NITROGEN MINERALIZATION IN THE ASPEN ECOSYSTEM 1 ABSTRACT

Transcription

1 NTROGEN MNERALZATON N THE ASPEN ECOSYSTEM 1 M.C. Amcher* 2, A.D. Johnson 2, T. Christopherson 2, D.E. Kutteref, D.L. Brtos 2, nd J. Kotuby-Amcher 2 USDA Forest Service Rocky Mountin Reserch Sttion, Logn, UT nd 3 Deprtment of Plnts, Soils, nd Biometeorology, Uth Stte Univ., Logn, UT ABSTRACT Aspen (Populus tremuloides) re in decline throughout the interior western U.S. becuse of conifer invion, fire suppression, nd overbrowsing by domestic livestock nd ntivil 1 e ungultes. Aspen restortion requires disturbnces such s fire or cutting nd exclosures to protect ginst overbrowsing. We mesurec!t the effects of the 1996 Pole Creek fire, Fishlke NF, UT on soil N minerlition/immobilition during the summer of 1997 using n ion exchnge membrne (EM) soil core incubtion method. Two incubtion periods were used: 6/25 to 7/22/97 (27 d) nd 7/22 to 9/23/97 (63 d). Net soil N minerlition/immobilition rtes in unburned spen nd unburned mixed spen/conifer res were very low. Amounts of NH 4 nd N 3 ccumulted by EM's nd in soil cores were significntly higher in the burned spen/conifer re thn in unburned res. More NH 4 ws ccumulted in the EM's nd soil cores during the first incubtion, Jheres more N 3 ws ccumulted in the EM's nd soil cores during the second incubtion indicting n increse in nitrifiction s summer progressed. Even though more soil cores in the burn re hd higher rtes of minerlition or immobilition thn those in unburned res, net minerlition/immobilition rtes in the burn re one yer fter the fire were not significntly different from unburned res becuse of the blnce of the two overll processes. The incresed mounts of soil NH 4 nd N 3 s result of fire provide source of N for sen regrowth until N minerlition/immobilition in lef litter cn gin become he dominnt process of N cycling in the spen ecosystem. Aspen provides mny benefits including wtershed protection, more wter yield thn conifers, wood fiber, biologicl diversity, wildlife hbitt, forge for domestic livestock nd ntive ungultes, recretionl sites, nd esthetic considertions (e.g., fll lef colors) (Brtos nd Cmpbell, 1998). Unfortuntely, spen re in decline throughout the interior westel U.S. (Brtos nd Cmpbell, 1998). About 6% of the 1 This pper ws written nd prepred by U.S. Government employees on officil time, nd therefore is in the public domin nd not subject to copyright. The use oftrdenmes is for informtionl purposes only nd does not constitute endorsement by the USDA-FS. 145 This file ws creted by scnning the printed publiction. Errors identified by the softwre hve been corrected; however, some errors my remin.

