Soil Respiration and N 2 O Flux Response to UV-B Radiation and Straw Incorporation in a Soybean Winter Wheat Rotation System

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1 Wter Air Soil Pollut (213) 224:1394 DOI 1.17/s z Soil Respirtion nd N 2 O Flux Response to UV-B Rdition nd Strw Incorportion in Soyben Winter Whet Rottion System Zhenghu Hu & Hiling Cui & Shuto Chen & Shunghe Shen & Hnmo Li & Ynping Yng & Cenzi Li Received: 8 April 212 / Accepted: 12 November 212 # Springer Science+Business Medi Dordrecht 212 Abstrct Field experiments were conducted in the soyben nd winter whet-growing sesons to ssess soil respirtion (SR) nd nitrous oxide (N 2 O) emission s ffected by enhnced UV-B rdition nd strw incorportion. The SR rte ws mesured using soil CO 2 flux system; the N 2 O flux ws mesured using sttic chmber gs chromtogrph technique. The results showed tht in the soyben nd winter whet-growing sesons, enhnced UV-B rdition significntly decresed the SR rtes nd tht strw incorportion incresed the SR rtes compred to the control tretment. The combined tretment of UV-B nd strw incorportion hd no obvious influence on the SR rtes. Enhnced UV-B rdition, strw incorportion, nd the combintion tretment incresed the temperture sensitivity of SR in the soyben-growing seson. The study lso showed tht N 2 O emissions were reduced by enhnced UV-B rdition nd tht strw incorportion hd no significnt effects on the men N 2 O emission fluxes in the soyben nd winter whet-growing sesons. Our findings suggest tht Z. Hu : S. Shen Yle-NUIST Center on Atmospheric Environment, Nnjing University of Informtion Science & Technology, Nnjing 2144, Chin Z. Hu (*) : H. Cui : S. Chen : H. Li : Y. Yng : C. Li School of Environmentl Science nd Engineering, Nnjing University of Informtion Science & Technology, Nnjing 2144, Chin e-mil: zhhu@nuist.edu.cn enhnced UV-B rdition my led to decrese in SR nd in N 2 O emissions, strw incorportion my increse SR, nd the combined tretment my hve no significnt influence on SR nd N 2 O emissions from soyben winter whet rottion systems. Keywords UV-B rdition. Strw incorportion. Soil respirtion. N 2 O fluxes. Soyben. Winter whet 1 Introduction Nitrous oxide (N 2 O) is n importnt greenhouse gs tht contributes to globl wrming nd strtospheric ozone depletion. Its globl wrming potentil in the 1-yer window is 298 times greter thn CO 2 nd hs longer tmospheric lifetime thn CH 4 (IPCC 27). Furthermore, N 2 O contributes to the depletion of the ozone lyer in the strtosphere (Wetherhed et l. 2). The tmospheric N 2 O concentrtion continued to increse with rte of.26 % per yer nd reched 319 ppb in 25 (IPCC 27). Agroecosystems re considered to be the principl source of tmospheric N 2 O, which ccounts for TgN 2 O N per yer nd 6 % of the globl nthropogenic N 2 O emission (IPCC 27b). Soil respirtion (SR), including the utotrophic respirtion of plnt root nd heterotrophic respirtion of soil microbes, plys n importnt role in the globl crbon cycle (Buchmnn 2; Schlesinger nd Andrews 2). The globl SR flux is gc/ yer (Schlesinger nd Andrews 2), ccounting for

