Soil Biology & Biochemistry

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1 Soil Biology & Biochemistry 42 (21) 944e92 Contents lists ville t ScienceDirect Soil Biology & Biochemistry journl homepge: Effects of wrming nd grzing on N 2 O fluxes in n lpine medow ecosystem on the Tietn plteu Yigng Hu,d, Xiofeng Chng,d, Xingwu Lin c, Ynfen Wng d, Shiping Wng,, *, Jichung Dun,d, Zhenhu Zhng,d, Xioxi Yng,d, Ciyun Luo,d, Gungping Xu,d, Xinqun Zho Key Lortory of Adpttion nd Evolution of Plteu Biot, Hiei Alpine Medow Ecosystem Reserch Sttion, Northwest Institute of Plteu Biology, Chinese Acdemy of Sciences, Xining 818, Chin Institute of Tietn Plteu Reserch, Chinese Acdemy of Sciences, Beijing 18, Chin c Institute of Soil Science in Nnjing, Nnjing 218, Chin d Grdute University, Chinese Acdemy of Sciences, Beijing 149, Chin rticle info strct Article history: Received 22 Octoer 29 Received in revised form 29 Jnury 21 Accepted 14 Ferury 21 Aville online 26 Ferury 21 Keywords: Glol wrming Grzing N 2 O flux Soil temperture Soil moisture Alpine medow FATE Tietn plteu A gret del of uncertinty is ssocited with estimtes of glol nitrous oxide (N 2 O) emissions ecuse emissions from rid nd polr climtes were not included in the estimtes due to lck of ville dt. In prticulr, very few studies hve ssessed the response of N 2 O flux to grzing under future wrming conditions. This experiment ws conducted to determine the effects of wrming nd grzing on N 2 O flux t different time scles for three yers under controlled wrming-grzing system. A free-ir temperture enhncement system (FATE) using infrred heters nd grzing significntly incresed soil tempertures for oth of growing (verge 1.8 C in 28) nd no-growing sesons (verge 3. C for 3-yers) within 2-cm depth, ut only wrming reduced soil moisture t 1-cm soil depth during the growing seson during the drought yer of 28. Generlly, the effects of wrming nd grzing on N 2 O flux vried with smpling dte, seson, nd yer. No interctive effect etween wrming nd grzing ws found. Wrming did not ffect nnul N 2 O flux when grzing ws moderte during the growing seson ecuse the trdeoff of the effect of wrming on N 2 O flux ws oserved etween the growing seson nd no-growing seson. No-wrming with grzing () nd wrming with grzing () significntly incresed the verge nnul N 2 O flux (7.8 nd 31.%) compred with no-wrming with no-grzing () nd wrming with no-grzing (), respectively, indicting tht wrming reduced the response of N 2 O flux to grzing in the region. Winter ccounted for 36e7% of nnul N 2 O flux for nd, wheres only for e8% of nnul N 2 O flux for nd. Soil temperture could explin e3% of nnul N 2 O flux vrition. Ó 21 Elsevier Ltd. All rights reserved. 1. Introduction Nitrous oxide (N 2 O) is one of the mjor greenhouse gses (IPCC, 27). Intensive studies in temperte grsslnd/steppe ecosystems nd griculturl lnd in Euro-Asi, North Americ, Austrli, nd New Zelnd hve reveled tht N 2 O fluxes vry with vegettion types, soil properties, climte conditions, nd lnd uses, nd tht the role of N 2 O emissions from grsslnds in the world is n importnt considertion in the glol N 2 O udget (Mosier et l., 1991, 1996, 1998, 22; Velthof nd Oenem, 199; Fless et l., 1996; * Corresponding uthor t: Institute of Tietn Plteu Reserch, Chinese Acdemy of Sciences, Beijing 18, Chin. Tel.: þ ; fx: þ E-mil ddress: wngship28@yhoo.cn (S. Wng). Mummey et l., 1997, 2; Bll et l., 1999; Breuer et l., 2; Billings et l., 22; Xu et l., 23,; Sggr et l., 24; Wng et l., 2; Du et l., 26; M et l., 26; Holst et l., 27; Mljnen et l., 27; Brton et l., 28; Brümmer et l., 28). Stehfest nd Bouwmn (26) recently clculted tht 1.8 Tg N 2 OeN yr 1 is emitted glolly from grsslnds. However, gret del of uncertinty is ssocited with estimtes of glol N 2 O emissions ecuse emissions from rid, polr, nd orel climtes were not included in the estimtes due to lck of ville dt, especilly dt during the winter (Bouwmn et l., 2; Stehfest nd Bouwmn, 26). In temperte ecosystems, prticulr interest hs focused on winter fluxes of N 2 O ecuse much of the nnul flux ppers to occur during winter nd during the trnsition from winter to spring, when freeze-thw events re common (Brumme et l., 1999; Groffmn et l., 2, 26; Butterch-Bhl et l., 22) /$ e see front mtter Ó 21 Elsevier Ltd. All rights reserved. doi:1.116/j.soilio

