Moïse YONI 1, Aboubakar SAKO 1, Luc ABBADIE 2 & Georges SERPANTIE 3. Université de Dédougou, BP 139 Dédougou, Burkina Faso

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1 Environment nd Nturl Resoures Reserh; Vol. 8, No. 4; 18 ISSN E-ISSN Pulished y Cndin Center of Siene nd Edution Cron nd Nitrogen Dynmis of Soil Orgni Mtter Frtions in Thikets nd Intergrowth Ares of Sudnin Svnnh Grsslnds, Bondoukuy, Western Burkin Fso Moïse YONI 1, Aoukr SAKO 1, Lu ABBADIE 2 & Georges SERPANTIE 3 1 Université de Dédougou, BP 139 Dédougou, Burkin Fso 2 UPMC - IEES Pris - Bât 44-45, 4 ple Jussieu Pris edex 5, Frne 3 IRD 911 Avenue d Agropolis BP 6451, Montpellier Cedex 5, Frne Correspondene: Moïse YONI, Université de Dédougou, BP 139 Dédougou, Burkin Fso. E-mil: yoni_moise@yhoo.fr Reeived: Novemer 1, 18 Aepted: Novemer 19, 18 Online Pulished: Novemer 3, 18 doi:1.5539/enrr.v8n4p16 URL: Astrt Soil orgni ron (SOC) nd totl nitrogen (TN) ply entrl role in physio-hemil fertility of soil, nd thus promoting griulturl produtivity. Yet little is known out SOC nd TN dynmis in tropil ferruginous soils of Su-Shrn Afri. In this study, thiket nd intergrowth soil smples, under oth ultivtion nd perennil grss fllow (Andropogon gynus), were olleted in Bondoukuy, western Burkin Fso. The smples were frtionted nd their SOC nd TN ontents in six orgno-minerl frtions were nlyzed. Beuse of the high lile orgni mtter pools in orse snd frtions, SOC (~63 µg C g 1 soil) ssoited with these frtions ppered to e more essile to soil miroes thn relitrnt nd oluded pools (~44 µg C g 1 soil) within the fine frtions of the fllow soils. The results lso indited tht ly frtions re likely to represent soure of the ville nitrogen to rop following long fllow periods (~ yers). In ontrst, the differenes in TN ontents were not signifint (p>.5) etween ploughed plots nd young fllow lnds (~1 yers). The sustntil derese in C/N rtios from orse prtiulte orgni mtter pools (C/N=68) to fine pools (C/N=1) suggested n inrese in the SOC deomposition rte in the fine frtions. This indites sustntil derese in miroil tivities following redution in prtiulte orgni mtter sizes. The SOC ontents were reltively high in orse (~93 µg C g 1 soil) frtions of the thiket soils ompred to those of the djent intergrowth soils (~6 µg C g 1 soil). A similr SOC distriution pttern ws lso oserved in fine frtions of the thiket nd the intergrowth soils. Totl nitrogen lso exhiited high distriution pttern in fine snd nd very fine snd frtions. The findings of this study demonstrted tht SOC nd TN restortion in semi-rid tropil svnnh soils is funtion of prtiulte orgni mtter sizes, vegettion type nd soil mngement prties. Keywords: Andropogon gynus, fllow lnds, prtiulte orgni mtter frtions, soil orgni ron, totl nitrogen 1. Introdution Although shrp inrese in plnted lnd res in the pst yers hs led to sequestrtion of lrge mounts of ron y vegettion round the gloe, in Sudnin tropil limti zones (i.e., semi-rid) of Afri, removl of vegettion through ploughing, grssing, rowsing nd urning not only hve impted the lol physil environment (e.g., soil degrdtion nd nutrient loss), ut hve lso drmtilly redued soil orgni mtter (soil orgni ron nd totl nitrogen) ontents s well s ron sequestrtion levels in soils (Mnn, 1986; Shlesinger, 1986). A deline in the ron sequestrtion levels hs negtive feedk mehnism for tmospheri CO 2 nd N 2 O ontents (Wilson, 1978; Sellers, 1997). Destrution of iomss hs lso dversely impted nitrogen fixtion rtes, whih, in turn, hs ffeted soil fertility nd griulturl prodution (Pimentel et l., 1995; Tiessen et l., 1984). To inrese soil orgni ron (SOC) nd totl nitrogen (TN) storge pity so tht the soil struture nd fertility n e restored, frmers in Burkin Fso use orgni fertilizers. However, in some res, lolized use of mnure hs very little effets on the soil fertility (Serpntié, 3). Furthermore, the limited vilility of long term fllow lnds nd inherent intensive griulturl prties ross the ountry indite tht the soil fertility is 16

