ASIA SPECIAL FEATURE

Soil Organic Carbon and its Fractions in Relationto Degradation and Restoration of Wetlandson the Zoigê Plateau, China

Junqin Gao & Xuewen Zhang & Guangchun Lei &Guangxing Wang

Received: 5 August 2012 /Accepted: 27 September 2013 /Published online: 10 October 2013# Society of Wetland Scientists 2013

Abstract Through restoration, degraded wetlands arethought to recover their carbon storage function over time.However, little is known about the dynamics of soil organiccarbon (SOC) and its fractions after restoration of degradedwetlands. In this study, we selected four wetlands, two de-graded (one grazed and one graze-released site), one restoredfor 6 years and one natural site from the Zoigê wetlands on theQinghai–Tibetan Plateau, China to investigate the dynamicsof SOC following restoration. The concentrations of SOC,dissolved organic carbon (DOC), light fraction organic carbon(LFOC), and heavy fraction organic carbon (HFOC) in thedegraded sites were significantly lower than those in thenatural wetland. In contrast, soil δ13C (i.e. 13C/12C ratio) inthe degraded sites was significantly higher than that in thenatural site. After 8 years of restoration, the restored and thenatural wetland sites did not differ significantly in SOC, DOC,LFOC, HFOC or δ13C. Therefore, restored wetlands regainedsome of their role in carbon storage over time. SOC turnoverwas slower in the natural wetland than in the degraded andrestored sites.

Keywords Labile organic carbon . Soil organic carbon .

Wetland degradation .Wetland restoration . Zoigê plateau

Introduction

Although wetlands occupy only 5–8 % of the Earth’s landsurface, they store 20–30 % or more of the world’s terrestrialsoil organic carbon (SOC) (Mitsch and Gosselink 2007).Thus, wetlands could play an important role in the reductionof carbon concentration in the atmosphere (MillenniumEcosystem Assessment 2005; Mitsch and Gosselink 2007).However, human activities such as wetland drainage andovergrazing are disturbing wetlands and degrading their bio-geochemical processes and ecosystem functions (Waddingtonand McNeil 2002). In order to recover the function of thedegraded wetlands for carbon storage, recent wetland restora-tion has occurred worldwide by means of restoring hydrologyand vegetation and limiting overgrazing (Waddington andPrice 2000; Badiou et al. 2011).

Wetland restoration provides the potential to allow thedegraded wetlands to function as a carbon sink (Tuittilaet al. 1999; Waddington and Price 2000; Yli-petäys et al.2007). For example, freshwater addition enhanced the accu-mulation of SOC in the Yellow River Delta, China (Wanget al. 2011). In a study of natural bogs in Canada, Waddingtonand Price (2000) also showed that the restoration of peatlandrecovered the potential for carbon sequestration, although netcarbon storage was not achieved in a short period of time. Therestoration of seasonally, semi-permanently and permanentlydrained wetlands could sequester a nearly equivalent amount

J. Gao (*) :X. Zhang :G. Lei (*)School of Nature Conservation, Beijing Forestry University,P.O. Box 159, Qinghuadonglu 35, Haidian District, Beijing 100083,Chinae-mail: wg638@163.come-mail: guangchun8099@gmail.com

G. WangDepartment of Geography and Environmental Resources, SouthernIllinois University Carbondale, Faner Hall, 1000 Faner Dr.,Carbondale 62901, USA

Wetlands (2014) 34:235–241DOI 10.1007/s13157-013-0487-9

of carbon to natural wetland when restored (approximately0.88MgC ha−1 year−1; Badiou et al. 2011). The conversion ofwetlands to agricultural lands resulted in SOC loss in NorthAmerica, but wetland restoration could provide the potentialto sequester abundant SOC over a period of 10 years (Eulisset al. 2006).

