Carbon, nitrogen and phosphorus allocation in agro-ecosystems of a West African savanna II. The soil...

Agriculture, Ecosystems and Environment 88 (2002) 215–232

Carbon, nitrogen and phosphorus allocation in agro-ecosystemsof a West African savanna

I. The plant component under semi-permanent cultivation

Raphaël J. Manlaya,∗, Maguette Kairéb, Dominique Massea, Jean-Luc Chottea,Gilles Ciorneia, Christian Floreta

a Institute for Research and Development (IRD, ex-ORSTOM), BP 1386, Dakar, Senegalb Senegalese Institute for Agricultural Research (ISRA), BP 2312, Dakar, Senegal

Received 4 February 2000; received in revised form 20 March 2001; accepted 27 March 2001

Abstract

Organic matter (OM) is both a commodity and a means of production in low-input farming systems of sub-Saharan Africa.Since this resource is becoming increasingly scarce in West African savannas (WAS), there is a need to assess OM, carbon(C), nitrogen (N), and phosphorus (P) allocation in local ecosystems related to land management. Carbon, N and P storageunder semi-permanent cultivation in savannas in southern Senegal was thus measured through a chronosequence including25 groundnut (Arachis hypogaea L.) crops and plots left to fallow for 1–26 years. The amounts of C, N and P in croppedplots were 5.5 t C, 106 kg N and 5.9 kg P ha−1, they increased to 17.7 t C, 231 kg N and 19.6 kg P ha−1 in fallow plots aged1–9 years. A threshold was reached after 10 years of fallow. Beyond it biomass amounts remained steady. Older fallow plotsstored 29 t C, 333 kg N and 33.8 kg P ha−1. Highest increases in woody components were found within the very first yearfollowing crop abandonment, and were achieved at the expense of the herbaceous layer. Carbon and nutrient allocation towoody below-ground biomass occurred only later. Massive nutrient losses were expected to occur at clearing due to bothburning and wood exportation. Because storage in woody and herbaceous biomass remained steady in fallows aged more then10 years, young fallows were found to have the highest productivity for wood and forage. However, plant productivity reliedon the high resprouting capacity of local tree species, and thus on the maintenance of long breaks of fallow needed for themaintenance of perennial rooting systems. One of the aims of programs to improve the management of fallows, or to replacethem with agroforestry techniques, should thus be to preserve perennial rooting systems by any means that are possible in thecropping systems of the WAS. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Plant biomass; Carbon; Nitrogen; Phosphorus; Root; Savanna; Senegal; Semi-permanent cultivation

∗ Corresponding author. Present address: Institute of Forestry,Agricultural and Environmental Engineering (ENGREF), BP44494, 34093 Montpellier Cedex 5, France.Tel.: +33-467-047-121; fax:+33-467-047-101.E-mail address: [email protected] (R.J. Manlay).

1. Introduction

In the West African savanna (WAS) belt, biophys-ical restrictions (rainfall patterns, poor nutrient statusand soil sensitiveness to erosion, weed and pest vital-ity) together with some human features (availabilityof labor, limited access to chemical inputs and com-mon land tenure), account for the local choices among

0167-8809/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-8809(01)00218-3

216 R.J. Manlay et al. / Agriculture, Ecosystems and Environment 88 (2002) 215–232

Fig. 1. Simplified ring organization of the mixed-farming system in a village in the West African savanna belt.

diversified, labor-saving, and extensive schemes ofcrop and animal production (Jones and Wild, 1975;Ker, 1995). Practices that sustained soil fertility in tra-ditional farming systems were mainly organic. In mostof the subregion, this was, and still is, the aim of fal-lowing and manuring. These techniques have provedto be suitable in sparsely populated areas, and haveled to the widespread practice of ring management inAfrican villages (Ruthenberg, 1971).

Three main rings can be distinguished (Fig. 1):

1. The savanna or forest ring: It has never beencultivated, or at least not in recent decades. Be-cause this is the farthest ring from the village,integration in the farming system remains slight,although it may provide wood and necessary pas-ture during the wet season. Unlike the other rings,this area has not usually been appropriated by thevillage.

2. The bush ring: It consists of a mosaic of bushfields—mainly cash crops—and young or old fal-lows, all appropriated by an individual or by thecommunity. Fertility is mainly sustained by fallow-ing. Fallow may also occur due to a shortage oflabor or of crop seeds.

3. The compound ring: It is devoted to food crops.Intensified continuous cultivation is enabled bymanuring (mostly during night corralling) andhousehold waste inputs throughout the dry season.

There are two main reasons for the growing needto assess carbon (C), nitrogen (N) and phospho-rus (P) storage in plant and soil of the savannaagro-ecosystems of West Africa related to soil char-acteristics and land management.

1.1. Sustainable management of local farmingsystems: the point of view of the peasant

For the last few decades, rural population dynamicsand tenurial and technical factors have encouraged ashift in traditional practices: continuous cultivationhas spread at the expense of the savanna ring whichhas almost vanished, and of the bush ring, which hasshrunk dramatically because of the growing need forland. Although this trend will not be reversed in thenear future, it is widely accepted that multi-purpose,improved fallows will continue to play a role insavanna farming systems, while in the compound ring,intensification will have to be maintained by better

R.J. Manlay et al. / Agriculture, Ecosystems and Environment 88 (2002) 215–232 217

crop–livestock integration and agroforestry practices(Ker, 1995).

However, a better understanding of agro-ecologicalpatterns of fertility conservation in existing semi-permanent and continuous cropping systems in thesavanna is a prerequisite for the safe adoption ofany technical innovation. This understanding mainlyrelies on the study of the organically mediated cyclesof C, N, and P at the plot scale as well as at the vil-lage farming scale. Indeed, in self-sufficient farmingsystems, plant biomass supplies peasants with food,wood, and forage (Floret et al., 1993), while closelyrelated below-ground organic matter pools influencethe chemical, physical and biological components ofsoil fertility (Jones and Wild, 1975). Nitrogen andP must also be included in such a study, becausethese elements are indicators of plant biomass qualityand of the most limiting chemical factors to plantproductivity in African savannas.

1.2. Controlling the global carbon balance: thepoint of view of society as a whole

Changes in land use in the tropics would be respon-sible for 29% of the total release of anthropogenic C(Fearnside, 2000), so there is an urgent need to ap-praise its impact on global change. In Senegal, morethan 40% of carbon dioxide emissions stems fromagriculture, land use changes and forests (Sokona,1995). However, C budget assessment in dry Africaremains rare compared to other inter-tropical zones(Desanker et al., 1995; Tiessen et al., 1998). Improv-ing West African demographic and economic contextshas of course priority over global change concerns;but C sequestration in African smallholder agricul-ture may also be a key to sustainable intensificationand recovery of farmers’ wealth (Woomer et al.,1998).

This work, which was conducted at the plot scale,is the first in a three-part study to assess current (de-scriptive approach) and future (modeling approach)amounts and flows of C and related nutrients in amixed farming system at the scale of a village terri-tory in southern Senegal (Manlay, 2000). An attempthas been made to address both agronomic and en-vironmental issues that have arisen previously. Thegeneral aims of the papers presented here are thustwo-fold:

1. Explanatory, i.e. understanding the ecological func-tioning of local agro-ecosystems as inferred fromthe study of the distribution of C and nutrients inplants and in soil, and defining sustainable rules forthe management of fallow or for its substitution.

