Effects of livestock grazing on soil nitrogen mineralization on Hulunber meadow steppe, China

Transcription

1 Vol. 62, 216, No. 5: Plnt Soil Environ. doi: /445/215-PSE Effects of livestock grzing on soil nitrogen minerliztion on Huluner medow steppe, Chin R. Yn 1, G. Yng 1, B. Chen 1, X. Wng 1, Y. Yn 1, X. Xin 1, L. Li 2, X. Zhu 1, K. Bi 1, Y. Rong 3, L. Hou 3 1 Institute of Agriculturl Resources nd Regionl Plnning, Chinese Acdemy of Agriculturl Sciences, Beijing, P.R. Chin 2 Stte Key Lortory of Vegettion nd Environmentl Chnge, Institute of Botny, Chinese Acdemy of Sciences, Beijing, P.R. Chin 3 Deprtment of Grsslnd Science, Chin Agriculturl University, Beijing, P.R. Chin ABSTRACT Soil nitrogen (N) cycling is n importnt fctor in terrestril ecosystems, including grsslnds. Understnding the effects of grzing on nitrogen cycling in grsslnd ecosystems is criticl for etter mngement nd for improving knowledge of the mechnisms underlying grsslnd degrdtion nd cn provide sic informtion for sustinle development in grsslnd ecosystems. In this study, in situ incution in intct soil cores ws used to mesure sesonl chnges in soil nitrogen minerliztion nd nitrifiction in the medow steppe of the Huluner grsslnds of northestern Chin. Soil plots were sujected to vrying intensities of cttle grzing, nd soil chrcteristics including severl spects of the nitrogen cycle were nlysed. The findings demonstrte tht soil inorgnic N pools nd nitrogen minerliztion peked in August nd tht moderte grzing intensity produced higher sesonl men net N minerliztion (A min ); net nitrogen minerliztion rte (R min ); net mmonifiction rte (R mm ) nd net nitrifiction rte (R nit ). Sesonl men net minerliztion rte ws incresed y 6 15% in the lightly nd modertely grzed plots ( AU cow/h) nd y 4 5% in the hevily grzed plots ( AU cow/h). Also it ws found tht soil moisture ws significntly positively correlted with inorgnic N, A min, R mm nd R min nd significntly negtively correlted with R nit, while soil temperture exhiited the opposite effect. The otined results demonstrted net nitrogen minerliztion nd mmonium rtes, which were strongly linked to grzing intensity, soil temperture nd soil moisture. Keywords: inorgnic nitrogen; terrestril ecosystem; climte; grzing intensity; nutrient cycling; environmentl fctors Soil nitrogen (N) minerliztion nd nitrifiction re the dominnt microil processes ffecting inorgnic N vilility in terrestril ecosystems nd re n importnt component of N cycling (Liu et l. 21). Soil nitrogen minerliztion shows high sptil nd temporl vriility nd is regulted y severl environmentl fctors, including soil temperture, moisture nd ph, nd y nthropogenic ctivities (Augustine nd McNughton 26). However, the effects of these fctors my vry in different ecosystems. Grsslnd ecosystems re n importnt component of terrestril ecosystems. Nitrogen vilility nd trnsformtion cused y sesonl nd internnul climte vrition hve significnt effect on N minerliztion nd the grsslnd productivity Supported y the Centrl Pulic Welfre Scientific Reserch Institutes, Project No ; y the Ntionl Nturl Science Foundtion of Chin, Grnt No ; y the Ntionl High-tech Reserch nd Development Projects, Projects No. 863 nd 212AA123-4; nd y the Specil Funding for Modern Agriculturl Technology Systems from the Chinese Ministry of Agriculture, Grnt No. CARS

2 Plnt Soil Environ. Vol. 62, 216, No. 5: doi: /445/215-PSE (Liu et l. 21). Nutrient cycling is different in grsslnd ecosystems thn in forest ecosystems, due to the disturnces nd inputs cused y livestock, including grzing, trmpling nd the deposition of excrement. These chnges cn directly or indirectly influence nitrogen minerliztion y ffecting plnt litter decomposition rtes nd soil microil iomss (Trcy nd Frnk 1998). Grzing is the most common humn ctivity in grsslnd ecosystems. Some studies hve indicted tht n sence of grzing reduces the net soil nitrogen minerliztion rte, while light grzing promotes soil nitrogen minerliztion, nd overgrzing inhiits soil