2 spen stnds cross the interior western U.S. hve been lost since the mid-19th century (Brtos nd Cmpbell, 1998b ). Decline of spen stnds is ttributed to nturl succession (e.g., invsion of conifers)(hrniss 1981 ), fire suppression (Jones nd Debyle, 1985b ), nd overbrol sing by domestic livestock nd ntive ungultes (Ky, 199). Western spen reprod yce primrily by suckering from lterl roots nd need disturbnces to stimulte the suckering response (Schier 1981 ). n the pst these disturbnces included nturl fires tht killed mture spen nd insects nd diseses tht wekened old decdent s nds prior to toppling by wind events. With the emphsis on suppression of nerly ll humn-cused nd most nturl fires throughout the twentieth century, one of tful e min forces of spen regenertion hs been lrgely removed from the lndscpe. Even when disturbnce such s fire or wind event does stimulte spen regenertion, overbrowsing of newly emerged spen suckers by domestic livestock nd ntive ungultes my quickly suppress regenertion. Tht coupled with conifer nd sgebrush invsion of declining spen stnds hs led to the precrious stte of spen in the interior western U.S. Cryer nd Murry (199h proposed n ecologicl model of spen to conifer succession tht indictes tht spen regenertion my be inhibited in lndscpes now dominted by conifers. This is becuse conifers my lter soil properties to suppress spen regenertion. Western U.S. soils tht developed under stble spen re Mollisols. The rpid brekdown of spen leves dded to the soil ech yer during lef fll contributes to formtion of the orgnic mtter enriched mollie horion (Jones nd DeByle, 1985; Cryer nd Murry, 1992). Aspen stnds usully hve significnt grss-forb understory. The rid brekdown of orgnic mtter from this understory lso contributes to the formtion n mintennce of the mollie horion. n declining spen stnds, decresing lef fll cn led to decrese in orgnic mtter ccumultion nd eventully decrese in mollie horion thickness (Cryer nd Murry, 1992). This my result in incresed wter percoltion through the soil profile nd formtion of n lbic horion. Conifer invsion cn ccelerte this process. n the theory of ecologicl successio proposed by Cryer nd Murry (1992), s conifers invde declining spen stnds, the mollie horion thins nd lbic nd rgillic horions form. As the mollie horion decreses nd the lbic horion increses, soil ph, orgnic mtter, exchngeble bses, ction-exchnge cpcity (CEC), nutrient levels, nd soil temperture ll decrese. ThJ lower ph nd nutrient levels re believed to suppress spen regenertion. Soils under mixed spen/conifer stnds re most often trnsition 1 type soils with properties intermedite between Mollisols nd Alfisols. n such cses, soil properties re mostly influenced by the vegettion tht occupied the site for the longest period of time (Jones nd DeByle, 1985). n preliminry study oft soil properties under spen nd mixed spen/conifer stnds in the Burnt Flt Anlysis Are of the Fishlke NF, UT, Brtos nd Amcher (1998) found tht conifer invsion hd not pprecibly ltered the nutrient sttus of these soils, lthough slight reduction in soil ph ws found under mixed spen/conifer stnds compred to pure speh stnds. These findings cst some doubt on whether 146

3 conifer invsion of declining spen stnds hs ltered soil properties s much s fered. The findings lso indidte tht these soils should support spen regenertion following tretment. Despite t ese findings, there remins considerble uncertinty bout whether fire is needed t<d return nutrients to the soil to support spen regrowth in res where conifers hve invded. Where nutrient return is not needed, cutting lone my be sufficient to stimulte egenertion. n either cse exclosures re needed to protect regenerting spen from overbrowsing. During June nd July, 1!996, n 8-cre nturl fire (the Pole Creek fire) occurred in the re of Betenson nd Grindstone Flts on the Bever Rnger District of the Fishlke Ntionl Forest, l.lth. This moderte-intensity fire burned in mixed stnds of older decdent spen nd invding conifers, primrily sub-lpine fir (Abies lsiocrp) nd Englemnn spruce (Pice eng/emnnit). After the fire, some cutting of fire-killed spen nd conifers ws done to thin the stnds nd four exclosures were constructed by Uth Division of Wildlife Resources nd Fish lke NF personnel to protect spen suckers from browsing by domestic livestock nd ntive ungultes. Two exclosures were built in the Grindstone Flt re nd two in the nerby Betenson Flt re. This pper will present results of the effects of nturl fire on soil N vilbility nd minerlition during the first summer fter the fire yer. OBJECTVES The objectives were to determine the effects of spen to conifer succession nd nturl fire on soil N vilbility nd minerlition/immobilition. METHODS We mesured soil nitrogen minerlition/immobilition during the first summer (1997) following the Pole Creek fire using n ion exchnge membrne ( EM) soil core incubtion technique. The method ws similr to tht developed by DiStefno nd Ghol (1986) except tht we used ion exchnge membrnes (lonics nc., Wtertown, MA) in plce of ion exchnge resin bgs. The EM's hve identicl ion exchnge properties to the more fmilir resin beds nd hve shown considerble promise for ssessing soil nutrient vilbility (Cooperbnd nd Logn, 1994 ). The ssembly for incubting the soil cores nd EM's is shown in Fig. 1. The soil core is enclosed in 2-in dimeter x 4-in long plstic liner (Forestry Suppliers, Jcskon, MS). End cps were used to hold the EM's ginst ech end of the soil core. Circulr holes (1.5-in dimeter) were cut in the center of the 2-in dimeter end cps. Ction (CEM) nd nion (AEM) exchnge membrnes were prepred by cutting 2-in dimeter circles from the originl membrne sheets nd drilling nine evenly-spced 1/8" holes in ech circle to llow pssge of wter nd ir through the soil cores. The GEM's nd AEM's were soked five times in 1 M HC nd 1 M NHC 3, respectively, rinsed with deionied wter, nd seled in ip-lock plstic bgs for trnsport to the field. To protect the EM's from brsion nd excessive desicction, porous screens were prepred by 147

