Chapter 2. CARBON, NITROGEN & PHOSPHORUS ALLOCATION IN

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3 Chpter 2. CARBON, NITROGEN & PHOSPHORUS ALLOCATION IN AGRO-ECOSYSTEMS A WEST AFRICAN SAVANNA - II. THE SOIL COMPONENT UNDER SEMI-PERMANENT CULTIVATION Sumitted to Agriculture Ecosystems & Environment Rphël J. MANLAY,, Dominique MASSE, Jen-Luc CHOTTE, Christin FELLER c, Mguette KAIRÉ d, Joel FARDOUX e, nd Roger PONTANIER IRD, BP1386, Dkr, Senegl on leve from ENGREF, BP 593, 3433 Montpellier Cedex 1, Frnce c CENA-USP, on leve from IRD, Cix Postl Pircic, SP, Brzil d ISRA, BP2312 Dkr, Senegl e IRD, BP 545, 3432 Montpellier, Frnce ABSTRACT The evolution of soil properties under semi-permnent cultivtion ws followed through groundnut crop-fllow chronosequence locted in the West Africn svnn elt. Specil emphsis ws put on contents nd mounts of cron (C), nitrogen (N) nd ville phosphorus s mesured y Olson modified y Din (1967) (noted P OD). Soil physicl properties (texture, pf, density) remined stedy mong ll plots. Significnt increses of C, N, nd mgnesium s consequence of crop ndonment were sttisticlly evidenced down to 2 cm deep only (texture eing the min fctor driving soil properties elow). The mounts of C in the -2 cm soil lyer were 12.2±.6 (stndrd error) t h -1 in groundnut cropped plots (n=6), 15.8±.8 t h -1 in young fllows (1-9 yers old; n=6) nd 14.9±.8 t h -1 in old fllows (n=5). Amounts of N were respectively 1.±.5, 1.3±.8 nd 1.28±.5 t h -1. P OD stocks mounted to 6.47±.56, 8.47±.82 nd 6.19±.64 kg h -1. The unexpected fst evolution of soil properties fter crop ndonment ws ttriuted to fst recovery of woody vegettion. Prctices of improvement of, or sustitution for fllowing tht should sve the resprouting cpcity of trees re thus needed. However, the stediness of soil chemicl properties in oldest fllows s compred to young fllows ws not expected too. It ws ttriuted to the poor protection of soil orgnic mtter from oxidtion y iologicl ctivity. This ws confirmed y mesh-g experiments,

4 which indicted tht more thn 4-6 % of decying woody root iomss disppered fter 6 months of in-situ incution. Biologiclly medited fst cycling ptterns of C, nd thus of N nd P orgnic inputs to the soil, could e etter indictors thn soil C nd nutrient storge to ssess the restortion of sndy soils fertility during the fllow period in West Africn svnns. KEY WORDS Cron, Nitrogen, Phosphorus, Root, Svnn, Senegl, Semi-permnent cultivtion, Soil Chpter 2

5 INTRODUCTION Mixed frming systems with low inputs re widespred in West Africn svnns (Ker, 1995). Their sustinility relies mostly on low popultion density nd on mngement of orgnic resources (mnuring nd fllowing). Under trditionl semi-permnent griculture field would e cropped during few yers, then left to fllow for 15 to 2 yers, depending on soil, climte nd humn context (Nye nd Greenlnd, 196). Fllow would e ment to replenish the fertility of the gro-ecosystem, nd provide food, wood nd forge (Floret et l., 1993). As result of fst demogrphic growth, uncertinty out lnd tenure, nd technologicl improvements, which increse the efficiency of humn lour, West Africn svnns experience growing need for lnd. Thus, trditionl orgnic prctices tht were fitted to low popultion density undergo shift towrds more intensified prctices, resulting in long to short fllow rottion nd in extension of permnent cultivtion. Becuse of the susequent nd progressive shortening of the fllow period encountered in tropicl Afric (Floret et l., 1993; Gleve, 1996), improvements or lterntives of the prctice of fllowing re needed; ut such proposls must tke into ccount the multiple functions of fllowing. Assessment of cron (C), nitrogen (N) nd phosphorus (P) lloction mong the different components of the ecosystem is prerequisite to ny proposl for the trnsformtion of existing cropping systems. The sustinility of these systems -like tht of locl nturl ecosystems- still mostly relies on proper cycling of endogenous energy nd nutrients. The present pper is involved with soil C, N nd P udgets during the crop-fllow cycle. A study of the dynmic of plnt iomss undertken in southern Senegl indicted tht young fllows (ged less thn 1 yers) experienced fst ccumultion of C, N nd P in vegettion, most of it hppening during the first yer of fllow (Chpter 1). Increse in old fllows ws much slower, ut mintining long periods of fllowing ws necessry to preserve the productivity recorded t erly stges of the succession, since it relied mostly on the cpcity of stump resprouting. While much of N nd P is lost from the system t clering due to urning nd wood hrvest, the evolution of soil properties must e ssessed s n indictor of the ecosystem fertility. In the chemiclly poor soils of the Tropics such s those of West Africn svnns, these properties lrgely hng on orgnic inputs (Swift nd Woomer, 1993; Brown et l., 1994). C, N nd P sttus hve een considered s relile indictors of the fertility of svnn soils for long time (Nye nd Greenlnd, 196; Jones nd Wild, 1975). In hevily leched sndy su-shrn soils indeed, chemicl (ph, ction exchnge cpcity or CEC) nd physicl (porosity, stility) properties rely hevily on soil orgnic mtter (SOM) content (de Ridder nd vn Keulen, 199; Feller, 1995; Asdu et l., 1997), even though soil orgnic cron storge cpcity is highly limited y corse texture (Pieri, 1989; Feller nd Bere, 1997). Moreover, orgnic mtter is the min source of energy for elow-ground iot, therey driving iologicl fertility (Herrick nd Wnder, 1998). Cron pools with different functions nd Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