nitrogen minerliztion (Biondini et l. 1998). Other studies hve shown tht moderte grzing cn enhnce microil decomposition, which is eneficil for soil nitrogen minerliztion (Xu et l. 27). However, the rnge of grzing intensities in different grsslnd types cn hve different influences on soil nitrogen minerliztion processes such s minerliztion nd nitrifiction (Zho et l. 21). On low grss steppe, moderte grzing ws shown to e most fvourle for soil N minerliztion, while on Serengeti grsslnd, net soil N minerliztion grdully incresed with n incresing grzing intensity (Segle et l. 1992). Reserch on nitrogen cycling in medow grsslnd ecosystems in Chin is n emerging field, nd mny knowledge gps remin. Few studies hve explored nitrogen minerliztion in grsslnd ecosystems. Therefore, the present study on the effects of grzing on soil N minerliztion in medow steppe region offers n importnt contriution to the understnding of grsslnd iogeochemicl cycles. The Huluner medow steppe in northestern Chin is representtive prt of the Eursi grsslnd ecosystem. Cttle grzing is the primry use of grsslnds in the regionl frming system. Therefore, understnding the effects of grzing on nitrogen cycling in these grsslnd ecosystems is criticl for etter mngement nd for improving knowledge of the mechnisms underlying grsslnd degrdtion nd cn provide sic informtion for sustinle development in grsslnd ecosystems. The ojectives of the present study were s follows: (1) to investigte the sesonl vrition of the inorgnic nitrogen pool nd the net nitrogen minerliztion rte in situ in surfce soils; (2) to compre the differences in soil nitrogen minerliztion rtes mong the different grzing intensities over growing seson; nd (3) to revel the reltionships etween the soil inorgnic nitrogen pool, the net nitrogen minerliztion rte nd other soil chrcteristics. MATERIAL AND METHODS Study site. This study ws conducted in the centre of the Huluner medow steppe (49 19'349''49 2'173''N, '521''119 57'854''E), in the northestern region of Inner Mongoli, Chin. The elevtion t the reserch sttion vries from m.s.l. The climte is temperte, semi-rid nd continentl, with n nnul verge of 11 frost-free dys. The verge nnul precipittion rnges from 35 4 mm, pproximtely 8% of which flls etween July nd Septemer. The nnul men ir temperture rnges from 5 C to 2 C, with mximum monthly men of C in July nd minimum of 48.5 C in Jnury. The monthly verge temperture nd precipittion t the study site in 214 re shown in Figure 1. The soil is chernozem or chestnut soil y type, nd the vegettion is chrcterized s typicl Leymus chinensis nd fors medow steppe. The min species re L. chinensis Stip. iclensis, Crex pediformis, Glium verum, Bupleurum scorzonerifolium nd Filifolium siiricum. Experimentl design. The grzing experiment ws estlished in 29 with 18 pddocks ech of 5 h (3 167 m) with six stocking densities (.,.23,.34,.46,.69 nd.92 AU/h; where 1 AU = 5 kg of dult cttle). Ech stocking rte ws replicted three times in rndomised lock design (Figure 2). The stocking rtes were chieved y using, 2, 3, 4, 6 or 8 young cows (25 3 kg) per plot. Continuous grzing lsted for 12 dys etween June nd Octoer on n nnul sis from The grzing cttle were kept in the grzing plots dy nd night, nd their drinking wter ws supplied from n outside wter source. Before eing fenced, the site ws prt of lrger re, under long-term free-rnging cttle grzing. The seline mesurements of vegettion nd soil trits were crried out using the 5 m 5 m grid method efore the grzing experiment. It ws found tht the soil profile is composed of humus lyer, leching lyer nd deposition lyer, nd the thickness of ech soil lyer is 48, nd 76 1 cm. The study res were clssified 23