4 cutting 2-in dimeter circles from 114-um Spectr/Mesh polyethylene filter screens. E ;r End cp Screen AEM CEM Fig. 1. lon exchnge membrne soil core configurtion We instlled the EM soil cores in the following mnner. Lef litter (unburned res) or sh lyer (buned res) ws first removed from the soil surfce nd set side. The soil coring hed with plstic liner ws driven. into the soil using slide hmmer ttchment (Forestry Suppliers, Jckson, MS) until flush with the top of the soil. The intct soil core ws removed from the coring hed, the ends of the core were trimmed, nd the excess soil ws returned to the soil hole. The end cps, plstic screens, nd EM's were ssembled s shown in Fig. 1, the EM soil core ws plced in the hole with the top end up, nd the core ws covered with lef litter or sh. Gore 16ctlons were mrked with flgged rebr stkes. An dditionl soil core ws tken djcent to the instlled EM soil core to determine initil soil exchngeble NH 4 nd N 3 1evels. The initil cores were removed from the coring tool nd plced in lbeled ip-lock plstic bgs. The cores were trnsported to the lbortory in cooler nd stored t< 4 oc until extrction. EM soil cores were instlled in three res on Grindstone nd Betenson Flts: four cores were instlled.in ech of the four exclosures in the burned mixed spen/conifer stnds, four cores were instlled in ech of three unburned mixed spen/conifer stnds ner the burned res, nd four cores were instlled in ech of two unburned spen stnds tht regenerted s result of fire in Thus, there were 16 replicte cores in the burned res, 12 in the unburned mixed spen/conifer, nd 8 in unburned pure spen. Soil temperture in ech re ws monitored t 4-in depth using Optic StowAwy temperture loggers (Onset Computer Corp., Bourne, MA). 148

