Rock phosphate-p enhances biomass and nitrogen accumulation by legumes in upland crop production systems in humid West Africa

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1 Biol Fertil Soils (26) 43: DOI 1.17/s SHORT COMMUNICATION Eklou A. Somdo. Knwr L. Shrwt. Ronld F. Kuehne Rock phosphte-p enhnces biomss nd nitrogen ccumultion by legumes in uplnd crop production systems in humid West Afric Received: 12 October 25 / Revised: 2 Februry 26 / Accepted: 6 Februry 26 / Published online: 5 April 26 # Springer-Verlg 26 Abstrct Experiments were conducted during in screen house nd in the field in the humid forest zone of Côte d Ivoire, to evlute the effects of phosphorus (P) from phosphte rock (PR) on the performnce of the root nodulting legume Crotlri micns grown for 8 weeks. The experimentl soils were cid Ultisols with <4 mg/kg extrctble Bry-1 P. Tilemsi PR from Mli nd triple superphosphte (TSP) were pplied t 6 kg P h 1 (screen house) nd 9 kg P h 1 (field) to the legume. Legume N-fixed (BNF) ws estimted by the 15 N-isotope dilution nd δ 15 N nturl bundnce methods, using Cssi obtusifoli L. s non-fixing legume reference plnt. Without P supply, nd under the field conditions, C. micns produced less thn 1 tonne of biomss nd ccumulted 29 kg N/h. The ppliction of PR-P enhnced legume N by bout fourfold over the unfertilised control. There ws no significnt difference between the effects of TSP nd PR. Phosphorus ppliction minly ffected the totl mount of N ccumulted rther thn the percentge derived from the tmosphere (%N df) per se. Furthermore, the cumultive effects of PR-P on the performnce of C. micns gretly improved with time in the screen house. This study confirms tht Tilemsi PR is n gronomiclly effective source of P for short-durtion legume green mnure (GM) even in the first yer of its ppliction to cid P-deficient soils in the West Africn humid zone. Keywords Acid soils. Crotlri. Phosphte rock. Biologicl nitrogen fixtion (BNF). West Afric E. A. Somdo (*) Afric Rice Center (WARDA), 1 BP 231 Cotonou, Benin, West Afric e-mil: somdo@hotmil.com K. L. Shrwt ICRISAT, Ptncheru, Andhr Prdesh, , Indi R. F. Kuehne University of Goettingen, Institute of Agronomy in the Tropics, Grisebchstr. 6, 3777 Goettingen, Germny Introduction Most trditionl stple food crop-bsed production systems in West Afric re subsistence-oriented, with no or low use of purchsed externl inputs. They rely on extended periods of bush-fllow to regenerte soil fertility nd prevent the build-up of weeds nd other pests. However, this prctice referred to s shifting cultivtion, is no longer vible option becuse of incresing pressure on lnd from growing popultions in the region. Becker nd Assigbé (1995) reported reduction from yers of forest fllow in the 198s to 3 7 yers in the 199s in uplnd ricebsed systems of seven countries in West Afric. Multiyer trils in frmers fields in uplnd rice-growing environments in Côte d Ivoire indicted tht this reduction in fllow length ws ssocited with 2 3% yield reduction nd incresed weed pressure (Becker nd Johnson 1999). Resource-limited frm households re unble to purchse the inputs necessry to reverse yield declines relted to lnd use intensifiction (e.g. N nd P fertilisers). Therefore, identifiction of ffordble cost technologies to improve soil fertility tht result in n increse in crop productivity is likely to be ttrctive nd redily dopted by the resource-limited frmers of the region. The use of N-fixing legumes s preceding short-durtion fllows offers the potentil to sustin food crop yields under intensified lnd use. Becker nd Johnson (1998 nd 1999) showed tht the N-fixing legume C. micns Link (Syn. C. ngyroides Kunth), grown s preceding crop, increses uplnd rice productivity nd suppresses weed growth under intensified lnd use in the humid forest zone of Côte d Ivoire. Also, the study reported tht rice yields fter legume crops were correlted with the legume P uptke. Likewise, synergy between legume-fixed N nd P supply hs been reported (Cssmn et l. 1993), nd ws suggested s mens to increse yields of crops grown in rottion with N-fixing legumes (Kirk et l. 1998). Tin et l. (1998) reported 9% P-induced increse in N ccumultion of tropicl leguminous cover crops grown on Alfisols in Nigeri. Biomss nd N ccumultion of