2 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e92 94 Most studies in glol chnge iology hve focused on the growing seson. However, there is mple evidence of mrked chnges in climtic conditions during winter (Schwrtz nd Reiter, 2; Dye, 22; Hodgkins et l., 23; Wolfe et l., 2) nd evidence tht this chnge hs implictions for fluxes in greenhouse gses in grsslnd ecosystems (Groffmn et l., 2, 26). In the originl DeNitrifictioneDeComposition (DNDC) model, N 2 O fluxes to the tmosphere were ssumed to e zero when the soil ws snow covered or frozen in ny of the lyers of e3 cm soil profiles (Li, 2). This ssumption is inconsistent with mny pulished field mesurements (Sommerfield et l., 1993; Röver et l., 1998; Xu et l., 23,), nd winter dynmics hve een shown to e prticulrly importnt for soil-tmosphere fluxes of greenhouse gses (Sommerfield et l., 1993; Brooks et l., 1997; Alm et l., 1998; Brumme et l., 1999; Groffmn et l., 26). Field mesurement results showed tht up to 4% of the nnul N 2 O losses occurred during the no-growing seson nd confirmed the importnce of spring nd utumn periods for ssessment of totl N 2 O losses from semi-rid temperte grsslnds (Xu et l. 23,; Groffmn et l., 26). Röver et l. (1998) lso reported significnt increse in N 2 O concentrtions elow the snow cover, which indictes restricted diffusion of N 2 O through the snow. The Tietn plteu ccounts for out 2% of the totl country re in Chin. Approximtely 4% of the Tietn plteu is lpine medow, which is widely used for grzing. Evidence shows tht the Tietn plteu is experiencing climtic wrming (Thompson et l., 1993, 2). Mny studies from tundr ecosystems (Jonsson et l., 1993; Schmidt et l., 1999, 22) suggest tht ltered N cycling in lpine ecosystems my e key response to climte nd grzing perturtions. However, there hve een very few studies ssessing the response of N 2 O flux to vrition in winter conditions (either nturl or mnipulted) (Schimel et l., 24; Groffmn et l., 21, 26), especilly to grzing in wrming conditions. To improve our knowledge of glol terrestril N 2 O losses, we need to understnd N 2 O emissions from grzing in wrming ecosystem in this region. To contriute to our understnding of N cycling nd N trce gs exchnge, nd especilly to the role of the lpine ecosystem in glol N 2 O udgets in future wrming conditions, study ws conducted in n lpine medow on the Tietn plteu. This study ws conducted with wrming nd grzing y mens of closedchmer mesurements of soiletmosphere N 2 O exchnge in the field from 26 to 29. The specific ims of this study were, for the first time, to (1) oserve temporl vrition in N 2 O flux t different time scles (i.e., dily, monthly, sesonlly, nd nnully), especilly for the contriution of N 2 O emission during the no-growing sesons; (2) evlute the effects of wrming nd grzing on N 2 O flux through controlled wrming with grzing experiment; nd (3) investigte the reltionships etween N 2 O flux nd soil temperture nd soil moisture. 2. Mterils nd methods 2.1. Controlled wrmingegrzing experiment A detiled description of the experimentl site, the design of the controlled wrming with grzing experiment heted y the free-ir temperture enhncement system (FATE), nd mesurements of soil temperture nd soil moisture cn e found in Zho nd Zhou (1999), Kimll et l. (28), nd Luo et l. (29,). In rief, in My 26 eight hexgonl rrys of Mor FTE (1 W, 24 V) infrred heters were deployed over vegettion cnopy tht hd previously een hevily grzed y sheep during cool sesons from Octoer to My of prior yers t the Hiei Alpine Medow Ecosystem Reserch Sttion with eight dummy rrys over reference plots. The heters were controlled using the proportionl-integrl-derivtive-outputs (PID) control system so s to ensure constnt wrming etween heted nd reference plots. The setpoint differences etween heted nd corresponding reference plots were 1.2 C during dytime nd 1.7 C t night during the growing seson (from My to Septemer). During no-growing seson, (from Octoer to April), ecuse some infrred thermometers were not working, the power outputs of the heters were mnully set t 1 W per plot. A two fctoril design (wrming nd grzing) ws used with four replictes of ech of four tretments: no-wrming with no-grzing (), no-wrming with grzing (), wrming with no-grzing (), nd wrming with grzing (). In totl,16 plots of 3-m dimeter were used in complete rndomized