SOC represents the balance between the input of organicmatter and output of the organic carbon losses through manyprocesses such as decomposition, leaching and dissolved car-bon export (Turetsky 2004; Wang et al. 2010). The storagepatterns of labile fractions of organic carbon, such asdissolved organic carbon (DOC) and light fraction organiccarbon (LFOC), also shift if the amount of organic matterchanges in the soil. Therefore, changes in these componentshave been suggested as early indicators of the resumption ofSOC dynamics in restored systems (Gregorich et al. 1994;Fang et al. 2009) and also as important indicators of soilquality. For example, LFOC was the most sensitive to thechanges of organic carbon following the abandonment ofcultivated lands in the northeast China (Zhang et al. 2007).Forest and grassland studies suggest that 13C abundance canbe used as an indicator of SOC dynamics (Fang et al. 2009).Because respired CO2 indicates the depletion of

13 C, a shift in13C vs. 12C ratio indicates a change in carbon storage dynam-ics. The carbon component of SOC includes 12C, 13C and 14C,with 12C the most abundant carbon type. During the decom-position of soil organic matter (SOM), carbon isotope frac-tionation occurs. Uptake and utilization of organic substancesby soil microorganisms may alter the isotopic composition ofe.g. microbial biomass, respired CO2, soil organic carbon anddissolve organic carbon (Werth and Kuzyakov 2010).

Little is known about the dynamics of SOC and the labilefractions of organic carbon in relation to the degradation andrestoration in peatlands in China. In this study, we hypothe-sized that SOC content decreases during wetland degradationand increases after restoration. Moreover, we presumed thatthe labile fractions of SOC such as DOC and LFOC changemore rapidly in degraded and restored wetlands in comparisonto natural wetlands. To test these hypotheses, we investigatedthe dynamics of soil carbon following the degradation andrestoration of Zoigê wetlands in China. Furthermore, we alsoanalyzed the turnover times of SOC in degraded, restored andnatural wetlands.

Materials and Methods

Site Characteristics and Sampling

The study area was located in the Zoigê wetlands (32°20′–34°00′N, 101°36′–103°30′E) at the eastern edge of Qinghai-Tibet Plateau in China. The average altitude of the study areais 3,400 m (Ding et al. 2004). The region is characterized by a

mean annual temperature of 0.6 °C. Mean monthly tempera-tures are −10.7 °C in January and 10.9 °C in July, respectively(Ding et al. 2004).

InMay 2011, four sites were selected in the Zoigê wetlands.The selected sites were spatially distributed within a radius of1 km (Fig. 1). Although the altitudes of these selected sitesranged from 3,441 m to 3,465 m, these sites were locatedwithin the same montane wetland (organic flat). Before the1930’s, the four sites were similar in soil, vegetation andhydrology, so that these wetlands were permanently inundatedwith about 30–50 cm average depth of water (Chai et al. 1965;Sun et al. 1998). The dominant species in this tussock peatlandis Carex spp. and the soil type is humus marsh soil (Chai et al.1965).

In the degraded wetland, a grazed site (grazed) was serious-ly degraded during the last several decades because of heavygrazing and invasion by Ochotona curzoniae . The surfacewater was drained with a ditch. The dominant species wereKobresia humilis and Potentilla anserina . Another site wasfenced in 2010, and was no longer grazed (graze-released).This site was seasonally inundated and the dominant speciesincluded Potentilla anserina and Trollius chinensis . A restoredsite (restored) had been restored by blocking ditches for about8 years before this study (i.e., 2005), and was also rotationallygrazed by cattle; it was seasonally inundated and dominated byBlysmus sinocompressus and Kobresia tibetica . A referencesite (natural wetland) was a tussock peatland in its naturalcondition, with rotational cattle grazing, all year inundation,and dominance by Carex muliensis .

Three 2×2 m plots were arbitrarily established within eachwetland type. Within each plot, nine soil samples were takenfrom three depths: 0–10, 10–20, and 20–30 cm. Each soilsample was split into three sub-samples in the field and placedinto separate plastic bags. One sub-sample was used to mea-sure bulk density and soil moisture. The second one wasrefrigerated at 4 °C for DOC analyses. The third one wasair-dried for SOC and density fractionation analyses.