2. Descriptive, i.e. to permit spatial integration and thetransfer to farm and village budgets by (a) yield-ing quantified relationships between land use, in-trinsic soil properties and C, N and P storage in allsoil–plant components and (b) providing parame-terized models linking C and nutrient amounts forboth easily measurable and barely measurable plantcomponents.

The first part of the study deals with C, N and P al-location in plant biomass during the crop–fallow cyclethat usually occurs in the bush ring. Soil allocation isanalyzed in Manlay et al. (2001a). Storage of C, Nand P in plant biomass and soil under continuous cul-tivation is examined in Manlay et al. (2001b).

This article (a) provides parameterized models forrecovery patterns of dry matter (DM), C, N and Pallocation in plants during fallow compared to initialcrops, and (b) evaluates the fate of above-ground plantbiomass and its related nutrients after conversion offallow to cropping.

2. Methods

2.1. Site characteristics

The study was conducted between 1993 and 1997in High Casamance, in southern Senegal. Crop andfallow plots were investigated in the village ofSare Yorobana (12◦49′N–14◦53′W). The climate isSudanian, tropical subhumid (Kowal and Kassam,1978). During the last 20 years, annual rainfall rangedbetween 570 and 1320 mm (mean 960 mm) and oc-curred from May to October (Fig. 2); temperatureaveraged 28◦C (Service de la Météorologie Na-tionale, Kolda station). Between 1977 and 1987, meanannual potential evapotranspiration was 1570 mm(Dacosta, 1989).

Ring-like, compound-centered management of thevillage distinguishes three main land use systems orunits along the typical, smooth toposequence (Fig. 3):

1. The up-slope plateau: This is still covered with vastareas of “woodlands with a well-developed tree


Fig. 2. Monthly patterns of rainfall, potential evapotranspiration and temperature at Kolda station, 1978–1997. Vertical bars stand for S.E.PET: potential evapotranspiration.

stratum and shrubby undergrowth” (de Wolf, 1998),secondary savanna and old fallows. ResproutingCombretaceae are the major component of woodyvegetation. Four woody species dominate the veg-etation of the study site, accounting for 70% ofwoody above-ground biomass (AGB):Combretumgeitonophyllum Diels, Combretum glutinosumPerr.,Piliostigma thonningii (Sch.) Miln.-Redh andTerminalia macroptera G. and Perr. Fallow herba-ceous plants are mostly annual, and are dominated

Fig. 3. Land use along a typical toposequence at Sare Yorobana, southern Senegal. Coding of sampled plots: FA, fallow; GN, groundnut.Asterisk (∗) denotes used in Manlay et al. (2001b): MI, millet; MA, maize; RI, rice.

byAndropogon pseudapricus Stapf andPennisetumpedicellatum Trin. Bush fields at the edge of theplateau are devoted to groundnut cropping (Arachishypogaea L.), a local cultivar of the Virginia type,usually in rotation with pearl millet (Pennisetumglaucum L.) if manured, but mostly with a shortfallow rotation. The soil is sandy, ferruginous(Baldensperger et al., 1967), and classified as fer-ric Lixisols (FAO, 1998). This unit encompassesthe forest/savanna and most of the bush ring.


2. The mid slope glacis: This is mainly coveredwith food crops such as late pearl millet, maize(Zea mays L.) and sorghum (Sorghum bicolor L.Moench), but also including some of the bushring. Permanent crops are usually manured, mostlyduring night tethering, at rates that depend on thesize of the owner’s herd. Plots adjoining the com-pounds also receive household wastes. Glacis soilsare haplic Lixisols.

3. The lowland: It is dedicated to rice (Oryza sativaL.) and palm plantations. Annual flooding and soiltexture explain the good chemical status of the soilsof this unit. Soils are haplic Gleysol (FAO, 1998).

Until now the village of Sare Yorobana has enjoyedrather good land availability and can thus be usedas a reference or starting point for comparison withmore densely populated study sites. Sedentary Peulhherdsmen have adopted diversified agriculture (rain-fed and flooded cereals, groundnut and cotton cashcrops) closely associated with extensive livestock rais-ing. In the bush ring, the rotation of groundnut, fallowand sometimes millet is only partly predictable, sinceit is influenced by strategies for fertility conservationand by regular labor or seed shortages, as well asgeographical constraints (distance to the compound).According to the traditional cropping pattern in thisring, plant biomass produced during fallowing is eitherexported as wood and forage use, or burnt during un-controlled fires, resulting in the massive disappearanceof plant biomass, with only the rooting systems surviv-ing (Floret et al., 1993). Traditional hand clearing maybe labor-intensive, but it allows resprouting stumps tosurvive several years before they start to disappear (asa result of repeated clearing, yearly shallow hoeingand sometimes ploughing). A stump is defined here asthe woody below-ground base of a tree that is left inthe ground after felling, and excludes the roots. Stumpweight and density range from 2 to 30 kg per individualand from 1500 to 2500 individuals ha−1 (Diao, 1995).The clearing of old fallows or savanna is usually fol-lowed by 2–4 years of continuous cropping consistingof a biennial rotation of groundnut with cereals (mil-let, sorghum). After rapid loss of initial fertility (4–5years), continuous cultivation can only be maintainedfor a few more years if sufficient manuring is carriedout prior to cereal cropping. However, biennial rota-tion of groundnut with fallow (“short- to medium-term

fallow” according to Ruthenberg (1971)) is usually in-troduced rapidly, if not straight after clearing and ismaintained as long as soil fertility and low weed com-petition allow good yields. The field is then returnedto fallow for one to several years, the length of fallowdepending on soil properties, labor availability or landtenure strategies (according to Senegalese tenure lawthe “right of the axe” can be claimed for a period ofnot more than 3 years, but for up to 15 years accordingto customary law). Generally, the farthest fallow plots(“permanent fallows”) located on the plateau were for-merly cropped when the village lay upslope, beforethe inhabitants moved down the glacis some 25 yearsago following several years of drought.

Crop–livestock integration is achieved thanks toconsumption of crop residues and the herbaceousbiomass of fallows by cattle, and by manuring dur-ing day straying and night tethering throughout thedry season (Fig. 3) (Ickowicz et al., 1997). Variableamounts of manure (0–10 t DM ha−1 per year) areapplied, mostly to cereal crops, depending on theavailability of cattle, the distance to the compoundand the crop to be grown. Although both savanna andbush rings provide 60% of the biomass consumedby livestock during the dry season, mineral balancerelated to livestock-mediated flows is only slightlynegative (e.g.−4 kg N ha−1 per year in the savannaring) or even positive (+3 kg N ha−1 per year in thebush ring), since it concerns large areas and manuringcompensates for some of the uptake (Manlay, 2000).

2.2. Sampling schemes

A time-saving, synchronic method was adopted:neighboring crop and fallow plots of different ageswere considered as representative of the same plotthroughout the succession, assuming they shared thesame initial soil properties and management history.

Sampling was carried out at the onset of the dry sea-son, very close to peak plant above-ground biomass.

