Effects of litter quality and climate change along an elevation gradient on litter mass loss in an alpine meadow ecosystem on the Tibetan plateau

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1 Plnt Ecol (21) 29: DOI 1.17/s Effects of litter qulity nd climte chnge long n elevtion grdient on litter mss loss in n lpine medow ecosystem on the Tietn plteu Gungping Xu Yigng Hu Shiping Wng Zhenhu Zhng Xiofeng Chng Jichung Dun Ciyun Luo Zengguo Cho Ailing Su Qioyn Lin Yingnin Li Mingyun Du Received: 19 Septemer 29 / Accepted: 8 Decemer 29 / Pulished online: 24 Decemer 29 Ó Springer Science+Business Medi B.V. 29 Astrct Knowledge of the response of litter mss loss to climte wrming nd litter qulity in lpine ecosystems is still sprse. Here, we conducted 57- dy litter decomposition experiment long n elevtion grdient from 32 to 38 m using different litter types to determine the influences of litter qulity nd climte chnge on the elementl mss losses nd on the temperture sensitivity of litter mss losses (nnul percentge decomposition (%) per 1 C temperture difference). Mss losses of C, nitrogen (N), phosphorus (P), potssium (K), sodium (N), clcium (C), nd Mgnesium (Mg) decresed with n increse in elevtion. In generl, N nd N concentrtions in litter nd rtios of C:N nd lignin:n were the est G. Xu Y. Hu S. Wng Z. Zhng X. Chng J. Dun C. Luo Z. Cho A. Su Q. Lin Y. Li Key Lortory of Adpttion nd Evolution of Plteu Biot, Northwest Institute of Plteu Biology, Chinese Acdemy of Sciences, 818 Xining, Chin G. Xu Y. Hu Z. Zhng X. Chng J. Dun C. Luo Z. Cho A. Su Q. Lin Grdute University of the Chinese Acdemy of Sciences, 149 Beijing, Chin S. Wng (&) Institute of Tietn Plteu Reserch, Chinese Acdemy of Sciences, 185 Beijing, Chin e-mil: wngship28@yhoo.cn M. Du Ntionl Institute for Agro-Environmentl Sciences, Tsuku , Jpn predictors of C mss losses. A higher N concentrtion nd C:N rtio in litter cused greter C mss losses, ut higher lignin:n rtio in litter resulted in lower C mss losses. Litter decomposition occurred in two-stge process. Cron mss loss in litter ws minly limited y soil temperture in the first growing seson of the decomposition period, wheres N concentrtion nd rtios of C:P nd N:P limited cron mss loss in the remining litter during the second growing seson of the decomposition period. Soil moisture ppered not to ffect litter mss loss nd the temperture sensitivity of litter mss loss of grss litter ws greter thn tht of shru litter in the lpine region. Keywords Litter qulity Mss loss Soil temperture Soil moisture Elevtion grdient Alpine medow Introduction Litter nutrient concentrtion, the concentrtion of cron frctions (e.g., lignin), cron:nitrogen (C:N) nd lignin:n rtios hve een widely used s litter qulity vriles due to their influence on microil ctivity nd litter decomposition rtes (Melillo et l. 1982; Tylor et l. 1989; Berg et l. 1996; Murphy et l. 1998). These vriles re lso used in some ecosystem cron models (Running nd Hunt 1993; Prton et l. 1994). However, there is still no universlly ccepted litter-qulity vrile (see cittions in Murphy et l.