lock distriution in the field. One dult Tietn sheep ws fenced in the grzing plots in the morning of 17 August 26 for pproximtely 2 h. The cnopy height ws out 8e9 cm efore grzing nd 4e cm fter grzing. The stocking rte roughly corresponded to moderte stocking rte in the region. Similrly, two dult Tietn sheep were fenced for pproximtely 1 h in the grzing plots in the mornings of 12 July, 3 August, nd 12 Septemer in 27, nd 8 July nd 2 August in 28. Men temperture nd totl rinfll during the growing sesons from 1 My to 2 Septemer in 26, 27, nd 28 were 8.4, 8., nd 8.1 C, nd 449.2, 397.6, nd mm, respectively. The sesonl rinfll distriution nd grzing time re shown in Fig. 1. At cm inside the edge of ech plot, type-k thermocouples (Cmpell Scientific, Logn, Uth, U.S.A) were used to utomticlly mesure soil temperture t depths of, 1, nd 2 cm every 1 min, nd 1 min verges were stored. Soil moisture t depths of 1, 2, 3, nd 4 cm ws mnully mesured though tue in the ground down to 4 cm depth using frequency domin reflectometer (FDR; Model Diviner-2, Sentek Pty Ltd., Austrli) t 8:, 14:, nd 2: every dy. The soil moisture ws expressed s volume percentge (%) or mm/1 cm. All dt were collected from 26 My 26 to 3 April N 2 O smpling nd nlysis N 2 O fluxes were mesured y opque, sttic, mnul stinless steel chmers (Lin et l., 29). The dimension (4 cm 4 cm 4 cm) ndrchitectureofthechmerswerethesmesthosereportedy M et l. (26). Gs smples were tken every 3e dys depending on wether conditions during the growing seson in 26 nd every 7e1 dys during the growing sesons from My to Septemer in 27 nd 28. There were 26 smpling occsions from 9 June to 17 Septemer in 26, nd 19 smpling occsions from My to Septemer for oth 27 nd 28. During the two no-growing sesons, there were 8 smpling occsions from Octoer 27 to April 28 nd smpling occsions from Octoer 28 to April 29, which were t lmost one-month intervls depending on wether conditions. The N 2 O flux etween 9:.m. nd 11:.m. locl time represents onedy verge flux (Tng et l., 26). Chmers were closed for hlf n hour nd gs smples (1 ml) were collected every 1 min using plstic syringes. Gs smples of N 2 O concentrtions were nlyzed with gs chromtogrphy (HP Series 489D, Hewlett Pckrd, USA) within 24 h following gs smpling. The gs chromtogrphy configurtions for nlyzing concentrtions of N 2 O nd the methods of clculting ech gs flux were the sme s those descried y Wng nd Wng (23) nd M et l. (26) Sttisticl nlysis Repeted-mesures nlyses of vrince (ANOVA), with wrming nd grzing s the min fctors (etween-suject fctors) nd with smple dte nd/or soil depth s within-suject fctors including interctions, ws pplied to test the effects of the min fctors on soil temperture, soil moisture, nd N 2 O fluxes

3 946 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e92 Dily rinfll (mm) nd ir temperture ( o C) Dily rinfll (mm) nd ir temperture ( o C) Dily rinfll (mm) nd ir temperture ( o C) Rinfll Air temperture Month-dy Fig. 1. Distriutions of rinfll, ir temperture, nd grzing time during growing sesons from 26 to 28. mens grzing time in the Figures. 3. Results 3.1. Soil temperture nd soil moisture Similr to the previous reports (Luo et l., 29,), wrming nd grzing significntly incresed verge soil temperture y 1.8 C t 2 cm during the growing seson in 28, in prticulr, the verge soil tempertures t 2 cm during the no-growing sesons were incresed y 3 C (rnge from 1.9 to 4 C) y heters in 27e28 nd 28e29 (Fig. 2) due to higher power outputs thn during the growing sesons. However, inconsistent with previous results of 26 nd 27, only in 28 wrming significntly decresed soil moisture t 1 cm y pproximtely 16% during the growing seson (Fig. 3). Both wrming nd grzing did not ffect soil moistures t ny soil depths during no-growing sesons ecuse the ground ws frozen (dt not shown) Effects of wrming nd grzing on temporl vrition in N 2 O fluxes Generlly, wrming, grzing, nd the interctions of wrming nd/or grzing nd smpling dte significntly ffected N 2 O fluxes, nd their effects vried with seson nd yer (Tle 1). The pek of dily N 2 O flux occurred during JulyeAugust for ll tretments except in 28 (Figs. 4 nd ). In 28, there ws urst of N 2 O flux during erly My (Fig. ). Wrming significntly decresed the verge N 2 O flux y 27.4% efore grzing compred with no-wrming (137.1 mgm 2 h 1 ), ut neither wrming nor grzing ffected N 2 O flux fter grzing during the growing seson in 26 (Fig. 4). However, wrming nd grzing significntly incresed N 2 O flux y 23.6 nd 28.