Effects of grassland conversion to croplands on soil organic carbon in the temperate Inner Mongolia

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1 RTICLE IN PRESS Journl of Environmentl Mngement 86 (28) Effects of grsslnd conversion to croplnds on soil orgnic crbon in the temperte Inner Mongoli Zhi-Ping Wng, Xing-Guo Hn, Ling-Ho Li Lbortory of Quntittive Vegettion Ecology, Institute of Botny, Chinese cdemy of Sciences, Nnxincun 2, Xingshn, Beijing 193, Chin Received 2 Mrch 26; received in revised form 2 November 26; ccepted 8 December 26 vilble online 2 Jnury 27 bstrct This study investigted the effects of grsslnd conversion to croplnds on soil orgnic crbon (SOC) in typicl grsslnd-dominted bsin of the Inner Mongoli using direct field smplings. The results indicted tht SOC contents decresed usully with incresing soil depth, with significnt differences between the upper horizons ( 3 cm) nd the underlying horizons (3 1 cm). lso, SOC densities decresed with n increse in the depth of soils. verge SOC densities in the upper horizons were nd kg C m 2 for desert grsslnd croplnd sites (sites 1 nd 2) nd medow croplnd sites (sites 3 nd ), respectively, with significnt differences between grsslnds nd croplnds (Po:5). However, the SOC densities in the underlying horizons did not significntly differ between the lnd uses. The SOC densities up to 1 cm depth were much higher in the medow croplnd sites thn in the desert grsslnd croplnd sites, reching pproximtely 16 nd 6 kg C m 2, respectively. The SOC: totl nitrogen (TN) rtios were pproximtely 1, with no significnt difference mong the soil horizons of grsslnds nd croplnds. The conversion of grsslnds to croplnds induced slight loss of SOC, with rnge of from % to 22% for the 1 cm soil depth over bout 35-yer period, in the temperte Inner Mongoli. r 27 Elsevier Ltd. ll rights reserved. Keywords: Soil orgnic mtter; Medow; Frmlnd; Lnd use; Semi-rid region 1. Introduction tmospheric crbon dioxide (CO 2 ) concentrtions hve incresed by 31% since 175, nd the current increse of pproximtely 1.5 mll 1 yr 1 is predominntly due to fossil fuel combustion nd lnd use chnges (IPCC, 21). Locl nd regionl humn-induced disturbnces to crbon stocks re ssocited with increses in tmospheric CO 2 concentrtion, ir temperture, nd extreme climtic events t the globl scle (Melillo et l., 1993). n importnt strtegy for controlling the rise in the tmospheric CO 2 concentrtions is to enhnce crbon sequestrtion in the terrestril ecosystems, which is cost-effective nd environmentlly beneficil. Soils my ct s source or sink of CO 2 in exchnge with the tmosphere; soils contin lrge stock of soil orgnic crbon (SOC), nd thus, even slight chnges in SOC stock Corresponding uthor. Tel.: E-mil ddresses: wngzp5@yhoo.com, zhipinw@clemson.edu (Z.-P. Wng). represent lrge CO 2 fluxes (Krogh et l., 23). Lnd uses cn induce the significnt chnges in SOC, lthough the involved processes in soils re incompletely understood. In the lst two centuries, lnd uses hve been significnt source of the tmospheric CO 2 through the conversion of nturl vegettion to frming (Wu et l., 23). Wherever the chnge in lnd uses decresed SOC, the reverse process usully incresed SOC, nd vice vers (Guo nd Gifford, 22). Using the SOC pool in current grsslnd s the bseline in comprison with tht in croplnd should be good indictor of either the SOC source contribution to the greenhouse effect or the potentil of SOC sequestrtion through the conversion between grsslnd nd croplnd (Tn nd Ll, 25). Hence, there is considerble potentil to enhnce SOC sequestrtion through efficient lnd mngements. Grsslnds re often susceptible ecosystem to chnges in lnd uses. Chnges in grsslnd soil crbon cn occur in response to wide rnge of mngement nd environmentl fctors (Tn nd Ll, 25). Chinese grsslnds covered 1.7% of its territory, reching 1 6 km 2 in /$ - see front mtter r 27 Elsevier Ltd. ll rights reserved. doi:1.116/j.jenvmn