2 258 Plnt Ecol (21) 29: ) ecuse severl sources of vrition oscure our understnding of the effects of different vriles, nd ecuse the process of litter mss loss depends on complex interctions etween soil properties, climtic fctors, nd litter types (Meentemeyer 1978; Hrmon et l. 199; Berg et l. 1993; Aerts 1997; Murphy et l. 1998; Hoie 2; Hoie nd Vitousek 2; Fierer et l. 25). There is prticulr lck of knowledge out how litter qulity nd its vriles ffect litter mss loss in lpine regions (Cornelissen et l. 27). A numer of studies hve shown tht the temperture sensitivity of litter mss loss cn vry with litter type nd extent of mss loss (Kirschum 1995; Ktterer et l. 1998; Dlis et l. 21; McTiernn et l. 23; Fierer et l. 25). Glol wrming nd ssocited environmentl chnges re predicted to ffect most regions of the northern hemisphere nd will e prticulrly pronounced t high northern ltitudes during this century (ACIA 25; IPCC 27). As the lrgest grsslnd re on the Eursin continent, the Qinghi-Tietn plteu is mostly situted t 3,5 m or more ove se level, nd covers n re of *2.5 million km 2 (Zheng et l. 2). Evidence shows tht the Qinghi-Tietn plteu is experiencing climtic wrming (Thompson et l. 1993, 2). Moreover, the region is predicted to experience much greter thn verge increses in surfce temperture in the future (IPCC 27). Despite the importnce of this region, little is known out the effect of glol wrming on litter decomposition, which could in turn ffect cron storge in these grsslnds. In this study, we conducted 2-yer litter decomposition experiment long n elevtion grdient from 32 to 38 m. We collected smples of four litter types from lpine medows free-grzed y sheep (two grss litter smples, shru litter smple, nd mixed litter smple) in My 27. The ims of this study were to investigte for the first time (1) the effect of litter qulity nd climte chnge on element mss losses, nd (2) the temperture sensitivity of litter mss loss on the Tietn plteu. fcility run y the Northwest Institute of Plteu Biology of the Chinese Acdemy of Sciences. HBAMERS is situted t ltitude N, longitude E. The sttion lies in the northest of the Tietn plteu in lrge vlley surrounded y the Qilin Mountins (Fig. 1). The sttion experiences typicl plteu continentl climte which is dominted y the southest monsoon from My to Septemer in summer nd high pressure from Sieri in winter. Summers re short nd cool, nd winters re long nd severely cold. Men nnul temperture is -2 C, nd men nnul precipittion is 5 mm, over 8% of which flls during the summer monsoon seson. Men elevtion of the vlley ottom is 3,2 m. The soil is Mt-Gryic Cmisol (Chinese Soil Txonomy Reserch Group, 1995), corresponding to Gelic Cmisol (WRB 1998). Bsic soil properties were orgnic cron 5.5 nd 3.3%, ph 7.3 nd 7.4, ulk density.75 nd 1.11 g cm -3 t 1 nd 2 cm soil depths, respectively (Co et l. 28). A detiled site description cn e found in Zho nd Zhou (1999). Litter smple collection nd decomposition The plnt community t the experimentl site t 3,2 m is dominted y Elymus nutns (Grise.), Po prtensis (Linn.), Festuc ovin (Linn.), Gentin strmine (Mxim.), Koresi humilis (C. A. Mey. ex Trutv.) (Serg.), Crex scrirostris (Kükenth.), Scripus distigmticus (Kükenth.) (Tng et Wng.), Potentill nive (Linn.), Gentin grumii (Kusnez.), Blysmus Mterils nd methods Experimentl site The experimentl site is locted t the Hiei Alpine Medow Ecosystem Reserch Sttion (HBAMERS), Fig. 1 The lndscpe of the experimentl site nd smpling loctions. Photo tken on 12 June 27. Filled circle, filled squre nd filled tringle show the sites t 32, 36, nd 38 m long the elevtionl grdient

3 Plnt Ecol (21) 29: sinocompressus (Tng et Wng.), Leontopodium nnum (Hook. f. et Thoms.) (Hnd.-Mzz.), nd Dsiphor fruticos (Linn.) (Ryd.). Litter smples of E. nutns, B. sinocompressus, D. fruticos (shru), nd mixed litter of the community were collected rndomly from this medow community grzed y sheep in My 27, irdried, nd stored indoors until the eginning of the decomposition experiment. Aout 3 km from HBAMERS, one m plot ws fenced in utumn 25 t ech elevtion of 32, 36, nd 38 m ove se level long the southern side of the Qilin Mountins. Air-dried litter smples (2 g oven-dry mss t 8 C) of single species nd mixed litter smples of the community were plced in cm littergs constructed from 1-mm mesh nylon cloth. We plced totl of 36 litter gs t the three sites t 32, 36, nd 38 m t spcings of 1 2 cm on June 1 27, nd 5 littergs ech species/ community mixture were rndomly selected to tke ck to the lortory on July 24 nd Octoer 14 in 27, nd April 24, June 24, July 24 nd Octoer 24 in 28, respectively, to mesure litter mss loss (i.e., 5 replictes 9 4 litter types 9 3 elevtions 9 6 smple dtes). All the smples t