2 125 Centrosem spp. incresed by 193 nd 259%, respectively, s P ws dded to n cidic P-deficient Oxisol (Cdisch et l. 1992). Applying P from regionlly vilble phosphte rock (PR) to N-fixing legumes my be n ffordble technology to resource-limited frmers to produce lrge mount of N-rich biomss, nd t the sme time enriches the soil through BNF. Lrge deposits of PR exist in West Afric (Buresh et l. 1997), nd their use efficiency cn be improved in P-deficient cid soils encountered in the West Africn humid forest zone (Mokwunye 1995). The objective of this study ws to evlute the effect of indigenous PR ppliction on the contribution of BNF to N ccumultion nd biomss production of short-durtion (8 weeks) preceding cover crops, for sustinble legumefood crop production system. The doption of this prctice by frmers in their frming clendr my be fcilitted by the fct tht preceding legumes re grown for green mnuring in the bsence of other mjor food or csh crops. Reluctnce of frmers to entirely devote their fields to green mnuring t the expense of csh or food crops is well-known (Becker et l. 1995). Mterils nd methods Experimentl sites, soils, plnt mteril nd phosphorus sources Experiments were conducted in in Côte d Ivoire t the min reserch centre of the Afric Rice Center (WARDA) t Mbé (screen-house trils, derived svnnh zone, 7.5 N, 5.1 W, 28 m ltitude), nd t field site in the humid forest zone in Dnné (7.3 N, 8.2 N, 336 m ltitude). The experimentl soils (Ultisols) were cidic (ph ) nd P-deficient (4 mg/kg Bry 1). They included: (1) topsoil ( 2 cm) from field under longrun nturl fllow in Dnné (soil 1) tht ws trnsported in sufficient quntity to Mbé, dried, sieved (<1 mm) nd used for screen-house study; nd (2) n uplnd soil in rice field in Dnné (soil 2). Some physicl nd chemicl chrcteristics of the soils re presented in Tble 1. Tble 1 Physicl nd chemicl properties of soil used in the screenhouse (1) nd field experiments (2) Chrcteristics Screen-house (Mbé) Field (Dnné) Soil order Ultisol Ultisol Texture Lom Cly-lom ph H 2 O (1:2.5) ph KCl Orgnic C ( %) Totl N (%).8.14 Avilble P (Bry 1) mg kg Exch. C (cmol (+) kg 1 ) CEC (cmol (+) kg 1 ) Exch. Acidity (cmol (+) kg 1 ).6.13 Crotlri micns Link (Syn. C. ngyroides Kunth) ws grown s N-fixing legume. This legume plnt is ntive to Southest Asi, nd hs been promoted s promising weed-suppressing green mnure in prts of West Afric (Becker nd Johnson 1998). Cssi obtusifoli L. ws used s non-n 2 -fixing reference plnt for BNF estimtion (Ldh et l. 1993). Tilemsi PR from the neighbouring Mli ws used s P source in the study. This PR hs been shown in certin environmentl conditions to be s effective nd economiclly profitble s the imported wter-soluble triple superphosphte (Btiono et l. 1997). The totl P content of the phosphte smple used in the experiments ws 13.7% P (3% P 2 O 5 ), with n vilble P of 4.2% P 2 O 5 s mesured in neutrl mmonium citrte. The molr PO 4 /CO 3 rtio ws 4.8. Wter-soluble triple super phosphte (TSP) ws used s reference P source in the study, nd it is mnufctured by treting PR with phosphoric cid (H 3 PO 4 ) nd is chrcterised by content of 19.6% wter-soluble P (44% P 2 O 5 ). Plnt culture Legume seeds were scrified for 3 min in concentrted sulphuric cid (commercil grde) to brek dormncy nd chieve high nd even germintion rte. They were then rinsed with tp wter nd ir-dried. To inoculte the legume plnt, fresh nodules of C. micns were collected in nerby field, nd squshed in distilled wter. The scrified seeds were soked overnight in the rhizobil suspension before plnting. Pre-treted seeds were dibble-seeded t density of 1 seeds m 2 (.1.1 m) in both the screen-house nd the field. Fixing nd non-fixing legumes were grown for 8 weeks t the sme spcing. Eight weeks correspond to the time vilble for short-durtion preceding green mnure crop grown in the trnsition period between the dry nd riny sesons, in the bsence of food or csh crops in the field. BNF determintion In the screen house, 15 N-lbeled mmonium sulphte fertiliser (1% tom excess) ws used to estimte the proportion of legume-n ccumulted tht ws derived from BNF. Due to insufficient 15 N-lbeled fertiliser mteril, BNF-N in the field ws mesured, using the δ 15 N nturl bundnce method. In the screen house, ech micro-plot sown to C. micns contined two open-ended PVC pipes (2 cm dimeter nd 4 cm long ech) where the soil hd been previously 15 N-lbelled. The 15 N-lbelled fertiliser ws pplied fter dissolution in 38 ml distilled wter t rte of 1 mg 15 Nm 2 (i.e. 1 kg 15 Nh -1 ) (Hrdrson nd Dnso 199). Ten milliliters of this solution were pipetted into beker nd filled with distilled wter up to the 2-ml mrk. The upper 15 cm of ech PVC-contined soil ws removed nd mixed mnully with the 2 ml 15 N solution before

3 126 returning the soil to the PVC pipe. The enriched mmonium sulphte ws dded only once t the beginning of the screen-house experiment. In the following cropping cycles, we expected some of the 15 N to remin in the vilble soil N pool for BNF estimtion (Preek et l., 199). One legume plnt ws grown in ech 15 N-isotope-lbelled PVC-pipe. Under field conditions, the sme reference plnt ws grown long with C. micns in the unlbelled soil of the plnted plot. Both legumes were hrvested 8 weeks fter sowing t the onset of flowering. Experimentl set-up nd mesurement In the screen house, concrete boxes mesuring m were filled with soil 1 (Tble 1) to depth of 4 cm. A 1-cm grvel lyer underneth the soil nd lterl outlet to the dringe cnl llowed for dringe nd void wter stgntion. Ech of the nine concrete-bsed microplots ws kept constntly erobic t field cpcity. Locl frmers hd repetedly plnted the field to rice. No record of previous fertilistion ws known. During the riny seson preceding the experiment, the field ws uniformly plnted to n unfertilised rice crop. At the beginning of the following experiment, lnd ws hndclered by slshing the vegettion, which ws then removed from the field. Ech plot in the field mesured 5 4 m. Bsl P ws pplied upon plnting of the legumes. Finely ground PR (<.3 mm) ws pplied t 6 kg P h 1 cycle 1 (screen house) nd 9 kg P h 1 (field), nd TSP (in grnulr form) ws brodcst t 6 kg P h 1 (both screen house nd field), nd mnully incorported into the upper 1 cm of the soil. All tretments received t the beginning of the vrious experiments included uniform dose of 1 kg K h 1 s potssium chloride (5% K). No minerl N fertiliser ws supplied. The three tretments (no P, PR nd TSP) were replicted three times nd rrnged in rndomized complete block design. At the onset of flowering, legume shoots were cut t the soil level from n re of.8 m 2 (screen house) or 12 m 2 (field), nd weighed to determine the fresh mtter yield of the legume plnts. Dry weight ws then determined fter oven drying t 7 C for 3 dys. Nodultion of six (screen house), or 15 (field) rndomly selected hills within ech repliction ws evluted t hrvest of the legume crop. The nodulted roots were removed by excvting circulr hole round ech hill. Nodules were then counted. Legume plnts were clered of dhering soil, seprted into stem plus leves nd root smples nd seprtely weighed. The smples were oven-dried nd then sub-smples were tken for totl N nd 15 N nlysis. Yield nd nutrient contents (N nd P) of the legume boveground biomss were determined t hrvest. The legume shoot biomss smples were nlysed for P by digesting the smples with 2:1 (v/v) mixture of concentrted nitric nd perchloric cid. The P concentrtion in the digests ws nlysed by colorimetry following the vndo-molybdte yellow colour method (Oklebo et l. 1993). The oven-dried mteril ws ground further into fine powder using bll-mill. Five-milligrm subsmples were weighed into smll tin cpsules, which were then closed nd rolled into bll. These smples were then nlysed for totl N nd 15 N using n elementl N nlyser connected to mss spectrometer (Reineking et l. 1993). Soil smples were collected t the initition of the trils nd their nlyses were crried out in three replictes using sub-smples from composite smples of five (screen house) nd ten (field) rndomly tken smples per plot. Ech composite smple of 5-g moist soil ws ir-dried nd nlysed for different prmeters. These included prticle size (Gee nd Buder 1986), ph (soil KCl solution rtio of 1:2.5), CEC (Chpmn 1965), sum of bses (Jckson 1967), orgnic C (Oklebo et l. 1993), totl Kjeldhl N, extrctble Bry-1 P nd exchngeble cidity (Oklebo et l. 1993). Clcultions nd sttisticl nlysis The proportion of plnt N, which ws derived from tmosphere (% N df ), ws clculted using the 15 N dilution nd the δ 15 N nturl bundnce methods, s pproprite. 