2 1394, Pge 2 of 12 Wter Air Soil Pollut (213) 224:1394 bout 8 % of the gross ecosystem respirtion (Longdoz et l. 2). Therefore, smll chnges in the SR rte my drmticlly lter tmospheric CO 2 concentrtions nd soil C sequestrtion (Iqbl et l. 29). The significnt increse in CFC concentrtions hs led to strtospheric ozone depletion, which hs resulted in the enhncement of ultrviolet-b (UV-B, nm) rdition reching the erth s surfce (Rozem et l. 25; Erickson et l. 2). The ozone recovery rte depends on mny fctors, including N 2 O emissions (Wetherhed et l. 2). It is well documented tht enhnced UV-B rdition decreses crop photosynthesis rtes (e.g., Yng et l. 27; Pndey nd Chplot 27), inhibits crop growth (e.g., Kkni et l. 23; Kdur et l. 27), nd reduces crop biomss (e.g., Yo et l. 26; Agrwl et l. 26). IthslsobeenreportedthtUV-Bcninfluence indirectly the quntity nd ctivity of soil microbil popultions (Johnson et l. 22, 23). Johnson et l. (23) suggested tht enhnced UV-B rdition cuses decline in the C/N rtio of soil microbil biomss. Crop growth nd soil microorgnisms re the min fctors ffecting SR nd N 2 O emissions in croplnd. Chen et l. (211) nd Hu et l. (21, b) reported tht enhnced UV-B rdition cn lter the SR rtes nd N 2 O emissions in frmlnd. Agriculturl ctivities produce lrge quntities of crop residues. Agriculturl residue is either removed from the field, burned in situ, or mulched in the field nd incorported into the topsoil (Vibol nd Towpryoon 21). Returning crop residue bck to the field is n importnt wy to increse soil fertility nd soil orgnic crbon (e.g., Nie et l. 27; Lennrt nd Jn 26). Therefore, strw incorportion cn hve n effect on SR nd N 2 Oemission in croplnd. M et l. (21) incorported rice strw into the topsoil of winter whet frm nd found tht the rice strw incorportion cn reduce N 2 O emission during the whet-growing seson. Ci et l. (21) nd Ho et l. (21) lso stted tht strw incorportion cn inhibit N 2 O emission from croplnds. On the contrry, Pthk et l. (26) found tht strw incorportion stimultes N 2 O emission from whet frm becuse of higher microbil ctivity. Previous reserch suggests tht UV-B rdition or strw incorportion cn lter the respirtion rtes nd N 2 O emissions in groecosystems. However, the combintion effect of enhnced UV-B nd strw incorportion on SR nd N 2 O emissions hs received little ttention. To our knowledge, only single-fctor effects of UV-B rdition or strw incorportion on SR nd N 2 O emissions hve been reported in the literture (e.g., Hu et l. 21, b; Zou et l. 25; Pthketl.26). In this study, we hypothesize tht the combintion impct of UV-B rdition nd strw incorportion on crop growth my result in potentil chnges in SR nd N 2 O emission fluxes. To test this hypothesis, we returned strw into the topsoil nd exposed the soil soyben nd soil winter whet system to enhnced UV-B rdition under field conditions using soil CO 2 flux system nd sttic chmber gs chromtogrph technique to mesure the SR rte nd the N 2 O flux. The objective of this study ws to investigte the effects of enhnced UV-B rdition nd strw incorportion on the SR rtes nd the N 2 O emission fluxes in the soyben- nd winter-whet growing sesons. Such informtion will be beneficil to develop mngement prctices tht minimize greenhouse gses emissions from croplnd. 2 Mterils nd Methods 2.1 Experimentl Site The field experiment ws crried out during the soyben (July October 28) nd winter whet (December 28 My 29) growing sesons. The experimentl field ws locted t the experimentl frm of Nnjing University of Informtion Science nd Technology (32 3 N, E) in Est Chin. Annul rottions such s soyben (Glycine mx) winter whet (Triticum estivum) nd pddy rice (Oryz stiv) winter whet re the min crop production regimes in the re. The nnul verge temperture ws 15.6 C nd the nnul rinfll verged t bout 1,1 mm. The soil ( 2 cm) ws clssified s hydromorphic nd contined 26.1 % cly, n initil ph(h 2 O) of 6.22, totl orgnic crbon of 19.4 gkg 1, nd totl nitrogen of 1.45 gkg Experimentl Design The field experiment ws designed with four tretments: control (C, mbient UV-B rdition nd no strw incorportion); cropping with the 2 % enhncement of UV-B (U); cropping with strw incorportion (S, whet strw in the soyben-growing seson nd