Due to vegetation patterns, three designs (three forfallows, two for crops) were adopted, depending onland use:

1. Cropped groundnut plots (coded as GN): Six fieldswere chosen up-slope that had borne groundnutin a biennial rotation with fallow—or sometimesmillet—for 4–15 years, and that had never beenchemically fertilized. Four were located on the


plateau and two on the upper glacis (GN04 andGN06). In each of them, four 4 m× 4 m subplotswere randomly defined (Design 1). In each square,vegetation was cut for plant biomass assessment,woody (stump regrowth) and herbaceous (weedand groundnut) layers being separated. Litterbiomass was nearly negligible, and thus not sam-pled. Fine roots were cored at each subplot corner.Coarse root biomass was estimated with a re-gression relationship established in fallows. Thisrelation proved to be satisfactory to link coarseroot biomass as measured by full excavation “rex”to that measured by the coring technique “rco”:rex = 1.73rco + 2.95 (in t ha−1); R2 = 0.6;P(F obs > F th) < 0.05; n = 9. Three out of thesix groundnut fields (GN02, GN03 and GN04)investigated in 1996 were re-sampled in 1997 as1-year-old fallows; stump biomass in these threecropped plots was assumed (and thus probablyslightly over-evaluated) to be that measured in thesame (uncropped) plots in 1997,

2. Fallow plots (FA), aged 1–26 years (age distri-bution is shown in Table 4): Woody AGB wasestimated in 17 plots (three 900 m2 square-likesubplots for each; Design 2). Woody AGB (trunk—any branch with a diameter above 4 cm, twig andleaf) and stump biomass (in fallows more than 4years of age) were estimated using a regression re-lationship derived from Kaıré (1999) based on thetree diameter at breast height, i.e. 1.3 m (Table 1).Since estimating the biomass of other plant com-ponents was very destructive and labor-intensive,it could not be performed using Design 2. Thesevariables were thus sampled at 1 m intervals in a0.5 m2 parallelepiped, along a 20 m long and 0.5 mwide transect (Design 3), in 11 plots close to theprevious ones and located on the plateau (exceptplot FA01a, which was located on the upper glacis).The herbaceous layer was harvested and comprisedliving and dead (but standing) plant biomass. Lit-ter was chosen as the A0 organic layer (> 2 mm).Fine roots were sampled with a core auger (=5.6 cm) in 10 cm increments. Coarse roots wererecovered by on-field manual separation from soilexcavated down to a depth of 40 cm. In fallowsaged 4 years or less, stumps were pulled out to-gether with their coarse roots. As mentioned abovein Section 2.1, it was hypothesized that reliable

estimates of stump biomass could be based on treediameter at breast height (in this case following theDesign 2 sampling scheme) in fallows more than 4years of age, while those in younger fallows couldonly be based on direct measurement (Design 3).

Below-ground plant biomass (BGB) was sampleddown to a depth of 40 cm in all plots. The choice ofthis limit was based on local observations (Diao, 1995)and other published works (Tomlinson et al., 1998).They indicate very low root activity below a depth of40 cm in ferruginous soils in West Africa.

2.3. Plant analyses

Stumps and coarse roots were hand washed andsorted according to their diameter (2–5, 5–10 and>10 mm). Fine roots were recovered from soil afterhydro-pneumatic elutriation on a 1 mm sieve (Webb,1995). All plant samples were oven-dried at 70◦Cto constant weight for dry matter content determina-tion. Samples were pooled and one analysis of eachcomponent was made for each plot. Carbon contentwas determined after dichromate oxidation, N withthe Kjeldahl method, P on ashes. All methods arefully described in Page et al. (1989). Carbon, N andP storage values were obtained by directly combiningDM stock values and the chemical content of plantbiomass (Tables 1–3; see Kaıré (1999) for the detailedcalculation of C, N and P amounts in woody biomass).

2.4. Modeling of plant biomass dynamics during thefallow period

In subhumid savannas, plants compete first fornutrients and light, and for water to a lesser extent(Scholes and Archer, 1997). The aim of this sec-tion was to check which biomass components couldbe fitted to the logistic model, a simple model thatsatisfactorily accounts for population dynamics withlimited resource availability (Pavé, 1994). Its basicmathematical expression is

S(t) = K

1 + ((K − S0)/S0)e−rt(1)

where S0 and K are the amounts of the element inthe plant component, respectively, att = 0 and whent → +∞.


Table 1Regression coefficients used for the estimation of woody biomass for the four main species found in Sare Yorobanaa

Species Component a b R2

Combretum geitonophyllum Above-ground biomass 0.283 2.17 0.97Trunk 0.103 2.44 0.97Twig 0.0947 2.15 0.96Leaf 0.0682 1.74 0.94Stump 0.205 1.63 0.89

Combretum glutinosum Above-ground biomass 0.149 2.33 0.98Trunk 0.0965 2.43 0.96Twig 0.0782 2.16 0.91Leaf 0.0676 1.99 0.94Stump 0.175 1.78 0.80

Piliostigma thonningii Above-ground biomass 0.157 2.27 0.97Trunk 0.0898 2.29 0.94Twig 0.0785 1.99 0.96Leaf 0.0554 1.79 0.89Stump 0.149 1.66 0.89

Terminalia macroptera Above-ground biomass 0.0979 2.40 0.96Trunk 0.0966 2.52 0.97Twig 0.0626 2.38 0.97Leaf 0.035 2.02 0.92Stump 0.155 1.69 0.92

Others Above-ground biomass 0.172 2.29Trunk 0.0965 2.42Twig 0.0785 2.17Leaf 0.0566 1.89Stump 0.1710 1.69

a Model: biomass (in kg DM)= aDb, whereD is the diameter (cm) at breast height. Number of replicates—above-ground biomass:n= 40; trunk, twig, leaf and stump:n = 15. Other species: parameters estimated as the mean of the parameters of the four main species.Derived from Kaıre (1999).

The form (1) of the model is best fitted to dy-namics relying on seedling recruitment. In fallows ofSare Yorobana, low cropping intensity saves manystumps from which trees can coppice. Consequently,modeling woody biomass recovery after crop aban-donment required an additional variable “b” thatenabled the model to account for a possible maxi-mum growth rate occurring within the first years offallow. The generalized form of the logistic modelwas thus

S(t) = K

1 + ((K − S0)/S0)e−rt+ b (2)

including values measured in crop plots for model pa-rameterization depending on the biological continuityor heritability of each compartment between crops andfallows (see Table 5 and Fig. 4a and b).

All statistical analyses were done using SAS soft-ware, Ver. 6.14 (Hatcher and Stepanski, 1994). NLINand REG procedures were used for the estimationof non-linear regression parameters. Model adequacywas estimated testingR2, slope and intercept of theregression of modeled versus observed data (Pavé,1994).