2 rrely mintined (Floret & Serpntié, 1993). In ontrst, in moist soils, the presene of plnts tends to hve onservtory effet on SOC nd TN ontents whih n lso hve positive impts on miroil tivities nd soil nutrient yling (Doormr, 199). Studies hve lso shown tht fllows following perennil grss speies suh s Andropogon gynus (A. gynus), n restore the soil fertility (De Bli & Somé, 1997; Fournier et l., ; Serpntié, 3). The perennil grss A. gynus is ommon plnt speies enountered in fllow lnds of the Sudnin tropil limti zone of Bondoukuy, western Burkin Fso. This grss is losely involved in ron nd nitrogen yles, nd therey influening physio-hemil properties of the fllow lnds (Yoni, 1995; 1997; Fournier et l., ). Before strting new frming session, the frmers in Bondoukuy determine if vegettion hs een properly restored using density nd height of A. gynus of the fllow lnds s inditors of improvement of the soil fertility (Yoni, 1997; Fournier et l., 1). Severl studies hve demonstrted strong negtive reltionship etween the presene of A. gynus nd soil prtile sizes (Cmpell & Souster, 1982). Thus, negtive reltionship hs een lso found etween SOC nd TN turnover nd orgno-minerl prtile sizes (Coote & Rmsey, 1983; Somé, 1996). Moreover, the stle orgni mtter pools re thought to e ssoited with fine soil prtiles, wheres the lile pools re generlly found in fresh plnt residues (Zhng et l., 1988). To identify quntittive vritions in SOC nd TN ontents within different orgno-minerl prtiles (i.e., prtiulte orgni mtter), the SOC nd the TN ontents were extrted through physil frtiontion of soil smples under fllow nd ultivted field onditions. We expet sustntil inrese of lile SOC ompounds in orse frtions nd more stle relitrnt SOC in the fine frtions. This suggests tht: i. soil orgni mtter would e more essile t the end of vegettion stge (fllow periods) ompred to the end of rop stge ii. nd (ii) the lterntion etween vegettion nd rop growth sttus would not only ffet the lile soil orgni mtter pools (i.e., more dynmi), ut lso those ssoited with fine frtions (i.e., less dynmi) tht nnot sustin rpid hnges of vegettion over. The ojetives of this study were therefore twofold: 1) to investigte the grin size effets on SOC nd TN umultion in fllow lnds round Bondoukuy nd 2) to determine the differenes in SOC nd TN ontents in vegettion tikets (lusters) nd sprsely vegetted intergrowth res. The findings of this omprehensive study re likely to shed some light on the SOC nd TN dynmis in rle lnds under semi-rid tropil limti setting nd provide the deision mkers roust mngement pln for effiient lnd use nd soil onservtion. 2. Environmentl nd Geologil Context Soil smples were olleted in Bondoukuy, situted in the northern order of the South Sudnin limti zone, known s otton frming zone (Figure 1). Bondoukuy lies etween N nd 3 45 W with n elevtion ove se level of 36 m. The verge nnul rinfll of the study re is 85 mm with the mximum rinfll ourring in August. The dily mximum temperture rnges etween 31 nd 39 C with n verge nnul potentil evpotrnspirtion of out 19 mm. The nturl vegettion over in the re is predominntly omposed of open woody svnnh, where s the dominnt grss iome is A. gynus, Pennisetum pedielltum nd Loudeti togoensis (Fournier et l., 1). The geology of the study re is omposed of sedimentry roks mde of sndstones nd shist of Pleozoi nd Infr mrin ges (Ouédrogo, 1998). The sndstone edrok in the plteu is omposed of qurtz, wheres in the glis (iron durirust) it is of shist-dolomite (Ldmirnt nd Legrnd, 1969). Different soil types re enountered long the lol pedotoposequene unit. At the plteu, pedologil over is predominntly omposed of sndy lom (i.e., ferri lixisols), whilst t the glis, soils re lomy (i.e., luvisols; Kissou, 1994; Outtr et l., 6). For enturies, humn presene in Bondoukuy hs mrkedly ffeted the lol eosystem. Susistene (erels) nd sh rop (otton) frming re the min tivities prtied in the re. Although the most fertile soils re regulrly ploughed, less fertile ones re inresing sujeted to short period fllowing (~5 yers). The nnul ploughing, sed on otton mize rottions, usully leds to the ollpse of soil struture, erosion nd redution of soil orgni mtter ontent. The seond most ommon tivity in the study re is extensive niml husndry. As result, the re is over grzed nd exposed to high demnds for eril iomss (tree rk, firewood nd perennils stems) y the lol rurl ommunities. 17

3 Figure 1. Phytogeogrphil mp of Burkin Fso, showing the study re (Modified from Guinko, 1984) 3. Mterils nd Methods Aording to the sptil vriility of the vegettion over due to environmentl heterogeneity, pedologil over nd humn tivities (Yoni et l., 5), the min riterion in seleting plots ws the presene of A. gynus with representtive density. The plots were on the sme soil type (i.e., ferruginous tropil soil) nd hd the sme ropping history, ut with different ges. Nine plots of 25 m 2 nd 5 m prt were used: 3 ultivted plots (CP), 3 ten yer fllow lnds () nd 3 twenty yer fllow lnds (). The CP (Figure 2) re onsidered s ontrol plots, wheres, the (Figure 2) orresponded to A. gynus set up time nd the (Figure 2) orresponded to A. gynus oloniztion time. Six topsoil ( 1 m) smples were rndomly olleted on ultivted fields, the thikets nd the intergrowth (Figure 2d) res of the fllow lnds, using n uger. The smpling pproh ws sed on 1 m in this study euse vritions in SOC nd TN ontent re likely to e gretest ner the soil surfe (Mills et l., 5). The soil smples were ir dried, sieved through 2 mm sieve nd stored for susequent nlyses. Orgno-minerl frtions (defined here s ll orgni nd minerl prtiles of the sme size) were seprted into different size lsses without orgni mtter destrution (Nro, 1997). Prtile size seprtion into six prtile size frtions (.5 µm) were isolted (Bize, 1988). The seprted orgno-minerl frtions were dried in n oven t 5 C nd then weighed. Eh frtion ws susequently ground in ermi mortr into very fine powder for hemil nlysis t the ENS-Pris Eologil Lortory (Frne). The results were expressed s perent (%) of sums of the frtions nd used for lultion of SOC nd TN ontents of eh frtion. Soil orgni ron nd TN ontents in susmples of individul frtions were nlysed using Crlo-Er CHN nlyser, nd they were expressed s µg C g -1 frtion nd µg N g -1 frtion. Soil orgni ron nd TN ontents (µg g -1 soil) within eh frtion were lulted from the soil texturl omposition nd SOC nd TN onentrtions of the orresponding frtions (µg g -1 frtion). In order to etter estimte SOC nd TN ontents in eh frtions during the rop-fllow rottion, we lulted the following vriles (Eq. 1 2): DC = HC-LC (1) Where DC = differene etween high nd smll orgni ron ontents; HC = high ron ontent nd LC = low ron ontent DN =HN-LN (2) 18