6 - 4 - turnover cn e isolted using simple size-frctiontion of SOM (Feller et l., 2). Menwhile, N nd P re recognised to e the most limiting nutrient fctors to plnt productivity in Africn svnns (Jones nd Wild, 1975; Bekund et l., 1997). Their cycle is closely linked to SOM dynmics: much of ville N nd P is stored in the vegettion nd mde ville to plnts fter microil oxidtion of fresh orgnic inputs. Pulished studies of C, N nd P dynmics in soil under semi-permnent or shifting cultivtion in dry West Afric re still few nd, except the work of Tiessen et l. (1998) nd Hrmnd et l. (2), rther concern humid climtes (Nye nd Greenlnd, 196; Kotto-Sme et l., 1997). On the other hnd, much hs een done on the impct of clering of primry vegettion over soil properties, ut little is known out tht impct on the dynmics of elow-ground plnt iomss. When fllow is mnully converted to crop, only rooting systems re spred (Floret et l., 1993). Woody stumps llowing for regulr resprouting survive severl yers efore they die (Bohringer et l., 1996). Removing stumps is however ecoming common precondition for cropping in West Africn svnns due to expnding mechnised tillge. This prctice clls into question the sustinility of semi-permnent cultivtion, since it hs proved to severely dmge soil physicl nd chemicl properties (Huluglle, 1994). But the fst decy of woody rooting systems tht my e suspected for such n evolution hs hrdly een quntified in slsh-nd-urn systems of the dry tropics. This work is the second of series of three chpters imed t quntifying cron (C), nitrogen (N) nd phosphorus (P) lloction in groecosystems under continuous nd semi-permnent cultivtion s prctised in villge of southern Senegl. The previous chpter delt with C, N nd P storge in plnt under semi-permnent cultivtion. In the third chpter, C, N nd P storge re ssessed in the plnt nd soil components under continuous cultivtion. Here we () compre ptterns of soil C, N nd ville P storge through crop-fllow chronosequence in West Africn svnn nd relte them to other soil properties () define temporl thresholds of evolution for these elements during the fllow period (c) quntify post-fllow decy of rooting systems nd relted N nd P inputs to the soil METHODS Site chrcteristics The study hs een undertken etween 1995 nd 1997 in the villge of Sre Yoron (12 49 N W), High Csmnce, southern Senegl. A detiled description of climte, vegettion nd griculturl prctices ws given in Chpter 1. The climte is Sudnin, tropicl dry (men nnul rinfll during the lst 2 yers: 96 mm from My to Octoer); temperture verged 28 C (Service de l Météorologie Ntionle, sttion of Kold). Men nnul potentil evpotrnspirtion ws 157 mm Chpter 2

7 etween 1977 nd 1988 (Dcost, 1989). Semi-permnent griculture is prctised t the top of the typicl, smooth toposequence. Two lnd use units re concerned: (1) plteu covered with mosic of woodlnds, svnn, fllows nd ush fields, nthropogenic formtions eing encountered mostly t the fringe of the plteu (Akpo, 1998). Soils were descried s sndy ferruginous y Bldensperger et l. (1967) nd re clssified s ferric Lixisols (FAO, 1998) (see Appendix 4 for detiled description of typicl soil profile). Anlyses mde in 17 plots of the crop-fllow succession (see Methods elow), indicte tht texture is snd-lomy in the -2 cm lyer, snd-cly-lomy in the 3-4 cm lyer. Cron content verged 5.5±.3 g kg -1 (± stndrd error) of soil in the -1 cm lyer, 4.1±.1 g kg -1 in the whole profile (-4 cm). Respective nitrogen contents were.44±.2 nd.36±.1 g kg -1. Aville phosphorus content ws 2.7±.2 nd 2.±.1 µg kg -1. Other min properties were: slight cidity (phh 2O:5.5±.1); CEC: 2.5±.1 nd 3.±.2 meq 1g -1 of soil in the -1 nd 3-4 cm lyers respectively, with sturtion rte decresing from 85±5 to 6±6 % from the -1 to the 3-4 cm lyer; pf 4.2: 3.1±.2 % in the -1 cm to 7.7±.6 % in the 3-4 cm lyer; ulk density: 1.5±.1 kg dm -3 in the whole profile, (2) glcis, with soils similr to those from the plteu (hplic Lixisols), ut with slightly less cly ccumultion elow 3-cm depth. This unit ers ll permnent crops nd the compounds. Sedentry Fulni herdsmen hve dopted diversified griculture (rinfed nd flooded cerels, groundnut nd cotton csh crops) closely ssocited with extensive livestock rising Smpling schemes A time-sving, synchronic method ws dopted: neighouring crop nd fllow plots of different ges cn e considered s the representtives of the sme plot during the succession, ssuming they shre similr soil properties nd lnd-use history (Snchez, 1987). The smpling ws done t the onset of the dry seson, close to pek ove-ground iomss in fllows, or just efore hrvest in the groundnut fields. Due to vegettion ptterns, two designs were dopted, depending on lnd use: (1) Groundnut plots (coded s GN): six fields tht hd een cropped with groundnut (Archis hypoge L.) in iennil rottion with fllow or sometimes perl millet for four to 15 yers nd tht hd never een chemiclly fertilised, were chosen. Four (GN1, GN2, GN3 nd GN5) of them were locted on the plteu, two (GN4 nd GN6) on the upper glcis. In ech of them, four 16-m² squre suplots were rndomly defined. After the clering of vegettion, soil smples were tken from smll pits dug t ech suplot corner for soil nlyses nd ulk density. Bulk density ws mesured using 1-cm 3 cylinder, Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