2 53 RTICLE IN PRESS Z.-P. Wng et l. / Journl of Environmentl Mngement 86 (28) , nd were distributed minly in the Inner Mongoli nd Tibet plteus (Ntionl Sttistics Bureu of Chin, 22). Previous works hve been conducted to estimte the SOC stocks in Chinese grsslnds using ntionl survey dtbses (e.g. Fng et l., 1996; Ni, 21, 22). However, the dtbses were insufficient, which ffected the estimted ccurcy of SOC stocks. lso, SOC densities for clculting the SOC stocks in the grsslnds of Chin were common vlues derived from globl ecosystems, wheres they were not perfectly suited to Chinese grsslnds (Ni, 22). On the other hnd, Guo nd Gifford (22) reviewed the effects of lnd uses on SOC stocks using met nlysis of the dt from 7 publictions, in which no report occurred in Chin. The Inner Mongoli steppes constitute n importnt component of the Eursin temperte grsslnds (Zhng et l., 1997; Wng et l., 27). To our knowledge, there is lck of informtion bout chnges in SOC induced by lnd uses in the temperte grsslnds of the Inner Mongoli. The Xilin River bsin is one of the most representtive geogrphic res in the Inner Mongoli steppe region (Tong et l., 2). In this study, we performed field smplings in the Xilin River bsin to ssess the effects of grsslnd conversion to croplnds on SOC. We then suggested efficient lnd mngements for protecting SOC loss from lnd uses in this region. 2. Mterils nd methods 2.1. Region nd site descriptions This work ws undertken in four grsslnd croplnd pired sites of the Xilin River bsin. The detiled description for this region cn be found in Tong et l. (2) nd Wng et l. (25). In brief, this region belongs to the semi-rid temperte climtic zone. The men nnul temperture is pproximtely.6 1C. The coldest monthly temperture is 21. 1C in Jnury, nd the wrmest is C in July. The region receives n verge of 35 mm precipittion nnully, pproximtely 1% of which flls s snow in winter. The plnts usully grow from lte pril to erly October. The soils re minly luvic kstnozem (previously nmed s chestnut soils) with the texture of pproximtely 2% cly, 2% silt, nd 6% snd. pired-site design ws tken to compre the chnges in SOC between grsslnds nd croplnds. Grsslnds re the most importnt lnd cover in the Xilin River bsin, while croplnds hve been derived from the reclmtion of grsslnds in the s. We determined mjor grsslnds of from desert psture to medow. They re representtive in the bsin. Hence, four pired sites with different soil nd vegettion chrcteristics re selected (Fig. 1). Site 1 is locted roughly in 1 km south of Xilin River. Croplnd ws reclimed from desert grsslnd round 197. The croplnd hs been plnted with spring whets Fig. 1. Four grsslnd croplnd pired sites in the Xilin River bsin, Inner Mongoli, Chin. nd highlnd brleys in fllow rottion system. Desert psture ws covered minly by Leymus chinensis, Stip grndis, Stip krylovii, gropyron cristtum nd rtemisi frigid with bout 2 5% cover. The site hs exhibited sndy chestnut soils with two distinct soil horizons, the upper horizon nd the underlying horizon. The distnce of smpling points between the grsslnd nd the croplnd ws bout 5 m. Site 2 is locted in pproximtely 2 km southest of Xier. Croplnd hs been plnted intermittently with mizes nd rpes for bout 2 yers. Grsslnd ws n bndoned lnd covered dominntly with Stip grndis, Stip krylovii, gropyron cristtum, rtemisi frigid nd Bupleurum bicule with 1 3% cover. The site hs existed brren brown clcic soils with grvel horizon below 6 cm of profile. The distnce of smpling points between the grsslnd nd the croplnd ws bout 1 km. Site 3 is locted in bout km north of Hiliut. Croplnd ws derived from the reclmtion of medow in The croplnd hs been plnted with spring whets in fllow rottion system but occsionlly plnted with rpes. Medow steppe ws minly composed of Stip grndis, Stip biclensis, Leymus chinensis, Bupleurum scorzonerifolium, Fillifolium sibiricum nd Festuc with the cover of 6 9% nd the plnt height of 6 cm. The medow hs been hrvested in utumns. The site hs been fertile chernozem soils with the upper humic horizon of bout 5 cm. The distnce of smpling points between the grsslnd nd the croplnd ws bout 1 m.