3 Wter Air Soil Pollut (213) 224:1394 Pge 3 of 12, 1394 rice strw in the winter whet-growing seson); nd cropping with the 2 % UV-B enhncement plus strw incorportion (US). Ech tretment hd three replicte plots (2 2-m 2 re of ech plot). Plots were rndomly rrnged in three blocks, ech block contining replicte plot of the four tretments. The blocks were seprted by.5-m strips. The intervl distnce of crop rows ws 3 cm. The min growth stges nd fertiliztion schedules of soyben (cv. Byuedou) nd winter whet (cv. Yngmi 14) re shown in Tble UV-B Tretments Supplementl UV-B rdition ws supplied with fluorescent UV lmps (4 W, Hude Instrument Fctory, Shnghi, Chin). Lmps were hung over nd perpendiculrly to the plnted rows nd were rrnged est westwrd to minimize shding. The UV-B lmps of the control group (C) were wrpped with polyester plstic film (Mylr-D, 125-μm thickness; DuPont Co., Wilmington, DE, USA) which filters off ll UV-B Tble 1 Min growth stges nd fertiliztion schedules of soyben nd winter whet Growing seson Dte Growth stges or fertiliztion schedules Soyben 6 July 28 Sow 9 July 28 Seedling 14 July 28 Trefoil 14 August 28 Brnching 23 August 28 Flowering 7 September 28 Pod 19 September 28 Grin-filling 13 October 28 Hrvest Winter whet 2 December 28 Sow, 1 kgnh 1 (ure), 78 kgph 1, 1 kgkh 1 18 December 28 Seeding 15 Jnury 29 5 kgnh 1,21kgPh 1, 27 kgkh 1 14 Februry 29 Turning green, 5 kgnh 1 23 Mrch 29 Elongtion 11 April 29 Booting 17 April 29 Heding 21 April 29 Flowering 27 April 29 Grin filling 31 My 29 Hrvest rdition nd UV-C rdition of the lmps (Fig. 1). The plstic film ws replced weekly to ensure uniformity in UV-B bsorption. Plnts under polyesterfiltered lmps received only mbient levels of UV-B rdition, wheres plnts beneth the exposed UV-B lmps received mbient nd supplementl levels of UV-B rdition. The UV-B rdint intensity ws recorded utomticlly with UV-B rdince mesurement instrument composed of UV-B rdition sensors (spectrl rnge, nm; SKU43, Skye Co., UK) nd dt loggers (Skye-Dthog, Skye Co.). The sensors were instlled t the vegettion level t the center of ech plot. Plnts were irrdited dily for 8 h (8: 16: hours) from the seedling to the hrvest stges. 2.4 Strw Incorportion The strw of winter whet nd rice were collected from whet nd rice fields. Dried strw ws chopped into 2-cm length. The totl C nd totl N of whet strw were 51.2 nd.9 %, respectively, nd the totl C nd totl N of rice strw were 46.7 nd 1.2 %, respectively. Strw ws spred evenly over the plots nd incorported into the plough lyer ( 2 cm). The pplied rte of strw ws 225 gm 2 in tretments S nd US. 2.5 SR nd N 2 O Flux Mesurements SR ws mesured using n utomted soil CO 2 flux mesurement system (Li-81, Li-Cor Inc., Lincoln, NE, USA) with n ttched chmber. PVC soil collrs, with height of 1 cm nd dimeter of 2 cm, were permnently inserted 3 cm into the soil for SR mesurements. There were three collrs in ech plot nd nine collrs for ech tretment. Aboveground vegettion within the soil collr ws erdicted by hnd severl dys prior to chmber plcement. Therefore, the SR we mesured did not include boveground respirtion from living plnts. The Li-81 chmber ws put on the PVC collr to mesure SR t ech plot nd then moved to the next plot. The nlyzer opticl bench mesured CO 2 concentrtion in the chmber; the CO 2 flux rtes (in micromoles CO 2 per squre meter per second) were clculted from the increse in CO 2 concentrtion over time. SR mesurements were mde once or twice weekly. On ech mesurement dy, SR ws mesured from 8: to 11: hours.