Density Fractionation

The light fraction (LF) and heavy fraction (HF) of the soilwere separated by flotation in a NaI solution of 1.7 g cm−3.The concentrations of LFOC and HFOC were determinedusing a FLASH1112 CNS Analyzer (Zhang et al. 2007).Each sample of 100 g of soil was placed in 500 ml of NaIsolution, and ultrasonicated at 400 J ml−1 with a calibratedVibracell VCX 600 probe-type model. The supernatant wasfiltered through a 0.45 μm membrane filter. The fractionrecovered on the filter was washed with 0.01 M CaCl2 anddistilled water. The sediment from the centrifuge tubes wasreplaced in the beakers and re-suspended in NaI. The aboveprocedure was repeated three times to clean the sediment. Theobtained three fractions were combined, and these combined

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fractions are referred to as the LF. The sediment from thecentrifuge tubes and the beaker, i.e., the heavy fraction (HF),was washed once with 0.01 M CaCl2 and ten times withdistilled water.

DOC Measurement

DOC in fresh soil was determined with a Shimadzu TOCanalyzer, which had a lower detection limit of 50 ug l−1

(Shimadzu Corp., Kyoto, Japan). Moist soil samples (equiva-lent to 10 g oven-dried weight) were extracted with 30 ml ofdistilled water for 30 min using an end-over-end shaker at

230 rpm, which was subsequently centrifuged for 20 min at 8,000 rpm. The supernatant was filtered through a 0.45 mmfilter into separate vials for carbon analysis (Ghani et al. 2003;Zhang et al. 2007).

δ13C Measurement

For analysis of δ13C in soil organic carbon, carbonates wereremoved from the soil samples within a period of 3 days in adesiccator, which contained 10 M HCl. The samples wereneutralized by adding deionized water and then dried. Theδ13 C (i.e. 13C/12C ratio) of the samples were then determined

China

Fig. 1 The position of the study sites on the Zoigê, Qinghai–Tibetan Plateau, China. (National Dynamic Atlas 2012)

Table 1 Results of repeatedmeasures ANOVA of the effectsof site type, soil depth and theinteraction on soil properties. Pvalues for significant effects andinteractions are in bold

Site Depth Site × depth

F3, 8 P F2, 7 P F6, 16 P

Bulk density 282.57 <0.001 42.92 <0.001 9.10 <0.001

pH 9.78 0.005 0.30 0.745 1.55 0.224

Soil moisture 312.20 <0.001 2.48 0.115 1.77 0.169

Soil organic carbon (SOC) 97.29 <0.001 5.45 0.016 1.18 0.366

Dissolved organic carbon (DOC) 143.28 <0.001 23.75 <0.001 5.61 0.004

Light fraction organic carbon (LFOC) 84.91 <0.001 0.74 0.490 1.11 0.400

Heavy fraction organic carbon (HFOC) 16.45 0.001 9.84 0.002 1.18 0.366

SOC/TN 6.21 0.017 5.36 0.017 1.07 0.420

δ13C 322.99 <0.001 19.14 0.001 3.03 0.075

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with an isotope ratio mass spectrometer (Delta Plus, ThermoFisher Scientific, Bremen, Germany) coupled with an elemen-tal analyzer (NC 2500, CE Instruments, Milano, Italy).

Data Analysis

To test the effects of sites type and soil depth on SOC, DOC,LFOC, HFOC and δ13 C (13C/12C ratio), we used ANOVAwith repeated measures, and in this model soil depth was usedas the repeated factor (Povin et al. 1990; Stuefer and Huber1998; Yu and Dong 2003). When significant effects werefound, Tukey’s HSD tests were used for multiple compari-sons. Regression analysis was used to examine the relation-ship between soil δ13C and log SOC. All the significant testswere made at a significant level of p =0.05. All the statisticalanalyses were conducted using SPSS 18.0 (SPSS, Chicago,IL, USA) and Sigmaplot 11.0 software package (SystatSoftware, San Jose, CA, USA).

Results

Soil Properties of the Selected Sites

There was a significant interaction of site type and soil depthon bulk density (Table 1). Soil bulk density at the 0–10 cmlayer differed significantly between the graze-released andrestored sites, but not between the restored and reference sites.In the soil layers of both 10–20 cm and 20–30 cm, bulkdensity was significantly different among the restored, refer-ence, and grazed or graze-released, but not between the grazedand graze-released sites (Fig. 2a). The pH values were signif-icantly higher in the grazed site than in the reference site, butnot significantly different among the grazed, grazed-release,and restored sites (Fig. 2b, Table 1). Soil moisture was thehighest in the natural wetland (Fig. 2c).