15 N-isotope dilution technique The following, Eq. 1 of Hrdrson nd Dnso (199), ws used to compute %N df. %N df ¼ 1 N fix N ref 1 ; (1) where: N fix is the 15 N tom % excess of the N-fixing plnt, nd N ref the 15 N tom % excess of the non-fixing reference plnt. δ 15 N nturl bundnce method The following, Eq. 2 of Sherer nd Kohl (1986), ws used to clculte %N df. %N df ¼ 1 δ 15 N ref δ 15 N fix δ 15 N ref B 1; (2) where δ 15 N ref is the δ 15 N of the non-fixing reference plnt, δ 15 Nfix is the δ 15 N of the fixing legume plnt nd B the δ 15 N of the fixing plnt grown hydroponiclly on N-free medi. B vlue for Crotlri spp. ws.73% (Ldh et l. 1993) δ 15 N ¼ 1 R smple R irn 2 RirN 2 R ¼ mss29=mss28 ¼ 15 N 14 N 14 N 2

4 127 The mount of totl N fixed from tmosphere ws estimted using the eqution: NðfixedÞ ¼ %N df 1 TNY (3) Totl N yield (TNY) ws clculted from the N concentrtion of the legume shoot nd dry mtter yield: TNY ¼ ðlsn =1ÞDM (4) where: TNY is the totl N yield (kg h 1 ), LSN is the legume shoot N concentrtion (%) nd DM is the dry mtter yield (kg h 1 ) of legume. Dt were nlysed using n nlysis of vrince (ANOVA) procedure of the SAS progrm (SAS 21). Unless otherwise indicted, the probbility level of 5% ws considered sttisticlly significnt. Results Phosphorus pplied in the screen house hd significnt positive effect on totl P uptke by C. micns. The increse ws 2.3 (TSP) nd 1.4-fold (PR) bove the unfertilised control (Tble 2). Phosphorus ccumultion by the legume ws even more impressive in the field when P ws pplied s TSP (8.6-fold) or PR (4.6-fold) ws pplied (Tble 2). Enhnced P uptke by C. micns resulted in significnt increse in legume biomss yield s P fertiliser ws supplied in the field. The increse ws in the rnge of 5.3-fold with TSP nd 4.3-fold with PR bove the no-p tretment (Tble 2). No significnt difference ws observed between the two P sources. The ppliction of PR nd TSP enhnced totl N ccumultion by C. micns grown in the screen house by 51 nd 84%, respectively, over the unfertilised control tretment (Fig. 1). In the field, the increse (p<.1) in legume totl N ccumultion ws 5- (TSP) nd 3.6-fold (PR) over the unfertilised tretment. There ws no C. micns N yield / N fixed (kg h -1 ) C. micns N yield / N fixed (kg h -1 ) % Ndf Screenhouse b b P PR TSP Field N yield N fixed b 62 28b 7 b P PR TSP Field C. micns 1 Tble 2 C. micns shoot dry weight nd P uptke s ffected by P sources in the screen-house (1st cropping cycle) nd field experiments P sources Legume shoot dry weight (kg h 1 ) Legume P uptke (kg h 1 ) Screen-house Field Screen-house Field P 2, PR 2,41 3, TSP 3,219 4, Men 2,543 2, LSD , Probbility P Unfertilised control, PR phosphte rock, TSP triple superphosphte P PR TSP Fig. 1 Legume N yield, mounts of N fixed nd proportion of N derived from tmosphere (%N df ) s ffected by P sources in screen house (1st cropping, 15 N isotope dilution) nd field (δ 15 N method). Brs cpped with the sme letters re not significntly different (LSD, p<.5) significnt difference between the two P fertiliser sources (Fig. 1). The mount nd percent of N fixed (N df ) were estimted by the 15 N-isotope dilution nd the δ 15 N nturl bundnce methods in the screen house nd the field experiments, respectively. Therefore, cution is required in the compr-

