Utilization of rock phosphate by crops on a representative toposequence inthe Northern Guinea savanna zone of Nigeria: response by Mucuna pruriens, Lablab purpureus and maize

The availability of P from rock phosphate (RP) is often too low to demonstr ate an immediate impact on cereal production. Legumes may improve the immed iate availability of P from RP and eventually benefit subsequent maize crop s. The ability of Mucuna pruriens (L.) var utilis (Wright) Burck and Lablab purpureus L. to use P from RP and the changes in selected plant and symbio tic properties and in the soil available P and particulate organic matter ( POM) pool as affected by the addition of RP were measured for a set of soil s on a representative toposequence ('plateau', 'slope' and 'valley' field) in the Northern Guinea savanna zone of Nigeria. At 18 weeks after planting (WAP), Mucuna accumulated significantly more N and P in the total biomass i n the plots treated with RP compared to the plots without RP addition on al l fields. Nitrogen accumulation of Mucuna reached 175, 177 and 164 kg N ha( -1) in the treatments with RP on the 'plateau', 'slope' and 'valley' fields , respectively. Phosphorus accumulation of Mucuna was highest at 18 WAP in all sites and reached 10, 14 and 10 kg P ha(-1) in the treatments with RP o n the 'plateau', 'slope' and 'valley' fields, respectively. Lablab accumula ted significantly more N and P at 18 WAP only on the 'plateau' field, but s ome of the potential differences in N or P accumulation may have been maske d by various pests especially affecting Lablab. P,highly significant negati ve correlation was observed between the aboveground biomass at 16 WAP and t he nematode population. The addition of RP significantly increased arbuscul ar mycorrhizal fungi (AMF) infection of the Mucuna (from 24 to 33%) and Lab lab roots (from 15 to 28%) to a similar extent in all fields. This increase d AMF infection was most likely caused by specific processes in the rhizosp here of the legumes as AMF infection of the maize roots (8%) was not affect ed by RP addition. Increases in nodule numbers and fresh weight were site- and species-specific and highest for the 'plateau' and 'slope' fields. The number of nodules increased on average from 8 to 19 (3 plants)(-1) and from 7 to 30 (3 plants)(-1) for Mucuna and Lablab, respectively, after RP addit ion. Although nearly all the aboveground legume biomass had disappeared fro m the soil surface at 51 WAP, both the Olsen-P status and POM N concentrati on were increased by the presence of legumes. Mucuna significantly enhanced the Olsen-P content of the soil after RP addition compared to the Lablab o r maize treatments on the 'plateau' and 'valley' fields. Due to the relativ ely high initial Olsen-P content of the 'slope' field (14 mg kg(-1)), diffe rences between treatments were not significant. The N concentration of the POM pool was significantly higher under legumes than under maize on the 'sl ope' and 'valley' fields, and indicates incorporation of part of the legume biomass in the POM pool. The addition of RP to herbaceous legumes was obse rved to lead to site- and species-specific changes in the tripartite legume -rhizobium-mycorrhizal fungus, driven by processes taking place in the rhiz osphere of the legumes, and in the soil available P pool. A cereal followin g these herbaceous legumes could benefit from this improvement in soil fert ility status. (C) 2000 Elsevier Science Ltd. All rights reserved.