4 Where DN = differene etween high nd smll totl nitrogen ontents; HN =high nitrogen ontent nd LN = low nitrogen ontent. Figure 2. Crop nd vegettion stges in Bondoukuy grsslnds: ) ultivted field (ontrol); ) young fllow lnd; ) old fllow lnd; d) vegettion luster (thiket) nd surrounding intergrowth re 19

5 3.1 Sttistil Anlysis The experimenttion design ws sed on rndomized plots in whih the min tretment ws the plot stge nd the su-tretment ws the grnulometri size frtion. All sttistil nlyses were onduted using the SttView 6. softwre (SAS Institute, 1996). ANOVA (Sheffe s test, 1959) ws performed to test the differenes in SOC nd TN ross fllow periods for individul frtions. Differenes were onsidered s sttistilly signifint t p < Results 4.1 Soil Orgni Cron nd TN Dynmis in different Frtions ANOVA onduted on the dt showed signifint effet of the plot stge on SOC nd TN ontents within the different grnulometri frtions (Sheffe s test, p<.5). Sheffe S test lso indited net inrese in SOC ontents within the orse frtions over time (Sheffe S test, p<.5; Tle1). In overll, the SOC ontents in old fllow lnds were higher in ll frtions (exept ly). Furthermore, the men TN onentrtions were signifintly higher (Sheffe S test, p<.5) in old fllow lnds ompred to young fllow lnds nd ultivted plots (Tle 1). As expeted, there were prtile size effets on C/N rtios of soil orgni mtter (Tle 2), resulting in higher rtios in orse frtions (> µm) ompred to the fine ones (< µm). In ddition, the C/N in frtions greter thn μm were signifintly (p<.5) lower in ultivted fields ompred to the two fllow lnds, ut they remined onstnt in fine silt (C/N=1) nd ly (C/N=8) frtions (Tle 2). Signifint (p<.5) vritions in SOC ontents were oserved in orse snd nd fine silt frtions (Figure 3). However, the silt frtion umulted more SOC thn the ly frtion during the rop-fllow rottions (Tle 1). Likewise, fine frtions showed higher vritions (p<.5) in TN during the rop-fllow rottions, wheres the highest TN onentrtions were found in the fine silt frtions (Figure 4). Tle 1. Soil orgni ron (SOC) (µg C g 1 soil) nd totl nitrogen (TN) (µg N g -1 soil) ontents in different frtions in the smples olleted in the intergrowth soils. The stndrd error is in prenthesis, n = 18. Numers with the sme letters y olumn re not signifintly different t the 5 % level (Sheffé's S test) SOC (µg g -1 soil) TN (µg g -1 soil) Grnulometri frtions (µm) Lnd use CP (496.82) (375.87) (9.92) (58.43) (17.74) (176.48) 13.3 (29.46) (31.72) 4.64 (2.67) 8.83 (5.9) 7.34 (4.24) 13.3 (7.52) (24.57) (39.61) (159.61) (82.98) 2.1 (127.7) (34.1) 99.7 (27.56) 58.7 (33.52) 5. (2.88) (7.35) 5.65 (3.26) 13. (7.62) (493.83) (574.74) (246.12) (153.41) (294.89) (423.12) 1.99 (69.85) (88.57) (16.61) (13.66) (12.29) (14.52) Tle 2. C/N rtios nlysed in the thikets nd in the intergrowth res in the different soil frtions. The stndrd error is in prenthesis, n = 18. Numers with the sme letter y olumn re not sttistilly signifint different t 5 % level (Sheffé's S test) Intergrowth soils thiket soils Grnulometri frtions (µm) Lnd use CP 8 (4) 12 (7), (12) 43 (25) 37 (21) 7 (4) (4) 6 (4) 12 (7) 1 (6) 24 (14) 18 (5) 5 (29) 164 (94) 166 (96) 45 (26) 174 (93) 175 (89) 7 (4.2) 8 (4.6) 11 (6.5) 12 (7.1) 19 (9.1) 17 (9.8) 16 (9.4) 18 (1.2) CP: ultivted plots; : Ten yers old fllow lnds; : Twenty yers old fllow lnds. 17 (9.8) 38 (21.9) 44 (25.4) 68 (59.1)