8 (2) Fllow plots (coded s FA), uncropped for one to 26 yers (see Chpter 1 for the distriution of smpled plots ccording to fllow length). Soil ws smpled for soil nlyses nd ulk density in pit every meter long 2-m long trnsect, in 11 plots. For ech sitution, soil nd ulk density were tken in 1-cm increments to 4-cm depth. Deeper smples would not hve modified our interprettions, since soil properties re not influenced y lnd use elow this depth (Detwiler, 1986; Feller, 1995) Soil nlyses Soil smples were cutiously sieved (t 2 mm) nd oven-dried t 15 C for 24 hours. Smples were pooled nd one nlysis ws mde per ech plot nd/or soil lyer. Soil ph ws mesured using 1:2.5 soil/wter or KCl solution. Totl C nd N from size frctions were determined y wet comustion (Fisons elementl nlyser N2 Crlo Er); totl C of non-frctionted (NF) soil ws mesured fter dichromte oxidtion. N ws determined with the Kjeldhl method, soil ville P with the Olsen method modified y Din (1967); soil totl-p ws not determined, nd P OD will stnd for soil ville-p (in the wy we mesured it) in wht follows (phosphorus in plnt iomss refers to totl P nd will e noted P t). Exchngele ctions were extrcted with CH 3COONH 4 t ph=7. CEC ws mesured y sturting soil with CCl 2, 2H 2O then exchnging C with K. Volumetric wter content ws determined t suction equivlent to pf2.5 (.322 tm) nd pf4.2 (14.5 tm). All methods re fully descried in Pge et l. (1989). Amounts of C, N nd P OD in soil were otined from soil ulk density mesures nd C, N nd P contents (Tle 2.1). Becuse no significnt differences in ulk density vlues etween tretments were found (see ), soil element udgets nd sttistics were clculted for soil equivlent depth only, nd not for soil equivlent mss s recommended y Ellert nd Bethny (1995). Size frctiontion of SOM ws performed on frctions [-5] nd [5-2] µm of smples of the -1 nd 1-2 cm lyers, ccording to the simplified, wet-sieving method from Gvinelli et l. (1995). In wht follows, the C content of frction will refer to the quntity of C per mss unit of soil, while the C concentrtion of frction will e defined s the quntity of C per mss unit of frction. C content of oth frctions llowed to estimte the prtitioning of C storge etween corse nd fine frctions, ssuming tht losses of wter-solule cron due to wet frctiontion were negligile nd tht ulk density of oth frctions ws equl. Chpter 2

9 Tle 2.1 Soil C, N, nd P OD content, C:N rtio nd modified [-2] µm ulk density in groundnut (GN) nd fllow (FA) plots.. C (g kg -1 ). N (g kg -1 ) Plot Soil lyer (cm) Plot Soil lyer (cm) GN GN GN GN GN GN GN GN GN GN GN GN FA FA FA FA FA1c FA1c FA FA FA FA FA FA FA FA FA FA FA FA FA FA FA FA c. P OD (mg kg -1 ) d. C/N Plot Soil lyer (cm) Plot Soil lyer (cm) GN GN GN GN GN GN GN GN GN GN GN GN FA FA FA FA FA1c FA1c FA FA FA FA FA FA FA FA FA FA FA FA FA FA FA FA e. Bulk density (frction [-2]µm) (kg dm -3 ) Plot Soil lyer (cm) GN GN GN GN GN GN FA FA FA1c FA FA FA FA FA FA FA FA Modified ulk density is the weight of the soil frction [-2] µm frction (dry sieving) per unit of volume. This modified ulk density hs een used for the clcultion of mounts of soil elements. The ge of the fllow plots is mentioned in plot coding Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

10 In situ root decomposition Post-fllow root dynmics fter stump removl ws ssessed with mesh-g decomposition experiment. Roots of Comretum glutinosum Perr., the most widespred tree species in fllows of the region, were smpled t the end of the dry seson, wshed, oven-dried t 7 C to constnt weight nd sorted in three dimeter-clsses ([-2] mm, [2-5] mm nd [5-1] mm). They were put in stinless-steel 3-mm mesh gs filled up with locl soil nd uried 15 cm deep in 15 yers old fllow plot t the onset of the riny seson. Vegettion ws clered nd soil left re during the whole experience. Twenty gs of ech dimeter clss hve een removed every six months, for two yers. Remining roots were wshed under wter nd oven dried efore weighting. Initil nd finl sh contents were mesured fter clcintion during three hours t 5 C. The decomposition rtes determined from this experiment were then pplied to estimte tree root decomposition occurring fter the clering of young nd n old fllow, ssuming tht (1) stumps were killed or removed (2) the decomposition rte remined constnt whtever the soil depth down to 4 cm nd the woody species to which the roots elonged (which ws confirmed in running experiment with roots of the three other min woody species of the ecozone, unpulished dt). The initil root iomss of these hypotheticl fllows ws set s the men vlue found for young fllows (ged 1-9 yers) nd old fllows (ged 1 yers nd more) on the study site (see Chpter 1) Dt nlyses Sttisticl nlyses were done using SAS softwre 6.14 (Htcher nd Stepnski, 1994) except principl component nlyses (PCA) tht were computed with ADE 4 softwre (Thioulouse et l., 1997). Multivrite nlyses were performed in ech soil lyer y computing Spermn R S correltion coefficients (proc CORR) for the following soil vriles: C (totl, nd in fine nd corse frctions for lyers -1 nd 1-2 cm only), N, P OD, ph in H 2 nd KCl, C, Mg, N, K, CEC, S, five-frction grnulometry + snd, cly, cly+fine silt, cly+silt, pf 2.5 nd pf 4.2. PCA were computed on the correltion mtrix of the tle contining 18 lines s cropped nd fllow plot replictes, nd columns s vriles listed ove. Using the synchronic method theoreticlly requires tht properties inherent to soil nd likely to drive the vlues of tested prmeters e the sme mong ll plots of the chronosequence. This condition is seldom fulfilled in field experimenttion. However, such vrile my e introduced s covrite in the liner model used for the nlysis of vrince (Anov), ssuming tht the rnge of vrition of the vrile is not too wide, so tht io-physicl processes remin roughly the sme etween plots nd differ only in their intensity. The works of Jones nd Wild (1975), Feller (1993) nd Zech et l. (1997) indicte tht prticulr ttention must e pid to texture s possile is for sttisticl interprettions trying to link SOM sttus to lnd mngement. Following the findings of Feller (1993), we introduced the cly+fine silt content s Chpter 2