3 RTICLE IN PRESS Z.-P. Wng et l. / Journl of Environmentl Mngement 86 (28) Site is locted in bout 25 km north of Modeng. Croplnd ws derived from the reclmtion of medow in The croplnd hs been plnted usully with rpes nd whets. Medow ws covered dominntly with Leymus chinensis, Stip krylovii nd rtemisi frigid with the cover of 5 8% nd the plnt height of 3 5 cm. The site hs been drk chestnut soils. The distnce of smpling points between the grsslnd nd the croplnd ws bout 5 m Soil smplings nd nlysis Field smplings were undertken during July ugust 25. In order to decrese soil heterogeneity in ech site, soils were smpled rndomly in the croplnd nd grsslnd on flt re. Soils were smpled in four horizons of 1, 1 3, 3 6 nd 6 1 cm using stinless steel corer (3.5 cm in dimeter). Ech site ws designed with three replictes for the croplnd or the grsslnd. The distnce ws bout 1 m between the replictes. For ech replicte, soils were smpled rndomly in circle of bout 2 m in dimeter nd bulked together for the sme depth intervls. Six smpling points were determined for the 1 nd 1 3 cm horizons, three points of which were further smpled for the 3 6 nd 6 1 cm horizons. ll soil smples were processed minimlly in field nd then tken to lbortory. Ech soil smple ws ir-dried nd sieved through 2 mm sieve, nd then ground to fine powder (mesh number 1) using pestle nd mortr. Soil bulk density (BD) ws determined using stinless steel cylinder of cm 3 (5 cm in dimeter, 5 cm in height). The soils did not contin the grvels with more thn 2 mm in dimeter, which ccounted for smll proportion, for determining the BD. For limiting lnd destruction, only one profile (5 cm in width, 1 cm in length, nd 1 cm in depth) ws dug in the croplnd nd the grsslnd of ech site. The soils for determining the BD were oven-dried t 15 1C to constnt weight. SOC nd totl nitrogen (TN) were nlyzed following the stndrd procedures (Liu, 1996). The SOC ws determined using the combustion method. First, 5 ml.8-m K 2 Cr 2 O 7 solution nd 5 ml H 2 SO were dded into the weighed soil nd then boiled t C for 5 min, nd then the remining K 2 Cr 2 O 7 ws titrted using.2-m FeSO. The TN ws determined by the Semi-micro Kjeldhl method, using 23 Kjeltec nlyzer Unit (FOSS, Sweden) Clcultion of SOC density nd sttisticl nlysis Since SOC content vries long soil profile, SOC density is clculted s follows: SOC Density ¼ Xn i¼1 D i B i O i, where n is the number of soil horizons, D i is the depth intervl (cm) of the horizon i from the top soil nd down on, B i is the BD (g cm 3 ) in the horizon i nd O i is the men SOC content (%) in the horizon i. Sttisticl nlysis ws performed using the SS progrm (SS Institute, 1999). Duncn s multiple comprison test ws used to determine significnt differences in men BD (g cm 3 ), SOC (%), SOC density (kg C m 2 ) nd TN (g kg 1 ) mong the depth intervls nd between the lnd uses in ech site t Po:5. 3. Results 3.1. Distributions of bulk density, soil orgnic crbon nd totl nitrogen Tble 1 lists the chrcteristics of BD, SOC nd TN in four pired sites. The bulk densities were pproximtely , , nd g cm 3 in the sites 1, 2, 3 nd, respectively. They did not significntly differ between the sme soil horizons of the croplnd nd grsslnd (P:5), with the exception of the 1 cm horizon in the site 2 nd the 3 1 cm horizon in the site. s result, there ws similr soil compction between the lnd uses in ech site. The SOC contents decresed generlly with incresing soil depth, with significnt differences between the upper horizons nd the underlying horizons (Po:5). The SOC contents in the sites 3 nd were higher thn those in the sites 1 nd 2, reching pproximtely 1.5% nd.5%, respectively. On dry soil mss bsis, the verge rtios of SOC: TN in four pired sites were pproximtely 1. The SOC: TN rtios were slightly higher in the sites 3 nd with fertile soils thn in the sites 1 nd 2 with brren soils, wheres they usully lcked significnt difference mong the soil horizons of the lnd uses (P:5). Hence, the SOC nd TN contents hd existed