5 Two incubtion periods were used: 6/25 to 7/22/97 (27 d) nd 7/22 to 9/23/97 (63 d). At the end of ech incubtion period, the lef litter or sh lyer ws removed from the tops of the cores nd the cores were removed from the ground nd plced in lbeled ip-lock plstic bgs. The cores were trnsported to the lbortory in cooler nd stored t< 4 oc until extrction. New initil cores for the second incubtion period were collected when the EM soil cores from the first period were removed nd the new EM soil cores were instlled. The EM soil cores were disssembled nd top nd bottom CEM-AEM pirs were rinsed with deionied wter to removed soil prticles nd plced in seprte lbeled petri dishes. The soil cores were removed from the plstic liners, returned to the ip-lock bgs, nd mixed by kneding the bgs. Ech CEM-AEM pir ws extrcted with 1 ml of 2 M KC in 25-mL wide-mouth plstic jrs by shking for 1 h on n orbitl shker t 15 rpm. The extrcts were froen until nlysis. Wter content of representtive subsmple of ech soil core ws determined grvimetriclly t 15 C. A representtive field-moist subsmple of ech soil core equivlent to 1 -g of oven-dry soil ws extrcted with 1 ml of 2 M KC in 25-mL wide-mouth plstic jrs by shking for 1 h on n orbitl shker t 15 rpm. The extrcts were filtered through Whtmn 4 filter pper nd froen until nlysis. The 2 M KC extrct of EM's nd soil cores were nlyed for NH 4 -N nd N 3 -N by the utomted phente nd Cd reduction methods, respectively using LACHAT flow injection nlysis system (Zellweger Anlytics, Milwukee, W). The remining soil from ech soil core ws ir-dried t room temperture nd weighed. The totl weight of ech soil core ws clculted by summing the oven-dry weight of the subsmple used for determining wter content, the oven-dry weight of the subsmple used for determining extrctble NH 4 nd N 3, nd the ir-dry weight of the remining soil in ech core. The ir-dried soils were sieved through 2-mm stinless steel sieve to remove corse frgments. Orgnic mtter content of the < 2-mm frction ws determined by loss on ignition (LO) for 16 h t 45 oc of n oven-dried (1 5 oc) subsmple. Totl C nd N contents of the< 2-mm frction were determined by LECO CHN nlyer. These soils do not contin free crbontes so totl Cis mesure of totl orgnic C. Amounts of NH 4 -N nd N 3 -N ccumulted by the EM's (ug/cm 2 ) were clculted by multiplying the NH 4 nd N 3 concentrtions in the KC extrcts by the extrct volume nd dividing by the surfce re of the EM (2.27 cm 2 ). Rtes of NH 4 or N 3 ccumultion by the cores (ug/cm 2 /d) were clculted by dividing the ccumulted mounts by the incubtion time in dys. The EM's t the top of the soil core remove NH 4 nd N 3 in precipittion or lechte pssing through the lef litter or sh lyers. The EM's t the bottom of the soil core remove NH 4 nd N 3 leched from the soil core. Concentrtions of NH 4 nd N 3 in the bottom EM's on soil weight bsis for the entire soil core (mg/kg) were clculted by multiplying the mounts on the EM's by the EM surfce re nd dividing by the dry weight of the soil core. Totl NH 4 nd N 3 concentrtions in the EM soil cores were obtined by summing the concentrtions of NH 4 or N 3 in the soil core plus the mounts extrcted 149

6 from the bottom EM's. Net mmonifiction is equl to totl NH 4 in the EM soil core NH4 in the initil soil core. Net nitrifiction is equl to totl N 3 in the EM soil core N 3 in the initil soil core. Net minerlition (or immobilition) is equl to net mmonifiction + net nitrifiction. Net mmonifiction, nitrifiction, nd minerlition rtes were clculted by dividing the net mounts by the incubtion time. An exmple of these clcultions for one of the soil cores from the burn re is shown in Tble 1. Tble 1. Net mmonifiction, nitrifiction, nd minerlition/immobilition for n EM soil core from the Betenson 1 exclosure (7/22-9/23/97). nitil soil core NH 4 -N, mg/kg FiniiEM soil core NH4-N, mg/kg Bottom CEM NH4-N, mg/kg Finl totl (bottom CEM + EM soil core) NH4-N, mg/kg Net (finl totl- initil) NH4-N, mg/kg Net mmonifiction rte (net NH4-N incubtion time), mg/kg/mo nitil soil core N 3 -N, mg/kg Finl EM soil core N 3 -N, mg/kg Bottom AEM N 3 -N, mg/kg Finl totl (bottom AEM + EM soil core) N 3 -N, mg/kg Net (finl totl- initil) N 3 -N, mg/kg Net nitrifiction rte (net N 3 -N incubtion time), mg/kg/mo Net minerlition/immobilition rte (net mmonifiction + net nitrifiction), mg/kg/mo Becuse the dt were non-normlly distributed, Kruski-Wllis nlysis of vrince on rnks ws run on ech mesured or derived N vrible for ech incubtion with stnd type s the fctor nd unburned spen, unburned mixed spen/conifer, nd burned mixed spen/conifer s levels within stnd type (SigmStt, ver. 2., SPSS Science, Chicgo, L). f sttisticlly significnt difference mong stnd types ws found (p <.5), pirwise multiple comprisons test ws conducted using Dunn's method to isolte which stnd type ws different from the others (p <.5). 15