11 covrite in Anovs. Results from Chpter 1 demonstrted tht threshold for the iomss of most plnt components ws reched fter 1 yers of fllowing on the study site. We thus clustered plot replictes in three groups: groundnut crop (GN), young fllows () ged less thn 1 yers, nd old fllows () under which slow plnt iomss ccumultion occurs. Proc GLM ws used on rnks of dt due to the smll numer of repeted mesures nd uncertinty out normlity of distriutions of dt nd residues (Potvin nd Roff, 1993). Pir-wise t-tests were performed on lest-squre mens in order to segregte tretments tht hd different effects on the level of the vrile tested (α=.5) RESULTS Soil properties & fllow succession Correltion nlysis indicted the following sttisticlly significnt links etween vriles for ech of the following soil lyers: - -1 cm: C (totl, nd in fine nd corse frctions) nd N were highly positively correlted (R S=+.69 ** to +.92 *** ) ( Appendix 9). Other chemicl vriles such s phkcl, C, Mg, CEC nd sturtion rte (S) were lso well positively relted to totl C nd N (R S=+.49 * to +.79 *** ). Cly + Fine silt ws positively correlted with totl C (R S=+.5 * ) nd to N (R S=+.63 ** ). pf4.2 ws positively relted to cly for every lyer, especilly the deepest ones. PCA showed tht the first principl component (PC) (reltive inerti or RI: 35 %) ws chrcterised y C, N, Mg nd CEC on the negtive side, nd snd on the positive side; old fllows (ssocited with good SOM sttus) were seprted from groundnut crops (Figure 2.1,). P OD ws the min contriutor to the second xis (RI: 16 %) nd isolted young fllows from other plots. The third xis (RI: 15 %) ws determined y texture ut did not llow to distinguish differences etween the three stges of succession (GN,, ) (Appendix 1), cm: neither C nor N were correlted to ny vrile, except CEC ( Appendix 9). But correltions were found for Cly+fine silt with CEC (R S=+.52 * ), nd S (R S=-.53 * ), nd ph (R S=-.67 ** to -.71 ** ). Cly, Cly+fine silt nd Corse snd were the min contriutors to the first PC lying on the negtive semi-xis, s opposed to ph nd C (Figure 2.1c). PCA performed in this lyer nd in deeper ones did not llow for ny possile distinction etween GN, nd plots (Figure 2.1d; Appendix 1), cm nd 3-4 cm: C, N, ph, CEC nd S were strongly (R S=-.55 * to +.93 *** ) relted to fine elements ( Appendix 9c,d). Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

12 Correltion circle. Plot replicte projection P FSAND PHKC L DE NS GN Lyer -1 cm CCorFr CA C S CFinFrc N CE C PF 42 PHH2O NA CLAY K CN SAND GN GN MG CLAYSI CLASI CSILT SI FSILT PF 25 CSAND Eigenvlues GN GN GN c. PF4 2 SAND N DENS FSAND S CA 8 6 Eigenvlues PHH2O PHKCL d. GN Lyer 1-2 cm CSAND CLAY PF2 5 CLASI MG C NA CFinFrc K CCorFr CEC FSILT P CN GN GN GN GN SI CSILT CLAYSI GN Figure 2.1 Principl components (PC) nlysis of the soil properties of chronosequence mde of six cropped plots nd 11 fllow plots. Correltion circles of the vriles nd projection of the plot replictes on plne PC 1x PC2:.-. lyer -1 cm c.-d. lyer 1-2 cm. Coding of vriles: C: cron. CA: clcium. CCorFr: cron content of the [5-2] µm frction. CEC: ction exchnge cpcity CFinFrc: cron content of the [-5]µm frction. CLAY: cly. CLASI: cly+fine silt. CLAYSI: cly+silt. CN: C:N rtio. CSAND: corse snd. CSILT: corse silt. DENS: ulk density. FSAND: fine snd. FSILT: fine silt. K: potssium. MG: mgnesium. N: nitrogen. NA: sodium. P: ville phosphorus. PHH2O: ph in wter. PHKCL: ph in KCl. PF25 nd PF42: volumetric wter content determined t suction equivlent to pf2.5 nd pf4.2. S: sturtion rte. SAND: snd. SI: silt. Coding of plot replictes: GN: groundnut crop. : young fllow (-9 yers). : fllow older thn 9 yers. See Appendix 11 for dt used in the PCA nd Appendix 1 for eigen vlues. Sttisticl nlyses (Tle 2.2) indicte no significnt influence of fllowing on soil physicl properties (fine texture, pf, nd ulk density). On the other hnd pf ws gretly ffected y texture elow 2-cm depth. Fllowing incresed C nd N only ove 2-cm depth, ut C contents were lower in old fllows thn in young ones (Tle 2.2). Highest P OD contents were found in young fllows t ny depth, ut vriility did not llow for sttisticlly significnt differences. Texture hd no influence on chemicl properties in the -1 cm lyer, except on Mg nd CEC. Below 2 cm, Cly+fine silt content strongly influenced C, N, ph, CEC nd S. Chpter 2

13 Tle 2.2 Effect of lnd mngement (fllowing) nd texture (cly+fine silt content) on soil properties.. physicl properties. Lyer Groundnut field Young fllow Old fllow F (cm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll Cly + fine silt content (%) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±1.7.4 pf2.5 (gh 2 1 g -1 soil) ± ± ± * ± ± ± * * ± ± ± ** 9.3 ** ± ± ± *** 9.1 ** ± ± ± * 19.4 *** 12.6 *** pf4.2 (gh 2 1 g -1 soil) ± ± ± * 4.1 * ± ± ± ± ± ± *** 13.2 *** ± ± ± *** 56.6 *** ± ± ± *** 15.1 *** Bulk density (kg dm -3 ) ± ± ± ± ± ± * ± ± ± ± ± ± ± ± ± p{ho: F os >F th =}: * <.5; ** <.1; *** <.1. Two men vlues with different letters differ significntly in their LS mens (α=.5; pir-wise T-test). See dt in Appendix 11. Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

