Soil amelioration and root symbioses ofParkia biglobosa (Jacq.) Benth. in West Africa

Parkia biglobosa is an important multipurpose tree from the savanna zone of West Africa. It has been reported to increase soil fertility and crop yields beneath its crown. However, no work has been conducted to determine the role of root symbioses in soil amelioration by this species. The existing reports of nodulation inParkia biglobosa are contradictory and the presence of mycorrhizae is not documented in the literature.Fieldwork was conducted at six sites in Burkina Faso and Nigeria, to investigate the amelioratory effect ofParkia biglobosa on soil fertility and to ascertain the occurence of root symbioses. Soil samples were collected from points close to the trunk, the mid-crown, crown edge and the open areas away from individual trees. The samples were analysed chemically to assess pH, organic matter, nitrogen, phosphorus and potassium content. Root samples were also collected from the field and examined for nodules and for evidence of mycorrhizal infection.The results of the soil analysis showed a significant amelioration for total nitrogen and available potassium with proximity to the tree, but organic matter, available phosphorus and soil pH showed no significant trend. This pattern was similar for all sites. The results also indicated that amelioration of nitrogen and potassium with proximity to the tree increased with tree size.Root nodules were absent from the roots of trees examined in the field, both in Burkina Faso and Nigeria. A pot experiment was set up using the soil collected from beneath the trees and a laboratory grown cowpea miscellany culture as rhizobial inoculum forParkia biglobosa seedlings. The roots of these seedlings also showed no evidence of nodulation. Ectomycorrhizae were not present in roots collected from the field, but endomycorrhizal infection was prolific.A study of root distribution in Burkina Faso suggested that one mechanism for increased soil fertility beneathParkia biglobosa is the widespread lateral root system, which redistributes nutrients from an extensive radius to a concentrated area beneath the crown. The root system is aided in the efficient uptake of nutrients by endomycorrhizae. Other possible mechanisms include interception of dust by the crown and deposition by throughfall and stemflow, defacation by animals beneath the tree and decomposition of leaf litter.