Variation of SOC and its Fractions

Site type significantly affected soil organic carbon (SOC) andits fractions (Table 1, F=6.2 to 323.0, p =0.017 to <0.001).SOC, dissolve organic carbon (DOC), light fraction organic Cconcentrations (LFOC) or heavy fraction organic C concen-trations (HFOC) did not differ significantly between the ref-erence and the restored wetland sites, but they were signifi-cantly higher than those in the grazed and graze-released sites(Fig. 3a, c, d). In the grazed and graze-released sites, thevalues of SOC were 186.7 and 293.0 g C kg−1 soil, respec-tively, significantly (p <0.05) lower than the correspondingvalues in the restored (418.5 g C kg−1) and reference site(491.8 g C kg−1; Fig. 3b). SOC, LFOC and HFOC were alsohigher in the graze-released than in the grazed site (Fig. 3a, c,d). SOC/TN did not differ significantly between the grazed,

graze-released and reference sites (p >0.05), but SOC/TN inthe restored site was significantly lower than that in the grazedsite (Fig. 3e).

Soil depth significantly affected SOC (F=5.45, p =0.016),DOC (F=23.75, p <0.001), HFOC (F=9.84, p =0.002) and

Fig. 2 a Bulk density, b pH and c soil moisture in the grazed, graze-released, restored and reference wetland sites on the Zoigê plateau

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SOC/TN (F=5.36, p =0.017, Table 1). Both SOC and HFOCwere significantly higher in the upper soil layer (0–10 cm)than in the lower layer (20–30 cm; Fig. 3a, d). SOC/TN wassignificantly higher in the lower layer (20–30 cm) than in theupper soil layers (0–10 cm and 10–20 cm; Fig. 3e).

The pattern of DOC differed across soil depths dependingon site type (Table 1, F=5.16, p =0.004). In the upper 0–10 cmlayer, dissolved organic carbon concentrations were 224.4±

27.8 mg kg−1 in the grazed site, significantly lower than that inthe graze-released, restored and reference sites (Fig. 3b, p <0.05). In the lower layers of 10–20 cm and 20–30 cm,dissolved organic carbon concentrations were 144.9±15.9and 163.3±27.1 mg kg−1 in the grazed and graze-released siteswhich were significantly lower than that in the restored andreference sites, 545.7±51.2 and 474.3±57.1 mg kg−1, respec-tively (Fig. 3b).

Fig. 3 a Soil organic carbon, b dissolved organic carbon, c light fraction organic carbon, d heavy fraction organic carbon, e SOC/TN and f δ13C in thegrazed, graze-released, restored and reference sites on the Zoigê plateau

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δ13C and its Relationship with SOC

Both site type (F=322.99, p <0.001) and soil depth (F=19.14,p =0.001) significantly affected δ13C (Table 1). Soil δ13C inthe reference and restored site did not differ significantly, butvalues in these types were significantly lower than those of thegrazed and graze-released site (Fig. 3f). Soil δ13C was alsohigher in the grazed than in the graze-released site (Fig. 3f).Soil δ13C was significantly lower in the upper soil layer (0–10 cm) than in the lower layer (10–20 cm and 20–30 cm;Fig. 3f).

Pooled soil δ13C was negatively correlated with the loga-rithm of SOC (Fig. 4). The reference sites had lower values ofδ13C than the grazed sites (Fig. 4).

Discussion

Changes of Soil Carbon due to Interference

Restored wetlands may regain carbon storage functions overtime, but little is known about how various carbon compo-nents change during recovery from disturbance. In our studyon the Zoigê plateau in China, carbon storage indicators suchas the concentrations of SOC, LFOC and HFOC were signif-icantly lower in the grazed and graze-released wetland sitesthan in the natural wetland site, but did not differ in therestored and the natural sites. SOC in the grazed, graze-released and restored sites was 40.4 %, 58.7 % and 89.2 %of that in the natural wetland, respectively. In the grazed,graze-released and restored sites, LFOC was 44.7 %, 60.2 %and 90.5%, and HFOCwas 25.9%, 56.0% and 86.2% of thatin the natural wetland, respectively. These comparisons im-plied that the trajectory of SOC, LFOC and HFOC recovery inthe grazed-released and restored wetlands was toward that of

natural wetlands. DOC in the grazed site at depths of 0–10 cm,10–20 cm and 20–30 cm layer was 50.2%, 38.1% and 26.8%of that in the natural wetland, respectively. Compared withSOC and LFOC, DOC was affected by the interaction be-tween wetlands site and soil depth (Table 1), indicated that theenvironmental factors, the carbon components in the soilprofile do not redevelop at exactly the same rate. These arelikely affected by processes related to soil water, microbialutilization and respiration (Zhang et al. 2007; Werth andKuzyakov 2010).