6 orgni C (µg C/g soil) d e grnulometris frtions (µm) Figure 3. Vritions in soil orgni ron (DC) distriution in different frtions from ropping to fllowing. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). Results expressed s µg C g -1 of soil 7 6 totl N (µg N/g soil) d grnulometris frtions (µm) Figure 4. Totl nitrogen vritions (DN) in different soil frtions during the trnsition from ropping to fllow lnds. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). Results expressed s µg N g -1 of soil 4.2 Reovery of SOC nd TN in Intergrowth nd in Thiket Soils The effets of frtiontion on SOC nd TN onentrtions were quntified y ompring the onentrtions of individul frtions to those otined in non-frtionted ulk soil smples (Tle 3). The results indited tht the frtiontion nd hemil nlysis only used smll losses of SOC ontents in the smples olleted in the intergrowth res. However, the sum of TN of individul frtions in the intergrowth soil smples ws higher thn those found in the ulk soil smples. Thus, t the end of the frtiontion nd hemil nlysis, the sums of SOC were.22,.27 nd.37 % in ultivted fields, young nd old fllow lnds respetively, s opposed to totl onentrtions of.25,.31 nd.38 % in the ulk soil smples. The frtiontion did not use ny losses of orgni ompounds in the thiket smples (Tle 3). In ontrst, the frtiontion lne of the smples from the sprsely vegetted intergrowth soils ws positive s the sum of 21

7 orgni ompound ontents in different frtions ws lower thn to those found in ulk soil smples. The sums of SOC in the different frtions were lso lower in young nd old fllow lnds (.27 nd.33%, respetively) thn its totl onentrtions in the ulk soils olleted from the sme plots (.36 nd.44%, respetively). Similrly, the sums of TN in the individul frtions were reltively lower in young nd old fllow lnds (.17 nd.25 %) thn the totl TN in the ulk soils (.23 nd.3 %, respetively). Tle 3. Qulittive lne of ron nd totl nitrogen ontents in different frtiontion of thikets nd intergrowth soils under different lnd uses. Stndrd error is in prenthesis, n = 18. Numers with the sme letters y line re not sttistilly signifint t 5 % level (Sheffé's test). Results express s µg g 1 soil Blne of the frtiontion Intergrowth soils Thiket soils Totl Nitrogen (µg g -1 soil) Orgni ron (µg g -1 soil) Totl Nitrogen (µg g -1 soil) Orgni ron (µg g -1 soil) Lnd use Bulk soil Amount Bulk soil Amount Bulk soil Amount Bulk soil Amount CP d (68.9) d (75.84) (8.94) (15.27) (111.81) 3.92 (173.58) ( ) 31. ( ) ( ) (84.4) (994.99) ( ) (114.41) (97.44) d (88.) d (146.49) CP: ultivted plots; : Ten yers old fllow lnds; : Twenty yers old fllow lnds (99.66) 443. ( ) ( ) ( ) 4.3 Distriution of SOC nd TN in Orgno-Minerl Frtions in the Intergrowth Soils Orgni ron nd TN ontents were minly high in fine nd orse frtions, with pronouned lower onentrtions in intermedite size frtions (fine snd, orse snd, very fine snd nd fine silt; Tles 1 nd 2). Thus, the results showed tht fine frtions (ly + fine silt) hd out 67 % of SOC in ultivted fields nd 54 % nd 49% in young nd old fllow lnds, respetively. With 83%, the fine frtions in ultivted fields hd the highest TN ontent ompred to young (81%) nd old (74%) fllow lnds. The soil orgni ron ontents in the orse frtions (orse snd + fine snd) vried etween 3 nd 8 %, wheres those of TN rnged from 4 to 22 %. 4.4 Soil Orgni Cron nd TN Dynmis in Thiket Soils The study of temporl vritions in the vegettion tikets showed tht SOC nd TN ontents vried in different frtions with SOC in orse snd ( = 562 µg C g 1 soil nd = 939 µg C g 1 soil) nd TN in fine silt ( = 15 µg N g 1 soil nd = 1 µg N g 1 soil) eing the most dynmi lyers s turnover time seems to e higher in these lyers. ANOVA performed on the dt demonstrted signifint (p<.5) fllow ge effet on SOC nd TN distriution ptterns in the vrious frtions (Tle 3). Aording to the Sheffé S test, there is net vrition (p<.5) in SOC distriution over time within ly (.5 2 µm) nd orse snd frtions (>25 µm) in whih SOC ontents inrese from young to old fllow lnds (Tle 4). However, in the intermedites frtions there were no signifint (p>.5) vritions in SOC ontents. Tle 4. Orgni ron (µg C g 1 soil) nd totl nitrogen (µg N g -1 soil) ontents in different frtions of soils olleted in thikets. The stndrd error is in prenthesis, n = 18. Numers with the sme letters y olumn re not sttistilly signifint t 5 % level (Sheffé's S test) C (µg g -1 soil) N (µg g -1 soil) Grnulometri frtions Lnd use (323.2) (5.6) (158.15) (83.3) (15.53) (34.23) (25.6) (21.49) (6.26) 8.77 (4.72) (15.5) (8.64) (5.58) (485.58) (118.4) (89.5) (141.21) (542.25) (57.17) (4.1) (.62) 9.15 (4.97) 1.21 (5.13) (.36) CP: ultivted plots; : Ten yers old fllow lnds; : Twenty yers old fllow lnds. The men omprison test showed tht TN ontents were higher (p<.5) in old fllow lnds thn in young ones for ll frtions exept for fine snd frtions (Tle 4). The C/N rtios were lso higher in orse frtions (i.e., 22