5 128 ison of the BNF-N estimtes under the vrious experimentl conditions. BNF-N mesured in the first cropping cycle in the screen house ws inconsistent, low nd not significntly ffected by P fertilistion, irrespective of the P source (Fig. 1). An ttempt ws mde, therefore, to compre tretment effects on N 2 fixtion by exmining the 15 N tom % excess differences in C. micns s n indictor of the reltive differences in N 2 fixtion by the N-fixing plnt induced by P fertilistion. This pproch hs the dvntge of excluding ny imprecision ssocited with the choice of n inpproprite non-fixing reference plnt. The differences in N 2 fixtionthenonlydependonthe reltive 15 N enrichments in the N-fixing crop s influenced by the imposed tretments (P fertilistion). The non-obligtory requirement for reference crop in compring tretment effects my be justifible when the precise quntifiction of N 2 fixtion is not compelling (Dnso et l. 1993). Eqution 1 gives vlid bsis for such methodology. Agin, dt showed no significnt differences in 15 N enrichments in the boveground biomss of the N-fixing legume (C. micns) s result of P fertilistion. The 15 N tom % excess in C. micns ws 2.839, 1.57 nd for the unfertilised control, PR nd TSP tretments, respectively, with LSD.5 vlue of 1,117. For the non-fixing legume (C. obtusifoli), these vlues were 2.429, 1.776, nd 1.84, respectively. In contrst, in the field, the mount of N 2 fixed (kg h 1 ) ws bout ninefold (p<.5) enhnced by TSP nd fourfold by ppliction of PR, s compred to the unfertilised control (Fig. 1). Agin, no significnt difference ws observed between the two P sources. However, phosphorus ppliction did not significntly ffect the percentge of N derived from tmosphere (%N df ) per se, the vlue of which ws <5% (Fig. 1). The cumultive effect of P on the performnce of C. micns is showed in Fig. 2. The ptterns of biomss yield, N df nd N nd P ccumultion during the second nd third cropping cycles showed stedily incresing trend. By the third cropping, legume biomss yield nd N ccumultion in the plots treted with PR stedily cught up with soluble P s P source. Nitrogen fixtion in the second cropping ws not determined becuse crosscontmintion of plnt mterils during sub-smpling for 15 N nlyses ws suspected. Estimtes of the mounts of N fixed (kg h 1 ) by C. micns showed n scendncy (p<.5) over the study period (cropping cycles effect). The pttern in legume totl P ccumultion ws similr to tht for the mount of N fixed. The performnce of the unfertilised C. micns improved over the sme period of study, though t less strong rte s compred with the ppliction of P fertilisers (Fig. 2). Legume shoot DM yield (kg h -1 ) LSD.5 Uplnd Legume N yield (kg h -1 ) LSD.5 Uplnd Legume P uptke (kg h -1 ) 35 Uplnd 3 LSD P P TSP Amounts of N fixed (kg h -1 ) 1 9 Uplnd 8 LSD Fig. 2 Effects of repeted P dditions on legume biomss yield, P nd N ccumultion over three cropping cycles under uplnd conditions in the screen house. LSD.5 vlues re ment to compre P sources over cropping cycles

6 Discussion The 8-week legume growth durtion ws chosen to coincide with the dry-to-wet seson trnsition period before the cropping seson in the humid forest zone of Côte d Ivoire. Adoption of legume-food crop rottions by frmers my be fcilitted by the fct tht the short (8 weeks) cropping cycle of the preceding cover legume would be completed in the bsence of other mjor food crops in the frming clendr. Reluctnce of frmers to devote their fields to green mnuring t the expense of csh or food crops hs been reported (Becker et l. 1995). Neither nodultion nor the percent of N derived from tmosphere (%N df ) per se by C. micns responded to P fertilistion, regrdless of the P source. This result supports suggestions mde by Ankomh et l. (1996) nd Snging et l. (1996) tht the effect of P on N 2 fixtion of rootnodulting legume species ws minly in the totl mount of N fixed rther thn on the percent of N derived from the tmosphere (%N df ) per se. On the other hnd, totl N ccumultion of C. micns ws enhnced s result of improved biomss yield s P ws dded. A reltively high shoot N concentrtion in both the fixing nd the non-fixing legumes (men N content in the boveground mtter ws 3 nd 3.3%, respectively) suggests tht soil N supply ws probbly dequte to meet legumes N requirements. For energetic resons (Grhm nd Ross 1979), uptke of soil minerl N substituted for or my even hve repressed N 2 fixtion (Becker et l. 1986). The trnsloction nd sieving of the experimentl soil might hve stimulted soil microbil ctivity nd N-minerliztion processes. However, no ttempt ws mde to