14 Tle 2.2 (continued). chemicl properties Lyer Groundnut field Young fllow Old fllow F (cm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll Cron (g kg -1 ) ± ± ± * * ± ± ± ** ** ± ± ± ** 5.9 ** ± ± ± *** 12.5 *** ± ± ± * * Nitrogen (g kg -1 ) ±.2.49 ±.4.5 ± * ** ±.2.37 ±.2.35 ± ±.2.33 ±.2.34 ± ** 5.3 * ±.3.32 ±.3.35 ± *** 22. *** ±.2.38 ±.2.38 ± * 7.6 * 6.2 ** Phosphorus OD (1-3 g kg -1 ) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ph (H2O) ± ± ± ± ± ± ** 6.9 ** ± ± ± *** 11.3 *** ± ± ± ** 4.3 * ± ± ± *** 7.3 ** ph (KCl) ± ± ± ± ± ± ** 5.4 * ± ± ± *** 9.1 ** ± ± ± *** 6.7 ** ± ± ± ** 6.2 ** C (meq 1g -1 soil) ± ± ± ± ± ± ± ± ± * ± ± ± ± ± ± Mg (meq 1g -1 soil) ±.3 c.48 ±.3.64 ± *** 4.7 * 23.1 *** ±.4 c.45 ±.5.6 ± ** 5.9 * 1. ** ±.4.48 ±.7.58 ± ±.5.5 ±.7.52 ± ±.3.48 ±.5.59 ± * * N (meq 1g -1 soil) -1.1 ±.5.2 ±.8.1 ± ±.2.1 ±.3.1 ± ±.3.2 ±.3.1 ± ±.5.2 ±.8.2 ± ±.2.2 ±.3.1 ± K (meq 1g -1 soil) -1.4 ±.4.6 ±.8.4 ± ±.4.6 ±.6.4 ± ±.4.5 ±.6.3 ± * * ±.3.5 ±.5.3 ± ±.3.5 ±.5.4 ± CEC (meq 1g -1 soil) ± ± ± * 6. ** ± ± ± * 4.2 * ± ± ± ** 6.2 ** ± ± ± * *** 48.5 *** ± ± ± * 74.8 *** 25.3 *** Sturtion rte (%) ± ± ± ± ± ± * 4.4 * ± ± ± * ± ± ± *** 8.3 ** ± ± ± * 2.9 Chpter 2

15 SOM qulity SOM qulity ws investigted using severl criteri: the C:N rtio of orgnic mtter of the nonfrctionted soil, nd the C concentrtion nd content, nd the C:N rtio of the [-5] nd [5-2] µm size-frctions. C:N rtio of the NF soil did not vry significntly during the fllow period nd verged 11.6±.3 (Tle 2.1 nd Tle 2.4). It slightly decresed with depth from 12.7 (-1 cm) to 1.7 (3-4 cm lyer). The mss of the [-5] µm frction verged 22.1±.7 nd 25.4±1. g 1g -1 of soil in the -1 nd 1-2 cm lyers (Tle 2.3,). C concentrtion of the [-5] µm frction verged 17.4±.7 g kg -1, the C content eing 3.8±.2g kg -1 in the -1 cm soil lyer. In the 1-2 cm lyer these vlues reched only 1.7±.3 gc kg -1 of frction nd 2.7±.1 gc kg -1 of soil. The [5-2] µm frction hd poorer C concentrtion nd content: 1.7±.1 gc kg -1 frction (tht is 1.3±.1 gc kg -1 soil) in the -1 cm lyer, 1.1±.2 gc kg -1 frction (.8±.1 gc kg -1 soil) in the 1-2 cm lyer. The comprison of the different Tle 2.3 SOM frctiontion in groundnut (GN) nd fllow (FA) plots.. -1 cm lyer Frction -5 µm Frction 5-2 µm Frctiontion Mss C content in g kg -1 of C/N Mss C content in g kg -1 of C/N recovery rte Plot (g 1 g -1 soil) frction soil (1) (g 1 g -1 soil) frction soil (2) ((1)+(2))/C t GN GN GN GN GN GN FA FA FA1c FA FA FA FA FA FA FA FA cm lyer Frction -5 µm Frction 5-2 µm Frctiontion Mss C content in g kg -1 of C/N Mss C content in g kg -1 of C/N recovery rte Plot (g 1 g -1 soil) frction soil (1) (g 1 g -1 soil) frction soil (2) ((1)+(2))/C t GN <.2 <.15 GN GN GN GN GN FA FA <.2 <.15 FA1c FA FA FA FA FA FA <.2 <.16 FA FA Frctiontion recovery rte computed s the sum of (1) nd (2) out of cron content of non-frctionted soil (from Tle 2.1). The ge of the fllow plots is mentioned in plot coding. Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

16 - 5 - situtions indicted tht, whtever the soil lyer, the C concentrtion of oth prticle-size frctions ([5-2] µm, [-5] µm) incresed significntly from the GN plots, to the nd situtions (Tle 2.4). The gin of C ws prticulrly evident for the corse frction [5-2] µm isolted from the 1-2 cm lyer. As mtter fct, this frction ws responsile for out 5 % of the totl C increse recorded etween GN nd plots. C:N rtio of fine nd corse frctions rised stedily nd significntly long the whole succession t oth depths; progression ws stronger in the corse frction thn in the fine one (Tle 2.3, nd Tle 2.4). Tle 2.4 Effect of lnd mngement (fllowing) nd texture (cly+fine silt content) on SOM qulity s ssessed y:. C:N rtio of NF soil. C concentrtion nd content, nd C:N rtio in fine nd corse soil frctions.. Lyer Groundnut field Young fllow Old fllow F (cm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll C/N on non-frctionted soil ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Lyer Frction Groundnut field Young fllow Old fllow F (cm) (µm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll Cron concentrtion (g kg -1 frction) ± ± ± ± ± ± ** ** ± ± ± ± ± ± Cron content (g kg -1 soil) ± ± ± * *** ± ± ± *** ** ± ± ± ± ±.25.9 ± C/N on frctions ± ± ± *. 4.7 * ±2.3 c 37.3 ± ± *** *** ± ± ± * 6.3 * 5.7 * ±4.6 c 51.1 ± ± ** 5.4 * 1.2 ** p{ho: F os >F th =}: * <.5; ** <.1; *** <.1. Two men vlues with different letters differ significntly in their LS mens (α=.5; pir-wise T-test). See dt in Tle 2.1 nd Tle 2.3. Chpter 2