8 orse snd nd fine snd) thn in fine frtions (i.e., ly, fine silt, orse silt nd very fine snd). Likewise, the C/N rtios of fine snd nd orse snd frtions were higher in old fllow lnds thn in young ones. In other frtions, the C/N rtios did not vry etween old fllow nd young fllow lnds (C/N~24) orgni C (µg C/g soil) ,-,5- grnulometris frtions (µm) Figure 5. Vritions in soil orgni ron (DC) ontents in different frtions during trnsition from young to old fllow lnds. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). Results expressed s µg C g -1 of soil The vriles DN nd DC showed tht: - for SOC ontents, DC ws higher in orse snd nd ly frtions thn in other frtions (Figure 5). These frtions were therefore the most dynmi ones in sequestrting SOC during the nturl suession of fllow lnds; - for TN ontents, DN ws more pronouned in fine frtions (i.e., ly nd fine silt frtions) thn in orse frtions (> µm) (Figure 6). Corse snd frtions umulted the lower proportion of TN reltive to fine snd, very fine snd nd orse silt frtions during the trnsition from young to old fllow lnds. totl N (µg C/g soil) d ,-,5-2 grnulometris frtions (µm) Figure 6. Totl nitrogen vritions (DN) in different soil frtions during the trnsition from young to old fllow lnd. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). Results expressed s 23

9 µg N g 1 soil C (µg/g soil) Frtion 25- µm C (µg/ g soil) Frtion 1-25 µm UT BT 1 Situtions Situtions C (µg/g soil) Frtion 5-1 µm Situtions C (µg/g soil) Frtion -5 µm Situtions UT BT d C (µg/g soil) Frtion 2- µm Situtions C (µg/g soil) 1 1 e f Figure 7.,,, d, e nd f. omprison of orgni ron ontents in soil frtions during fllow trnsition. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). Results expressed s µg C g -1 soil. Where : Ten yers old fllow lnd; : Twenty yers old Fllow lnd. UT = thiket soils nd BT = intergrowth soils Frtion.5-2 µm Situtions UT BT 24

10 N (µg/g soil) Frtion 25- µm Situtions N (µg/g soil) Frtion 1-25 µm d Situtions UT BT N (µg/g soil) Frtion 5-1 µm Situtions N (µg/g soil) Frtion -5 µm d Situtions UT BT d N (µg/g soil) Frtion 2- µm d Situtions N (µg/g soil) Frtion.5-2 µm Situtions UT BT e f Figure 8.,,, d, e nd f: omprison of totl nitrogen ontents in soil frtions during fllow trnsition. Mens with the sme letters re not sttistilly signifint t 5% level (Sheffé's S test). : Ten yers old fllow lnd; : Twenty yers old Fllow lnd. UT: thikets; BT: intergrowth res 25

11 4.5 Soil Orgni Cron nd TN Distriution in Orgno-Minerl Frtions in the Thiket Soils Most of SOC ws found in ly nd orse snd frtions, wheres TN ws minly found in fine frtions (Tle 4). Corse frtions ontriution to SOC ontents vried from 44 to 52 % in young fllow lnds nd out 49 % in old ones, ut those of fine frtions were high (48 56 % nd 51 % for young fllow lnds nd old fllow lnds, respetively). Fine frtions ontriuted up to 7 % of TN ontents in oth young nd old fllow lnds s ompred to the low ontriution of orse frtions (22 32 % nd % for young fllow lnds nd old fllow lnds, respetively). 4.6 Sptil Aundne of SOC nd TN in the Tiket nd Intergrowth Soils Trnsition from young to old fllow lnds showed reltively high SOC in ll frtions in the intergrowth res, wheres only the orse frtions exhiited reltive inrese of SOC in the thikets (Figure 7). The sme trnsitionl pttern ws oserved in TN ontents with high ontents in ll frtions in the intergrowth res nd two distint distriution ptterns in the thikets (Figure 8): i. TN ontents dereses in fine snd frtions (1 25 µm) nd remins onstnt in very fine snd frtions (5 1 µm) nd ii. TN ontents inresed in orse snd (25 µm), orse silt ( 5 µm), fine silt (2 µm) nd ly frtions (.5 2 µm). 5. Disussion 5.1 Soil Orgni Cron nd TN Contents in Different Frtions Soil physil frtiontion is omplex proess, whih sometimes results in loss of soil orgni ompounds. Furthermore, nlysis of SOC nd TN my e ffeted y the minerl mtries whih vry within frtions nd etween frtions nd ulk soil. Thus, mny uthors hve oserved orgni ompounds loss during soil physil frtiontion (Feller, 1994; Nro, 1997; Lehmnn et l., 1998; Stemmer et l., 1999; Mnly et l., 2, ; Blesdent et l., ). This loss is expeted in wter-solule ompound exporttion (out 5 1% of SOC ontents) during suessive wet sieving nd entrifugtions. As result, orgni ompounds ould e trnsferred from orse to fine frtions. In ft, soil shking n led to orse orgni ompound division (snd rsive effet) whih my trnsfer smll quntities of orgni ompounds to smll frtions (Feller, 1994).The importne of this trnsfer depends on orse orgni mtter onentrtions, whih re prtiulrly smll in the present study. Blesdent et l. (1991) hve shown tht orgni mtter trnsfer from orser to finer frtions is possile, nd it n e even signifint when ulk soil is exposed to sonition. However, when ultrsoni sounds re pplied on less thn 5 µm frtions, s in the present study, the mounts of orgni mtter trnsferred re negligile. 5.2 Temporl Vritions in SOC nd TN Contents Beuse of the lile ondition of orse frtions, prtiulrly orse snd, the orgni mtter pools re more prone to deomposition. This finding is onsistent with severl studies rried out on tropil soils (e.g., Feller, 1994; Nro, 1997; Six et l., 1999; Six et l., ; Six, Pustin et l., ; Mnly et l., 2 ; Devine et l., 14; Peter, 18). It n e therefore suggested tht, despite their low orgni mtter ontent, the orse frtions re likely to ontriute more to the soil orgni mtter minerliztion in ultivted fields. Although the fine frtions re quntittively fr more importnt thn the orse frtions, the turnover time of soil orgni mtter in these frtions is lower during fllowing. This suggests tht fllowing my hve redued the stility of the fine frtion lyers in the present study re. In ontrst, severl studies hve demonstrted tht lthough orgni mtter turnover time is low in the fine frtions, these frtions re more stle thn orser frtion (Feller, 1994; Puget et l. ; Mills et l., 5). The low stility found in this study is likely due to ioturtion of the A. gynus root systems. By ontrst, the proesses tht ontrol SOC undne in the ly lyer re more diffiult to explin euse of the low SOC ontents in young fllow lnds reltive to ploughed fields nd old fllow lnds. The reent hnge in the vegettion over under young fllow lnds my hve resulted in rpid orgni mtter deomposition nd its stiliztion in the ly frtions. A negtive trnsitory effet of fllow lnds on SOC ontents, the so-lled priming effet, in the ly frtions my lso exist. In the priming effet, stimultion of soil orgni mtter minerliztion is rried out through ontinuous fresh orgni mtter inputs (Fontine, 2). The high DN vritions in TN ontents oserved in the fine silt frtions ould e sried to the unertinties produed during the physil frtiontion. As fr s nitrogen vilility for upoming rops is onerned, the results showed tht fine silt, ly nd orse frtion lyers re likely to ply ruil role in soil TN replenishment. In overll, fllow lnds hd positive effet on soil TN ontents, ut this effet is only notiele in 26