% during the growing seson in 27, respectively. Wrming did not significntly ffect N 2 O flux ut grzing incresed it y.% during the growing seson in 28 (Fig. ). Averge sesonl N 2 O fluxes were 4.8 (rnged 4.e.1 mg m 2 h 1 ), 7.1 (rnged 6.9e7.2 mg m 2 h 1 ),.7 (4.6e6.8 mg m 2 h 1 ), nd 7.6 (rnged 7.e8.2 mg m 2 h 1 ) mg m 2 h 1 during the two growing sesons ( months from My to Septemer) in 27 nd 28 (Fig. ), nd 3.3 (rnged 1.8e4.8 mg m 2 h 1 ),. (rnged.2e.7 mg m 2 h 1 ),.2 (rnged.7e1.1 mg m 2 h 1 ), nd. (rnged.8e1.8 mg m 2 h 1 ) mg m 2 h 1 during the two no-growing sesons (7 months from Octoer to April) in 27e28 nd 28e29 (Fig. 6) for,,, nd, respectively. The contriutions of totl N 2 O emissions during no-growing sesons to totl nnul N 2 O emissions were 49. (rnged 3.9e6.9%), 2. (rnged 44.9e.6%), 4.8 (repeted-mesures) y dte nd soil depth s descried y Klein et l. (27) using SPSS Version 12.. Multi-comprison of lest stndrd difference (LSD) ws conducted for ll mesured vriles within ech smpling dte nd ech soil depth using two-wy ANOVA. Becuse no-grzing tretment ws pplied on ll plots efore 16 August 26, the dt during the growing seson efore nd fter 16 August 26 were nlyzed seprtely. The influence of wrming nd grzing on men monthly, sesonl, nd nnul N 2 O fluxes during growing sesons nd no-growing sesons were investigted using two-wy ANOVA, in which wrming nd grzing were crossed (Klein et l., 27). All sttisticl nlyses were performed with SPSS using the GLM procedure nd type III sum of squres. Simple correltion nlysis ws used to mesure the reltionships etween N 2 O fluxes nd the corresponding men soil temperture nd soil moisture t different soil depths etween 9:.m. nd 11:.m. during growing sesons nd no-growing sesons from 27 to 29, respectively, due to different grzing times in 26. All significnt differences were t p ¼. level. Soil temperture t 2 cm ( o C) Fig. 2. Dynmics of soil temperture t 2 cm under different tretments from Octoer 27 to April 29. : no-wrming with no-grzing, : no-wrming with grzing, : wrming with no-grzing, nd : wrming with grzing. Brs were stndrd error. Note: the soil tempertures t nd 1 cm re not shown here ecuse of the sme trends s tht t 2 cm under different tretments.

4 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e Soil moisture t 1 cm during growing seson (%) Month Fig. 3. Dynmics of soil moisture t 1 cm during growing seson under different tretments in 28. : no-wrming with no-grzing, : no-wrming with grzing, : wrming with no-grzing, nd : wrming with grzing. Brs were stndrd error. Note: there were no effects of wrming nd grzing on soil moistures elow 1 cm (dt not shown). ( 27.1e18.%), nd 8.4% ( 19.e23.%) for,,, nd tretments, respectively. Therefore, wrming with grzing or without grzing gretly reduced the contriution of N 2 Oemission during the no-growing seson to nnul emission, wheres grzing Tle 1 Summry of the nlysis of vrince on N 2 O fluxes from repeted-mesure ANOVAs using month nd dy s repeted-mesures conducted seprtely for growing nd no-growing seson from 26 to 29. Yer Period Model F P 26 Before grzing Wrming (W) Dte (D) <.1 W D 2.88 <.1 After grzing W.36.7 Grzing (G) W G D 8.64 <.1 W D G D W G D e29 During growing seson W.3.4 G 4.86 <.1 W G Yer (Y) W Y G Y W G Y..823 D 24. <.1 W D 4. <.1 G D 7.42 <.1 W G D 3.9 <.1 Y D <.1 W Y D 3.9 <.1 G Y D 9.4 <.1 W G Y D During no-growing seson W G W G.3.91 Yer (Y) <.1 W Y G Y W G Y D W D 7.93 <.1 G D W G D Y D W Y D 6.6 <.1 G Y D 3.89 <.1 W G Y D NW W NW 2 W did not ffect its contriution during the no-growing seson to nnul N 2 O emission, regrdless of wrming in the region. Bsed on monthly scle (Fig. 7), the effects of wrming nd grzing on N 2 O flux were opposite (i.e., wrming nd grzing decresed nd incresed monthly verge N 2 O flux y 37.2 nd 6.4%, respectively), nd there ws no interction etween them when excluding the dt during the growing seson in 26. In prticulr, there ws no significnt difference etween (monthly verge 3.1 mg m 2 h 1 ) nd (monthly verge 3.2 mg m 2 h 1 ), wheres nd incresed y 7. nd 34.4% of monthly N 2 O flux compred with, respectively. Averge nnul N 2 O fluxes were 4.2, 6., 4.2, nd. mgm 2 h 1 for,,, nd tretments. Wrming did not ffect verge nnul N 2 O flux, wheres grzing significntly incresed verge nnul N 2 O fluxes, nd no interctions etween wrming nd grzing on N 2 O flux were found for ll yers. Moreover, nd significntly incresed verge nnul N 2 O flux 7.8 nd 31.