2.5. Study of the fate of post-fallowabove-ground biomass

The fate of woody AGB (burnt on site or broughtback to the compound) at clearing depends on branchdiameter. Inquiries among farmers indicated that inSare Yorobana 4 cm was the threshold beyond whichtree biomass was exported to fulfill wood need, whilesmaller twigs and leaves (available biomass for


Table 2Carbon, nitrogen and phosphorus content (g per 100 g DM) ofplant components of the four main woody speciesa

Species Component C N P

Combretum geitonophyllum Trunk/stump 38.8 0.45 0.04Twig 35.8 0.29 0.04Leaf 35.8 1.48 0.09

Combretum glutinosum Trunk/stump 37.6 0.16 0.03Twig 38.5 0.32 0.02Leaf 39.7 1.18 0.05

Piliostigma thoningii Trunk/stump 37.3 0.23 0.02Twig 37.0 0.37 0.03Leaf 37.5 1.55 0.08

Terminalia macroptera Trunk/stump 36.5 0.16 0.02Twig 37.6 0.18 0.02Leaf 36.1 1.15 0.07

a Values for “other species” were estimated as the mean con-tents measured for the four main species. Derived from Kaıre(1999).

burning or ABB) were burnt on site. Combustionefficiency was not measured, and depends on localpractices. Taking into account the work of Stromgaard(1985), two scenarios were considered, depending onthe burning efficiency (50 and 90%) of remainingplant biomass. They were applied to two hypotheticalfallow plots: a young (aged less than 10 years) andan old fallow plots (aged 10 years or more). Theirwoody and herbaceous AGB were computed as the

Table 3Carbon, nitrogen and phosphorus content (g per 100 g DM) of above-ground (non-woody) and below-ground plant components of croppedand fallow fieldsa

Land use Component C N P

Groundnut crop Herbaceous layer 37.5± 0.4 1.93± 0.02 0.11± 0.00Fine root 34.1± 0.5 1.66± 0.03 0.07± 0.01Coarse root 38.0b 0.35b 0.02b

Stump 38.0b 0.35b 0.02b

Fallow Herbaceous layer 34.4± 0.9 0.72± 0.05 0.07± 0.01Litter 33.1 ± 0.5 0.51± 0.03 0.03± 0.00Fine root 34.3± 0.4 0.78± 0.05 0.04± 0.00Coarse root 36.6± 0.4 0.40± 0.03 0.03± 0.00Stump 37.0± 0.8c 0.41 ± 0.06c 0.02 ± 0.01c

a Standard error (groundnut plots:n = 6; fallow plots: n = 11). Fine roots: diameter ranging from 0 to 2 mm. Coarse roots: diameterabove 2 mm (stump not included).

b Estimated as the mean content of coarse root in FA1a, FA1b and FA1c.c Estimated as the content of coarse root in fallows aged 1–4 years; used for the calculation of C, N and P stock values in fallows

aged 1–4 years only.

mean value for the young and old fallow plot pools(Table 4).

In what follows, all biomass data are expressed indry matter units.

3. Results

3.1. Plant C, N and P content

Carbon contents were very stable among treespecies and other plant biomass components (Tables 2and 3). Nitrogen and P contents were highest in treeleaves, the herbaceous layer and fine roots of cropvegetation.

3.2. Trends in the whole plant system

The plant biomass of the whole ecosystem averaged49 t ha−1 in the crop and fallow plots investigated. Thiscorresponded to 18 t C, 250 kg N and 21 kg P ha−1, ofwhich more than 40–50% were stored in woody AGB(Table 4 and Fig. 4a). Stumps were the second contrib-utors to the storage of C and nutrients in the ecosys-tem. Dry matter storage in woody AGB was morethan three times higher in old fallows (43 t ha−1) thanin 1-year-old stands (14 t ha−1); C, N and P amountsshowed similar temporal patterns. Stump biomass dou-bled, reaching 15 t ha−1 and storing nearly a fourth of


N and P of the whole woody vegetation. Herbaceousbiomass (3.0 t ha−1 or 1.1 t C ha−1 in crops) doubledin youngest fallows, but was lowest in the two old-est fallows. Organic and nutrient amounts in fine and

Fig. 4. Dry matter, carbon, nitrogen and phosphorus storage in plant biomass during a crop–fallow succession, and fitting to amodified, logistic-like model: (a) above-ground components; (b) below-ground components. Mathematical expression of the model:S(t) = K/(1+((K−S0)/S0) e−rt)+b. Plain dots: data used for model adjustment. All below-ground values calculated for the 0–40 cm layer.Fine roots: diameter ranging from 0 to 2 mm. Coarse roots: diameter above 2 mm (stump not included). Solid line:P(F obs > F th) < 0.05(see Table 5). Dotted line:P(F obs > F th) < 0.05 andP(H0: intercept= 0) < 0.05 andP(H0: slope= 1) < 0.05 (see Table 5).

coarse roots increased four- (N) to six-fold (P) betweencrops and oldest fallows. Fine roots represented 14%of root biomass (not including stumps). Their distribu-tion among soil layers did not differ much through the


Fig. 4. (Continued ).

chronosequence; half were stored in the upper 10 cm.The shoot:root ratio averaged 0.22 in crops. It washigher in fallows aged less than 10 years than in theolder plots of the succession.

3.3. Adjustment of temporal dynamics of C, N and Pin plant biomass to the logistic model during fallow

Best fits to the model were found for data of DMand C in woody and herbaceous AGB (Table 5 andFig. 4a). A threshold was observed from 10 yearsof fallow on, irrespective of the variable and thecomponent, except for stumps, whose initial biomasswas already substantial. However, different patternswere noted below this threshold, depending on thevariable and the component. Models adjusted to dataof AGB components showed no point of inflectionsince maximum growth occurred in the very firstyear of fallow. By contrast, curves for coarse root

DM and C, and fine root N and P were S-shaped(Fig. 4b).

Thus, after considerable shift within the first 10years of fallowing during which AGB reacted fasterthan BGB, biomass and nutrient amounts for nearlyall plant components converged towards asymptoticvalues.

Taking into account these results, data were clus-tered in three groups: groundnut fields (GN); youngfallows (YF) aged less than 10 years; old fallows(OF) aged 10 years or more. Mean amounts for plantbiomass and computed net productivity (annual vari-ation in stocks) are summarized in Table 6. Fastestaccumulation of elements generally occurred duringthe first year of fallow. This was not the case forbelow-ground N and P; the fastest increases for theseelements occurred between 1 and 10 years of fallow.Dry matter, C, N and P accumulation was 10–20 timesslower beyond 10 years of fallow.


Table 4Dry matter storage (t per year) in plant components under crop–fallow successiona