17 Ptterns of soil C, N & P OD storge long the succession Temporl ptterns of C, N nd P OD storge during the crop-fllow cycle re shown in Figure 2.2 (detiled soil dt; see Tle 2.1 for clcultion) nd Figure 2.3 (ggregte dt for the ecosystem). Soil C, N nd P incresed within the very first yer of fllow (+4.4 tc, +43 kgn, nd +5.7 kgp OD h -1 in the whole profile, tht is gin of 2, 22 nd 5 % reltively to mounts in soil of cropped fields). Verticl distriution of C, N nd P OD remined quite constnt etween the tretments. The t h Cron concentrtion of C, N nd P OD in the upper soil lyer (-1 cm) ws rely pronounced (33 % of the whole mount in the -4 cm profile). C storge in the whole profile ws 22.3±.7 t h -1 in the groundnut fields ut reched 25.6±.8 t h -1 in oldest fllows. N verged 192±11 nd 23±81 kg h -1 kg h Nitrogen in plots of cropped fields nd old fllows, respectively. P OD storge in these clusters reched 11.3±.9 kg h -1 nd 9.7±1.2 kg h -1 respectively. Highest C nd P OD mounts were found in young fllows. Fllowing influenced significntly the vlue of C nd N stocks in the -1, 1-2 nd -4 cm lyers (Tle 2.5). C storge Phosphorus OD eqully incresed in fine nd corse frctions 25 in the -1 nd 1-2 cm lyers (Tle kg h Length of fllow (y) 2.5). But significnt differences were found in the [5-2] µm frction only, ecuse initil C contined in the corse frction ws only fourth of tht in the fine frction. In the -1 cm lyer, highest mounts of totl C were reched in old fllows; this hppened in the young fllows when -4 cm -1 cm considering lyers 1-2 nd -4 cm. Figure 2.2 Evolution of soil C, N nd P OD storge in the -1 nd -4 cm lyers long the crop-fllow succession. See dt in Appendix 13. However, C nd N did not increse significntly in the old fllows s compred Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

18 to the young ones. Fine elements were etter predictor for C nd N storge thn stge of succession in the 2-3 nd 3-4 cm lyers. No sttisticlly significnt evolution of P OD storge long the succession could e evidenced, lthough P OD storge ws 3 % higher in young fllows thn in other plots. Tle 2.5 Effect of lnd mngement (fllowing) nd texture (cly+fine silt content) on soil C (totl nd in frctions), N nd P OD storge.. non-frctionted soil Lyer Groundnut field Young fllow Old fllow F (cm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll Cron storge (t h -1 ) ± ± ± * * ± ± ± ** ** ± ± ± ** 5.2 * ± ± ± *** 12.8 *** ± ± ± ** ** Nitrogen storge (kg h -1 ) ±28 74 ± ± * ** ± ±27 53 ± ** * ± ± ± ** 4.5 * ± ± ± *** 11.6 *** ± ± ± * 5.7 * 5.5 * Phosphorus OD storge (kg h -1 ) ± ± ± ± ± ± ± ± ± ±.2 3. ± ± ± ± ± soil size frctions Lyer Frction Groundnut field Young fllow Old fllow F (cm) (µm) Men (±SE) (n=6) Men (±SE) (n=6) Men (±SE) (n=5) Mngement Texture Overll Cron storge in frctions (t h -1 ) ± ± ± ± ± ± **. 1.2 ** ± ± ± ± ± ± * p{ho: F os >F th =}: * <.5; ** <.1; *** <.1. Two men vlues with different letters differ significntly in their LS mens (α=.5; pir-wise T-test). See dt in Appendix 13 nd Appendix 14. When considering the whole ecosystem (Figure 2.3), the contriution of soil to C storge decresed from 8 % in the cropped fields to 47 % in old fllows. The sme trends were recorded for N, lthough lloction to soil remined high in ll tretments (95-87 % of the whole mount in the plnt-soil system). A shrper evolution ws evidenced for soil P OD tht represented 66 % of the mount of iologiclly ctive P (plnt P t+soil P OD) in the fields nd only 22 % in old fllows. Chpter 2

19 Cron 6 5 t h Plnt Soil -2 cm corse frction Soil -2 cm fine frction Soil 2-4 cm 3 Nitrogen 25 2 kg h Plnt Soil -2 cm Soil 2-4 cm Phosphorus 5 kg h Groundnut field Young fllow Old fllow Plnt Soil -2 cm (OD) Soil 2-4 cm (OD) Figure 2.3 C, N nd P storge in the plnt-soil system t three min stges of the crop-fllow succession. Figures for plnt iomss derived from Chpter 1. Young fllow: ged -9 yers. Old fllow: older thn 9 yers. OD: ville phosphorus (soil) s mesured y Olsen s method modified y Din (1967). See dt in Appendix 12. Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

20 Post-fllow dynmics of root iomss Ded tree root iomss disppered quickly fter eing dug in the soil of the fllow clered (Figure 2.4). After the six first months of in situ incution, highest initil decomposition rtes were recorded for the finest roots: 61 % vs. 5 % nd 41 % of mss loss for the [2-5] nd [5-1]mm dimeter clsses. As incution went on, the rtes decresed nd remined constnt eyond the second yer. 1 [-2] mm dimeter 8 ** c c % Root mss remining ** c [2-5] mm dimeter d 1 [5-1] mm dimeter 8 6 ** 4 2 c c Months Figure 2.4 Root decomposition dynmics of Comretum glutinosum Perr. fter clering of 15 yers old fllow s mesured during mesh-g experiment. Verticl rs stnd for SE. Results of Z- nd SNK-tests (α=.5; n=2). See dt in Appendix 15. Chpter 2

21 These results were pplied to simulte the decy of root iomss occurring fter the clering of two hypotheticl young nd old fllows in which stumps would hve een removed (Figure 2.5). Estimted disppernce (oxidtion nd sptil redistriution) of dry mtter (DM), C, N nd P t relted to decying root iomss would e mssive during six months following the clering of fllow plots nd mount to 4./9.3 tdm, 1.4/3.3 tc, 2/43 kgn nd 1.1/3.4 kgp t h -1, depending on the ge of fllow (young/old). During the second cropping seson these vlues would drop y nerly two thirds. Dry mtter Cron t.h t.h Nitrogen Phosphorus kg.h kg.h Months fter clering Months fter clering Figure 2.5 Estimted remining mounts of dry mtter, cron, nitrogen nd phosphorus from the decying root component fter clering of young () nd old () fllow (stumps removed). See dt in Appendix DISCUSSION Generl trends in soil C, N & P dynmics Generl results of this study on soil orgnic cron compre firly well with the findings of Jones nd Wild (1975), t lest for the upper soil lyer. These uthors report the men cron content of 245 ferruginous soils to e 6.2 g kg -1 (5.5 g kg -1 in our study). Nye nd Greenlnd (196) found C vlues to verge 7.6 g kg -1 in the upper soil lyer (-5 cm) of vrious uncropped ecosystems of the dry tropics. Figures from the Seneglese cse study detiled y Tiessen et l. (1998) re close to ours, though lower Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