3 Vol. 62, 216, No. 5: Plnt Soil Environ. doi: /445/215-PSE Rinfll (mm) rinfll 214 rinfll verge (21 214) temperture (214) Temperture ( ) Figure 1. Monthly rinfll nd temperture in 214 nd verges from for the grzing experimentl sites. Vlues on ech pnel re totl nnul rinfll nd men temperture Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec 4 s chernozem or chestnut soil nd lom in terms of texture. The plnt roots were decresed with the increse of soil depth. The soil ph vlue ws 6 7. The totl soil nitrogen ws g/kg nd the orgnic cron concentrtion of the surfce soil ws g/kg; these vlues did not vry significntly mong the plots with different grzing intensities. Soil smpling nd nlysis. Net N minerliztion rtes were mesured using n in situ soil core incution method. Within ech replicted plot, five 2 2 m qudrts were rndomly demrcted on the first soil smpling dte nd were repetedly smpled throughout the experiment. Two PVC tues (7.5 cm in dimeter nd 12 cm in length) were driven into the soil 1 cm from where the plnts nd litter were clipped nd removed. Two soil cores were otined. One soil core ws returned to the lortory nd stored in refrigertor t 4 C for lter nlysis of the initil concentrtions of NH 4 + -N, NO 3 -N nd inorgnic N. The other soil core ws incuted in situ fter seling the top of the core with piece of plstic film tht prevented wter permetion nd llowed gs exchnge. The core ws incuted for pproximtely 3 dys during the wrm, wet seson from June to Octoer in 214. In the lortory, following the removl of roots nd stones, ech soil core ws well mixed y hnd to form homogenous smple nd then pssed through 2 mm sieve. To nlyse inorgnic nitrogen, 1-g susmple ws tken from oth the initil nd incuted soil cores, nd 5 ml of 2 mol KCl solution ws dded. The soil nd extrctnt were shken for 1 h in reciprocl shker. After shking, the soil suspension ws filtered through No. 1 filter pper. The concentrtions of NH 4 + -N nd NO 3 -N were determined using flow injection utonlyzer (Fistr 5 Anlyzer, Foss Tector, Sweden). The net minerliztion nd nitrifiction rtes were expressed on dry mss sis. The soil moisture content nd ulk soil density of the composite smple from ech plot were determined grvimetriclly vi oven-drying t 15 C for 24 h. Rinfll nd temperture dt were collected from n utomtic meteorologicl sttion (AU/h) (AU/h) (AU/h) Repliction 1 Repliction 2 Repliction 3 Figure 2. Experimentl design digrm of different cttle grzing stocking rte 24

4 Plnt Soil Environ. Vol. 62, 216, No. 5: doi: /445/215-PSE (Milos 52, Visl, Finlnd) t intervls of 3 min. The soil temperture (t 1-cm depth) ws recorded using n utomtic soil temperture nd humidity sensor (Bet Them1K3A1 Thermistor, Cmpell Scientific Inc., USA) plced in ech plot. Clcultions nd dt nlysis. Net N minerliztion (nitrifiction) rtes during the incution period were clculted from the lnce etween the inorgnic N contents of the initil nd incuted smples. Cumultive N minerliztion ws determined y summing the mount of net N minerliztion t ech incution stge during the growing seson. Net N minerliztion nd nitrifiction rtes were clculted using the following equtions: For time intervl of Δt = t i+1 t i ; A mm = [NH 4 + -N] i+1 [NH 4 + -N] i ; A nit = [NO 3 -N] i+1 [NO 3 -N] i ; A min = A mm + A nit ; R min = A min /Δt; R mm = A mm /Δt; R nit = A nit /Δt. Where: t i nd t i+1 initil nd finl incution dtes, respectively; c [NH 4 + -N] i nd c [NH 4 + -N] i+1 men NH 4 + -N contents of the initil nd finl incuted smples, respectively; c [NO 3 -N] i nd c [NO 3 -N] i+1 men NO 3 -N contents of the initil nd finl incuted smples, respectively; N mmonium mount (A mm ); N nitrifiction mount (A nit ); nd N minerliztion mount (A min ) ccumultion of NH 4 + -N, NO 3 -N nd totl inorgnic N; nd soil net N minerliztion rtes (R min ); soil net N mmonium rtes (R mm ) nd soil net N nitrifiction rtes (R nit ) re the net N minerliztion, mmonium nd nitrifiction rtes. For ech selected smpling dte, mens nd stndrd error (SE) of soil nitrogen minerliztion nd other fctors were clculted nd the plot vlues represent mens (n = 3) ± SE. One-wy ANOVAs nd the lest significnt difference (LSD) tests were used to exmine the effects of grzing tretments on sesonl nd men soil nitrogen minerliztion nd other fctors, with effects of P <.5 eing significnt (ANOVA; SPSS softwre, Version 21., IBM compny, USA). () Soil moisture (%) G. G.23 G.34 G.46 G.69 G.92 Soil ulk density (g/cm 3 ) () Soil ulk density (g/cm 3 ) Jun. Jul. Aug. Sep. Men. Jun. Jul. Aug. Sep. Men (c) 35 Soil temperture ( ) Jun. Jul. Aug. Sep. Men Figure 3. Chnges in () soil moisture content; () soil ulk density nd (c) soil temperture in the top 1 cm soil lyer under different grzing intensities. Six stocking densities (.,.23,.34,.46,.69 nd.92 AU/h, where 1 AU = 5 kg of dult cttle 25