synchronous chnge in soils Chnges in soil orgnic crbon density The SOC densities were pproximtely , , , nd kg C m 2 in the upper horizons of the sites 1, 2, 3 nd, respectively (Fig. 2). There were significnt differences between the upper horizons of the croplnds nd grsslnds t Po:5, with the exception of those in the site 2. However, the SOC densities in the underlying horizons lcked significnt difference between the lnd uses (P:5). The SOC densities up to 1 cm depth were pproximtely 16 kg C m 2 in the sites 3 nd nd 6 kg C m 2 in the sites 1 nd 2, wheres they lcked significnt difference between the lnd uses (P:5).. Discussion Cultivtions redily induce soils compcted. For exmple, the 2 cm soil BD in cultivted lnds ws significntly higher thn those in psture nd forest soils (Evrendilek et l., 2; Celik, 25). In this study, however, the bulk densities showed similr chnge

4 532 RTICLE IN PRESS Z.-P. Wng et l. / Journl of Environmentl Mngement 86 (28) Tble 1 Chrcteristics of bulk density, soil orgnic crbon nd totl nitrogen in four pired sites Horizon depth (cm) Site 1 Site 2 Site 3 Site BD (g cm 3 ) SOC (%) TN (g kg 1 ) BD (gcm 3 ) SOC (%) TN (g kg 1 ) BD (gcm 3 ) SOC (%) TN (g kg 1 ) BD (gcm 3 ) SOC (%) TN (g kg 1 ) b b bc b b b b c b b b b c c bc.7. b d c c c d.177. e d.27. c e d d.7.1 d.87.1 Croplnd b b b b b b c cd c.37.1 b d c b bc c cd.27.1 de d c e e b c c 1.7. Grsslnd Men vlue7se is shown for ech determintion, n ¼ 3. Mens with different letters identify significnt differences (Po:5) between horizons of the lnd uses for ech vrible using Duncn s multiple rnge tests. bbrevitions: BD, bulk density; SOC, soil orgnic crbon; TN, totl nitrogen. between the croplnd nd the grsslnd in ech site (Tble 1). This is explined to be prtly due to moderte cultivtion in the croplnd by fllow rottion system. In ddition, the soils were smpled fter cultivtion in the croplnd, nd thus the surfce soils were loose with lower BD. In the site 2, the croplnd hd less BD thn the grsslnd due to the ltest cultivtion. The SOC densities were slightly over- or under-estimted in the croplnds when compred with those in the grsslnds. Consequently, the soil compction induced by cultivtions did not significntly ffect the SOC estimtes. This ws consistent with results reported by Grnett et l. (21), who suggested tht the effect of compction on the finl SOC storge estimte ws smll. The SOC densities were determined seprtely in two distinct soil horizons: (i) the upper horizon ( 3 cm) nd (ii) the underlying horizon (3 1 cm) (Fig. 2). The upper soils were susceptible to chnges in the lnd uses nd climte. In the sites 1, 3 nd, the SOC densities were significntly lower in the croplnds thn in the grsslnds t Po:5. In the site 1, the lower SOC density in the croplnd resulted indirectly from possible SOC ccumultion in the grsslnd. Moderte grzing often incresed surfce SOC storges in grsslnds compred to un-grzed grsslnds (Derner et l., 1997; Schumn et l., 1999). It is thought tht well-mnged grzing stimultes growth of herbceous species nd improves nutrient cycling in grsslnd ecosystems, nd thus my contribute to the observed increse in SOC storges (Schumn et l., 22). The cultivtions tend to ccelerte SOC decomposition, brek up soil ggregtes, nd decrese soil porosity. For the upper fertile soils of the croplnds in the sites 3 nd, the significnt SOC losses re explined to be prtly due to cultivtions. In the site 2, no significnt SOC loss in the croplnd reltive to the grsslnd is becuse of the brren soils with very low SOC content of bout.5.6%. Hence, the conversion of grsslnds to croplnds often induced slight loss of SOC in the temperte Inner Mongoli. The SOC hs turnover cycle of decdl order or even less in the surfce horizons while tht is on the order of hundreds/thousnds yers or more in the underlying horizons (Schimel et l., 199). The underlying horizons usully exist the SOC with more reclcitrnt component. lso, the underlying horizons re rrely ffected by moderte cultivtions. Hence, the SOC