4 1394, Pge 4 of 12 Wter Air Soil Pollut (213) 224:1394 Fig. 1 Spectrum of UV-B tubes () nd Mylr filmfiltrted UV-B tubes (b) Rdition intensity (µw cm -2 nm -1 ) UV-C UV-B UV-A+PAR Wvelength (nm) Rdition intensity (µw cm -2 nm -1 ) UV-A+PAR b Wvelength (nm) The Li-81 system included soil temperture nd moisture sensors. The soil temperture nd moisture sensors were inserted into the soil t the time of SR mesurement. Soil temperture nd moisture t the 5- cm depth were monitored djcent to ech PVC collr. The N 2 O emission flux ws mesured using sttic chmber gs chromtogrph technique (Zou et l. 25). During the crop growing seson, bordwlks were instlled to reduce soil nd crop disturbnce during gs smpling. Circulr bse frmes for the gs chmber were instlled in the plots. Ech bse frme ws 8 cm high nd hd 2.5-cm width groove on the top edge. The smpling chmber ws 1-cm-high PVC cylinder with dimeter of 25 cm, wrpped in lyer of sponge nd luminum foil to minimize the effect of solr rdition on the internl temperture. During the mesurement, the chmber ws plced over the vegettion with its rim mtching the groove of the bse frme. The chmber ws seled by filling wter in the groove on the top edge of the bse frme. Three gs smples were tken by syringes t, 1, nd 2 min fter closing the chmber. In ech tretment, gs smples were collected from three chmbers to serve s replictes. N 2 O flux mesurements were mde once or twice weekly. Air temperture inside the chmber ws recorded for ech flux mesurement. Soil temperture nd moisture t 1-cm depth were mesured djcent to the bse frmes t the time of gs smpling, with W.E.T. monitor (Delt-T Devices Ltd., Burwell, Cmbridge, UK). The gs smples were nlyzed for N 2 O with gs chromtogrph (Agilent 689N, Agilent Co., USA) equipped with n electron cpture detector (ECD) (Wng nd Wng 23). The oven ws operted t 55 C nd the ECD t 33 C. The N 2 O flux ws determined from the slope of the chnges in the

5 Wter Air Soil Pollut (213) 224:1394 Pge 5 of 12, 1394 mixing rtio with durtions t, 1, nd 2 min following chmber closure (Chen nd Hung 29). Almost ll the smple sets yielded liner regression vlues of R 2 >.9 for the mixing rtio time reltionship. 2.6 Crop Biomss Mesurements At the end of ech cropping seson, root biomss nd boveground biomss (shoot biomss) were hrvested nd determined by oven drying to constnt weight t 7 C. 2.7 Sttisticl Anlysis The verge SR rtes nd N 2 O flux nd their stndrd errors were clculted bsed on replicted mesurements. The primry nd interction effects of enhnced UV-B rdition nd strw incorportion on SR nd the N 2 O flux were evluted by ANOVA. Significnce referred to in the text is t the p=.5 level, unless otherwise stted. All sttisticl nlysis ws conducted using SPSS 13. (SPSS Inc., Chicgo, IL, USA). 3 Results 3.1 Chnges of Soil Temperture nd Moisture The sesonl chnges of soil temperture nd moisture re shown in Fig. 2. Soil temperture (Fig. 2, b) shows the sesonl ptterns following sesonl vrition in ir temperture. No significnt difference in soil temperture ws observed mong the C, U, S, nd US tretments (p>.5). Soil moisture (Fig. 2c, d) hs no cler sesonl vrition, nd no differences in soil moisture mong different tretments were observed. In this cse, soil moisture of different tretments hd the sme effects on the respirtion rte nd the N 2 O emission (see lso Cook nd Orchrd 28). 3.2 Effects of Enhnced UV-B Rdition nd Strw Incorportion on SR Temporl Dynmics of SR The SR rte shows similr sesonl ptterns for different tretments (Fig. 3, b). It is cler tht enhnced UV-B rdition nd strw incorportion did not lter Fig. 2 Sesonl vrition in soil temperture (T s ) nd moisture (M s )., b T s in the soyben- nd winter whet-growing sesons, respectively. c, d M s in the soyben- nd winter whet-growing sesons, respectively