7 RESULTS The Pole Creek fire hd n effect on the post-fire rtes of ccumultion of soil NH4 nd N 3 by the top nd bottom EM's in the incubted soil cores (Fig. 2). Although there were significnt differences in the rtes of NH 4 nd N 3 ccumultion by the EM's between burned nd unburned res, there were no significnt differences in ccumultion rtes between spen nd mixed spen/conifer res. During the first incubtion period (6-7/97), rtes of NH 4 ccumultion by top nd bottom GEM's in burn re soil cores were significntly greter thn those by soil core CEM's in unburned spen nd mixed spen/conifer res. However, differences mong rtes of NH4 ccumultion by both top nd bottom GEM's in burned nd unburned res were not significnt for the second incubtion period (7-9/97). Furthermore, there ws higher rte of NH4 ccumultion by the GEM's during the shorter (27 d) first incubtion thn during the longer (63 d) second incubtion. During the first incubtion period, N 3 ccumultion rtes by the top AEM's in the burn re were not significntly different from those in the unburned res. However, N 3 ccumultion rtes by bottom AEM's were significntly greter in the burn re thn in unburned res during the first incubtion period. During the second incubtion period, N 3 ccumultion rtes by both top nd bottom AEM's were significntly greter in the burn re thn in the unburned res. Rtes of N 3 ccumultion by the AEM's during the second incubtion were much greter thn those during the first incubtion. There ws shift in the reltive distribution of inorgnic N species in the burn re from the first to the second incubtion period. More NH4 ws dsorbed by the CEM's during the first incubtion, but more N 3 ws dsorbed by the AEM's during the second incubtion. This shift in the reltive distribution of inorgnic N species from the first to the second incubtion period is n indiction of n increse in soil nitrifiction in the burn re s the summer progressed. The distribution of NH4 nd N 3 mong the top nd bottom EM's tended to follow the reltive mobility of the ions. Nitrte is more mobile thn NH4 so more N 3 ws found in the bottom thn top AEM's in the burn re. On the other hnd, more NH4 ws found in the top thn bottom GEM's. During the first incubtion, significntly more NH 4 nd N 3 were extrcted from initil nd EM soil cores nd bottom EM's from burned res thn those from unburned res (Fig. 3). Net mounts of NH4 nd N 3 in burned nd unburned res were not significntly different, however, becuse of higher levels of NH4 nd N 3 in the initil soil cores from the burn re thn from unburned res. However, more of the EM soil cores from the burn re tended to show net decrese in NH4 nd net increse in N 3 thn those from unburned res even though the medin vlues were not significntly different. Within unburned res, no significnt differences were found between spen nd mixed spen/conifer stnds for ny of these vribles. During the second incubtion, only the initil soil cores hd significnt difference in extrctble NH4 between burned nd unburned res (Fig. 4 ). Extrctble 151

8 NH 4 in EM soil cores nd bottom CEM's nd net chnges in extrctble NH 4 mounts were not significntly different between burned nd unburned res during the second incubtion. n contrst, significntly more N 3 ws extrcted from initil nd EM soil cores nd bottom AEM's in the burned re thn those from unburned res (Fig. 4 ). However, even though more EM soil cores in the burn re hd lrger net increse thn decrese in N 3, medin net N 3 1evels were not significntly different between burned nd unburned res. No significnt differences were found between spen nd mixed spen/conifer stnds for extrctble soil or EM NH 4 or N 3. There ws shift in the distribution of N 3 between the soil nd AEM's from the first to the second incubtion. n the first incubtion, most of the mesured N 3 ws extrcted from the soil cores. The bottom AEM's ccounted for only smll percentge of N 3 in the EM soil cores. n the second incubtion, most of the mesured N 3 ws extrcted from the bottom AEM's indicting tht N 3 ws being leched from the soil cores nd ws being ccumulted by the bottom AEM's. Net mmonifiction, nitrifiction, nd minerlition/immobilirion rtes were not significntly different between burned nd unburned res or between spen nd mixed spen/conifer stnds (Fig. 5). Even though medin rtes were not significntly different, there tended to be greter net loss of NH 4 from soil in the burn re during the erly prt of summer (first incubtion) thn lter in the summer (second incubtion). Net gins in soil N 3 in the burn re tended to be greter during the ltter prt of summer thn in the erly prt. The loss in NH 4 ws lrgely blnced by the gin in N 3 (nitrifiction), lthough some loss of NH 4 my hve been from direct microbil utilition of this form of N DSCUSSON n undisturbed (unburned) spen nd mixed spen/conifer soils, the vrious microbil minerlition nd immobilition rections tend to be t stedy stte. There is little free NH 4 nd N 3 in the soils (Figs. 2, 3, 4) becuse it is quickly utilied by microorgnisms or tken up by plnt roots. Thus, net minerlition/immobilition rtes tend to be very low (Fig. 5) even though there is ctive mmonifiction, nitrifiction, nd immobilition occurring in the soil. Depending on durtion nd intensity, fire lters mny soil properties including exerting strong influence on soil microbil minerlition/immobilition processes (Schlesinger, 1997). Losses of gseous nd prticulte N occur during combustion of lef litter nd soil orgnic mtter, microbil popultions nd enyme ctivity my decrese, but vilble nutrients re lso returned to the soil by the residul sh (Schlesinger, 1997). Even though totl soil N my be lower becuse of combustion, residul N in the sh is subjected to rpid minerlition nd nitrifiction provided the fire is not too intense nd microbil nd enyme popultions cn recover (Schlesinger, 1997). Amcher nd Brtos (1998) found tht lthough totl C nd totl N in -6-inch soil cores from the Lst Chnce nd Betenson/Grindstone Flts burn res were lower 152