Plot AGBb Litter Root ABBc

Woody Herbaceous Fine (per sampling depth in cm) Coarse Stump

Trunk Twig Leaf Total 0–10 10–20 20–30 30–40

GN01 0.13 2.37 0.19 0.15 0.11 0.07 5.2GN02 0.02 3.41 0.20 0.17 0.10 0.07 3.0 13.8GN03 0.29 2.34 0.25 0.16 0.12 0.08 3.1 5.0GN04 1.20 3.45 0.33 0.18 0.11 0.07 3.0 3.5GN05 0.24 2.76 0.21 0.09 0.08 0.05 2.9GN06 0.03 3.42 0.29 0.18 0.10 0.07 3.0FA1a 6.51 1.84 0.78 0.28 0.20 0.14 5.4 3.5FA1b 5.51 2.10 1.17 0.16 0.25 0.26 8.6 13.8FA1c 4.99 1.53 0.81 0.22 0.19 3.4 5.0FA1d 2.2 1.3 0.8 4.3 4.3FA1e 8.0 5.3 3.6 16.9 1.69FA1f 8.4 6.3 5.0 19.8 1.98FA2a 6.01 0.57 0.72 0.27 0.30 0.20 5.9 11.7FA2b 11.5 7.5 5.1 24.1 20.8FA3a 5.9 3.2 1.4 10.5 8.0FA3b 14.7 9.1 5.8 29.6 23.0FA3c 12.4 8.2 4.5 25.1 19.8FA4 9.45 2.77 0.90 0.28 0.21 0.16 4.7 4.7FA6a 15.4 9.0 6.2 30.6 11.5 19.2FA6b 15.8 9.0 4.4 29.2 15.6 1.87FA7a 3.32 2.97 1.11 0.57 0.44 0.34 13.8FA7b 16.2 8.5 3.3 28.0 11.1 17.3FA10a 19.8 11.4 6.7 37.8 11.8 21.2FA10b 21.6 11.4 6.0 39.0 17.7 22.1FA12 1.93 3.86 1.19 14.3FA13a 0.96 1.19 1.82 1.19 0.74 0.68 19.6FA13b 21.5 10.5 3.9 35.9 12.5 1805FA15a 24.0 13.3 7.7 45.0 12.0 22.5FA15b 22.3 11.8 6.2 40.4 18.6 20.5FA17 2.15 2.65 1.32 15.8FA18a 1.81 1.75 1.22 0.74 0.62 0.53 14.1FA18b 27.0 11.5 5.8 44.3 15.4 21.0FA25 22.5 12.1 6.4 41.0 13.8 17.6FA26 1.43 1.73 1.21 0.80 0.69 0.82 19.9

a GN, groundnut crop; FA, fallow; the associated number stands for the age of fallow (in years). Fine roots: diameter ranging from 0to 2 mm. Coarse roots: diameter above 2 mm (stump not included).

b Living above-ground biomass.c Available biomass for burning.

3.4. Post-fallow dynamics of above-ground plantbiomass

Proposed estimates do not take into account pos-sible translocation that occurs during the dry season.Thus, N and P returns from AGB to the soil might beoverestimated.

3.4.1. Herbaceous above-ground biomassEstimates of potential returns of C, N and P

from herbaceous biomass to the soil were 2.1 t C,41 kg N and 3.9 kg P ha−1 derived from a young fal-low; 0.5 t C, 13 kg N and 1.3 kg P ha−1 from an oldfallow. However, these are upper estimates, sincereturns of C and N to the soil at clearing rely


Table 6Simplified budget and annual increase in dry matter, carbon, nitrogen and phosphorus in above- and below-ground plant biomass during acrop–fallow successiona

Total storage Annual variation (according to years of fallow)c

Groundnutfield

Young fallow Old fallow 0–1 1–10 10–25

DM (t ha−1) AGBa 3.3 29.7 44.4 17.9 2.1 0.0BGBb 11.4 18.3 34.8 3.4 1.6 0.2

C (t ha−1) AGBa 1.2 11.0 16.4 6.6 0.8 0.0BGBb 4.3 6.7 12.6 1.3 0.6 0.0

N (kg ha−1) AGBa 59 151 186 59.2 9.0 −0.7BGBb 47 80 147 4.0 6.9 0.5

P (kg ha−1) AGBa 3.3 15.0 22.1 7.8 1.0 0.0BGBb 2.6 4.6 11.6 0.3 0.7 0.1

a Above-ground biomass.b Below-ground biomass.c Annual increases were estimated by using mean values calculated from the adjusted logistic-like model when available (see Section

3.2), from a simple linear regression otherwise. See Table 4 for the size of the population sampled.

Fig. 5. Fate of dry matter, carbon, nitrogen and phosphorus in above-ground woody biomass after clearing of a young (YF) and old (OF)fallow. Two scenarios are considered: 50 and 90% of available plant biomass for burning is burnt. Assumptions: ashes are DM-, C- andN-free; all P, except that exported for wood requirements, is returned to the soil both as ash or non-burnt biomass. Young fallow: agedless than 10 years; old fallow: 10 years and more.


heavily on the occurrence of fire during the dryseason.

3.4.2. Woody above-ground biomassDry matter returns from a young fallow were es-

timated to be 8.4 t ha−1 when combustion efficiencywas set at 50% of ABB, or 1.7 t ha−1 under the 90%hypothesis. Values computed for old fallows were 18.4and 10.2 t DM ha−1. This would mean an on-site recy-cling of 0.6–4.0 t C and 9–59 kg N ha−1 depending onburning efficiency and the duration of fallow. Phospho-rus returns to the soil would amount to 6.9–7.0 kg ha−1

(YF and OF). Wood harvest during clearing of an oldfallow would result in the export of 50% of DM andC, 27% of N and 66% of P contained in initial woodyAGB (Fig. 5).

Finally, potential returns of above-ground C and nu-trients stored in woody and herbaceous AGB to soilafter conversion to cropping would not differ muchbetween young, biomass-accumulating fallows, andolder stands close to equilibrium, and could not ex-ceed 5 t C, 90 kg N and 10 kg P ha−1.

4. Discussion

Plant biomass components showed contrasteddynamics during the crop and fallow cycle. Aftercrop abandonment, woody and root componentsincreased at the expense of the herbaceous layer,while litter showed no clear pattern of evolu-tion. However, amounts of carbon storage in plantbiomass, which reached at best 29 t ha−1, werelower than those generally reported for savannasand dry tropical forests, respectively, 50 and118 t C ha−1 (Allen-Diaz et al., 1996; Tiessenet al., 1998). Annual increments of carbon in SareYorobana fallows (7.9 t C ha−1) compared wellwith those in savannas and dry forests (5 t C ha−1;Tiessen et al., 1998) during the first year of fallowonly.

In local fallows disturbed mostly by fire, the presentstudy demonstrates the existence of an ecologicalthreshold around 10 years after crop abandonment.A similar value had already been recorded in similarecozones (Stromgaard, 1986; Donfack et al., 1995);however, it may vary considerably, depending onclimate and cropping history.

Swift and Anderson (1994) distinguish the produc-tive biota—the community of plants and animals thatprovide peasants with valuable goods such as food orconstruction materials—and the resource biota—thebody of “organisms which contribute positively to theproductivity of the system”. Fallowing also providesboth production and resource services.

4.1. Fallow as a productive ecosystem

Wood supplies 90% of the needs of rural popula-tions in the WAS for domestic energy and construc-tion material (Breman and Kessler, 1995). However,amounts of standing wood and of the productivityof fallows and woodlands in the subregion remainrare and are difficult to estimate because of uncon-trolled fire, grazing and tree felling. Estimates forwoody biomass can thus range from 20 to 60 t ha−1 insub-Saharan savanna woodlands (Breman and Kessler,1995). The highest values of woody biomass foundfor fallows (45 t ha−1) were consistent with thesevalues, but were nevertheless much lower than thoseusually reported for dry tropical forests (90 t ha−1)(Martinez-Yrizar, 1995). Similarly, wood productivity1 to 10 years after crop abandonment (2 t ha−1) waslower than that of West African secondary successions(2.5–4.7 t ha−1 per year) (Alexandre and Kaıré, 2000).For oldest fallows, wood productivity (0.1 t ha−1 peryear) equaled at the most one-tenth of that estimatedfor dry mature forests of the region (1.2 t ha−1) (FAO,1989).