22 thn our findings, mostly due to the different environmentl constrints of their study zone(drier climte nd higher humn pressure). Our results report N content to e.44 g kg -1 in the -1 cm lyer, which compres well with the review of Nye nd Greenlnd (196) nd is close to the vlue predicted y the multiple regression reltion computed y Jones (1973) linking N content to cly content nd nnul rinfll. As suggested y this uthor, it must however e kept in mind tht high sptil nd temporl vriility is feture of C nd N soil content in West Africn svnns. Compring ville-p content (s defined y P OD) with other studies is questionle, since results re highly relint on the nlyticl method. The phosphorus sttus of the soils of our study my e prticulrly poorer thn expected for the ecozone. Indeed, totl P reported y Mnly (1994) for four fllow plots ged one to 15 yers verged 6 µg kg -1, which is only hlf of tht reported y Jones nd Wild (1975) in 181 tropicl sndy soils. Nye nd Greenlnd (196) lso provide much higher ville-p content (7.2 µg kg -1, Truog method) under soil nd climte conditions similr to those of south Senegl. Their review reports higher contents of Mg, K nd C in soils under fllow thn those found in our study except for humid svnn fllows Influence of intrinsic soil properties The undnce of fine elements seemed to ply key role in explining vritions of chemicl nd physicl properties mong which SOM content-, especilly elow 2 cm deep. For the -2 cm lyer, our results indicte tht the soil properties (including C, N nd P OD content) re driven mostly y the durtion of the fllow in the crop-fllow rottion, proly due to the low vrition of the soil fine elements content. Fllowing hs only little impct, if ny, on soil wter retention cpcity, nor does it improve density, lthough repeted tillge in cropped fields might lso hve decresed soil density nd thus ised the tests. On methodologicl point of view, our results suggest tht studies imed t connecting fllow mngement nd soil properties could restrict smpling to the -2 cm lyer only Nutrient lnce of the crop-fllow system Annul increse of N nd P (plnt P t + soil P OD) for the whole system ws highest fter one yer of fllow nd nerly nil eyond the 1-yers threshold (Figure 2.3). Efficiency of young fllows nd pstures for chieving fst recovery of sustntil mounts of soil N nd ville P hs lredy een underlined y Jones (1971), Friesen et l. (1997), Brnd nd Pfund (1998), nd Hrmnd nd Njiti (1998), lthough nutrient pumping y trees might not e significnt in the region (Bremn nd Kessler, 1995). Under high soil chemicl constrint, svnns hve developed nutrient-conservtive strtegies (Myers et l., 1994), nd spontneous vegettion exhiits prticulr efficiency in recycling nitrogen from litter nd extrcting ssimille phosphorus from soil (Adie et l., 1992; Friesen et l., 1997). The decrese of soil P OD content in the oldest fllows is nother illustrtion of this feture, s it might e linked with Chpter 2

23 immoilistion in woody live iomss (Bremn nd Kessler, 1995). Dt from Mnly (1994) report indeed stle P t:p OD rtio (28.3±2.9; n=4) in soil during the fllow succession in Sre Yoron, indicting limited relloction of P from the ville pool to more unville forms during fllowing. We thus suggest tht P OD, together with wter vilility, my e the min limiting fctor responsile for the symptotic property of the curve of tree iomss s function of the length of fllow estlished in Chpter 1 (Murphy nd Lugo, 1986; Akpo, 1998; Sirois et l., 1998). In nery Guine, Sirois et l. (1998) found similr, decresing trends for soil extrctle-p nd exchngele K during the first eight yers of fllow. However, these uthors recorded decreses for Mg, CEC nd ph, while our study did not revel such ptterns Soil orgnic sttus fter crop ndonment Minerl fertiliztion without orgnic mendments rely leds to sustinle productivity in West Afric, s it often results in SOM minerliztion nd susequent soil structure disruption, ph decrese nd luminic toxicity increse (Pieri, 1989; Mokwunye nd Hmmond, 1992). The study of plnt iomss dynmics indictes tht fllowing rises elow-ground C stored in plnt iomss up to 12.6 t h -1 nd sets C input fluxes up to t lest 3.6 t h -1 y -1 in old fllows (see Chpter 1). It is commonly ccepted tht clering primry forests or woodlnds nd susequent cropping leds to drmtic decline of soil cron content in dry nd wet tropics (Brms, 1971; Sind, 1974; Feller nd Milleville, 1977; Tiessen et l., 1992; Juo et l., 1995). Resons put forwrd for this re the (1) drop of orgnic inputs to the soil due to lower elow-ground productivity nd plnt iomss removl t hrvest, (2) modifiction of soil wter nd temperture conditions, (3) erosion nd leching enhncement, (4) tillge (Brown et l., 1994). However, the potentil of fllowing to reverse SOM losses due to prolonged cultivtion remins much more controversil. Mny uthors relte significnt increses of soil cron contents following crop ndonment (Greenlnd nd Nye, 1959; Ain, 1979; Areol et l., 1982; Tiessen et l., 1992; Feller, 1993). But from their works no consensus rises out the shpe of the curve of cron ccumultion s function of the length of fllow, nd the miniml period of fllow required for significnt improvement of SOM content. At lest s mny studies held under vrious tropicl climtes report no significnt evolution of cron storge fter crop ndonment (Bew nd Lejoly, 1993; Bremn nd Kessler, 1995; Jiyeo, 1995; Juo et l., 1995; Kotto-Sme et l., 1997; Sirois et l., 1998; Denich et l., 2 mong others). Whtever their findings, these works seldom include texture s covrite in their model of cron dynmics s inferred from chronosequences. Nevertheless, s hypothesised y Nye nd Greenlnd (196), the cpcity of fllowing to restore the orgnic content of soil should e judged regrding how fr the C content stnds from the equilirium level under ntive vegettion. This is rely documented in the previously cited studies. Recent findings of Hrmnd et l. (2) suggest tht cron storing cpcity of fllowing could lso hng upon tree species composition of the secondry vegettion. Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion

24 The present work revels low -though significnt- cpcity of cron storge in the sndy soils of fllows of West Africn svnn. This ptitude is detectle within the very first yer fter crop ndonment, which is feture of corse textured soils (Feller nd Bere, 1997). Yet, lter pprent inerti of SOM storge does not gree with most of the previously cited studies; usully these show stedy increse of soil cron even in old fllows. In ddition, more thn hlf of the rise recorded in the -2 cm lyer fter crop ndonment occurs in the corse frction [5-2] µm. Feller (1995) showed tht this frction hd high turn-over rte (.4 to 1. over 1 yers), nd ws restricted to iologicl functions such s C, N nd P supply to microflor nd funl communities. Thus, the effect of fllowing is rther trnsient nd hs only little impct on the improvement of soil chemicl properties. Severl fctors my explin the wek nd frgile response of locl soils to fllow mngement. Jones nd Wild (1975) nd Feller (1995) hve shown tht cly minerlogy, nd rinfll to lesser extent, were the est predictors for cron content in tropicl soils. For plteu soils of High Csmnce, lrge corse-snd frction nd limited cly content do not llow for efficient SOM protection ginst microil oxidtion, leching nd erosion losses (Feller nd Bere, 1997). Rinflls, constrining the durtion of the plnt growth period, soil moisture nd temperture ptterns unfvourle to humifiction re the other iotic resons for limited SOM storge (Moureux, 1967). Hrvest of dedwood y people, s well s CO 2 minerliztion induced y fire might lso e put forwrd, lthough the effect of fire on SOM content of young fllows hs recently een questioned (Msse et l., 1997) Fllowing s tool for the recovery of iologicl control over ecosystem fertility As mtter of fct, we put forwrd the growing control exerted y iologicl ctivity over SOM dynmics s the ultimte fctor limiting cron storge cpcity of soils under fllow nd suggest tht this control confers prticulr ecologicl role to SOM in svnn sndy soils. Becuse the study site lies in region of recent humn settlement leving wide res of non-clered svnn, soil living popultions re quite similr to those found in wekly distured svnn or under more humid climte (Derourd et l., 1998). Mssive root decy following fllow clering, oserved y Hrmnd et l. (2) in Northern Cmeroon too, testifies intense iologicl ctivity even during the dry seson. Such n ctivity would e constrined more y ville C shortge thn y temperture (Kiser, 1983), nd even thn y soil humidity in the cse of termites. Termites re the min conveyors of orgnic inputs to the soil in wooded Africn svnns throughout the yer nd my re-disptch more thn 9 % of net cron production of the ecosystem (Jones, 199). Erthworms improve physicl nd chemicl vilility of SOM to micro-orgnisms s well s soil porosity through nnul remixing of the whole soil upper lyer (Lvelle et l., 1998). As consequence of climte nd of the priming effect y mcrofun, minerliztion ctivity y soil microflor is much higher in tropicl thn in temperte soils (Jenny et l., Chpter 2

25 ; Jenkinson nd Ayn, 1977), resulting in more mture humic compounds in SOM of tropicl soils (Grisi et l., 1998). For instnce, yerly, in situ emissions of C-CO 2 from soil respirtion hve een reported to rech even up to 75 % of soil orgnic cron stored in sndy soil ering suhumid svnn (Schefer, 1974). But we suggest tht mssive grzing over soil cron y the soil heterotrophic community is not out of enefit for the fllow ecosystem-s--whole. Recent findings of reserch studies held in West Africn fllows report incresed iomss nd diversity of soil microflor (Wick et l., 1998) nd termites (Blck nd Okwkol, 1997; Srr et l., 1998 nd Fll, 1998 in Senegl) fter crop ndonment. Fllowing modifies funl iodiversity too (Derourd et l., 1998), thus ltering the structure nd wekening the pthogenicity of nemtode popultion (Pâte, 1997; Mnly et l., 2). It might lso stimulte mycorrhizl soil infectivity (Duponnois, pers. comm.). In fct, so-clled elow-ground heterotrophic engineers (Jones et l., 1994; Lvelle et l., 1997) together with rooting systems contriute to the ptitude of the fllow ecosystem to uffer the effects of the pronounced hrshness of climte nd of soil nutrient poverty nd physicl instility (Menut et l., 1985; Perry et l., 1989; Brown et l., 1994). This is chieved through efficient nutrient conservtion strtegies, structurl integrity relying on live plnt iomss, nd functionl stility through iodiversity enhncement (Odum, 1969; Giller et l., 1997). The wek potentil of fllow soils to stilise orgnic inputs into humic compounds should thus e viewed s the energetic cost to py for the self-orgnising process occurring in the fllow ecosystem, which is necessry for the replenishment of soil fertility in svnn griculturl systems with low inputs West Africn fllow mngement in glol chnge perspective On glol chnge perspective, this study provides informtion for cron sequestrtion potentil in svnn s n ttempt to mitigte nthropogenic greenhouse emissions, of which 2 % would stem from lnd use chnge in the tropics (Schimel, 1995). When integrting results from Chpter 1 into the discussion, C dynmics exhiit vrious trends, depending on the plnt or soil component considered (Figure 2.3). Wht hs lredy een oserved under more humid contexts (Toky nd Rmkrishnn, 1983; Toky nd Rmkrishnn, 1983; Kotto-Sme et l., 1997) is eing confirmed in the present study: during the crop-fllow succession most rective DM, C, N nd P reservoirs re lso the most iologiclly ctive ones, while mounts in soil remin quite stle. After crop ndonment, woody nd root iomsses increse, while those of the herceous lyer drop, nd mounts of litter nd SOM show no cler trend of evolution. According to current slsh-nd-urn prctices, clering of mture fllow in south Senegl would led to the relese of 27 tc h -1 through immedite urning, lter comustion of wood for energetic needs nd on-site decomposition of roots, stumps nd remining unurnt twigs nd leves, nd minerliztion of smll prt of SOM. Nerly hlf of this vlue (12.3 tc h -1 ) cn e recovered during the first yer of fllow. Then, nnul rte of storge is only 2 tc h -1 during the 1 following yers of fllow. However, this Cron, nitrogen & phosphorus lloction in gro-ecosystems of West Africn svnn - II. The soil component under semi-permnent cultivtion