5 Vol. 62, 216, No. 5: Plnt Soil Environ. doi: /445/215-PSE RESULTS AND DISCUSSION Chnge of soil environmentl fctor. The sesonl dynmics of soil environmentl fctor re shown in Figure 3. The soil moisture content showed similr ptterns under ll grzing intensities, pek soil moisture content occurred in August, ut the men soil moisture content in the ungrzed tretment ws 14% higher thn in the G.92 tretment (P <.5). Soil ulk density showed grdul positive trend with n incresing grzing intensity throughout the incution period (Figure 3). The monthly soil temperture ws highest in June nd July nd lowest in August nd Septemer in ll of the grzing tretments (Figure 3c). Long-term grzing ws ssocited with significntly higher soil temperture compred with the ungrzed plots (P < 9.95). Sesonl vrition of soil inorgnic N pools under different intensity. During the incution period, soil NH 4 + -N, NO 3 -N nd soil inorgnic N showed similr temporl ptterns under different grzing intensities, incresing through August, then declining in Septemer (Figure 4). These results re consistent with the conclusions of Liu et l. (211), who showed tht in semi-rid grzing grsslnd on the Loess Plteu of northern Chin during growing seson, pek of nitrogen minerliztion occurred in August, nd sustntil mount of nitrogen immoiliztion in Septemer when soil moisture content nd temperture were low (Gleeson et l. 28, Liu et l. 211). Compred with the ungrzed plots, the light grzing tretment G.34 resulted in higher concentrtion of NH 4 + -N, NO 3 -N nd lrger soil inorgnic N pool (P <.5), while the intermedite grzing tretment G.46 nd the hevy grzing tretment G.92 led to lower concentrtions of NH 4 + -N, NO 3 -N nd smller soil inorgnic N pool (P <.5) (Figure 4). Chnge of soil net N minerliztion nd nitrifiction rtes under different grzing intensity. The soil net N minerliztion, mmonium nd nitrifiction rtes exhiited significnt temporl vritions during the growing seson. The net N minerliztion rtes rnged from.73 to 5.33 mg/kg/dy, nd the net N minerliztion rtes nd net mmonifiction rtes were highest in August nd lowest in June nd July (Figure 5). () 5 () 5 NH 4 + -N (mg/kg) G. G.23 G.34 G.46 NO -N (mg/kg) NO 3 -N (mg/kg) G.69 1 G.92 1 (c) NH 4 + -N + NO3 -N (mg/kg) 26 Jun. Jul. Aug. Sep. Jun. Jul. Aug. Sep. Jun. Jul. Aug. Sep. Figure 4. Sesonl ptterns of () NH 4 + -N; () NO 3 -N nd (c) totl minerl nitrogen (NH 4 + -N + NO 3 -N) in the top 1 cm soil lyer under different grzing intensities. Ech point is the men from three replicted plots. Error rs represent ± stndrd error. Six stocking densities (.,.23,.34,.46,.69 nd.92 AU/h, where 1 AU = 5 kg of dult cttle

6 Plnt Soil Environ. Vol. 62, 216, No. 5: doi: /445/215-PSE Nitrte nitrogen immoiliztion ws oserved in August in ll tretments, with the gretest net nitrte nitrogen immoiliztion occurring in the ungrzed tretment G. nd the light grzing tretment G.23 (P <.5). It ws oserved tht the grzing tretments tended to decrese oth R min nd R mm compred with the ungrzed control plots in the erly growing seson, in June nd July. However, in August, grzing incresed oth R min nd R mm compred with the ungrzed control plots nd decresed R nit (P <.5). These results re consistent with others demonstrting tht grzing decreses N minerliztion in the erly growing seson (Augustine nd McNughton 26). However, these results re opposite to those of previous works showing tht light grzing positively ffected net N minerliztion in the erly growing seson (Le Roux