6 1394, Pge 6 of 12 Wter Air Soil Pollut (213) 224:1394 Fig. 3 Effects of enhnced UV-B nd strw incorportion on SR rtes., b SR rtes in the soyben- nd winter whet-growing sesons, respectively. Dt re the men vlues. Error brs re SEs Soil respirtion rte (µmol m -2 s -1 ) C S U US Dte (mm-dd-yy) Soil respirtion rte (µmol m -2 s -1 ) Dte (mm-dd-yy) b the sesonl ptterns of SR in the soyben- nd winter whet-growing sesons, which generlly corresponded to the sesonl vrition of soil temperture Sesonl Men SR Rtes The sesonl men SR rtes of the four tretments re shown in Fig. 4. In the soyben-growing seson, the SR rtes were significntly reduced by 3.3 % (p=.2) under the U tretment nd incresed by 14.5 % (p=.5) under the S tretment compred with tht under the C tretment. No difference existed between the SR rtes of C nd US (p=.193), suggesting tht there my be n ntgonism between enhnced UV-B rdition nd strw incorportion. In the winter whetgrowing seson, the SR rtes were significntly reduced by 29.6 % (p=.7) under the U tretment nd incresed by 25.1 % (p=.18) under the S tretment compred with tht under the C tretment. No Soil respirtion rte (µmol m -2 s -1 ) b c Soyben C S U US Winter whet Growing seson Fig. 4 Sesonl men SR rtes. Dt re the men vlues±se. Different lowercse letters denote significnt difference mong different tretments t p.5 b

7 Wter Air Soil Pollut (213) 224:1394 Pge 7 of 12, 1394 significnt difference in SR rtes ws observed between the C nd the US tretments (p=.669). The sesonl men SR rtes for the C, U, S, nd US tretments were 3.86, 2.69, 4.42, nd 3.5 μmolm 2 s 1 in the soyben-growing seson, respectively, nd 2.91, 2.5, 3.64, nd 3.8 μmol m 2 s 1 in the winter whet-growing seson, respectively (Fig. 4). During the two growth sesons, the greter inhibition effect of UV-B nd the greter promotion effect of strw incorportion on the SR rtes were found in the soybengrowing seson nd the winter whet-growing seson, respectively Temperture Sensitivity of SR Regression nlysis shows n extremely significnt exponentil reltionship between the SR rtes nd soil temperture (p<.1; Fig. 5). Soil temperture itself cn explin bout 5 % of the sesonl vritions in the SR rtes (Fig. 5). The regression b vlues were used to clculte the SR temperture sensitivity coefficient Q 1 (=e 1b ). In the soybengrowing seson, the Q 1 vlues of the C, U, S, nd US tretments were 1.55, 1.91, 1.8, nd 1.71, respectively. The U, S, nd US tretments incresed the Q 1, indicting tht UV-B rdition nd strw incorportion incresed temperture sensitivity of SR in the soyben-growing seson. 3.3 Effects of Enhnced UV-B Rdition nd Strw Incorportion on N 2 O Emission The men N 2 O emission fluxes of the different growth stges in the soyben- nd winter whetgrowing sesons re shown in Fig. 6. In the soyben-growing seson, the men N 2 O fluxes were lower in the U tretment thn in the C, S, nd US tretments. The U tretment reduced N 2 O fluxes by 37.9 % (p=.1), 24.6 % (p=.38), 48.4 % (p=.7), nd 34.2 % (p=.1) in the trefoil brnching stge, the flowering podding stge, the seed filling mture stge, nd the whole growth stge, respectively, compred with tht under the C tretment. Enhnced UV-B rdition hd significnt inhibition on the N 2 Oemissionfrom the croplnd. Compred to the C tretment, both S nd US incresed the men N 2 O fluxes by 19.4 nd 2.6 % in the flowering podding stge nd reduced the men N 2 O fluxes by 26.4 % (p=.6) nd 22.3 % (p =.15) in the trefoil brnching stge. According to the men N 2 O fluxes in the whole growth stge, S nd US hd no significnt effect on the N 2 O emission compredtothectretment. In the winter whet-growing seson, compred to the C tretment, the U tretment reduced N 2 O fluxes by 23.4 % (p=.22), 35.7 % (p=.), 39.7 % (p=.1), nd 24.7 % (p=.) in the elongtion Fig. 5 Reltionship between SR rtes nd soil temperture in the soybengrowing seson