% compred with nd, respectively, indicting tht wrming reduced the response of nnul N 2 O flux to grzing on the lpine medow ecosystem in the region Reltionships etween N 2 O flux nd soil temperture nd soil moisture Averge N 2 O flux Generlly, lthough the correltions etween N 2 O flux nd soil temperture t different soil depths were significnt, their vlues Averge N 2 O flux Dte (Month-dy) Fig. 4. Effects of wrming nd grzing on N 2 O fluxes efore nd fter grzing during the growing seson in 26. : no-wrming with no-grzing, : no-wrming with grzing, : wrming with no-grzing, nd : wrming with grzing. Pnels inside the Figures were verge vlues of N 2 O fluxes under different tretments. Brs were stndrd error. Different letters men significnt differences t p ¼. level under different tretments.

5 948 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e Averge N 2 O flux Dte (Month-dy) Averge N 2 O flux were smll except in 27. In this cse, soil temperture explined out 3% of the vrition in N 2 O flux during the growing seson (Tle 2). The effects of soil moisture t different soil depths on N 2 O flux were smll during the growing seson (r 2 <.), nd there were no significnt correltions etween N 2 O flux nd soil temperture nd soil moisture during the no-growing seson (dt not shown). 4. Discussion 1 1 c Dte (Month-dy) 28 Fig.. Effects of wrming nd grzing on N 2 O fluxes during the growing seson in 27 nd 28. : no-wrming with no-grzing, : no-wrming with grzing, : wrming with no-grzing, nd : wrming with grzing. Pnels inside the Figures were verge vlues of N 2 O fluxes under different tretments. Brs were stndrd error. Different letters men significnt differences t p ¼. level under different tretments. The processes of N 2 O formtion re very complicte. N 2 Ois formed during the nitrifiction process when O 2 is limiting nd during denitrifiction (Sggr et l., 24; Pérez et l., 26). Nitrifiction explined out 64e88% of the vrition of N 2 Oin Inner Mongoli steppe (Xu et l., 23). The intensity of nitrte reduction in soils depends minly on soil prmeters tht control the oxygen stte of soils. Among these prmeters, ville C, temperture, nd the soil wter content seem to e the most importnt (Mg nd Vinther, 1999). Generlly, soil wter content cuses increse in denitrifiction (Mg nd Vinther, 1999; Xu et l., 23). However, in our study, we did not mesure these processes. Therefore, it is little difficult to discuss them deeply ecuse some processes re mixed ech other. For exmple, some studies show tht oth of increses of soil wter nd temperture incresed denitrifiction (Mg nd Vinther, 1999), wheres usully soil wrming cuse decrese of soil wter content which will increse eroic stte in sols, in this cse, we do not know which fctor is min controlling fctor for N 2 O formtion in our study Similrly, grzing increses excrete ptches which increse NO 3 en in soils (M et l., 26; Lin et l., 29). Thus, denitrifiction of NO 3 en increses N 2 O emission. Menwhile, grzing lso increses soil temperture in our study. All of these processes re most importnt for us to clerly explin why nd how temperture nd grzing ffect N 2 O emission Temporl vrition of N 2 O fluxes Averge N 2 O flux (ug Averge N 2 O flux 2O flux Averge N m -2 h -1 ) Dte (Month-dy) Fig. 6. Effects of wrming nd grzing on N 2 O fluxes during the no-growing seson in 26e27, 27e28, nd 28e29. : no-wrming with no-grzing, : no-wrming with grzing, : wrming with no-grzing, nd : wrming with grzing. Pnels inside the Figures were verge vlues of N 2 O fluxes under different tretments. Brs were stndrd error. Different letters men significnt differences t p ¼. level under different tretments. Similr to the results of Lin et l. (29), gret yerly vrition in N 2 O fluxes etween 26 (mximum mg m 2 h 1 ) nd