9 thn those from unburned res, the differences were not significnt. n contrst, orgnic mtter (LO), totl C, nd totl N in the -4-inch soil cores (this study) were higher in the Betenson/Grindstone Flts burn re thn those from unburned res (dt not shown). Thus, combustion losses seem to be low. Although tree mortlity from the fire ws high, the Pole Creek fire left drk colored sh, lso n indiction of incomplete combustion. Thus, the Pole Creek fire ws probbly of moderte intensity overll in terms of its impct to the soil. The source of the high mounts of NH 4 ccumulted by the GEM's nd in the soil cores in the burn re the first summer fter the fire my include NH 4 s combustion product in the sh lyer, relese from minerl sources of NH 4, nd rpid mmonifiction of combustion products. Much of this vilble NH 4 ws probbly crried over in the soil from the summer in which the fire occurred ( 6-7/96). Nitrifiction (microbil oxidtion of NH 4 to N 3 ) on the other hnd occurred mostly in the first summer fter the fire (6-9/97) becuse of the observed decrese in NH 4 nd increse in N 3 ccumulted by the EM's nd soil cores from the first to the second incubtion period. The EM soil core incubtion method provides snpshot of soil N minerlition/immobilition for the incubtion periods studied, but since we did not hve soil cores in plce immeditely fter the fire, we could not quntify soil N trnsformtions in lte summer nd fll of the fire yer (7-9/96). Nitrifiction is influenced by mny fctors including temperture, moisture, nd the substrtes NH 4, 2, nd C 2 (Stevenson, 1986). The high mounts of vilble NH 4 found in burn re soils provide redy substrte for surviving or post-fire invding popultions of nitrifying microorgnisms. Summertime soil temperture rnges nd mens in the burn re were higher thn in unburned res becuse the reltively open cnopy nd drk colored sh served to trp het (dt not shown), thus helping to ccelerte nitrifiction. There ws net decrese in men soil core wter content for ll res in the first incubtion (25.1 %to 16.4 %), but net increse in the second (16.4% to 37.5 %). Although men soil core wter contents for both incubtions from unburned mixed spen/conifer res (31.3 %) were higher thn those from unburned spen (23.5 %) or the burned mixed spen/conifer res (24.1 %), dequte soil moisture ws vilble in the burn res for nitrifiction to occur throughout the summer. Although denitrifiction nd more especilly leching cn potentilly remove much of the N 3 produced by post-fire nitrifiction, the lrge mounts of soil NH 4 nd N 3 tht were vilble during the growing seson provide redy source of minerl N for uptke by spen suckers. A strong spen suckering response ws observed on Betenson nd Grindstone Flts nd in other burned res on the Fishlke NF the first summer fter the fires (Amcher nd Brtos, 1998). We do not know how much N 3 remining in the burn re soils in the fll will be vilble for spen uptke during subsequent summers so this study will continue for two more summers. A longer residence time of post-fire soil N 3 provides more opportunity for regenerting spen clones to exploit this source of N. Thus, vilble N from sh my be n importnt source of N to young spen until N minerlition/immobilition in lef litter cn gin 153

