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Titre : Nutritional characterization of the rhizosphere of symbiotic cowpea and maize plants in different cropping system

Auteur : Ndakidemi Patrick Alois

Université de soutenance : Cape Peninsula University of Technology

Grade : DOCTORAL Degree in Chemistry 2005

Résumé
A 2-factorial experiment, involving 3 levels of phosphorus (0, 40, and 80 kg P.ha-I ) as main treatment and 4 cropping systems (mono crop, maize/cowpea inter-row, maize/cowpea intra-row, and maize/cowpea intra-hole cropping) as sub-treatment was conducted in the field for 2 consecutive years in 2003 and 2004 to assess i) the effects of exogenous P supply and cropping system on the concentrations of plant-available nutrients in the rhizosphere of cowpea and maize ; ii) the effect of exogenous P supply on tissue concentrations of minerals in nodulated cowpea and maize in mixed plant cultures iii) the effects of exogenous P supply and cropping system on plant growth and N2 fixation, and iv) the effects of exogenous P supply and cropping system on phosphatase activity and microbial biomass in the rhizosphere of cowpea and maize. At harvest, it was found that applying 40 or 80 kg P.ha-I significantly increased cowpea grain yields by 59-65% in 2003 and 44-55% in 2004. With maize, the increases in grain yield were 2037% in 2003 and 48-55% in 2004 relative to zero-P control. In both cropping seasons, the number of pod-bearing peduncles per plant, the number of pods per plant, the number of seeds per pod, and seed yield per cowpea plant were significantly increased with the application of exogenous P. In contrast, these parameters were all significantly depressed by mixed culture relative to mono crop cowpea. Intercropping maize with cowpea produced higher total yields per unit land area than the sole crop counterpart. Supplying 40 or 80 kg P.ha-I to cowpea and maize significantly decreased the soil pH, increased soil organic matter and concentrations of extractable P, Ca, Mg, S, Na, Zn, and Fe in rhizosphere soil in 2003 and 2004. However, the concentrations of P, K, Ca, Mg, S, Zn, Cu, Mn, S, and Fe were significantly lower in the rhizosphere soil of cowpea in 2003 relative to maize, while in 2004 only P, K, Mg, Zn, Mn and Fe were decreased in the legume’s rhizosphere compared with the cereal. There was no effect of cropping system on the rhizosphere pH of cowpea and maize, except for intra-hole planted cowpea, where pH was higher relative to the other cropping systems. In both Year I and 2, P application significantly increased the concentrations of P, Mg, Ca and K in shoots and pods of cowpea but decreased those of Zn, Cu and B. P supply also increased P, K, S and Na in roots, but decreased Zn, Cu, Mn and B in the two years of study. Similarly, P elevated nodule concentrations ofP, Mg and S, but decreased those ofZn, Cu, Mn and Al in Year I, just as nodule P and S-increased in Year 2 with a decrease in Zn concentration. At whole-plant level, tissue concentrations of P, Ca and S rose with exogenous P supply, while those of Zn, Cu and B were decreased. There was also a significant increase in the concentrations ofP, Ca, Mg, K, S, Na, Cu, Zn, Mn, Fe and Al in shoots, roots and whole plants of the (cowpea) legume relative to the (maize) cereal in the both years of study. The cropping system also differentially affected nutrient concentrations and amounts in the test species. For example, shoot levels of P, K, Cu and B in cowpea were significantly greater in monoculture compared with mixed cultures in both years of experimentation. Furthermore, the co-planting ’ :’f cowpea and maize in one hole increased shoot Fe (Year 1), Mg, Ca and B (Year 2), as well as whole-plant Fe and B (Year I) compared with either monoculture or the other mixed cultures. Exogenous supply of P to cowpea plants numerically, but not significantly, increased growth of all organs and whole plants in Year 1. However, in Year 2, shoots, pods, roots, nodules and whole-plants of cowpea were significantly increased with exogenous P application at either 40 kg or 80 kg P.ha-I relative to zero-P control. Applying mineral P to maize also significantly increased growth of shoots and whole plants in both Year I and 2 relative to zero-P control. The dry matter yield of organs and whole plants of cowpea was numerically, but not significantly, decreased in mixed culture relative to monoculture in Year 1. However, in Year 2, biomass of shoots, pods, roots, nodules and whole plants were markedly decreased in mixed culture relative to monoculture. But with maize, plant growth was not affected by planting pattern in both Year I and 2. Intercropping increased N concentrations in whole plants of maize relative to sole culture in Year I. Cowpea was unaffected in Year 1. In Year 2, mixed culture, especially intra-hole planting, significantly increased N concentrations in roots and whole plants of cowpea relative to monoculture. Similarly, intercropping increased N concentrations in shoots and whole plants of maize relative to sole culture. Isotope analysis showed that the 1)1’N values of intercropped cowpea shoots, pods, roots, nodules and whole plants were much lower when compared to those in monoculture. The mixed culture, especially intra-hole planting, also markedly decreased the 1) I’N values of shoots, roots and whole plants of maize relative to those in sole culture. As a result, the Ndfa of shoots, pods and whole cowpea plants were increased with intercropping relative to monoculture. Not only was the % Ndfa in shoots, pods, roots and whole cowpea plants increased by mixed culture relative to sole culture, but also the magnitude ofthis increase was greater in Year 2 compared to Year I. Applying exogenous P to the cropping system decreased the alkaline phosphatase activity in the rhizosphere soil of cowpea and maize in Year 1. The acid phosphatase activity was however not significant for both species in Year 1. The provision of P also decreased both acid and alkaline phosphatase activity in the rhizosphere of both cowpea and maize especially at the 80 kg P.ha-1 in Year 2. Although the cropping system had no effect on the acid and alkaline phosphatase activity in the rhizosphere of both cowpea anda maize in Year I, acid phosphatase activity in the rhizosphere of cowpea was significantly increased by mixed culture, especially in the intra-hole planting, relative to monoculture in Year 2. Alkaline phosphatase activity was also significantly higher in the rhizosphere of inter-row planted cowpea relative to monoculture.

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