnds t the Hrvrd Forest Longterm Ecologicl Reserch Site Chronic Nitrogen Amendment Study over the growing seson, 21. Rtes (within ech stnd) with the sme letter re not significntly different. nd Frey et l. (24) oserved tht ctive fungl iomss ws lower in fertilized plots thn in control. Frey et l. (24) lso detected significnt reduction in the ctivity of the enzyme phenol oxidse, lignindegrding enzyme produced y white-rot fungi. Along N deposition grdient in the Cliforni costl scrulnd, N fertiliztion reduced mycorrhizl infection rtes nd survivl (Egerton-Wrurton nd Allen, 2), nd Ruhling nd Tyler (1991) oserved tht following N dditions, mycorrhizl species nerly cesed producing fruiting odies. Altertions in the microil community my lso chnge enzyme production nd decomposer efficiency, reducing rtes of respirtion. It is lso possile tht decreses in soil ph hve reduced microil ctivity. In oth stnds, soil ph mesured in the O-horizons in 22 ws lower in the fertilized plots thn in control plots. In the hrdwood stnd control ph ws 3.35, nd the high N plot mg C kg-soil -1 h Control Low N High N Fig. 7. Respirtion rtes (per grm of soil dry weight) of lortory-incuted, root-free soils from control nd fertilized hrdwood nd red pine forest stnds t the Hrvrd Forest Longterm Ecologicl Reserch Site Chronic Nitrogen Amendment Study. Rtes (within ech stnd) with the sme letter re not significntly different. ws In the pine stnd, control ph ws 3.54, nd the high N ws 3.15 (A. Mgill, University of New Hmpshire, personl communiction). Additions of proton-producing NH 4, long with the cidity generted y ccelerted rtes of nitrifiction in the fertilized plots, my well hve reduced soil microil ctivity s others hve suggested (Aerts nd de Cluwe, 1999). After 13 yers, root ctivity my lso e reduced. Root iomss (A. Mgill, personl communiction) in orgnic horizons nd minerl horizon to 2 cm in 1999 ws highest in the control plot nd lowest in the high N plot; iomss in the pine stnd, however, did not show discernle pttern. In yer 13, tree mortlity ws noticele in the pine stnd, nd sustntil crown dieck ws oserved in the hrdwood stnd. Insofr s microes preferentilly use the short-lived frctions of soil orgnic mtter (Prton et l., 1987; Schimel

12 54 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) et l., 1994), nd tht the verge ge of C contriuting to soil respirtion t the Hrvrd Forest is 8 yers (Trumore, 2), loss of oth oveground nd elowground litter would reduce the microil contriution to soil respirtion. Furthermore, s shown y Uselmn et l. (2), rhizodeposition is rpidly used y soil microes, nd loss of this orgnic mtter input due to declines in root ctivity would lso depress soil respirtion. This study indictes tht the microil community hs undergone long-term chnge in response to chronic N dditions, strongly ffecting n importnt cron flux. Our work suggests tht different processes within forest soils my e ltered over the course of N deposition, nd tht initil, short-term mesures of soil respirtion in response to N fertiliztion will indequtely indicte long-term soil responses to ltered resource vilility. Given the importnce of soil respirtion in net ecosystem C lnce, it is criticl to understnd nd to e le to predict long-term responses of soil processes to N deposition. Acknowledgements This study is contriution to the Hrvrd Forest LTER study, nd ws funded y the NSF Ecosystem Studies (DEB ), LTER (BSR-11764, DEB- 8592, DEB ), nd REU progrms (DBI ). We thnk Alison Mgill, Mrk Cstro, Molly Sperduto, Dn Sperduto, nd Michelle Miliefsky for field nd lortory ssistnce. We pprecite thoughtful reviews provided y Jcqui Aitkenhed-Peterson nd Kte Ljth. A Techer-Scholr Chir provided y Allegheny College to R. Bowden fcilitted this pper. The contriutionydvidsonndsvgewsfundedprtly y the Ntionl Institute for Glol Environmentl Chnge (NIGEC) through the US Deprtment of Energy (Coopertive Agreement No. DE-FC3-9ER611). Opinions, findings, conclusions or recommendtions expressed in this puliction re those of the uthors nd do not necessrily reflect the views of the DOE. References Aer, J.D., Mgill, A., Boone, R., Melillo, J.M., Steudler, P., Bowden, R.D., Plnt nd soil responses to three yers of chronic nitrogen dditions t the Hrvrd Forest, Petershm, MA. Ecol. Appl. 3, Aer, J.D., Ndelhoffer, K.J., Steudler, P., Melillo, J.M., Nitrogen sturtion in northern forest ecosystems hypotheses nd implictions. BioScience 39, Aerts, R.C., de Cluwe, H., Nitrogen deposition effects on cron dioxide nd methne emissions from temperte petlnd soils. Oikos 84, Bloom, A.J., Sukrpnn, S.S., Wrner, R.L., Root respirtion ssocited with mmonium nd nitrte sorption nd ssimiltion y rley. Plnt Physiol. 