10 be the dominnt N cycling process in the spen ecosystem. ACKNOWLEDGMENTS This work ws supported by USDA-CSREES Rngelnds Specil Grnt. We thnk Scott Wlker of the Uth Division of Wildlife Resources nd Bob Cmpbell, Dndy Pollck, nd Doug Sorenson of the Fishlke NF for their ssistnce throughout the project. REFERENCES Amcher, M.C., nd D.L. Brtos Soil properties ssocited with spen to conifer succession nd spen restortion tretments. p. 55. n: Abstrcts, 51st Annul Meeting, Society for Rnge Mngement Feb., Gudljr, Jlisco, Mexico. Brtos, D.L., nd M.C. Amcher Soil properties ssocited with spen to conifer succession. Rngelnds 2: Brtos, D.L., nd R.B. Cmpbell, Jr Decline of quking spen in the interior west- exmples from Uth. Rngelnds 2: Brtos, D.L., nd R.B. Cmpbell, Jr. 1998b. Wter depletion nd other ecosystem vlues forfeited when conifer forests displce spen communities. p n: D.F. Potts (ed.) Rngelnd mngement nd wter resources My, Reno, NV. Americn Wter Resources Assoc., Herndon, VA. Cooperbnd, L.R., nd T.J. Logn Mesuring in situ chnges in lbile soil phosphorus with nionexchnge membrnes. Soil Sci. Soc. Am. J. 58 : Cryer, D.H., nd J.E. Murry Aspen regenertion nd soils. Rngelnds 14: DiStefno, J.F., nd H.L. Ghol A proposed use of ion exchnge resins to mesure nitrogen minerlition nd nitrifiction in intct soil cores. Commun. Soil Sci. Plnt Anl. 17: Hrniss, R.O Ecologicl succession in spen nd its consequences. p n: N.V. Debyle (ed.) Sitution mngement of two intermountin species: spen nd coyotes. Vol. 1. Aspen April, Uth Stte Univ., Logn, UT. Jones, J.R., nd N.V. Debyle Soils. p n: N.V. Debyle nd R.P. Winokur (eds.) Aspen: ecology nd mngement in the western United Sttes. GTR RM-119. Rocky Mountin Forest nd Rnge Experiment Sttion, Ft. Collins, CO. Jones, J.R., nd N.V. Debyle. 1985b. Fire. p n: N.V. Debyle nd R.P. Winokur (eds.) Aspen : ecology nd mngement in the western United Sttes. GTR RM-119. Rocky Mountin Forest nd Rnge Experiment Sttion, Ft. Collins, CO. Ky, C.E Yellowstone's northern elk herd: criticl evlution of the "nturl regultion" prdigm. PhD Disserttion. Uth Stte Univ., Logn, UT. Schier, G.A Aspen regenertion. p n: N.V. Debyle (ed.) Sitution mngement of two mngement species: spen nd coyotes. Vol. 1. Aspen April, Uth Stte Univ., Logn, UT. Schlesinger, W.H Biogeochemistry. An nlysis of globl chnge. 2nd ed. Acdemic Press, Sn Diego. Stevenson, F.J Cycles of soil. John Wiley & Sons, NY. 154