ech smpling period were dried t 8 C to mesure their mss losses, nd then ground to pss through 1-mm sieve to determine concentrtions of cron (C), nitrogen (N), phosphorus (P), potssium (K), clcium (C), nd mgnesium (Mg) s per the methods descried in AOAC (1984). Litter chemistry ws mesured y sequentilly digesting mteril into frctions tht correspond with solule cell contents, cellulose, hemicellulose, lignin, nd cid insolule sh (Vn Soest 1963; Ryn et l. 199) on forge fier nlyzer (ANKOM 2, Mcedon, New York, USA). All the nutrient concentrtions were clculted on the sis of orgnic mtter (i.e., dry mtter minus crude shes). Soil temperture nd soil moisture long the elevtion grdient At the center of ech plot, wether sttions (Onset Computer Corportion, Jpn) were used to monitor soil temperture nd soil moisture t 5-cm soil depth. The sensors were connected to CR1 dtlogger, nd soil temperture nd soil moisture were mesured every 1 min, nd then 3-min verges were stored. Dt clcultion nd nlysis Repeted-mesures nlysis of vrition of mss losses of different elements in litter ws used with elevtion nd species s etween-suject effects nd with smpling dte s the within-suject effect, using GLM nlysis in SPSS (Version 12, SPSS Inc. Chicgo, Illinois, USA, 25). For ech species nd smpling dte, the significnt difference etween elevtions ws ssessed y One-wy ANOVA nd lest significnce difference (LSD) tests. Temperture sensitivity of litter mss loss ws defined s the reltive differences in litter mss losses (%) etween two different elevtions divided y the nnul verge temperture difference ( o C) of the corresponding elevtions. Liner regression nlysis ws performed to test the dependency of the nnul verge percentge mss losses of litter on differences in nnul verge soil temperture t 5-cm depth mong elevtions. The significnce of temperture sensitivity of mss losses (i.e., the slopes of the regression equtions) mong different litter types ws tested using pired comprisons. Prtil correltion nlysis ws performed to test the dependency of C mss losses in ech smpling period on the proposed litter qulity vriles nd verge soil temperture nd soil moisture t 5 cm. All the significnces mentioned in the text were t the.5 level. Results Soil temperture nd moisture long the elevtion In generl, lthough there ws no significnt difference etween 32 nd 36 m for nnul soil temperture nd soil moisture t 5 cm, they decresed with n increse in elevtion (P \.5). Their nnul verge vlues over two yers were 3.3, 2.8, nd.1 C for soil temperture (Fig. 2), nd 18.6, 17.8, nd 8.1% for soil moisture t 32, 36, nd 38 m, respectively (Fig. 2). However, the ptterns of the differences in soil temperture nd soil moisture t 5 cm mong elevtions differed significntly etween the growing seson (My to Octoer) nd nongrowing seson. For exmple, during the growing seson, the soil tempertures t 5 cm were 9.5, 7.5, nd 5.8 C t

4 26 Plnt Ecol (21) 29: Fig. 2 Dynmics of soil temperture t 5 cm () nd soil moisture t 2 cm ()t different elevtions ove se level from August 26 to July 28 A Annul soil temperture t 5 cm ( o C) Elevtion (m) 1 Soil temperture t 5 cm ( C) Aug. 6 Nov. Fe. My Aug. Nov. Fe. My Soil moisture t 5 cm (%) B Elevtion (m) Aug.6 Nov. Fe. My Aug. Nov. Fe. My Annul soil moisture t 2 cm (%) 32, 36, nd 38 m, respectively, wheres during the nongrowing seson they were -2.9, -1.8, nd -5.6 C, respectively, indicting tht during the nongrowing seson, the verge soil temperture t 5 cm ws significntly higher t 3,6 m due to temperture inversion on the foot of the mountin. During the growing seson, soil moisture ws 26.2, 27.6, nd 14.4% t 32, 36, nd 38 m, respectively, indicting tht the men soil moisture t 5 cm ws lowest t 3,8 m during the growing seson, especilly from June to July in 27 nd 28 (Fig. 2) due to the effect of topogrphy on rinfll. Initil litter chemicl components nd their mss losses The initil litter chemistry of E. nutns, B. sinocompressus, D. fruticos, nd mixed litters of the communities showed significnt differences (Tle 1). When sorting the species nd mixed litters from high to low litter qulity, ech of the potentil litter-qulity vriles clssified the litter types differently. The shru species, D. fruticos, ws n exception, hving significntly lower litter qulity when mesured y ll of the vriles except percentges of phosphorus nd clcium. The other litter types displyed more