Low woody biomass at equilibrium and net pro-ductivity of Sare Yorobana fallows are likely toexpress soil constraints rather than climate condi-tions. Shallow (30–40 cm deep) clay accumulationin plateau and glacis soils generates seasonal hydro-morphy and a physical barrier (Baldensperger et al.,1967), which, together with limited availability ofnutrients, seriously restricts root development andtotal plant biomass accumulation (Jones and Wild,1975; Manlay et al., 2001a). Tree felling by farmers,the total amount of which was estimated at 2 t ha−1

since crop abandonment in local fallow fields (Kaıré,1999), has only a small impact on woody productiv-ity. Peasants economize on labor by harvesting deadwood rather than felling living trees. Dead wood pro-duction in Sare Yorobana fallows may be estimated at0.4–0.7 t ha−1 in young and old fallows (Shackleton,


1998). In fact, massive removal of fresh wood onlyoccurs before conversion to cropping.

The herbaceous vegetation of subhumid savannascan control woody plants for a short period dur-ing seedling recruitment only (Scholes and Archer,1997). Since the rapid decrease in herbaceous stand-ing biomass as early as 5 years after crop abandon-ment is concomitant with woody layer closure inSare Yorobana, the present work confirms that treescontrol the herbaceous layer through competition fornutrients and light in this region (Akpo, 1998). Irre-spective of the quality of forage provided by sometree foliage, pastoral usage of fallow thus usually par-tially conflicts with the need for woody products. Theherbaceous layer of local fallow fields is an essentialsource of fodder for cattle, contributing up to 75% ofthe livestock diet (Delacharlerie, unpublished data).In fact, fallows of intermediate age that ensure bothforage quality and quantity may have the best pastoralvalue among all vegetation facies of uncropped plotsin Sare Yorobana (Ickowicz et al., 1997).

4.2. Role of plant biomass in sustainingagro-ecosystem fertility

4.2.1. Cycling of nitrogen and phosphorus in plantbiomass

From an agro-ecological point of view, fallow-ing has been described as an intricate process oforganic matter and nutrient accumulation (Table 6)(Juo and Manu, 1996). The removal of wood andgrass during the fallow period—for energy and foragerequirements—remains negligible in Sare Yorobana(Masse, unpublished data), but dramatic minerallosses from the ecosystem occur at clearing throughfelling or burning of woody and grass layers. Suchpractices are responsible for the loss of 40–90% of Ncontained in above-ground plant biomass (dependingon the efficiency of combustion and the fate of theherbaceous layer), and of 20–60% of P (dependingon the age of fallow). In the bush ring, the impact ofsuch N losses on crop yield might be of little signifi-cance, since part of these losses may be alleviated bygroundnut-induced, N-fixation processes. However,this would not be true for phosphorus. Slash and burnpractices can thus be justified only in the context oflow labor availability (Nye and Greenland, 1960),or when shifting to a whole farming system scale.

In Sare Yorobana for instance, ashes from fuelwoodwould provide 3 kg P ha−1 per year to plots in thecompound ring (Manlay, 2000).

Though often overlooked, the root component infact accounts for more than a third of total N and Pstorage in plant biomass. Considering P immobiliza-tion in harvested groundnut biomass (2.8 kg P ha−1),the clearing of a 1-year-old fallow could provide al-most enough P for 3 years of cropping, thanks toP returned to the soil both from cleared AGB anddecaying root biomass (7.4 kg P ha−1). As a matterof fact, groundnut is never cropped for more than 1year in High Casamance, which indicates possible (1)pre-eminence of pest and weed problems, (2) asyn-chrony between nutrient release and plant needs in thefirst years of cropping, due to massive mineralizationof plant biomass and soil organic matter (Myers et al.,1994) and (3) fast conversion of P back to less avail-able forms in the soil. Internal recycling (transloca-tion) in trees was not taken into account in the presentstudy, which might have resulted in an overestimationof nutrient accumulation in woody biomass at the endof the dry season (Breman and Kessler, 1995).

4.2.2. Carbon dynamicsOn its own, mineral fertilization is not sustain-

able in the tropics, especially in West Africa (Pieri,1989). Associated organic amendments are needed.Among the different organic components, roots, and,to a lesser extent, litter, play key ecological roles,since they provide most of carbon inputs to the soil.There is, however, a paucity of quantitative root dy-namics studies in tropical savannas, especially inWest Africa. The highly variable estimates found forfine root biomass (1.4–4.4 t ha−1) in Sare Yorobanawere similar to findings in other studies under similarclimates (2–5.3 t ha−1; Charreau and Nicou, 1971;César and Coulibaly, 1993). Unlike AGB, the totalroot biomass measured in young and old fallows(30–44 t ha−1) was higher than is usually reported fordry to subhumid tropical successions (3–27 t ha−1;Menaut and César, 1979; Martinez-Yrizar, 1995).As a result, relative DM allocation to plant AGBcompared to BGB (shoot:root value) found in SareYorobana was 10 times lower than that of tropical dryforests (Martinez-Yrizar, 1995). Regular occurrenceof fire and low nutrient availability favor strategiesinvesting in BGB, and might both account for such a


discrepancy. On the other hand, physiognomic climax(which would correspond to high shoot:root ratio)might not have been reached after 25 years of fal-low succession; though this is doubtful, since sacredwoodlands close-by exhibited physiognomy similarto oldest fallows (unpublished observations).

A time-lag in the maximum growth rate betweenabove- and below-ground components of the woodylayer (Section 3.2 and Fig. 4) suggests that priorityis given by woody plants to the development of au-totrophic organs over rooting systems at the onsetof the succession. Thanks to the preservation of agood rooting system, more carbon is allocated tobelow-ground biomass only later. This process maybe seen as a conservative strategy for fast immobi-lization of a limiting resource such as P in the woodybiomass (storage function of the trunk and twigs). Agood rooting system in secondary vegetation wouldthus only be built up several years after crop aban-donment. This has implications for the definition ofsustainable rules for fallow management. Such rulesshould aim at preserving the resprouting capacityof woody plants, since this capacity influences plantproductivity recorded shortly after crop abandon-ment, as well as below-ground biological activity andfood-web organization.

In a more dynamic approach, roots might accountfor 40–85% of the net primary production of plantcommunities (Fogel, 1985). Root carbon originatingfrom aerial parts returns to the soil through root exuda-tion and decay. Quantifying these inputs is a difficulttask. Optimistic root turnover estimates under tropicaldry climate range from 0.5 to 1.2 per year (Menautand César, 1979; Lamotte and Bourlière, 1983; Brownet al., 1994). Thus, using a value of 0.5 per year (thefine root biomass of local fallows decreases by halfduring the dry season; Manlay, unpublished data), Cinputs from fine and coarse roots to the soil of youngfallows would reach 1.6–3.6 t C ha−1 per year. This isprobably an underestimation of the actual carbon in-flow, since it does not take into account root exudation.