11 First ncubtion ( 6-7/97) 1.5 Top CEM.6 Top AEM 1. -".5 i - " N E b (...) (...) > ::J N E -> ::J 1.5 Bottom CEM.6 Bottom AEM C') 1..4 b.5.2 '<t b Aspen A/C Burned Aspen A!C Burned Second ncubtion (7-9/97).6 Top CEM 8 TopAEM.4 6 l 4 b ".2 " N - E!ill + N 2 - E (...). (...)..._... > ::J > ::J - 8 b.6 Bottom CEM - Bottom AEM '<t (Y") 6 OA _... Aspen A/C Burned Aspen A/C Burned Fig. 2. Box plots of rtes of NH 4 nd N 3 ccumultion by ction (CEM) nd nion (AEM) exchnge membrnes t the top nd bottom of EM soil cores for two incubtion periods in unburneq spen, unburned mixed spen/conifer (A/C), nd burned mixed spen/conifer. The 25 1 h nd 75th percentiles re shown s box centered bout the medin (5th percentile), the 1oth nd goth percentiles re shown s error brs, nd the 5th nd 95th percentiles nd outliers re shown s points. Different letters bove the box plots indicte tht the medins re significntly different (p <.5). 155

12 CJ) First ncubtion (6-7/97) 1 nitil soil core 4 nitil soil core t EM soil core Bottom CEM CJ) E - -.:::t b Soil+ CEM Net - -5 ' -1 Aspen A/C Burned CJ) 2 1 b i b EM soil core 3 b Bottom AEM CJ) E 1..5 C'? Soil+ AEM Net b Aspen A/C Burned Fig. 3. Box plots of initil nd finiiem soil core NH 4 nd N 3, bottom EM NH 4 nd N 3, finl totl soil + EM NH 4 nd N 3, nd net NH 4 nd N 3 in EM soil cores from the first incubtion (6/25-7/22/97) in unburned spen, unburned mixed spen/conifer (A/C), nd burned mixed spen/conifer. The 25th nd 75th percentiles re shown s box centered bout the medin (5th percentile), the 1oth nd goth percentiles re shown s error brs, nd the 5th nd 95th percentiles nd outliers re shown s points. Different letters bove the box plots indicte tht the medins re significntly different (p <.5). 156

13 Second ncubtion (7-9/97) 1 nitil soil core 4 nitil soil core 75 b 3 5 i b - =...::+:- 1 EM soil core 4 EM soil core b $ Ol 1.5 Ol Bottom CEM 6 Bottom AEM b --Ol E Ol - '<t i E 4.5 C 2 -!;:.;;;;;;;;;P. 1 Soil+ CEM 6 Soil+ AEM = 1 Net 75 Net Aspen A/C Burned -! Aspen A/C Burned i Fig. 4. Box plots of initil nd finl EM soil core NH 4 nd N 3, bottom EM NH 4 nd N 3, finl totl soil+ EM NH 4 nd N 3, nd net NH 4 nd N 3 in EM soil cores from the second incubtion (7/22-9/23/97) in unburned spen, unburned mixed spen/conifer (A/C), nd burned mixed spen/conifer. The 25th nd 75th percentiles re shown s box centered bout the medin (5th percentile), the 1oth nd goth percentiles re shown s error brs, nd the 5th nd 95 1 h percentiles nd outliers re shown s points. Different letters bove the box plots indicte tht the medins re significntly different (p <.5). 157

14 First ncubtion (6-7/97) Ammonifiction Nitrifiction Minerlition 3 3 "' 3 -) "' 2 "' ) ) ) 1 1 ) ) - i - C'l "<t C'l E - E - $ E 1! "<t Aspen NC Burned Aspen NC Burned Aspen NC Burned Second ncubtion (7-9/97) Ammonifictl-on Nitrifiction Minerlition 3 3 "' 3 -) "' 2 "' ) ) ) 1 1 E ) -- ) - = - C'l - -1 i -1-1 "<t C'l "<t As pen AJC Burned As pen AJC Burned Aspen AJC Burned E.. E.. Fig. 5. Box plots of net mmonifiction, nitrifiction, nd minerlition/immobilition rtes in EM soil cores for two incubtion periods in unburned spen, unburned mixed spen/conifer (NC), nd burned mixed spen/conifer. The 25th nd 75th percentiles re shown s box centered bout the medin (5th percentile), the 1oth nd goth percentiles re shown s error brs, nd the 5th nd 95th percentiles nd outliers re shown s points. Different letters bove the box plots indicte tht the medins re significntly different (p <.5). 158