12 the long term fllowing sine the differenes in ontents were not signifint etween ploughed plots nd young fllow lnds. The derese in C/N rtios from orser to finer frtions suggests n inrese in soil orgni mtter deomposition levels nd, thus derese in soil orgni mtter iodegrdility following prtile size redution (Gom-Thimkl, 9; Suhrjit et l., 1; Plz-Bonill et l., 14). The high C/N rtios found in orse frtions indite the reent origin of soil orgni mtter whih is hrteristi of fresh orgni mtter inputs rih in ellulose (Feller, 1994; Nro, 1997). During deomposition, soil orgni mtter prtile sizes re redued progressively, nd prt of this orgni mtter ends up eing ssoited with fine frtions with low C/N rtio (Ling et l., 14). The ft tht the high C/N rtios is found within orse frtions of young nd old fllow lnds rther thn in ploughed plots n e explined y n importnt quntity of ded roots in the soil nd often y the inorportion of unurned oveground plnt mterils. 5.3 Soil Orgni Cron nd TN Distriution in Soil Orgno-Minerl Frtions Regrdless of lnd-use prties, the reltive SOC nd TN ontents tend to inrese from orse to fine frtions despite the low ontent of fine minerl prtiles. Compring the reltive SOC nd TN ontents, it n e notied tht for frtions greter thn µm (i.e., snds nd orse silt frtions), old fllow lnds present more signifint SOC nd TN ontents thn young fllow lnds nd ultivted fields. In frtions smller thn µm (i.e., ly nd fine silt frtions), the differenes etween SOC nd TN ontents re less signifint etween ploughed plots nd fllow lnds. Fllow lnds ffet positively nd signifintly orse frtions ontriution to soil orgni mtter restortion s orroorted y mny studies (Ling et l., 14; Sheng et l., 15; Yu et l., 17). Climti onditions suh s humidity nd high tempertures n lso explin the smll ontriution of orse frtions to the soil orgni mtter ontents in the present study. It is possile tht, t the eginning of the riny seson, the minerliztion rte of plnt litters within orse frtions is high. This will strongly limit the orgni mtter umultion in lyers hving prtile sizes greter thn µm. This hypothesis is reinfored y mny uthors (Blesdent et l. ; Vonir et l., 6; Feyss et l., 11; Rtnyke et l., 11; Guimres et l., 13; Udom et l., 15) who hve demonstrted tht orgni mtter renewl rte in orse frtions is higher thn in the fine frtions. The high minerliztion rte of fresh soil orgni mtter oserved in the present study, with high snd ontents, is due to the ft tht the ulk soil orgni mtter ontents re lwys ssoited with fine prtiles whih n physilly protet them ginst miroil ttks (Feller, 1994). Beuse SOC onstitutes lrge reservoir nd supplier for nutrients, it n e onluded tht the soil is hrterised y essentil nutrient puity. 5.4 Soil Orgni Mtter Dynmis in the Different Fllow Stges Comprison of SOC nd TN ontents in thiket soils nd in the surrounding intergrowth soils highlights n unexpeted mehnism. In ft, SOC ontents re higher in the thikets thn in the intergrowth res in orse snd frtions. Likewise, nitrogen ontents tend to e higher in orse silt frtions in the thiket soils thn in the intergrowth soils. This n e ttriuted to high perentge of roots in the thikets ompred to the intergrowth res. In generl, orse frtions ontin orgni prtiles (e.g., root litters) whose sizes re greter thn 25 µm. High SOC ontents found in the thikets my e sried to n tive ontriution of A. gynus to the soil orgni mtter restortion. It n e suggested tht orgni prtiles trnsfer from A. gynus roots hs lredy een tken ple from orse frtions in the thikets to fine frtions in the intergrowth res. Consequently, orgni mtter inorportion during the fllow period ourred rpidly in orse frtions with grin size less thn 25 µm. The orgni mtter inorportion ws lso ompnied y trnsfer of orgni mtter from the thikets to the intergrowth res. Soil orgni ron ontents in fine silt frtions (< µm) nd orse ftions (< 25 µm) followed similr distriution ptterns, hrterised y high ontents in the intergrowth res ompred to those in the thikets, suggesting progressive inorportion of SOC through frtion overtime. In ontrst, in the ly frtion, SOC ontents found in the thikets re higher thn those found in the intergrowth res. The orgni mtter ontents oserved in young nd old fllow lnds suggested tht orgni mtter ontents inrese with deresing soil texturl sizes, nd they were lwys high in old fllow lnds thn in young ones. However, the high orgni mtter restortion dynmis in orse frtions of the thiket soils is n indition of possile ontriution of A. gynus iomss to SOC ontents. However, orgni mtter ontents in the thiket soils re not lwys higher thn those in intergrowth soils, s orgni mtter distriution is funtion of frtions. The presene of A. gynus lone nnot justify orgni mtter restortion, ut the fllow proess ppers to ply ruil role in the soil orgni mtter restortion. It n lso e notied tht irrespetive of SOC nd TN 27