5 Plnt Ecol (21) 29: Tle 1 Initil chemistry of litter smples on the sis of orgnic mtter in different litter Litter %C %N %P %K %N %C %Mg %HC %Cellu %Lig C:N C:P N:P Lig:N EN 24.5c 1.1c c 23.9c 4.6d c 6.6c 4.1c BS 24.7c 1.3.1c 1.1c.23.7c d 18.3d c DF d c d 2.8d 15.5 Mixed d c 19.8c c EN Elymus nutns, BS Blysmus sinocompressus, DF Dsiphor fruticos, Mixed mixed litter of the community under free-grzing. HC hemicellulose, Cellu cellulose, nd Lig lignin. Vlues followed y different letters for the sme column men significnt differences t.5 level vrition in reltion to one or nother vrile. E. nutns hd the lowest N concentrtion nd highest C:N rtio (low litter qulity), ut the most fvorle litter qulity reltive to severl other vriles (e.g., percentge lignin, lignin:n rtio). Mixed litter hd the highest litter qulity ccording to N concentrtion nd C:N rtio. However, when evluting the other vriles (e.g., percentge of cellulose nd hemicellulose), the litter qulity of the mixed litter diminished compred to the other litter types. Vlues for B. sinocompressus were generlly intermedite mong the litter types. Since ech of the litter-qulity vriles clssifies the litter qulity differently in reltion to ech other, there exists rnge of possiilities for utilizing litter qulity s predictor of mss losses. All element (C, N, P, K, N, C, nd Mg) mss losses were ffected significntly y elevtion, smpling dte, litter type, nd their interctions (Tle 2 nd Figs. 3, 4 nd 5). In generl, their mss losses decresed significntly with n increse in elevtion, ut the differences ecme smller with decomposition time. For exmple, the verge cumultive C mss losses for ll litter types were 28.1, 19.9, nd 14.5%, nd 6.8, 57.3, nd 54.7% during 54-dy nd 57-dy decompositions t 32, 36, nd 38 m, respectively. The verge reltive differences of C mss losses were greter y 93.8 nd 37.2% t 32 nd 36 m compred with tht t 3,8 m during 54-dy decomposition, wheres the differences were only 11.2 nd 4.8% during 57-dy decomposition, indicting tht the potentil effect of climte vriles (temperture, moisture) on litter decomposition decreses with decomposition time (Tle 3). The verge cumultive mss losses of grss litter (i.e., E. nutns nd B. sinocompressus) were greter thn tht of shru litter (D. fruticos) for C, P, K, N, nd Mg, wheres the opposite ws oserved for N during the decomposition period. The verge cumultive mss losses of C nd N of the mixed litter were the gretest during the decomposition period. The effect of litter qulity on litter decomposition vried with decomposition time. For exmple, the verge cumultive C mss losses for ll elevtions were 18.2, 21., 27.1, nd 17.1%, nd 58.6, 59.5, 63.2, nd 49.1% for E. nutns, B. sinocompressus, mixed litter, nd D. fruticos during 54-dy nd 57-dy decompositions, respectively. The reltive differences of C Tle 2 Summry of repeted-mesures nlysis of vrition in mss losses of different elements in litter using elevtion nd species s etween-suject effects nd smpling dte s within-suject effect with GLM nlysis in SPSS (Version 12) Element E D S E 9 D E 9 S D 9 S E 9 D 9 S C \.1 \.1 \.1 \.1 \.1 \.1 \.1 N \.1 \.1 \.1 \.1 \.1 \.1 \.1 P \.1 \.1 \.1 \.1.34 \ K \.1 \.1 \.1 \.1 \.1 \ N \.1 \.1 \.1 \.1 \.1 \ C \.1 \.1 \.1 \.1 \.1 \.1 \.1 Mg \.1 \.1 \.1 \.1.3 \.1.91 E elevtion, D smpling dte, S species/community mixture

6 262 Plnt Ecol (21) 29: Fig. 3 Cumultive cron mss loss (%) of different litter types t different elevtions ove se level. Men ± SE is shown in the figure Cumultive cron mss loss (%) A Elymus nutns 7 Blysmus sinocompressus Dys of decomposition Dys of decomposition Cumultive cron mss loss (%) B Cumultive cron mss loss (%) Dsiphor fruticos C Dys of decomposition Cumultive cron mss loss (%) Mixed litter D Dys of decomposition Fig. 4 Cumultive nitrogen mss loss (%) of different litter types t different elevtions ove se level. Men ± SE is shown in the figure Cumultive nitrogen mss loss (%) Elymus nutns A Percentge of nitrogen mss loss (%) Blysmus sinocompressus Dys of decomposition Dys of decomposition B Percentge of nitrogen mss loss (%) Dsiphor fruticos C Dys of decomposition Percentge of nitrogen mss loss (%) Mixed litter D Dys of decomposition mss losses were 6.4, 22.8, nd 58.5% during 54-dy decomposition, nd 19.3, 21.2, nd 28.7% during 57-dy decomposition compred with tht of D. fruticos (Tle 3), indicting tht the potentil effect of litter qulity on C mss loss my e controlled y different litter qulity vriles with decomposition time. In the erly stges of litter decomposition (i.e., during the 54-dy decomposition period), the effect of climte vriles on litter decomposition ws greter thn tht of litter qulity sed on the verge reltive differences (%) of C mss losses t different elevtions compred with tht of 3,8 m nd the verge reltive differences (%) of C mss losses of different litter types compred with tht of D. fruticos, wheres the opposite ws the cse over the longer decomposition period (i.e., during 57-dy decomposition) (Tle 3). In order to determine whether litter decomposition occurs in two-stge process, we evluted the differences in C mss loss of ech species/community