5. Conclusion

In the agro-ecological context of Sare Yorobana, thedynamics of plant biomass throughout the crop–fallowcycle occur in three main stages: (1) the cropping

period, (2) an initial dynamic period lasting 10 yearsafter crop abandonment during which the fallowecosystem experiences drastic modifications in C andnutrient storage in plant biomass and (3) a subse-quent phase characterized mainly by the stagnationof organic matter storage. The work reported hereunderlines the preeminent role of the tree layer in thecontrol of plant dynamics in the whole ecosystem andyields a simple model of this dynamics. However,further work—involving considerable methodologicaldifficulties—is required to evaluate how such eco-logical dynamics (especially in stumps) is related tothe way local practices are applied at clearing, theintensity of cropping, and the duration of human set-tlements. These preliminary results will enable thedevelopment of a model of man–vegetation dynamicswhere both cropping patterns and wood and forageuptake are taken into account.

From a more pragmatic perspective, and from astrictly “productivist” point of view, appraisal of theoptimal length of fallow in southern Senegal dependson conflicting human needs. It is suggested here thatkeeping fallow stands for longer than 10 years for theproduction of useful plant biomass may only be rel-evant when the population density is low, since thesestands are more productive than young fallows fordead wood only. Based on current demographic dy-namics, it can be argued that the growing needs forwood and forage would be best met with fallows aged6–10 years. Nutrient budgets (N and P) indicate thatthese fallows might also be efficient enough to meetthe needs of cropped plants for a period of severalyears. However, local clearing practices result in se-rious nutrient losses, and weed competition and pesthazards may considerably reduce crop yields.

On the other hand, strictly “nutrient” and “produc-tivist” considerations are fairly restrictive. Thewell-known decline in yields in the overcrowdedgroundnut belt of central Senegal (just 200 kmfrom Sare Yorobana) after several decades of min-ing agriculture, stresses how seldom the shortfallow–groundnut rotation is, in fact, sustainable(Pieri, 1989). When unavoidable, substitutes for longbreaks of fallow must at least ensure the developmentof a good rooting system as a tool for soil fertilityconservation. Consequently, recommendations forintensification of agriculture, or alternatives to pro-longed fallowing should aim at (1) increasing plant


production (both forage and wood) during fallowby harvesting wood biomass every 6–10 years, (2)accelerating plant succession with improved fallows(Peltier and Pity, 1993), (3) mimicking the ecologicalmechanisms of soil fertility conservation that occur infallows under continuous cultivation systems (adop-tion of “dispersed tree” plantation in cropped fieldsor live-hedge systems, and stump-saving, no-till prac-tices) and (4) recycling nutrients through slash andmulch techniques and hay making. However, theseproposals require varying degrees of labor input to thesystem, and should thus be selected with reference tothe local demographic background.

Acknowledgements

The authors acknowledge critical and helpful com-ments by two anonymous reviewers, J. Aronson,C. Feller, A.-M. Izac, C. Millier, J.-C. Remy andD. Richard. This work received financial support fromthe EU Project “Reduction of the Fallow Length,Biodiversity and Sustainable Development in Centraland West Africa” (TS3-CT93-0220, DG12 HSMU)(Floret, 1998), the Inter-institutional Inciting ActionORSTOM-CNRS-CIRAD-INRA “Bio-functioning oftropical soils and sustainable land management”, theFrench Institute for Research and Development (IRD,ex-ORSTOM), the Senegalese Institute for Agricul-tural Research (ISRA) and the French Institute ofForestry, Agricultural and Environmental Engineering(ENGREF).

References

Akpo, L.E., 1998. Effet de l’arbre sur la végétation herbacée dansquelques phytocénoses au Sénégal. Variations selon un gradientclimatique. PhD Thesis, Univ. Cheikh Anta Diop, Dakar.

Alexandre, D.-Y., Kaıré, M., 2000. Les productions des jachèresafricaines à climat soudanien (bois et produits divers). In: Floret,C., Pontanier, R. (Eds.), La Jachère en Afrique Tropicale. Vol.II. De la Jachère Naturelle à la Jachère Améliorée: Le Point desConnaissances, Dakar, Senegal, 13–16/04/1999. John Libbey,Paris, pp. 169–199.

Allen-Diaz, B., Chapin, F.S., Diaz, S., Howden, M.,Puigdefabregas, J., Stafford Smith, M., 1996. Rangelands ina changing climate: impacts, adaptations, and mitigation. In:Watson, R.T., Zinoywera, M.C., Moss, R.H. (Eds.), ClimateChange 1995. Impacts, Adaptations and Mitigation of Climate

Change: Scientific–Technical Analysis. Cambridge UniversityPress, New York, pp. 131–158.

Baldensperger, J., Staimesse, J.P., Tobias, C., 1967. NoticeExplicative de la Carte Pédologique du Sénégal au 1/200000—Moyenne Casamance. ORSTOM, Dakar.

Breman, H., Kessler, J.J., 1995. Woody plants in agro-ecosystemsof semi-arid regions. In: Advanced Series in AgriculturalSciences, Vol. 23. Springer, Berlin.

Brown, S., Anderson, J.M., Woomer, P.L., Swift, M.J., Barrios,E., 1994. Soil biological processes in tropical ecosystems. In:Woomer, P.L., Swift, M.J. (Eds.), The Biological Managementof Tropical Soil Fertility. Wiley, Chichester, pp. 15–46.

César, J., Coulibaly, Z., 1993. Conséquence de l’accroissementdémographique sur la qualité de la jachère dans le Nord de laCôte d’Ivoire. In: Floret, C., Serpantié, G. (Eds.), La Jachère enAfrique de l’Ouest. Atelier International, Montpellier, France,2–5 December 1991. ORSTOM, pp. 415–434.

Charreau, C., Nicou, R., 1971. L’amélioration du profil culturaldans les sols sableux et sablo-argileux de la zone tropicale sècheouest-africaine et ses incidences agronomiques. III. Les facteursbiologiques: faune et végétation et leur influence sur le profilcultural et la productivité agricole. L’Agronomie Tropicale 26,565–631.

Dacosta, H., 1989. Précipitations et écoulements sur le bassin dela Casamance. PhD Thesis, Univ. Cheikh Anta Diop, Dakar.

de Wolf, J., 1998. Species composition and structure of the woodyvegetation of the Middle Casamance region (Senegal). For.Ecol. Manage. 111, 249–264.

Desanker, P.V., Frost, P.G.H., Justice, C.O., Scholes, R.J. (Eds.),1995. The Miombo Network: Framework for a TerrestrialTransect Study of Land-Use and Land-Cover Change in theMiombo Ecosystems of Central Africa, IGBP Report, Vol. 41.The International Geosphere–Biosphere Programme (IGBP),Stockholm.

Diao, O., 1995. Comportement des systèmes racinaires des ligneuxdurant le cycle culture-jachère en Afrique soudanienne. Etudesur un terroir de la région de Kolda, Haute-Casamance, Sénégal.Master Thesis, ENCR, Bambey.

Donfack, P., Floret, C., Pontanier, R., 1995. Secondary successionin abandoned fields of dry tropical northern Cameroon. J. Veg.Sci. 6, 499–508.

FAO, 1989. Studies on the volume and yield of tropical foreststands. Dry forest formations. Forestry Papers, Vol. 51. Foodand Agricultural Organisation, Rome.

FAO, 1998. World reference base for soil resources. World SoilResources Reports, Vol. 84. Food and Agricultural Organisation,Rome.

Fearnside, P.M., 2000. Global warming and tropical land-usechange: greenhouse gas emissions from biomass burning,decomposition and soils in forest conversion, decompositionand soils in forest conversion, shifting cultivation and secondaryvegetation. Clim. Change 46, 115–158.