13 distriution ptterns through different frtions, the inflow of fresh orgni mtter seems to our in orse snd frtions, wheres its long term storge tkes ple in ly frtions. 6. Conlusion This study showed tht fllowing of intensively ploughed soils in tropil limti setting n restore SOC nd TN ontents. The study lso identified the vrious proesses through whih the soil orgni mtter restortion tkes ple. Hene, the results showed tht lthough fllowing leds to orgni mtter umultion within the soil orser frtions, whih n signifintly mke essentil nutrient ville to rops during ultivtion phses, the finer frtions onstitute the sites of the orgni mtter restortion nd improvement of physil properties. An extended fllowing (1 yers) n produe two importnt effets: i. umultion of orgni ompounds in orser frtions nd restortion of SOC in the short term so tht rop needs in essentil nutrients during ultivtion n e stisfied nd ii. umultion of orgni ompounds in finer frtion nd restortion of SOC in the long term to mke nutrients ville to rops during ultivtion. The optiml fllowing period remins n open question s fr s the investigted system is onerned. In other words, in sndy soil, orgni mtter soures re minly sed on sptilly heterogeneous perennil grsses with n ggregted shllow root system. The otined results on grnulometri frtions of the soils in thikets nd intergrowth res suggested tht long fllowing period is not lwys neessry. Aknowledgements We re grteful to Benest D. nd Verdier B. from the Lortoire d Eologie t ENS Pris, for the ron nd nitrogen nlyses nd for the grnulometri hrteriztion. This study ws finned through n inter-stte o-opertive reserh progrmme set up y the Frenh Institut de Reherhe pour le Développement. Referenes Bize, D. (1988). Guide des nlyses ourntes en pédologie. Inr édition Pris, 172 p. Blesdent, J., Besnrd, E., Arrouys, D., & Chenu, C. (1998). The dynmis of ron in prtile-size frtions of soil in forest-ultivtion sequene. Plnt nd Soil, 1, Blesdent, J., Chenu, C., & Blne, M. (). Reltionship of soil orgni mtter dynmis to physil protetion nd tillge. Soil & Tillge Reserh, 53, Cmpell, C. A., & Souster, W. (1982). Loss of orgni mtter nd potentilly minerlizle nitrogen from Sskthewn soils due to ropping. Cnd Journl of Soil Chnge, 62, Coote, D. R., & Rmsey, J. F. (1983). Quntifition of the effets of over 35 yers of intensive ultivtion on four soils. Cndin Journl of Soil Siene, 63, De Bli, P. H., & Somé, N. A. (1997). Étt struturl d'horizons superfiiels sleux sous ulture ou jhère herée en Afrique de l'ouest (Burkin Fso). Étude et gestion des sols, Devine, S., Mrkewitz, D., Hendrix, P., & Colemn, D. (14). Soil ggregtes nd ssoited orgni mtter under onventionl tillge, no-tillge, nd forest suession fter three dedes. Plos ONE, 9 (1), 1-12, e Devineu, J. L., Fournier, A., & Klog, B. (1997). Les sols et l végéttion de l région de Bondoukui (Ouest urkiné). Présenttion générle et rtogrphie préliminire pr télédétetion stellitire (SPOT). Pris, Orstom: 117 p + plnhes et rtes. Dormr, J. F. (199). Effet of tive roots on the deomposition of soil orgni mterils. Biology nd Fertility of Soils,1, Feller, C. (1994). L mtière orgnique dns les sols tropiux à rgile 1:1 : reherhe de omprtiments orgniques fontionnels. Une pprohe grnulométrique. Dotort de sienes de l vie et de l terre. Institut de Géologie Strsourg, Université Louis Psteur :393p. Feyss, D. H., Njok, J. T., Asfw, Z., & Nyngito, M. M. (11). Physio-hemil soil properties of semirid Ethiopi in two lnd use systems: implitions to rop prodution. Interntionl Journl of Agriulturl Reserh, 6, Floret, C., & Serpntié, G. (1993). L jhère en Afrique de l'ouest. Atelier interntionl Jhère. Montpellier, Orstom : 494 p. 28