8 1394, Pge 8 of 12 Wter Air Soil Pollut (213) 224:1394 Fig. 6 Effect of enhnced UV-B rdition nd strw incorportion on the men N 2 O fluxes in different growth stges., b N 2 O fluxes in the soyben- nd winter whet-growing sesons, respectively. Dt re the men vlues. Error brs re SEs N2O fluxes (µg m -2 h -1 ) C S U US N2O fluxes (µg m -2 h -1 ) Trefoil brnching stge Flowering podding stge Growth stges Seed filling mturity stge Whole growth stge b Turning-green stge Elongtionbooting stge Heding-grin filling stge Growth stges Mturity stge Whole growth stge booting stge, the heding grin filling stge, the mturity stge, nd the whole growth stge, respectively. AccordingtothemenN 2 O fluxes of the whole growth stge, compred to C, S hd no significnt effects on the N 2 O emission, while US reduced the men N 2 O fluxes by 44.5 % (p=.16). 4 Discussion 4.1 Soil Respirtion Our study suggests tht enhnced UV-B rdition reduced SR rtes in the soyben-growing seson nd the winter whet-growing seson. It hs been shown tht enhnced UV-B rdition cn inhibit photosynthesis nd growth of crop, leding to inhibited root development nd the ccumultion of dry mtter (Pl et l. 26). In the present study, we found tht enhnced UV-B rdition significntly decresed root biomss, shoot biomss, nd totl biomss of soyben by 68.3, 73.9, nd 73.6 %, respectively (Tble 2). The reltionship between soyben biomss nd the SR rtes ws y=.3191x (R 2 =.938), indicting tht the men SR rte hd significntly positive reltionship with biomss. In the winter whetgrowing seson, enhnced UV-B rdition decresed shoot biomss nd totl biomss by 14.1 nd 12.1 %, respectively (Tble 2). Enhnced UV-B hd more negtive effect on soyben thn on winter whet (Tble 2), explining the greter UV-B s inhibition effect on the SR rtes in the soyben-growing seson. Since UV-B rdition cn penetrte, below 5 mm, into the soil (Moorhed nd Cllghn 1994), UV-B rdition my impose n effect on microbil communities vi direct influence on plnt growth nd physiologicl metbolism nd therefore decrese the respirtion of root nd soil microorgnisms. It hs lso been well recognized tht enhnced UV-B rdition cn indirectly influence soil microbil popultions nd

9 Wter Air Soil Pollut (213) 224:1394 Pge 9 of 12, 1394 Tble 2 Effect of enhnced UV-B rdition nd strw incorportion on biomss of soyben nd winter whet (grms per bse frme) Tretments Root biomss Shoot biomss Totl biomss Soyben Winter whet Soyben Winter whet Soyben Winter whet C 2.87± ± ± ± ± ±.41 U.91±.1b 2.44±.4b 13.21±1.17b 25.33±.64b 14.12±1.26b 27.77±.63b S 5.39±.86c 2.76±.18b 81.27±12.8c 3.15± ±13.63c 32.92±.89 US 4.87±.7c 2.11± ±6.63c 29.24± ±7.18c 31.34±.47 Dt re the men vlues±se. Different lowercse letters denote significnt difference mong different tretments t p.5 ctivities (Serles et l. 21; Johnson et l. 22, 23; Robson et l. 24),whichmyddtoour understnding of the differences in SR between C nd U tretments. In this study, strw incorportion cn promote SR rtes (s shown by compring the S with the C tretment), consistent with the observtions of Montoy- González et l. (29). Curtin et l. (1998) conducted n experiment under controlled conditions (constnt 2 C) nd lso found tht whet strw incorportion significntly increses soil CO 2 flux. The resons my be twofold: (1) strw incorportion cn increse the mount of orgnic mtter in soils (Nie et l. 27), fcilitte the minerliztion of soil crbon (Duong et l. 29; Jcinthe et l. 22), nd increse the light constituent of orgnic crbon tht cn be esily decomposed nd utilized by soil microbe (Mlhi nd Lemke 27). Xu et l. (211) found tht totl C minerliztion, microbil biomss C, prticulte orgnic C, nd lbile orgnic C re significntly incresed under strw incorportion fter 2-yer durtion. Roldán et l. (23) lso found similr results in short-term (1 yer) residue mngement experiments. (2) Strw incorportion cn promote the root/shoot biomss rtio of crop plnt (Li et l. 28), which