The labile carbon (LFOC and DOC) has been suggested asearly indicators of the effects of land use on the dynamics ofSOC (Gregorich et al. 1994; Zhang et al. 2007). In our study,the change of LFOC was most highly correlated with SOC(R2=0.92), indicating that LFOC was the most sensitive frac-tion for detecting the dynamics of SOC after the restoration ofthe wetland (Freixo et al. 2002; Swanston et al. 2002; Roscoeand Buurman 2003; Zhang et al. 2007).

The Turnover Time of SOC

Based on the regression models between log SOC and soilδ13C, we found that SOC turnover was much slower in thereference site than in the degraded and restored sites (Fig. 4).This was partially attributed to permanent inundation andhigher water content in the natural site of the wetland(Fig. 2). Due to permanent inundation, the diffusion of atmo-spheric oxygen into the saturated wetland soil was limited,leading to a low oxygen environment that suppressed micro-bial activities and thus greatly affected the decomposition ofsoil organic matter (Daulat and Clymo 1998; Chimner andCooper 2003). In contrast, in unflooded soils, oxygen diffusesmore readily and aerobic soils accelerate the decomposition ofsoil organic matter (Silvola et al. 1996).

Conclusions

In the freshwater marsh region on the Zoigê plateau in China,the degradation of wetland resulted in a decrease in SOCcontent and its fractions, while the restoration of wetlandincreased the accumulation of SOC and its fractions. SOCturnover was slower in the natural wetland than in the degrad-ed or restored wetland. LFOC was the most sensitive fractionfor the detection of changes in SOC during wetland recovery.Results of this study suggest that restoration can influence thecarbon dynamics of freshwater marsh on the Zoigê plateau,and that the initial regeneration of soil carbon pools is consid-erably rapid after restoration. Further work is required toaddress the long-term rate of recovery in the wetlands of thisregion in relation to land-use change.

Fig. 4 The relationship of δ13C with log [soil organic carbon (SOC)] forthe four sites

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Acknowledgments The authors gratefully thank two anonymous re-viewers for their critical and constructive comments. We also thank Dr.Fei-Hai Yu and Dr. Fang-Li Luo for their help on improving the manu-script. This research is supported by the Forestry Commonweal Program(200804005) and NSFC (41071329, 30700108).

References

Badiou P, McDougal R, Pennock D, Clark B (2011) Greenhouse gasemissions and carbon sequestration potential in restored wetlands ofthe Canadian prairie pothole region. Wetlands Ecology and Man-agement 19:237–256

Chai X, LangHQ, Jin SR, ZuWC,MaXH, Zhang ZY (1965)Marshes onZoige plateau. Science Press, Beijing

Chimner RA, Cooper DJ (2003) Influence of water table levels on CO2

emissions in a Colorado subalpine fen: an in situ microcosm study.Soil Biology and Biochemistry 35:345–351

Daulat W, Clymo R (1998) Effects of temperature and watertable on theefflux of methane from peatland surface cores. Atmospheric Envi-ronment 32:3207–3218

Ding W, Cai Z, Wang D (2004) Preliminary budget of methane emissionsfrom natural wetlands in China. Atmospheric Environment 38:751–759

Euliss NH, Gleason RA, Olness A, McDougal RL, Murkin HR, RobartsRD, Bourbonniere RA, Warner BG (2006) North American prairiewetlands are important nonforested land-based carbon storage sites.Science of the Total Environment 361:179–188

Fang HJ, Yu GR, Cheng SL, Mo JM, Yan JH, Li SG (2009) 13Cabundance, water-soluble and microbial biomass carbon as potentialindicators of soil organic carbon dynamics in subtropical forests atdifferent successional stages and subject to different nitrogen loads.Plant and Soil 320:243–254