Floret, C. (Ed.), 1998. Raccourcissement du temps de jachère,biodiversité et développement durable en Afrique Centrale(Cameroun) et en Afrique de l’Ouest (Mali, Sénégal): FinalReport. Comm. des Communautés Européennes. ContratTS3-CT93-0220 (DG 12 HSMU), IRD, Paris.


Floret, C., Pontanier, R., Serpantié, G., 1993. La Jachère en AfriqueTropicale. Man and Biosphere, Vol. 16. UNESCO, Paris.

Fogel, R., 1985. Roots as primary producers in below-groundecosystems. In: Fitter, A.H. (Ed.), Ecological Interactionsin Soil, Plant, Microbes and Animals. Blackwell ScientificPublications, Oxford, pp. 23–36.

Hatcher, L., Stepanski, E.J., 1994. A step-by-step approach tousing the SAS system for univariate and multivariate statistics.SAS Institute Inc., Cary, NC.

Ickowicz, A., Usengumuremyi, J., Bastien, D., de Choudens, N.,1997. Spatial analysis of land use by cattle herds in a village ofthe Sudanese zone in Senegal. Application for grazing systemimprovement. In: XVIIIth International Grassland Congress,Winnipeg-Saskatoon, Canada, 8–17/06/1997, Vol. 1, pp. 29–30.

Jones, M.J., Wild, A., 1975. Soils of the West AfricanSavanna. Technical Communications, Vol. 55. CommonwealthAgricultural Bureaux, Farnham Royal.

Juo, A.S.R., Manu, A., 1996. Chemical dynamics in slash-and-burnagriculture. Agric. Ecosyst. Environ. 58, 49–60.

Kaıré, M., 1999. La production ligneuse des jachères et sonutilisation par l’homme au Sénégal. PhD Thesis, Universitéd’Aix-Marseille I, Marseille.

Ker, A., 1995. Farming Systems in the African Savanna:A Continent in Crisis. International Development ResearchCentre (IDRC), Ottawa.

Kowal, J.M., Kassam, A.H., 1978. Agricultural Ecology ofSavanna: A Study of West Africa. Clarendon Press, Oxford,UK.

Lamotte, M., Bourlière, F., 1983. Energy flow and nutrient cyclingin tropical savannas. In: Bourlière, F. (Ed.), Tropical Savannas.Elsevier, Amsterdam, pp. 583–603.

Manlay, R.J., 2000. Organic matter dynamics in mixed-farmingsystems of the West African savanna: a village casestudy from south Senegal. PhD Thesis, ENGREF, Paris,http://www.engref.fr/ANGLAIS/Accueil.htm.

Manlay, R.J., Masse, D., Chotte, J.-L., Feller, C., Kaıré,M., Fardoux, J., Pontanier, R., 2001a. Carbon, nitrogenand phosphorus allocation in agro-ecosystems of a WestAfrican savanna-II. The soil component under semi-permanentcultivation. Agric. Ecosyst. Environ., in press.

Manlay, R.J., Chotte, J.-L., Masse, D., Laurent, J.-Y., Feller,C., 2001b. Carbon, nitrogen and phosphorus allocation inagro-ecosystems of a West African savanna-III. The plant andsoil components under continuous cultivation. Agric. Ecosyst.Environ., in press.

Martinez-Yrizar, A., 1995. Biomass distribution and primaryproductivity of tropical dry forests. In: Bullock, S.H., Mooney,H.A., Medina, E. (Eds.), Seasonally Dry Tropical Forests.Cambridge University Press, Cambridge, pp. 326–345.

Menaut, J.C., César, J., 1979. Structure and primary productivityof Lamto savannas. Ivory Coast Ecol. 60, 1197–1210.

Myers, R.J.K., Palm, C.A., Cuevas, E., Gunatilleke, I.U.N.,Brossard, M., 1994. The synchronisation of nutrient

mineralisation and plant nutrient demand. In: Woomer, P.L.,Swift, M.J. (Eds.), The Biological Management of Tropical SoilFertility. Wiley-Sayce Publication, Chichester, pp. 81–116.

Nye, P.H., Greenland, D.J., 1960. The Soil Under ShiftingCultivation, Vol. 51. Technical Communications, FarnhamRoyal.

Page, A., Miller, R., Keeney, D. (Eds.), 1989. Methods of SoilAnalysis. Part 2. Chemical and Microbiological Properties, Vol.9. Agronomy, Madison, WI.

Pavé, A., 1994. Modélisation en Biologie et en Ecologie. Aléas,Lyon.

Peltier, R., Pity, B., 1993. De la culture itinérante sur brûlis aujardin agroforestier en passant par les jachères enrichies. Boiset Forêts des Tropiques 235, 49–57.

Pieri, C., 1989. Fertilité des Terres de Savanes. Ministère dela Coopération—CIRAD, Paris (English Transl.: Pieri, C.,1992. Fertility of soils: a future for farming in the WestAfrican Savannah. In: Springer Series in Physical Environment.Springer, Berlin).

Ruthenberg, H., 1971. Farming Systems in the Tropics. ClarendonPress, Oxford.

Scholes, R.J., Archer, S.R., 1997. Tree–grass interactions insavannas. Annu. Rev. Ecol. Syst. 28, 517–544.

Shackleton, C.M., 1998. Annual production of harvestabledeadwood in semi-arid savannas. South Africa For. Ecol.Manage. 112, 139–144.

Sokona, Y., 1995. Greenhouse gas emission inventory for Senegal,1991. Environ. Monit. Assess. 38, 291–299.

Stromgaard, P., 1985. Biomass, growth, and burning of woodlandin a shifting cultivation area of South Central Africa. For. Ecol.Manage. 12, 163–178.

Stromgaard, P., 1986. Early secondary succession on abandonedshifting cultivator’s plots in the Miombo of South CentralAfrica. Biotropica 18, 97–106.

Swift, M.J., Anderson, J.M., 1994. Biodiversity and ecosystemfunction in agricultural systems. In: Schulze, E.D., Mooney,H.A. (Eds.), Biodiversity and Ecosystem Function. Springer,Berlin, pp. 15–41.

Tiessen, H., Feller, C., Sampaio, E.V.S.B., Garin, P., 1998. Carbonsequestration and turnover in semiarid savannas and dry forest.Clim. Change 40, 105–117.

Tomlinson, H., Traore, A., Teklehaimanot, Z., 1998. Aninvestigation of the root distribution ofParkia biglobosa inBurkina Faso, West Africa, using a logarithmic spiral trench.For. Ecol. Manage. 107, 173–182.

Webb, N., 1995. Installation Guide to the Root Washer BasicSystem. Delta-T Devices Ltd., Cambridge.

Woomer, P.L., Palm, C.A., Qureshi, J.N., Kotto-Same, J.,1998. Carbon sequestration and organic resource managementin African smallholder agriculture. In: Lal, R., Kimble,J.M., Follett, R.F., Stewart, B.A. (Eds.), Management ofCarbon Sequestration in Soil. CRC Press, Boca Raton,pp. 153–173.

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Carbon, nitrogen and phosphorus allocation in agro-ecosystems of a West African savanna II. The soil...

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