is benefit to root respirtion. Previous studies suggest tht bout 7 % of the totl crbon llocted to the root is used for respirtion nd only 3 % totl crbon is used for growth (Högberg et l. 22). Tble 2 shows tht strw incorportion significntly improved the root/shoot biomss rtio by 21.5 nd 35.3 % in the soyben- nd the winter whetgrowing sesons, respectively. Thus, strw incorportion hd the greter promotion effect on the SR rtes in the winter whet-growing seson. Compred to C tretment, the US tretment hd no significnt effect on the SR rtes in the soyben- nd winter whet-growing sesons. In the winter whetgrowing seson, the US tretment hd no significnt effect on the root, shoot, nd totl biomss nd the rtio of root/shoot biomss (Tble 2). In the soybengrowing seson, the US tretment incresed the root, shoot, nd totl biomss (Tble 2), but did not chnge the rtio of root/shoot biomss (.56 nd.53 of the C nd US tretments, respectively). It is well known tht crop growth is the min fctor influencing the SR rtes, thereby explining the no significnt difference in the SR rtes between the C nd US tretments. Tble 2 lso shows tht enhnced UV-B rdition hs significnt negtive influence on plnt biomss nd tht strw incorportion hd positive influence on biomss, which mens tht the ppliction of strw cn effectively llevite, even eliminte, the negtive influence brought by enhnced UV-B rdition. 4.2 Soil Temperture, Moisture, nd SR Soil temperture nd moisture re two crucil fctors in controlling the SR rtes. Our results show the expected exponentil correltion of the SR rtes with soil temperture. Soil temperture controls the rte of biologicl rection through its influence on enzyme kinetics (Dvidson nd Jnssens 26; Dvidson et l. 26). Soil moisture is nother fctor driving the sesonl vritions in the SR rtes. If wet site dries substntilly or dry site is wet substntilly, lrge vribility of soil respirtion my occur (Dvidson et l. 1998; Jssl et l. 28; Lee et l. 24). Our site experienced gret vritions of soil moisture during the experimentl period (Fig. 2), which contributed substntilly to the significnt effects of soil moisture on the SR rtes. Q 1 hs been commonly used to estimte the SR rtes from temperture (Curiel et l. 24). Q 1 is

10 1394, Pge 1 of 12 Wter Air Soil Pollut (213) 224:1394 relted to mny fctors including temperture, orgnic mtter, microbil biomss, soil vilble mtter, nd plnt LAI (Dvidson et l. 1998). In this study, we did not discuss the Q 1 of SR in the winter whet-growing seson, for there were lrge influences on Q 1 not only from soil temperture but lso plnt senescence nd lower soil moisture in the lter growth stges of winter whet (Fig. 2), nd the effect of soil moisture on Q 1 is complex (Qi et l. 22). In our study, UV-B rdition cn promote the Q 1 of SR in the soyben-growing seson. There re severl potentil resons for this: (1) UV-B rdition cn inhibit plnt photosynthesis, root growth, nd dry mtter ccumultion (Pl et l. 26), which re relted to the Q 1 vlue. Dvidson et l. (1998) suggested tht Q 1 responses not only to temperture but lso plnt root biomss, litter inputs, nd microbil popultions. (2) UV-B rdition hs n indirect positive effect on the mount of soil bcteri, which my be sensitive to temperture chnge. Previous reserch suggested tht UV-B rdition cn increse the biomss C nd N in soil microbes (Hu et l. 21b), which my ccelerte the decomposition of crbon nd nitrogen compounds s temperture rises, resulting in the rise of the Q 1 vlues. Therefore, enhnced UV-B my modify the vlue of Q 1 by chnging the growth of crop nd the structure nd the quntity of microbil communities. On the other hnd, strw incorportion lso incresed Q 1 in the soyben-growing seson, which my be ttributed to incresed soil microbil biomss (Jcinthe et l. 22) nd the bundnce nd the ctivity of soil microbes. The Q 1 vlue will chnge s the proportion of temperture-sensitive microbe increses. Further studies on plnt chemistry nd the decomposition processes in soil re needed to understnd the mechnisms of the effects of UV-B nd strw incorportion on the temperture sensitivity of SR. Further studies ttempting to investigte the temperture sensitivity of SR should chrcterize the different responses of the utotrophic nd the heterotrophic respirtion to UV-B rdition nd strw incorportion. 