6 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e Monthly men Annul men N 2 O Month Fig. 7. Effects of wrming nd grzing on monthly nd nnul N 2 O fluxes. Brs were stndrd error. Different letters men significnt differences t p ¼. level under different tretments. 27e28 (mximum 2. mg m 2 h 1 ) ws found in our study (Figs. 4 nd ), proly ecuse the study site ws winter grzing re until June 26. Some ptches of fresh sheep dung covered the medow evenly efore the experiment in 26, which my results in greter soil NO 3 en content in 26 compred with 27 nd 28 (Lin et l., 29). Therefore, the sustntive N 2 O emissions my come from the sheep excret ptches nd the process of denitrifiction of NO 3 1 in the soil in 26 (M et l., 26; Lin et l., 29). The results suggest tht hevy grzing intensity during the no-growing seson (Zhou et l., 2), which removes lmost ll litters nd deposits mny excret ptches on the medow, could cuse gret N 2 O emissions during the growing seson in the region (Lin et l., 29). However, the experimentl site ws fenced fter 26, nd grzing intensity for the grzing tretment ws moderte for two or three dys during the growing seson, which gretly reduced excret ptches in the grzing plots. For nd tretments in our study, fluxes of N 2 O showed no cler sesonl pttern, with production during the nogrowing seson similr to fluxes during the growing seson (i.e., winter ccounted for pproximtely 36e7% of nnul N 2 O flux), which ws consistent with other reports (Brumme et l., 1999; Xu et l., 23, ; Groffmn et l., 26). Mosier et l. (1996) lso mesured high N 2 O fluxes (> mg-n m 2 h 1 ) during winter in Colordo short grss steppe. Even Kiser et l. (1998) nd Röver et l. (1998) found higher N 2 O fluxes (c. mg-n m 2 h 1 )in frozen soils (soil temperture 4 C) under rle mngement in Germny. Therefore, the fct tht pproximtely 4% of the verge nnul N 2 O losses occurred during the no-growing seson confirmed the importnce of spring nd utumn periods for Tle 2 Simple correltion etween N 2 O flux nd soil temperture nd soil moisture t different soil depths during the growing seson. Time Soil temperture Soil moisture cm 1cm 2cm 4cm 1cm 4cm **.214**.173**.148** 27.6**.87**.92**.46**.141** ** 26e28.281**.283**.274**.24** **Men significnt t p ¼.1. Only significnt correltions were shown. ssessment of totl N 2 O losses in the region. High fluxes during these periods hve een ttriuted to: (1) ccumultion nd relese of N 2 O from eneth frozen soil lyers (Goodrod nd Keeney, 1984; Burton nd Beuchmp, 1994; vn Bochove et l., 21); (2) freezing-induced microil mortlity followed y rpid regrowth nd high rtes of microil trnsformtions of N (Edwrds nd Killhm, 1986; Christensen nd Tiedje, 199; Deluc et l., 1992; Schimel nd Clein, 1996; Brooks et l., 24; Dörsch et l., 24); nd (3) freezing-induced disruption of soil ggregtes nd relese of ville cron tht stimultes N 2 O emission (Groffmn nd Tiedje, 1989; vn Bochove et l., 2). However, our study indicted tht wrming ltered the sesonl pttern of N 2 O flux ecuse wrming significntly reduced N 2 O flux during the no-growing seson; winter ccounted for only out e8% of nnul N 2 O flux for nd Effects of wrming on N 2 O fluxes We found tht the effect of wrming on N 2 O flux vried with yer nd seson (Tle 1). For exmple, wrming reduced, incresed, nd did not ffect N 2 O fluxes during the growing seson in 26, 27, nd 28, respectively. These differences my hve een ttriuted to 1) incresed plnt uptke to NO 3 en(xu et l., 24) leding to decresed denitrifiction rte (Zk et l., 199; Groffmn et l., 1993, 26) due to incresed plnt production y wrming (Luo et l., 29); 2) ccumultion of litter nd incresed decomposition rte y wrming (Luo et l., 29), which my increses releses of ville C nd N into soils; nd 3) decresed soil moisture y wrming during the growing seson in 28 (Fig. 2). In 26, incresed plnt uptke for the wrming tretments my e dominnt effect which reduced N 2 O fluxes ecuse of norml rinfll (449 mm) during the growing seson (Fig. 1), no effect of wrming on soil moisture in 26 (Luo et l., 29), nd little litter ccumultion. In prticulr, wrming mde dung ptches dry more quickly fter rin, which lso gretly decresed N 2 O emission (M et l., 26; Lin et l., 29). However, in 27 little drought (398 mm rinfll during the growing seson) (Fig. 1) my hve limited plnt uptke for N; moreover, much litter ccumultion nd incresed decomposition y wrming my hve incresed minerl N return to the soil (Xu et l., 23,), which could e the