7 Plnt Ecol (21) 29: Fig. 5 Cumultive phosphorus mss loss (%) of different litter types t different elevtions ove se level. Men ± SE is shown in the figure Cumultive phosphorus mss loss (%) Elymus nutns A Cumultive phosphorus mss loss (%) Dys of decomposition Dys of decomposition Blysmus sinocompressus B Cumultive phosphorus mss loss (%) Dsiphor fruticos Dys of decomposition C Cumultive phosphorus mss loss (%) Mixed litter D Dys of decomposition Tle 3 Reltive differences (%) in C mss losses of different litter types t different elevtions compred with tht t 3,8 m, nd of different species litter for Elymus nutns (EN), Blysmus sinocompressus (BS), nd mixed litter (ML) compred with tht of Dsiphor fruticos (DF) during 57-dy nd 57- dy decomposition periods Compred with 3,8 m Compred with DF 54-dy 57-dy 54-dy 57-dy Elevtion (m) 3,2 3,6 3,2 3,6 3,2 3,6 3,8 3,2 3,6 3,8 EN BS ML DF / / / / / / mixed litter etween elevtions during the first growing seson, during the nongrowing seson nd during the second growing seson (Fig. 6). We found tht for ll litter types, the reltive cumultive C mss losses during the first growing seson s proportion of totl cumultive C mss losses over the 57-dy decomposition period significntly decresed with n increse in elevtion. During the second growing seson, however, it ws lowest t 3,2 m nd even significntly incresed with n increse in elevtion for ll litter types except D. fruticos, lthough there ws no significnt difference etween 36 nd 38 m (Fig. 6). The verge cumultive mss losses of C, N, P, K, N, C, nd Mg for ll litter types nd elevtions during the 57-dy decomposition period were 57.6, 46.2, 78.3, 58.9, 75., 76.6, nd 63.8%, respectively, indicting tht N mss loss ws less thn tht of other elements. Fctors ffecting cron mss loss In generl, sed on prtil correltion nlysis of results from 6 smplings over the 57-dy decomposition period, we found tht high C concentrtion nd high rtios of N:P nd lignin:n in initil nd remining litter reduced C mss losses, wheres positive correltions were oserved etween C mss losses nd concentrtions of N, P, N, cellulose, nd rtios of C:N nd N:P in initil nd remining litters.

8 264 Plnt Ecol (21) 29: Fig. 6 Cumultive cron mss loss under specified decomposition periods s percentge of totl cumultive cron mss loss over the whole decomposition period. FGS first growing seson (114 dys), NGS nongrowing seson (181 dys), nd SGS second growing seson (212 dys). Different letters men significnt difference t.5 level nd rs in the figure show stndrd errors Cumultive C mss loss (%) c Elymus nutns FGS NGS SGS Decomposition period c A Cumultive C mss loss (%) c Blysmus sinocompressus FGS NGS SGS Decomposition period c B Cumultive C mss loss (%) Dsiphor fruticos C Cumultive C mss loss (%) c Mixed c D FGS NGS SGS Decompositon period FGS NGS SGS Decomposition period Soil temperture nd soil moisture did not ffect C mss losses (Tle 4). However, if one excludes the nongrowing seson from 24 Octoer 27 to 24 June 28, C mss losses incresed significntly with increses in soil temperture nd concentrtions of N, P, N, cellulose nd C:N rtio (Tle 4). There were no significnt prtil correltions etween C mss losses nd ny vriles in Tle 1 during the nongrowing seson (dt not shown). During the first growing seson of the decomposition experiment, soil temperture, N concentrtion nd rtios of C:N nd lignin:n were the min fctors tht ffected C mss losses in litter, wheres prtil correltion ws not significnt etween C mss losses nd soil temperture during the second growing seson of the decomposition experiment (Tle 4). These results indicte tht C mss losses in litter ws minly controlled y soil temperture during the erly phse of the Tle 4 Prtil correltion efficiencies of ech listed vrile in Tle 1 for ech smpling dte with cron mss losses during specified decomposition periods controlling for the other listed vriles DP # C% N% P% N% Cellu% C:N C:P N:P Lignin:N ST ODP -.159**.243**.14**.278**.166**.262**.14** -.141** -.252** ODPEW.132*.132*.4**.169**.144* -.195**.35** FGS.292**.423** -.463**.699** SGS.419**.197** -.216** DP decomposition period, ODP over the decomposition period, ODPEW over the decomposition period except winter from 14 Octoer 27 to 24 April 28, FGS first growing seson from 1 June 27 to 14 Octoer, nd SGS second growing seson from 24 April to 24 Octoer 28. Cellu cellulose, ST soil temperture t 5 cm. Only significnt prtil correltions re shown in the Tle *, ** men significnt t.5 nd.1 level, respectively