99, Bowden, R.D., Newkirk, K.M., Rullo, G., Cron dioxide nd methne fluxes y forest soil under lortory-controlled moisture nd temperture conditions. Soil Biol. Biochem. 3, Bowden, R.D., Rullo, G., Stevens, G.R., Steudler, P.A., 2. Soil fluxes of cron dioxide, nitrous oxide, nd methne t productive temperte deciduous forest. J. Environ. Qul. 29, Bowden, R.D., Steudler, P.A., Melillo, J.M., Aer, J.D., 199. Annul nitrous oxide fluxes from temperte forest soils in the northestern United Sttes. J. Geophys. Res. 95, Brumme, R., Beese, F., Effects of liming nd nitrogen fertiliztion on emissions of CO 2 nd N 2 O from temperte forest. J. Geophys. Res. 97, Burton, A.J., Pregitzer, K.S., Reuss, R.W., Hendrick, R.L., Allen, M.F., 22. Root respirtion in North Americn forests: effects of nitrogen concentrtion nd temperture cross iomes. Oecologi 131, Burton, A.J., Pregitzer, K.S., Crwford, J.N., Zogg, F.P., Zk, D.R., 24. Simulted chronic NO 3 deposition reduces soil respirtion in northern hrdwood forests. Biogeochemistry. Co, M., Woodwrd, F.I., Dynmic responses of terrestril ecosystem cron cycling to glol climte chnge. Nture 393, Cstro, M., Peterjohn, W.T., Melillo, J.M., Steudler, P.A., Gholz, H.L., Lewis, D., Effects of nitrogen fertiliztion on fluxes of N 2 O, CH 4 nd CO 2 from soils in Florid slsh pine plnttion. Cn. J. For. Res. 24, Compton, J., Wtrud, L.S., Porteus, L.A., DeGrood, S., 24. Response of soil microil iomss nd community composition to chronic nitrogen dditions t Hrvrd forest. For. Ecol. Mnge. 196, Dvidson, E.A., Belk, E., Boone, R.D., Soil wter content nd temperture s independent or confounded fctors controlling soil respirtion in temperte mixed hrdwood forest. Glol Chnge Biol. 4, Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., Wisniewski, J., Cron pools nd flux of glol forest ecosystems. Science 263, Egerton-Wrurton, L.M., Allen, E.B., 2. Shifts in the diversity of rusculr mycorrhizl fungi long n nthropogenic nitrogen deposition grdient. Ecol. Appl. 1, Fisk, M.C., Fhey, T.J., 21. Microil iomss nd nitrogen cycling responses to fertiliztion nd litter removl in young northern hrdwood forests. Biogeochemistry 53,

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14 56 R.D. Bowden et l. / Forest Ecology nd Mngement 196 (24) Schimel, D.S., Brswell, B.H., Hollnd, E.A., McKeown, R., Ojim, D.S., Pinter, T.H., Prton, W.J., Townsend, A.R., Climtic, edphic, nd iotic controls over storge nd turnover of cron in soils. Glol Biogeochem. Cycles 8, Schimel, D., Melillo, J., Tin, H., McGuire, A.D., Kicklighter, D., Kittel, T., Rosenloom, N., Running, S., Thornton, P., Ojim, D., Prton, W., Kelly, R., Sykes, M., Neilson, R., Rizzo, B., 2. Contriution of incresing CO 2 nd climte to cron storge y ecosystems of the United Sttes. Science 287, Schlesinger, W.H., Andrews, J.A., 2. Soil respirtion nd the glol cron cycle. Biogeochemistry 48, 7 2. Schlesinger, W.H., Biogeochemistry: An Anlysis of Glol Chnge. Acdemic Press, Sn Diego, CA. Schulze, E.D., Air pollution nd forest decline in spruce (Pice ies) forest. Science 244, SPSS Softwre Products, SigmStt Version 2. for Windows. SPSS, Inc., Chicgo, IL. Townsend, A.R., Brswell, B.H., Hollnd, E.A., Penner, J.E., Sptil nd temporl ptterns in terrestril cron storge due to deposition of fossil fuel nitrogen. Ecol. Appl. 6, Trumore, S., 2. Age of soil orgnic mtter nd soil respirtion: rdiocron constrints on elowground C dynmics. Ecol. Appl. 1, Uselmn, S.M., Qulls, R.G., Thoms, R.B., 2. Effects of incresed tmospheric CO 2, temperture, nd soil N vilility on root exudtion of dissolved orgnic cron y N-fixing tree (Roini pseudocci L.). Plnt Soil 222, Vitousek, P.M., Aer, J.D., Howrth, R.W., Likens, G.E., Mtson, P.A., Schindler, D.W., Schlesinger, D.H., Tilmn, D., Humn ltertion of the glol nitrogen cycle: sources nd consequences. Ecol. Appl. 7, Vose, J.M., Elliot, K.J., Johnson, D.W., Wlker, R.F., Johnson, M.G., Tingey, D.T., Effects of elevted CO 2 nd N fertiliztion on soil respirtion from ponderos pine (Pinus ponderos) in open-top chmers. Cn. J. For. Res. 25, Wring, R.H., Running, S.W., Forest Ecosystems: Anlysis t Multiple Scles. Acdemic Press, Sn Diego, CA. Wigley, T.M.L., Richels, R., Edmonds, J.A., Economic nd environmentl choices in the stiliztion of tmospheric CO 2 concentrtions. Nture 379, Zogg, G.P., Zk, D.R., Burton, A.J., Pregitzer, K.S., Fine root respirtion in northern hrdwood forests in reltion to temperture nd nitrogen vilility. Tree Physiol. 16,