7 9 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e92 dominnt contriution to sesonl incresed N 2 O flux for the wrmed plots. On the other hnd, in 28, hevy drought yer (only 339 mm rinfll during the growing seson), wrming significntly decresed soil moisture, which my hve limited the response of N 2 O flux to wrming. It is lso possile tht incresed levels of inorgnic N or lower soil moisture y wrming ltered the product rtios (NO:N 2 O:N 2 ) during denitrifiction not to fvor N 2 O (Dvidson nd Verchot, 2). Therefore, the effect of wrming on N 2 O flux will depend on the offsets etween the positive nd negtive effects of wrming on N 2 O production processes. Xu et l. (23) reported tht denitrifiction ws min process for N 2 O formtion which explined out 64e88% of the vrition of totl N 2 O emission in the Inner Mongoli steppe. The intensity of nitrte reduction in soils depends min on soil prmeters tht control the oxygen stte of soils. Among these prmeters, ville C, temperture, nd soil wter content seem to e the most importnt (Beuchmp et l., 1989; Aulkh et l., 1992; Mg nd Vinther, 1999). High temperture enhnced oth eroic respirtion nd denitrifiction, nd eroic respirtion further enhnced denitrifiction y consuming oxygen, resulting in strong sensitivity of denitrifiction to temperture (Mg nd Vinther, 1999). Wrming did not ffect N 2 O flux during the no-growing seson in 26e27, ut wrming significntly reduced N 2 O flux during the two no-growing sesons in 27e28 nd 28e29. We oserved tht wrming decresed the snow cover nd cused the snow to melt erlier compred with the no-wrming plots due to incresed soil temperture (Fig. 3; Luo et l., 29,). Relese of N 2 O trpped eneth ice lyers in the soil, one mechnism y which soil freezing hs een reported to increse N 2 O flux (Goodrod nd Keeney, 1984; Burton nd Beuchmp, 1994; vn Bochove et l., 2, 21, 26), did not pper to e mjor fctor, s we did not oserve ny mrked ursts of N 2 O production during the no-growing seson in 26e27 (Fig. 6), wheres some ursts were found during the no-growing sesons in 27e28 nd 28e29 for nd tretments (Fig. 6). Groffmn et l. (26) lso suggest tht winter climte chnge tht decreses snow cover nd induces soil freezing will increse soil-tmosphere N 2 O fluxes from northern hrdwood forests. Our results suggest tht in wrmer world with less soil freezing, N 2 O emission from the lpine medow on the Qinghi-Tietn plteu my decrese, perhps y s much s 1e2 times. Therefore, generlly sed on nnul scle, wrming did not significntly ffect nnul N 2 O flux ecuse of the offset etween positive nd negtive effects on N 2 O flux etween growing nd no-growing sesons in the region Effects of grzing on N 2 O fluxes In our study, grzing consistently incresed N 2 O flux oth during growing nd no-growing sesons, except during the growing seson in 26 ecuse of only one grzing period during the first yer of the experiment. Although grzing cn led very quickly to chnges in nutrient pools nd fluxes (Ross et l., 1999; Augustine nd Frnk, 21), in vegettion cover (Pruelo et l., 21), nd in plnt community composition in grsslnds (O et l., 21; Wng et l., 23), the contriution of grzing to N 2 O flux my minly come from the excret ptches which increse NO 3 en in soils (M et l., 26; Lin et l., 29). However, no significnt differences in N trce gs fluxes were found etween grzing histories nd grzing systems in the Inner Mongoli steppe (Holst et l., 27). Although Wng et l. (2) lso found differences in N 2 O emission rtes etween grzed nd ungrzed plots in the Inner Mongoli steppe t certin times during the growing seson, especilly during the flowering of the grsses, they found tht grzing decresed the N 2 O emission rte. As the result of microil relted nitrifiction nd denitrifiction processes, production of N 2 O ws usully restricted y soil temperture nd moisture (Zk et l., 199; Groffmn et l., 1993). In the lpine medow on the Tietn plteu, lower soil temperture nd higher soil moisture were oserved compred with tht in the Inner Mongoli steppe (M et l., 26; Lin et l., 29); therefore, different climte conditions my result in different grzing effect on N 2 O flux. In our study, grzing incresed soil temperture (Fig. 2). Both incresed temperture nd high NO 3 en induced y excrete ptches in soils my enhnced the denitrifiction (Mg nd Vinther, 1999) which enhnced N 2 O emission Reltionships etween N 2 O flux nd soil temperture nd soil moisture Although N 2 O fluxes were significntly correlted with soil moisture nd soil temperture, the correltions explined