9 Plnt Ecol (21) 29: Litter mss losses (%) decomposition period nd then y litter qulity during the lter phse of the decomposition period, nd tht soil moisture ppered to hve no significnt effect on C mss losses in litter. Temperture sensitivity of litter mss losses We mesured the reltionships etween the percentges of the litter mss losses during the first yer (i.e., over 386-dy decomposition period) nd the differences of nnul soil temperture t 5 cm cross the elevtion grdient. The reltionships were wek for the mixed litter (P =.52) nd D. fruticos (P =.9). The temperture sensitivities of the litter mss loss of the mixed litter, B. sinocompressus, E. nutns, nd D. fruticos were pproximtely 4.9, 5.6, 8.7, nd 3.4, respectively (Fig. 7), which were significntly different (P =.16). Discussion M EN BS DF Difference in nnul soil temperture t 5 cm etween elevtions ( C) Fig. 7 Reltionships etween litter mss loss (%) for 386 dy decomposition period cross the elevtion grdient nd differences in nnul verge soil temperture t 5 cm depth mong elevtions. The regression equtions re EN: y = x (r 2 =.99, P =.2); BS: y = x (r 2 =.99, P =.13); M: y = x (r 2 =.97, P =.52); nd DF: y = x? (r 2 =.86, P =.9), respectively, where y is the litter mss loss for 386-dy decomposition period cross the elevtion grdient, nd x is the difference in nnul verge soil temperture t 5 cm mong elevtions. M: mixed litter, BS: Blysmus sinocompressus, EN: Elymus nutns, DF: Dsiphor fruticos Mny decomposition studies hve identified lignin concentrtion nd the rtios of lignin:n nd C:N s the most relile predictors of decomposition (Berg et l. 1982; Stohlgren 1988; Tylor et l. 1989; Running nd Hunt 1993; Prton et l. 1994; Murphy et l. 1998). In prticulr, Murphy et l. (1998) suggested tht litter mss loss ws limited y C sustrtes rther thn y nutrient content, including N concentrtion. However, our results indicted tht N concentrtions nd rtios of C:N nd lignin:n in litter re the est predictors of C mss losses, lthough the reltionships etween C mss loss nd litter nutrient concentrtion or rtios of C:N, C:P, N:P, nd lignin:n vried with decomposition periods (Tle 4). Significnt effects of litter N content on litter mss loss hve een oserved in other mesic systems (Tylor et l. 1989). High N concentrtion in litter consistently promoted fster C mss loss during the summer decomposition periods (Tle 4), wheres the chemicl components in litter did not ffect C mss losses during the winter decomposition period due to slow decomposition with low temperture. Along with the chnges in decomposition dynmics, some fctors controlling C mss loss switched from the rtios of C:N nd lignin: N during the first growing seson to the rtios of C:P nd N:P during the second growing seson (Tle 4). It is likely tht vrition of resource rtios in litter with decomposition time nd stoichiometric constrints result in vrition of the decomposer community (Cherif nd Loreu 27), ecuse vrition in the elementl composition of cteri nd fungi is very smll (Mkino et l. 23). It is known, for exmple, tht cteri nd fungi hve different C:P nd C:N rtios nd tht the sustrte C:N rtio cn hve n influence on the fungl/cteri iomss rtio (Eilnd et l. 21). We found tht N mss loss ws less thn tht of other elements in litter, cusing lower C:N rtios nd higher N:P rtios in the remining litter over decomposition time. Similrly, the vst numer of limiting resources creted y the comintion of vrious essentil elements (e.g., P nd N) in orgnic resources with different possile elementl rtios my ffect the structure of the decomposer community (Cherif nd Loreu 27). Inconsistent with previous reports tht low C:N rtio promotes fster rte of decomposition (Berg et l. 1982), or reports tht there is no reltionship etween the C:N rtio in litter nd mss losses (Murphy et l. 1998), we found tht there were positive correltions etween C:N rtios nd C mss losses during different decomposition periods, except during winter nd the second growing seson

10 266 Plnt Ecol (21) 29: (Tle 4). Two resons cn e proposed to explin this difference. First, the sttisticl nlysis method used ws different. Significnt negtive correltions (P \.5) were found etween C mss losses nd C:N rtios in litter when simple correltion nlysis ws used (dt not shown). However, positive prtil correltions were found etween C mss losses nd C:N rtios in litter during the decomposition periods when prtil correltion nlysis ws used (Tle 4), indicting tht simple negtive correltions etween C mss losses nd C:N rtios in litter were cused y other correlted vriles rther thn the C:N rtio itself (Tilmn nd Downing 1994). Second, lthough initil higher C:N rtio for Dsiphor fruticos (Tle 1) cused lower C mss loss (i.e., negtive correltion), C:N rtios in the remining litter decresed with decomposition time due to N mss losses which mtched the vrition in C mss losses (i.e., positive correltion). Therefore, it is