Freixo AA, Machado PLOA, Dos Santos HP, Silva CA, Fadigas FS(2002) Soil organic carbon and fractions of a Rhodic Ferralsol underthe influence of tillage and crop rotation systems in southern Brazil.Soil and Tillage Research 64:221–230

Ghani A, DexterM, Perrott K (2003) Hot-water extractable carbon in soils:a sensitive measurement for determining impacts of fertilisation,grazing and cultivation. Soil Biology and Biochemistry 35:1231–1243

Gregorich E, Monreal C, Carter M, Angers D, Ellert BH (1994) Towardsa minimum data set to assess soil organic matter quality in agricul-tural soils. Canadian Journal of Soil Science 74:367–385

Millennium Ecosystem Assessment (2005) Ecosystems and human well-being: wetlands and water synthesis. Island Press, Washington

Mitsch WJ, Gosselink JG (2007) Wetlands. Wiley, HobokenNational Dynamic Atlas (2012) http://www.webmap.cn/index.php?

&embeded=&map=chinaadmin

Povin C, Lechowicz MJ, Tardif S (1990) The statistical analysis ofecophysiological response curves obtained from experiments in-volving repeated measures. Ecology 71:1389–1400

Roscoe R, Buurman P (2003) Tillage effects on soil organic matter indensity fractions of a Cerrado Oxisol. Soil and Tillage Research 70:107–119

Silvola J, Alm J, Ahlholm U, Nykanen H, Martikainen PJ (1996) CO2

fluxes from peat in boreal mires under varying temperature andmoisture conditions. Journal of Ecology: 219–228

Stuefer JF, Huber H (1998) Differential effects of light quantity andspectral light quality on growth, morphology and development oftwo stoloniferous Potentilla species . Oecologia 117:1–8

Sun G, ZhangW, Zhang J (1998) The mire and peatland of the HengduanMountainous region. Science Press, Beijing

Swanston CW, Caldwell BA, Homann PS, Ganio L, Sollins P (2002)Carbon dynamics during a long-term incubation of separate andrecombined density fractions from seven forest soils. Soil Biologyand Biochemistry 34:1121–1130

Tuittila ES, Komulainen VM, Vasander H, Laine J (1999) Restored cut-away peatland as a sink for atmospheric CO2. Oecologia 120:563–574

Turetsky MR (2004) Decomposition and organic matter quality in conti-nental peatlands: the ghost of permafrost past. Ecosystems 7:740–750

Waddington J, McNeil P (2002) Peat oxidation in an abandoned cutoverpeatland. Canadian Journal of Soil Science 82:279–286

Waddington J, Price J (2000) Effect of peatland drainage, harvesting, andrestoration on atmospheric water and carbon exchange. PhysicalGeography 21:433–451

Wang XW, Li XZ, Hu YM, Lv JJ, Sun J, Li ZM, Wu ZF (2010) Effect oftemperature and moisture on soil organic carbon mineralization ofpredominantly permafrost peatland in the Great Hing’anMountains,Northeastern China. Journal of Environmental Sciences 22:1057–1066

Wang H, Wang RQ, Yu Y, Mitchell MJ, Zhang LJ (2011) Soil organiccarbon of degraded wetlands treated with freshwater in the YellowRiver Delta, China. Journal of Environmental Management 92:2628–2633

Werth M, Kuzyakov Y (2010) 13C fractionation at the root-microorganisms-soil interface: a review and outlook for partitioningstudies. Soil Biology and Biochemistry 42:1372–1384

Yli-petäys M, Laine J, Vasander H, Tuittila ES (2007) Carbon gasexchange of a re-vegetated cut-away peatland five decades afterabandonment. Boreal Environment Research 12:177–190

Yu FH, Dong M (2003) Effect of light intensity and nutrient availabilityon clonal growth and clonal morphology of the stoloniferous herbHalerpestes ruthenica . Acta Botanica Sinica 45:408–416

Zhang J, Song C, Wang S (2007) Dynamics of soil organic carbon and itsfractions after abandonment of cultivated wetlands in northeastChina. Soil and Tillage Research 96:350–360

Wetlands (2014) 34:235–241 241