et l. 23, Shn et l. 211). On verge, during the study period, intermedite grzing tretment G.46 produced higher sesonl men A min, R min, R mm nd R nit (Figure 5), which is consistent with numer of previous studies (Segle et l. 1992, Xu et l. 27). For exmple, Xu et l. (27) showed tht cumultive net N minerliztion rtes t the nnul time scle tend to e higher under the moderte grzing intensity of 4. sheep/h thn under either lower or higher grzing intensity, supporting intermedite disturnce hypothesis (Connell 1978). The reson my e tht moderte grzing could further enhnce microil degrdtion, which ws fvourle for soil N minerliztion (Shriff et l. 1994). But in different types of grsslnds, the effects of grzing on soil N minerliztion hve een found to e different. For exmple, in low grss pririe, moderte grzing ws shown to e the most conducive to soil N minerliztion, wheres in tll grss pririe, the soil net N minerliztion rte grdully incresed with n incresing grzing intensity (Segle et l. 1992). This study confirms their findings nd provides possile explntion for differences etween the results otined nd the results from studies in which the N minerliztion rte incresed in response to grzing (McNughton et l. 1997). The role of soil N minerliztion wrrnts further investigtions. Reltionships etween soil chrcteristics nd N minerliztion nd nitrifiction. Person s correltion nlysis demonstrted tht soil mois- () Minerliztion rte (mg/kg/dy) G. G.23 G.34 G.46 G.69 G.92 Ammonium rte (mg/kg/dy) () Ammonium rte (mg/kg/dy) /11 7/7 7/7 8/12 8/12 9/11 6/11 7/7 7/7 8/12 8/12 9/11 (c) Nitrifiction rte (mg/kg/dy) /11 7/7 7/7 8/12 8/12 9/11 Figure 5. Chnges in () soil net nitrogen minerliztion rte; () mmonium rte nd (c) nitrifiction rte in the top 1 cm soil lyer under different grzing intensities. Ech column is the men from three replicted plots. Error rs represent ± stndrd error. Tretments with different letters re sttisticlly different t P <.5 level. Six stocking densities (.,.23,.34,.46,.69 nd.92 AU/h, where 1 AU = 5 kg of dult cttle 27

7 Vol. 62, 216, No. 5: Plnt Soil Environ. doi: /445/215-PSE Tle 1. Person correltion coefficients (r) of seson soil inorgnic nitrogen (N); soil net N minerliztion (A min ); soil net mmonium rtes (R mm ); soil net nitrifiction rtes (R nit ) nd soil net N minerliztion rtes (R min ) with other soil chrcteristics including soil moisture, soil temperture nd soil ulk density on Huluner medow steppe r Soil moisture Soil temperture Soil ulk density Inorgnic N.819**.384**.296* A min.736**.273*.141 R mm.756**.268*.78 R nit.51** R min.725** *P <.5; **P <.1 ture ws significntly positively correlted with inorgnic N, A min, R mm nd R min nd significntly negtively correlted with R nit, while soil temperture exhiited the opposite effect (Tle 1). The results indicte tht the net N minerliztion rte ws significntly ffected y soil temperture nd soil moisture, with reltively low soil temperture nd reltively wet soil eing relted to greter soil minerliztion, n incresed soil inorgnic N content nd soil N minerliztion. These findings re consistent with the results of other grsslnd studies (Shn et l. 211) ut do not gree with the results of previous studies showing tht high temperture nd reltively dry environment encourge more soil minerliztion nd increse the soil inorgnic N content nd tht etween 5 ~ 25 C, the soil net N minerliztion rte nd nitrifiction rte increse with n incresing temperture nd humidity. The soil net minerliztion nd nitrifiction rtes decline when the soil temperture exceeds 25 C nd soil shows high wter content (Wng et l. 26). Some studies hve indicted tht net N minerliztion rtes re positively correlted with sesonl temperture nd re less sensitive to soil moisture (Sierr 1997). Other reserchers hve reported tht soil N trnsformtion is generlly sensitive to temperture in wet ecosystems (Pul et l. 23) ut sensitive to soil moisture in rid nd semi-rid ecosystems (Xu et l. 27). These oservtions could e explined y the different