confirm this hypothesis on the experimentl soil. Dt (not shown) on minerl N performed on in situ soil smples could not corroborte this suggestion. It is possible tht the suspected enhnced microbil ctivity s result of the soil trnsloction nd sieving might not hve tken plce in situ. Furthermore, it cnnot be excluded tht rhizobil seed inocultion with indigenous isoltes from West Afric ws inefficient, s C. micns origintes from Southest Asi (Becker, personl communiction) nd hs so fr not been cultivted in West Afric (no efficient rhizobil strins were present in the soil). It is noteworthy tht by the third consecutive legume cropping, N 2 fixtion incresed on verge from 9 to 36 kg h 1 or 12 to 33% N df (Fig. 2). This my be relted to the dpttion or the build-up of the rhizobil popultion in the soil with time. Results (Fig. 2) suggest tht the effect of PR on the performnce of C. micns ws improved with time nd PR effectiveness ws equl to the soluble P source by the third cropping cycle. A similr high effectiveness of pplied PR-P in reltion to soluble P ws reported from mucun trils in Vietnm (Ng Thi Tsiung 1993) nd with soybens in Indonesi (Wjnnwt 1993). Our results could be explined by exmining the chemicl sttus of the soil in which the legume crop ws grown. The exchngeble cidity (Al 3+ +H + ) of the soil ws low nd less thn 2.5% of CEC (Tble 1). Thus, P-fixtion my not hve been n importnt mechnism ffecting PR-P vilbility in this soil. The importnce of P-fixtion in most West Africn cid Ultisols my hve been lrgely overestimted (Mokwunye nd Hmmond 1992). In fct, these soils hve low cpcity for P dsorption nd, frequently, rther low Al sturtion (Buresh et l. 1997). The improvement in the performnce of the unfertilised legume crop with the successive cropping cycles (Fig. 2) might be the result of dditionl phosphte ions relesed from the decomposition of incorported legume residues. De Swrt nd vn Diest (1987) demonstrted tht in cid soil, solubiliztion of Tilemsi PR (the sme mteril ws used in the present study) proceeds rpidly enough to supply sufficient P to young plnts of Puerri jvnic. This my explin the observed positive response of the legume biomss yield to pplied PR-P, which ws sttisticlly similr to tht of TSP (Tble 2). Although the %N df by C. micns, s mesured in the field (Fig. 2) ws not significntly ffected by ppliction of PR-P, it ws similr (28%) to the 36% reported by Becker nd Johnson (1998) using the sme mesurement method in the humid forest zone in Côte d Ivoire. Conclusion Under P-deficient cid soil conditions in the West Africn humid forest zone of Côte d Ivoire, significnt positive response to rock-p ppliction ws observed for biomss nd N ccumultion by the root-nodulting N-fixing C. micns. The significnt increse in the totl mount of N ccumulted by C. micns ws the effect of higher biomss yield response to PR-P ppliction. This study suggests tht C. micns, when grown s preceding legume crop, cn meet the N demnd of the subsequent food crop, provided it is fertilised with phosphorus. The indigenous unprocessed Tilemsi rock-p from Mli ppers to be n effective source in this regrd. Acknowledgements The present study ws prt of collbortive reserch project between the Afric Rice Center (WARDA), Côte d Ivoire nd the Göttingen University s Institute of Agronomy in the Tropics, Germny. The finncil nd logistic support of both institutions is highly pprecited. The fellowship grnted by the Deutscher Akdemischer Austusch Dienst (DAAD) to Eklou A. Somdo during the reserch work is lso cknowledged. References 129 Ankomh AB, Zpt F, Hrdrson G, Dnso SKA (1996) Yield, nodultion, nd N2 fixtion by cowpe cultivrs t different phosphorus levels. Biol Fertil Soils 22:1 15 Btiono A, Ayuk E, Bllo D, Koné M (1997) Agronomic nd economic evlution of Tilemsi phosphte rock in different groecologicl zones of Mli. Nutr Cycl Agroecosyst 48: Becker M, Assigbé P (1995) Rice-bsed cropping systems reserch in West Afric. In: Cheneu-Loquy A, Leplideur A (eds) Quel venir pour les rizicultures de l Afrique de l Ouest. Proc. Int. Coll., 4 7 April 1995, Bordeux, Frnce. 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