Utilization of rock phosphate by crops on a representative toposequence in the 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 demonstrate an immediate impact on cereal production. Legumes may improve the immediate availability of P from RP and eventually bene®t subsequent maize crops. 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 symbiotic 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 soils on a representative toposequence (`plateau', `slope' and `valley' ®eld) in the Northern Guinea savanna zone of Nigeria. At 18 weeks after planting (WAP), Mucuna accumulated signi®cantly more N and P in the total biomass in the plots treated with RP compared to the plots without RP addition on all ®elds. Nitrogen accumulation of Mucuna reached 175, 177 and 164 kg N ha in the treatments with RP on the `plateau', `slope' and `valley' ®elds, respectively. Phosphorus accumulation of Mucuna was highest at 18 WAP in all sites and reached 10, 14 and 10 kg P ha in the treatments with RP on the `plateau', `slope' and `valley' ®elds, respectively. Lablab accumulated signi®cantly more N and P at 18 WAP only on the `plateau' ®eld, but some of the potential differences in N or P accumulation may have been masked by various pests especially affecting Lablab. A highly signi®cant negative correlation was observed between the aboveground biomass at 16 WAP and the nematode population. The addition of RP signi®cantly increased arbuscular mycorrhizal fungi (AMF) infection of the Mucuna (from 24 to 33%) and Lablab roots (from 15 to 28%) to a similar extent in all ®elds. This increased AMF infection was most likely caused by speci®c processes in the rhizosphere of the legumes as AMF infection of the maize roots (8%) was not affected by RP addition. Increases in nodule numbers and fresh weight were siteand species-speci®c and highest for the `plateau' and `slope' ®elds. The number of nodules increased on average from 8 to 19 (3 plants) and from 7 to 30 (3 plants) for Mucuna and Lablab, respectively, after RP addition. Although nearly all the aboveground legume biomass had disappeared from the soil surface at 51 WAP, both the Olsen-P status and POM N concentration were increased by the presence of legumes. Mucuna signi®cantly enhanced the Olsen-P content of the soil after RP addition compared to the Lablab or maize treatments on the `plateau' and `valley' ®elds. Due to the relatively high initial Olsen-P content of the `slope' ®eld (14 mg kg), differences between treatments were not signi®cant. The N concentration of the POM pool was signi®cantly higher under legumes than under maize on the `slope' and `valley' ®elds, and indicates incorporation of part of the legume biomass in the POM pool. The addition of RP to herbaceous legumes was observed to lead to siteand species-speci®c changes in the tripartite legume±rhizobium± mycorrhizal fungus, driven by processes taking place in the rhizosphere of the legumes, and in the soil available P pool. A cereal following these herbaceous legumes could bene®t from this improvement in soil fertility status. q 2000 Elsevier Science Ltd. All rights reserved.