4.3 N 2 O Fluxes Shrp vritions in the N 2 O emission between the C nd U tretments were observed in the soyben- nd winter whet-growing sesons (Fig. 6, b). The potentil reson for the reduced N 2 O emission under U tretment is tht UV-B rdition cn inhibit the growth of plnt nd the ccumultion of orgnic mtter nd therefore reduce the biomss (Tble 2). Some studies suggest tht incorportion of strw into the soil is n effective wy to reduce N 2 O emission. M et l. (21) reported tht rice strw incorportion in which strw ws evenly incorported into the topsoil decreses N 2 O emission by 3 18 %. Our study shows tht strw incorportion hd no significnt effect on the men N 2 O emission fluxes in the soyben- nd the winter whet-growing sesons. Ci et l. (21) reported tht strw ddition does not ply significnt role in N trnsformtion compred to the effect of the inherent soil orgnic crbon content. Our results re lso consistent with the result of Ho et l. (21) who observed no enhnced N 2 O emission from strw incorportion into the soil, even in the sme plce for over 1 yers, indicting tht the proportion of strw N converted to N 2 O my be less thn the proportion of fertilizer N converted to N 2 O. According to the integrted effects of UV-B nd strw incorportion (US) on the men N 2 O fluxes in the winter whetgrowing seson, enhnced UV-B rdition plyed lrger role in N 2 O emission thn strw incorportion. Strw incorportion cn stimulte the denitrifiction rtes by providing vilble C for denitrifying bcteri (with dequte NO 3 nd moisture). Additionlly, the decomposition of strw will consume oxygen, which stimultes denitrifiction rtes nd N 2 O production (Groffmn 1985; Ho et l. 21). On the other hnd, the ddition of high C/N rtio strw will induce N immobiliztion nd decrese the vilble N for nitrifiction nd denitrifiction, which reduces N 2 O production (Ho et l. 21). In other words, the mount of N 2 O production is determined by two compensting processes. 5 Conclusions Enhnced UV-B rdition, strw incorportion, nd the combintion tretment did not chnge the sesonl pttern of the SR rtes in the soyben- nd winter whet-growing sesons. Over the growing sesons, enhnced UV-B rdition significntly reduced the SR rtes, strw incorportion incresed the SR rtes, nd the combined tretment of UV-B nd strw incorportion hd no obvious influence on the SR rtes. Enhnced UV-B rdition, strw incorportion, nd

11 Wter Air Soil Pollut (213) 224:1394 Pge 11 of 12, 1394 the combintion tretment incresed the temperture sensitivity of SR in the soyben-growing seson. In the whole growth stge, enhnced UV-B rdition significntly decresed the men N 2 O emission fluxes, nd strw incorportion hd no significnt effect on the men N 2 O emission fluxes in the soyben- nd winter whet-growing sesons. The combintion tretment of UV-B nd strw incorportion hd no significnt effect on the men N 2 O emission fluxes in the soyben-growing seson, but decresed the men N 2 O emission fluxes in the winter whet-growing seson. Acknowledgments This work ws conducted under the finncil supports from the Ntionl Nturl Science Foundtion of Chin ( , 41588), the Ministry of Eduction of Chin (grnt PCSIRT), the Key Lbortory of Arid Climtic Chnge nd Disster Reduction of Gnsu Province (CMA; IAM 214), nd the Qing Ln Project of Jingsu Province (21). References Agrwl, S. B., Rthore, D., & Singh, A. (26). 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