densities lcked significnt difference between the underlying horizons of the lnd uses in ech site. The croplnds hd slight SOC losses with rnge of from % to 22% compred to the grsslnds, prticulrly in the 3 cm upper soil horizons (Fig. 2). This rnge ws minly consistent with previous report by Wu et l. (23), who estimted SOC loss of 1 % in the cultivted soils reltive to their non-cultivted counterprts in Chin. On the other hnd, our SOC loss percentges were lower thn those in other studies. For exmple, using met nlysis, Guo nd Gifford (22) found tht soil crbon stocks, t globl scle, declined by 59% fter lnd

5 RTICLE IN PRESS Z.-P. Wng et l. / Journl of Environmentl Mngement 86 (28) Crbondensity(KgCm -2 ) b Site c Site Site1 B d Site B B Croplnd Grsslnd -3cm 3-1cm -1cm Fig. 2. Distributions of soil orgnic crbon densities (kg C m 2 ) in the soil horizons of pired sites. Vlue is men7se (n ¼ 3). Different letters (cpitl for 3 cm; lowercse for 3 1 cm; itlic cpitl for 1 cm) represent significnt difference between grsslnd nd croplnd for ech site using Duncn s multiple rnge tests (Po:5). In this study, the SOC losses up to 1 cm depth rnged from.25 to 3.69 kg C m 2 in the croplnds reltive to the grsslnds, with n verge of 1.36 kg C m 2 over bout 35-yer period. The rnge ws minly consistent with previous estimtion in the grsslnd-dominted region of Inner Mongoli by Wng et l. (23), who found decrese in SOC density of from to 2.15 kg C m 2 due to desert expnsion nd soil degrdtion. Wind erosion cn result in considerble mount of soil loss nd deposition. Potentil soil crbon losses nd gins from wind erosion depend on the crbon content of erodible prticles nd ggregtes (Eynrd et l., 25). In recent decdes, vegettion degrdtion (Tong et l., 2), soil desertifiction (Feng et l., 21) nd wind erosion (Li et l., 25) were severe in the semirid Inner Mongoli. ll these vribles would disturb the ssessment for SOC in this region. griculturl nd grsslnd soils hve substntil potentil to sequester crbon (Ll et l., 1998). Improved lnd mngement during the next 2 5 yers would provide considerble potentil to increse the SOC stock in Chin (Wu et l., 23). However, the effects of grzing mngements on SOC sequestrtion hve not been sufficiently evluted in the grsslnds of the Inner Mongoli. The evlution requires further works to test. 5. Conclusions Soils ply n importnt role in crbon cycling in terrestril ecosystems. Lnd uses ffect both SOC storge nd CO 2 exchnge between soils nd the tmosphere. This study investigted the effects of grsslnd conversion to croplnds on SOC in typicl grsslnd-dominted bsin of Inner Mongoli. On n verge, the conversion of grsslnds to croplnds induced slight loss of SOC in the 1 cm soil depth over bout 35-yer period in the temperte Inner Mongoli. The croplnds were reclimed from grsslnds round 197. During such long period, the sites were not disturbed severely. Potentil frequent cultivtions should be strictly voided. It could be proposed tht fllow system, dequte fertiliztion nd conservtion tillge re expected to significntly prevent SOC loss nd enhnce SOC sequestrtion in the croplnds, while moderte grzing is used to increse SOC stock in the grsslnds of Inner Mongoli. use chnges from psture to crop. Evrendilek et l. (2) nd Celik (25) lso found tht the conversion of the psture into the cultivted lnd decresed soil orgnic mtter pool for the 2 cm soil depth by 9% in Mediterrnen plteu. Cultivtion of lpine grsslnd soils in Chin for 8, 16, nd 1 yr decresed SOC by 25%, 39%, nd 55%, respectively (Wu nd Tiessen, 22). The discrepncy in bove SOC losses is due to the vrious edphic conditions nd lnd uses. cknowledgments This study ws funded by n Innovtive Reserch Group Project of Ntionl Nturl Science Foundtion of Chin (NSFC) (Grnt no ). We gretly thnk Mr. Yu- Zhng Dong, Yu Zhng, Zhn-Jun Liu, Jin-Jun Chen, nd Guo-Ling Zhng for ssistnce with field smplings. Helpful comments of two nonymous referees re grtefully cknowledged.

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