importnt to exercise cution when directly compring the reltionship etween C mss loss nd C:N rtio in litter in the literture since the nlysis method used to clculte the correltions cn vry significntly cross studies. The effect of soil temperture on C mss losses vried with decomposition period (Tle 4). Soil temperture explined pproximtely 67% of vrition in C mss loss during the first growing seson, ut it did not ffect C mss loss during the second growing seson. These results indicte tht with the decomposition time the effect of qulity of the remining litter on C mss loss my override the effect of soil temperture during the second growing seson. Soil moisture did not ffect litter mss losses in the region. However, Murphy et l. (1998) found t elow 2,5 m elevtion in the Southwest USA tht litter mss loss ws minly controlled y lignin concentrtion nd precipittion rther thn temperture. Therefore, in our colder nd wetter region t more thn 3,2 m elevtion, temperture my e n importnt fctor controlling C mss loss during the first summer, nd future summer wrming my increse C mss loss from litter. Litter mss loss nd its temperture sensitivity for the low litter qulity shru, D. fruticos, ws low compred with grsses (Fig. 7), which is consistent with previous reports (Cornelissen et l. 27). As wrming increses invsion of shrus to the community in the lpine nd tundr regions (Klein et l. 27, 28; Post et l. 28), future wrming my reduce the mount of cron relesed to the tmosphere from litter in the region. The temperture sensitivity of litter mss loss will influence rtes of ecosystem cron sequestrtion in wrmer world (Jones et l. 23; Fierer et l. 25), in prticulr in our study re. Bostt nd Agren (1999) hve hypothesized tht the temperture sensitivity of litter mss loss is governed y microil enzyme kinetics nd will e influenced y the qulity of the litter C eing consumed y microorgnisms. This hs een clled the C qulity-temperture hypothesis y Fierer et l. (25), who first determined the hypothesis nd suggested tht the temperture sensitivity of microil decomposition should e inversely relted to litter C qulity. However, we found tht there were no significnt correltions etween the temperture sensitivity of litter mss loss nd the initil chemicl components in litter, even though shru with the highest lignin concentrtion (Tle 1) hd the lowest temperture sensitivity (Fig. 6). This is proly due to different pproches used to chrcterize temperture sensitivity (Q 1 used in Fierer et l. 25). Conclusion Higher tempertures significntly incresed element litter mss losses, which suggests tht glol wrming will decrese C storge in the study site, unless shru encrochment is extensive. The N concentrtions nd rtios of C:N nd lignin:n in litter were the est predictors of C mss loss. The effect of soil temperture on C mss losses ws greter thn tht of litter nutrient qulity during the first growing seson of the decomposition period, wheres C mss loss in litter ws minly controlled y N concentrtion nd the rtios of C:P nd N:P in remining litter during the second growing seson of the decomposition period. The mss losses of grss litter nd their temperture sensitivity were greter thn tht of shru litter. Therefore, our study on the temperture sensitivity of litter mss losses my hve importnt implictions for predictions out future contriutions of lpine nd possily lso other cold regions to cron dynmics worldwide.

11 Plnt Ecol (21) 29: Acknowledgments This reserch ws funded y the 1- Tlent Progrm of the Chinese Acdemy of Sciences, the Knowledge Innovtion Progrms (KZCX2-YW-N-4/XB2-6-1), the Chinese Ntionl Nturl Science Foundtion Commission ( ), nd Qinghi Science nd Technology Bureu. References ACIA (25) Arctic climte impct ssessment. Cmridge University Press, Cmridge Aerts R (1997) Climte, lef litter chemistry, nd lef litter decomposition in terrestril ecosystems: tringulr reltionship. Oikos 79: AOAC (1984) Officil methods of nlysis of the Assocition of Officil Anlyticl Chemists, 14th edn. Assocition of Officil Anlyticl Chemists, Wshington, DC Berg B, Wessen B, Ekohm G (1982) Nitrogen level nd decomposition in Scots pine litter. Oikos 38: Berg B, Berg MP, Bottner P, Box EO et l (1993) Litter mss loss rtes in pine forests of Europe nd estern United Sttes: some reltionships with climte nd litter qulity. Biogeochemistry 2: Berg B, Ekohm G, Johnsson ME, McClugherty CA, Rutiglino F, Snto AV (1996) Mximum decomposition limits of forest litter types: synthesis. Cn J Bot 74: Bostt E, Agren GI (1999) Soil orgnic mtter qulity interpreted thermodynmiclly. Soil Biol Biochem 31: Co G, Xu XL, Long RJ, Wng QL, Wng CT, Du YG, Zho XQ (28) Methne emissions y lpine plnt communities in the Qinghi-Tiet plteu. Biol Lett 4: Cherif M, Loreu M (27) Stoichiometric constrints on resource use, competitive interctions, nd elementl cycling in microil decomposers. Am Nt 169: Chinese Soil Txonomy Reserch Group (1995) Chinese soil txonomy. Science Press, Beijing, pp Cornelissen JHC, vn Bodegom PM, Aerts R et l (27) Glol negtive vegettion feedck to climte wrming responses of lef litter decomposition rtes in cold iomes. Ecol Lett 1: Dlis P, Anderson J, Bottner P, Couteux M (21) Temperture responses of cron minerliztion in conifer forest soils from different regionl climtes incuted under stndrd lortory conditions. Glo Chng Biol 6: Eilnd F, Klmer M, Lind AM, Bth E (21) Influence of initil C/N rtio on chemicl nd microil composition during long term composting of strw. Micro Ecol 41: Fierer N, Crine JM, McLuchln K, Schimel JP (25) Litter qulity nd the temperture sensitivity of decomposition. Ecol 86: Hrmon ME, Bker GA, Spycher G, Greene E (199) Leflitter decomposition in the Pice/Tsug forests of Olympic Ntionl Prk, Wshington, USA. For Ecol Mnge 31:55 66 Hoie SE (2) Interctions etween litter lignin nd soil nitrogen vilility during lef litter decomposition in Hwiin montne forest. Ecosystems 3: Hoie SE, Vitousek PM (2) Nutrient limittion of decomposition in Hwiin forests. Ecol 81: IPCC (27) Climte Chnge 27: summry for policymker. Vlenci, Spin Jones CD, Cox P, Huntingford C (23) Uncertinty in climte-cron-cycle projections ssocited with the sensitivity of soil respirtion to temperture. Tellus B Chem Phys Meteorol 55: Ktterer T, Reichstein M, Andren O, Lomnder A (1998) Temperture dependence of orgnic mtter decomposition: criticl review using literture dt nlysed with different models. Biol Fert Soil 27: Kirschum MUF (1995) The temperture dependence of soil orgnic mtter decomposition, nd the effect of glol wrming on soil orgnic C storge. Soil Biol Biochem 27: Klein JA, Hrte J, Zho XQ (27) Experimentl wrming, not grzing, decreses rngelnd qulity on the Tietn plteu. Ecol Appl 17: Klein JA, Hrte J, Zho X-Q (28) Decline in medicinl nd forge species with wrming is medited y plnt trits on the Tietn plteu. Ecosystems 11: Mkino W, Cotner JB, Sterner RW, Elser J (23) Are cteri more like nimls thn plnts? growth rte nd resource dependence of cteril C:N:P stoichiometry. Funct Ecol 17: McTiernn KB, Coüteux MM, Berg B et l (23) Chnges in chemicl composition of Pinus sylvestris needle litter during decomposition long Europen coniferous forest climtic trnsect. Soil Biol Biochem 35: Meentemeyer V (1978) Mcroclimte nd lignin control of litter decomposition rtes. Ecol 59: Melillo JM, Aer JD, Murtore JF (1982) Nitrogen nd lignin control of hrdwood lef litter decomposition dynmics. Ecol 63: Murphy KL, Kloptek JM, Kloptek CC (1998) The effects of litter qulity nd climte on decomposition long n elevtion grdient. Ecol Appl 8: Prton WJ, Ojim DS, Cole CV, Schimel DS (1994) A generl model for soil orgnic mtter dynmics: sensitivity to litter chemistry, texture, nd mngement. In: Quntittive modeling of soil forming processes. Soil Science Society of Americn Specil Puliction 39, pp Mdison, Wisconsin, USA Post E, Pedersen C, Wilmers CC, Forchhmmer MC (28) Phenologicl sequences revel ggregte life history response to climtic wrming. Ecol 89: Running SW, Hunt ER Jr (1993) Generliztion of forest ecosystem process model for other iomes, BIOME-BGC, nd n ppliction for glol-scle models. In: Ehleringer JE, Field CB (eds) Scling physiologicl processes: lef to glol. Acdemic Press, New York, pp Ryn M, Melillo J, Ricc A (199) A comprison of methods for determining proximte cron frctions of forest litter. Cn J For Res 2: Stohlgren TJ (1988) Litter dynmics in two Sierrn mixed conifer forests. II. Nutrient relese in decomposing lef litter. Cn J For Res 18:

12 268 Plnt Ecol (21) 29: Tylor BR, Prkinson D, Prsons WFJ (1989) Nitrogen nd lignin content s predictors of litter decy rtes: microcosm test. Ecol 7:97 14 Thompson LG, Mosley-Thompson E, Dvis M et l (1993) Recent wrming: ice core evidence from tropicl ice cores with emphsis on Centrl Asi. Glo Plnet Chnge 7: Thompson LG, Yo T, Mosley-Thompson E et l (2) A high-resolution millennil record of the South Asin monsoon from Himlyn ice cores. Science 289: Tilmn D, Downing JA (1994) Biodiversity nd stility in grsslnds. Nture 367: Vn Soest PJ (1963) Use of detergents in nlysis of firous feeds: rpid method for the determintion of fier nd lignin. J Assoc Off Anl Chem 46: WRB (1998) World reference se for soil resources. FAO/ ISRIC/ISSS, Rome Zho XQ, Zhou XM (1999) Ecologicl sis of lpine medow ecosystem mngement in Tiet: Hiei lpine medow ecosystem reserch sttion. Amio 28: Zheng D, Zhng QS, Wu SH (2) Mountin geoecology nd sustinle development of the Tietn plteu. Kluwer Acdemic, Norwell