less thn 2% of the vrince of the mesured fluxes (Holst et l., 27). Groffmn et l. (26) suggested tht correltions etween gs flux nd soil temperture were higher for CO 2 nd CH 4 thn for N 2 O, wheres there were no significnt correltions etween ny gs flux nd soil moisture. However, Lin et l. (29) found tht oth soil temperture nd soil moisture explined 34e6% of N 2 O flux vrition. Our results show tht soil temperture only explined less thn 1% of N 2 O flux vrition except in 27, which explined out 3% of N 2 O flux vrition. Therefore, N 2 O flux in the lpine ecosystem could depend more on climte conditions thn in temperte grsslnds/steppes ecuse other fctors contriuting to N 2 O flux vrince could not e identified in these studies. 4.. Loss of N s N 2 O emission Reported men emission rtes re mostly elow 6. mg N 2 OeN m 2 h 1 or smller thn.6 kg N h 1 y 1 (Mosier et l., 1991, 1996, 22; Epstein et l., 1998; Mummey et l., 1997, 2). The verge nnul N 2 O emission ws in the rnge of.3e.28 kg N 2 OeN h 1 y 1 in the Inner Mongoli steppe (Xu et l., 23, ; Wng et l., 2; Holst et l., 27). Du et l. (26) reported men totl nnul N 2 O flux of.73.2 kg N 2 OeNh 1 y 1 in the sme region. In our study, verge nnul vlues were 2.7, 4.1, 2.7, nd 3.- mg N 2 OeN m 2 h 1 or.24,.36,.24 nd.31 kg N h 1 y 1 for,,, nd when grzing ws moderte during the growing seson, respectively. Only e8% of nnul 7.2 kg N deposition input (Zhng nd Co, 1999) my e offset y N 2 O flux from soil to the tmosphere. Thus, the N 2 O formtion does not pper to e significnt pthwy in the N cycle of such ecosystems which ws consistent with previous report (Billings et l., 22). However, Driscoll et l. (23) reported tht pproximtely 2% of this N deposition input ws cused y emission y N 2 O flux from the soil to the tmosphere. In our study, the dt in 26 indicted tht hevy grzing in winter cused much higher nnul N 2 Oemission(verged 8.2 kg N h 1 y 1 ) from the lpine medow due to the high density of excret ptches Conclusions Generlly, the effects of wrming nd grzing on N 2 O flux vried with yer, seson, nd smpling dte. There ws no interctive effect etween wrming nd grzing. Wrming did not ffect nnul N 2 O flux due to the offset etween positive effect (i.e., incresed N 2 O flux) during the growing seson nd negtive effect (i.e., decresed N 2 O flux) during the no-growing seson when grzing ws moderte during the growing seson. Grzing significntly incresed N 2 O flux for oth growing nd no-growing sesons, ut wrming reduced the response of N 2 O flux to

8 Y. Hu et l. / Soil Biology & Biochemistry 42 (21) 944e92 91 grzing. Fluxes in N 2 O only for the no-wrming tretments showed no cler sesonl pttern. There were significnt correltions etween N 2 O flux nd soil temperture t different depths, nd soil temperture could explin the highest (3%) N 2 O flux vrition in 27. Annul emission of n verge.3 kg N h 1 y 1 ws found when grzing ws moderte during the growing sesons regrdless of wrming. However, nnul emission could e verged t 8.2 kg N h 1 y 1 when hevy grzing occurred in the winter of 26. These results imply tht grzing intensity my e min control fctor to N 2 O flux in the region. Therefore, our study hs importnt implictions for predictions out future contriutions of lpine, nd possily other cold regions, to the glol N 2 O udget under grzing with future wrming conditions. Acknowledgements This reserch ws funded y the 1-Tlent Progrm of Chinese Acdemy of Sciences, Ntionl Bsic Reserch Progrm of Chin, the Knowledge Innovtion Progrms of Chinese Acdemy of Sciences, nd the Chinese Ntionl Nturl Science Foundtion Commission. References Alm, J., Srnio, A.J., Nyknen, H., et l., Winter CO 2,CH 4 nd N 2 O fluxes on some nturl nd drined orel petlnds. Biogeochemistry 44, 163e186. Augustine, D.J., Frnk, D.A., 21. Effects of migrtory grzers on sptil heterogeneity of soil N properties in grsslnd ecosystem. Ecology 82, 3149e3162. Aulkh, M.S., Dorn, J.W., Mosier, A.R., Soil denitrifiction significnce, mesurement, nd effects of mngement. Advnce of Soil Science 18, 1e7. Bll, B.C., Scott, A., Prker, J.P., Field N 2 O, CO 2 nd CH 4 fluxes in reltion to tillge, compction nd soil qulity in Scotlnd. Soil nd Tillge Reserch 3, 29e39. Brton, L., Kiese, R., Gtter, D., Butterch-Bhl, K., Buck, R., Hinz, C., Murphy, D.V., 28. 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