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15 Pimentel, D. C., Hrvey, C., Resosudrmo, P., Sinlir, K., Kurtz, D., MNir, M., Crist, S., Shpritz, L., Fitton, L., Sffouri, R., & Blir, R. (1995). Environmentl nd eonomi osts of soil erosion nd onservtion enefits. Siene, 267, Plz-Bonill, D., Álvro-Fuentes, J., & Cntero-Mrtínez, C. (14). Identifying soil orgni ron frtions sensitive to griulturl mngement prties. Soil nd Tillge Reserh, 139, Puget, P., Chenu, C, & Blesdent, J. (). Dynmis of soil orgni mtter ssoited with prtile-size frtions of wter-stle ggregtes. Europen Journl of Soil Siene, 51, Rtnyke, R. R., Senevirtne, G., & Kulsooriy, S. A. (11). The effet of ultivtion on orgni ron ontent in the ly minerl frtion of soils. Interntionl Journl of Soil Siene, 6, SAS. (1996). Proedures in the SAS/STAT guide personl omputers. Version 6, SAS Institut In.,Cry, NC. Sheffe, H. (1959). The nlysis of Vrine. New York: Wiley édition. Shlesinger, W. H. (1986). Chnges in soil ron storge nd ssoited properties with disturne nd reovery. In J. R. Trlk, & D. E. Reihle (Eds.), The hnging ron yle: A glol nlysis (pp ). New York, USA. Sellers, P. J. (1997). Modelling the exhnges of energy, wter, nd ron etween ontinents nd the tmosphere. Siene, 275, Serpntié, G. (3). Persistne de l ulture temporire dns les svnes otonnières d'afrique de l'ouest. Etude de s u Burkin Fso. Thèse de Dotort d Agronomie, Institut Ntionl Agronomique, Pris-Grignon, 342+nnexes. Sheng, H., Ping, Z., Yngzhu, Z., Ykov, K., Qing, Z., Tid, G., & Cuihong, W. (15). Loss of lile orgni ron from susoil due to lnd-use hnges in sutropil Chin. Soil Biology nd Biohemistry, 88, Six, J., Elliot, E. T., & Pustin, K. (1999). Aggregte nd soil orgni mtter dynmis under onventionl nd no-tillge systems. Soil Siene Soiety of Ameri Journl, 63, Six, J., Elliot, E. T., & Pustin, K. (). Soil mroggregte turnover nd miroggregte formtion: mehnism for C sequestrtion under no-tillge griulture. Soil Biology nd Biohemistry, 32, Six, J., Pustin, K., Elliot, E. T., & Comrink, C. (). Soil struture nd orgni mtter: I. distriution of ggregte-size lsses nd ggregte-ssoited ron. Soil Siene Soiety of Ameri Journl, 64, Somé, A. (1996). Les systèmes éologiques post-ulturux de l zone soudnienne (Burkin Fso) : struture sptio-temporelle des ommunutés végétles et évolution des rtères pédologiques. Thèse de Dotort d Eologie, Université Pierre et Mrie Curie, Pris 6, 212p + nnexes. Stemmer, M., Von Lützow, M., Kndeler, E., Pihlmyer, F., & Gerzek, M. H. (1999). The effet of mize strw plement on minerlistion of C nd N in soil prtile size frtions. Europen Journl of Soil Siene, 5, Suhrjit, S. K., Rmhndrn, N. P. K., Viml, N. D., & Mohn, K. B. (1). Cron storge in reltion to soil size-frtions under tropil tree-sed lnd-use systems. Plnt Soil, 328, Tiessen, H., Stewrt, J. W. B., & Hunt, H. W. (1984). Conepts of soil orgni mtter trnsformtions in reltion to orgno-minerl prtile size frtions. Plnt nd Soil, 76, Udom, B. E., Benwri, A. O., & Osro, E. T. (15). Orgni Cron nd Nitrogen Distriution in Prtile-size Frtions of Soils Under Cssv, Plntin nd Ruer Bsed Lnd Use. Interntionl Journl of Soil Siene, 1(4), Vonir, N., Kprmwng, T., Am, A. A., & Hssn, A. M. (6). Vrition in morphologil properties nd prtile size distriution of Alfisols, Ineptisols nd Entisols in the Gui Soil Series, Buhi, Nigeri. Journl of Applied Sienes, 6, Wilson, A. T. (1978). Pioneer griulture explosion nd CO 2 levels in the tmosphere. Nture, 273, Yoni, M. (1995). Étude du stde à Andropogon gynus dns l reonstitution des jhères soudniennes à Bondoukuy, Ouest du Burkin Fso. Mémoire IDR Eux et Forêts, Université de Ougdougou 92 p. + nnexes. 3

16 Yoni, M. (1997). Les jhères à Andropogon gynus en svnes soudniennes. Influene du sol et des prtiques ulturles (s de Bondoukuy, Burkin Fso). Mémoire de DEA, Eologie et Biologie Végétles, FAST-IRD, Université de Ougdougou, 98 p. + nnexes. Yoni, M., Hien, V., Adie, L., & Serpntié, G. (5). Dynmis of soil orgni mtter in the Sudnin svnnhs of Burkin Fso. Chiers Agriultures, 14(6), Yu, P., Kexin, H., Li, Q., & Zhou, D. (17). Soil orgni ron frtions re ffeted y different lnd uses in n gro-pstorl trnsitionl zone in Northestern Chin. Eologil Inditors, 73, Zhng, H., Thompson, M. L., & Sndor, J. A. (1988). Compositionl differene in orgni mtter mong ultivted nd unultivted rguiudolls nd hpludlfs derived from loess. Soil Siene Soiety of Ameri Journl, 52, Copyrights Copyright for this rtile is retined y the uthor(s), with first pulition rights grnted to the journl. This is n open-ess rtile distriuted under the terms nd onditions of the Cretive Commons Attriution liense ( 31