sesonl ptterns of temperture nd precipittion etween the different study regions, which my directly ffect soil temperture nd moisture dynmics nd led to different sesonl ptterns of soil inorgnic N nd net N minerliztion. However, cumultively, these results suggest tht iotic fctors (e.g., soil moisture nd temperture) could regulte the sesonl ptterns of soil inorgnic N nd soil N minerliztion. Therefore, the temporl scling pplied in this study provides theoreticl references for how soil conditions ffect the soil inorgnic N pool nd net N minerliztion under different grzing pressures, which my hve importnt implictions for grzing mngement. Acknowledgements We re grteful to mny collegues from the Huluner Grsslnd Ecosystem Reserch Sttion for ssistnce with field oservtions nd smple collection. REFERENCES Augustine D.J., McNughton S.J. (26): Interctive effects of ungulte herivores, soil fertility, nd vrile rinfll on ecosystem processes in semi-rid svnn. Ecosystems, 9: Biondini M.E., Ptton B.D., Nyren P.E. (1998): Grzing intensity nd ecosystem processes in northern mixed-grss pririe, USA. Ecologicl Applictions, 8: Connell J.H. (1978): Diversity in tropicl rin forests nd corl reefs. Science, 199: Gleeson D.B., Herrmnn A.M., Livesley S.J., Murphy D.V. (28): Influence of wter potentil on nitrifiction nd structure of nitrifying cteril communities in semirid soils. Applied Soil Ecology, 4: Le Roux X., Brdy M., Loiseu P., Louult F. (23): Stimultion of soil nitrifiction nd denitrifiction y grzing in grsslnds: Do chnges in plnt species composition mtter? Oecologi, 137: Liu T.Z., Nn Z.B., Hou F.J. (211): Grzing intensity effects on soil nitrogen minerliztion in semi-rid grsslnd on the Loess Plteu of northern Chin. Nutrient Cycling in Agroecosystems, 91: Liu X.R., Dong Y.S., Ren J.Q., Li S.G. (21): Drivers of soil net nitrogen minerliztion in the temperte grsslnds in Inner Mongoli, Chin. Nutrient Cycling in Agroecosystems, 87: McNughton S.J., Bnyikw F.F., McNughton M.M. (1997): Promotion of the cycling of diet-enhncing nutrients y fricn grzers. Science, 278:

8 Plnt Soil Environ. Vol. 62, 216, No. 5: doi: /445/215-PSE Pul K.I., Polglse P.J., O Connell A.M., Crlyle J.C., Smethurst P.J., Khnn P.K. (23): Defining the reltion etween soil wter content nd net nitrogen minerliztion. Europen Journl of Soil Science, 54: Segle S.W., McNughton S.J., Ruess R.W. (1992): Simulted effects of grzing on soil nitrogen nd minerliztion in contrsting Serengeti grsslnds. Ecology, 73: Shn Y.M., Chen D., Gun X.X., Zheng S.X., Chen H.J., Wng M.J., Bi Y.F. (211): Sesonlly dependent impcts of grzing on soil nitrogen minerliztion nd linkges to ecosystem functioning in Inner Mongoli grsslnd. Soil Biology nd Biochenistry, 43: Shriff A.R., Biondini M.E., Grygiel C.E. (1994): Grzing intensity effects on litter decomposition nd soil nitrogen minerliztion. Journl of Rnge Mngement, 47: Sierr J. (1997): Temperture nd soil moisture dependence of N minerliztion in intct soil cores. Soil Biology nd Biochemistry, 29: Trcy B.F., Frnk D.A. (1998): Herivore influence on soil microil iomss nd nitrogen minerliztion in northern grsslnd ecosystem: Yellowstone Ntionl Prk. Oecologi, 114: Wng C.H., Wn S.Q., Xing X.R., Zhng L., Hn X.G. (26): Temperture nd soil moisture interctively ffected soil net N minerliztion in temperte grsslnd in Northern Chin. Soil Biology nd Biochemistry, 38: Xu Y.Q., Li L.H., Wng Q.B., Chen Q.S., Cheng W.X. (27): The pttern etween nitrogen minerliztion nd grzing intensities in n Inner Mongolin typicl steppe. Plnt nd Soil, 3: Zho H.T., Zhng X.L., Xu S.T., Zho X.G., Xie Z.B., Wng Q.B. (21): Effect of freezing on soil nitrogen minerliztion under different plnt communities in semi-rid re during nongrowing seson. Applied Soil Ecology, 45: Received on July 7, 215 Accepted on April 19, 216 Corresponding uthor: Prof. Dr. Xioping Xin, Chinese Acdemy of Agriculturl Sciences, Institute of Agriculturl Resources nd Regionl Plnning, Beijing, P.R. Chin; e-mil: xinxp@sin.com 29